**1. Introduction**

The shoulder complex is constituted with some joint. It is understood that glenohumeral joint (GHJ) and scapulothoracic joint (STJ) are important for shoulder complex and they make up a large fraction of shoulder movement. Especially, scapulothoracic joint has been focused recently. It was reported that patients with shoulder disease show abnormal scapular move‐ ment and scapular muscle activation during shoulder motion, so to normalize the scapular movement, it is important to improve the symptom [1–3]. Clinically, the therapist usually pre‐ scribes the scapular exercise to the patients. So, we will be able to provide the more effective rehabilitation program if we understand more about the scapulothoracic joint. Some func‐ tions of the scapula were reported in previous studies. These include contributing to the range of shoulder motion synchronizing the motion of humerus, supporting the humerus, keeping the muscle tension, maintaining the appropriate muscle length on glenohumeral joint, and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

getting off the impingement raising the glenoid fossa [4]. However, not only these, scapulo‐ thoracic joint is also related to the "exertion of muscle power" on shoulder joint, so we have to consider keeping rehabilitation in mind. However, there remain some ambiguities regarding the function of scapular muscle. In previous studies, scapular muscles with high activation during specific exercises for scapular muscles [5, 6] have been reported. And it was reported that the combination of standard rehabilitation interventions and scapulothoracic joint con‐ trol training exercises were effective treatments for the shoulder joint dysfunction [7, 8]. But the change of shoulder muscle strength by only one scapular muscle exercise is not examined in exact detail, not by a number of exercises. In other words, the evidence is not enough to choose the best exercise from a number of exercises for each patient. Additionally, it is not revealed how the mechanism runs for scapular stability. Wherefore, which exercise is the best for the improvement of scapular movement for each patient remains a matter of research. And so, I suggest some studies in this section to understand the function of scapulothoracic joint. From this, I expect upgrading your ability about the clinical rehabilitation.

## **2. The study**

#### **2.1. The changes of muscle strength on shoulder joint after strengthening of serratus anterior**

The muscle weakness is usually involved in the rehabilitation of patients with shoulder disease. So, we have to have the skill to improve this. The reason for muscle weakness on shoulder joint is not only due to glenohumeral joint like the weakness on deltoid and rotator cuff muscle but due to scapular instability with scapular muscle weakness. The patient with scapular instability usually have "winging scapula." The winging scapula was also caused by long thoracic nerve paralysis and the patients with this have weakness of shoulder muscle strengths. Serratus anterior innervated by long thoracic nerve is scapu‐ lar protractor muscle and it plays a prominent role in scapular stability in consort with trapezius. Clinically, the exercise for serratus anterior has an effect on improved strength on shoulder joint. But the study which investigated how the change in the muscle strength on shoulder joint by the strengthening for serratus anterior is modest in size. Wherefore, I examined the changes of muscle strength on shoulder joint after the exercise for serratus anterior.

#### *2.1.1. Materials and methods*

Thirty‐six healthy young men who had no orthopedic and neurological abnormality partici‐ pated in this study. The subjects were separated into two groups randomly, one was the train‐ ing group where members were exercised for serratus anterior, and the other was the control group where members did not exercise. The mean anthropometric characteristics ± standard deviation (SD) of the training group were age, 22.1 ± 1.5 years; height, 172.2 ± 6.5 cm; and weight, 62.7 ± 8.7 kg. Those of the control group were age, 21.6 ± 1.5 years; height, 171.4 ± 5.6 cm; and weight, 62.4 ± 6.3 kg.

First, the subjects in the training group and control group were measured for muscle strength. Then, in the training group, they performed exercise every day for a week. The exercise was established at the modified elbow pushup plus, which was known as the typical exercise for serratus anterior. In control group, they spent without exercise. After 1 week, they were mea‐ sured for the muscle strength in common for the first time.

The measurement of muscle strength was composed by the muscle strengths of scapular pro‐ traction, shoulder flexion, abduction, external rotation, and internal rotation. The measure‐ ment was done using handheld‐dynamometer (MicroFET2, Hoggan Health Industries, Inc., USA). Regarding the muscle strength of shoulder flexion, abduction, external rotation, and internal rotation, the measurement values (*N*) were multiplied by the lever arm length (m), and the muscle strength was represented by torque values (Nm). The measurement position of shoulder flexion and abduction was the same as manual muscle testing, and that of shoul‐ der external and internal rotation was at 0° abduction. The measurement position of scapular protraction was defined at supine and 90° shoulder flexion and 90° elbow flexion (**Figure 1**). From this position, the subjects thrusted their upper limb vertically upwards. The measurer measured the power by handheld ‐ dynamometer on olecranon. The measurement of each muscle strength was done three times and the average was adopted as the muscle strength value.

The exercise was established at the modified elbow pushup plus (**Figure 2**). The pushup plus indicated high serratus anterior activity by Decker et al. and Moseley et al. [9, 10]. The basic position of this exercise was on elbow, and the femoral area was grounded. Then, the subjects moved their scapula to maximum scapular retraction position like moving their body close to the floor. And they moved their scapular to the maximum scapular protraction position like getting their body away from the floor. Additionally, they returned to the maximum scapular retraction position and this process was defined one time. The subjects in the training group performed this exercise in three sets: one set was 20 times, each day for 1 week.

Each muscle strength for the first time was normalized as 100% and the muscle strengths after 1 week were computed as percentage. The muscle strengths between the first time and after 1 week were compared for each group using paired *t*‐test.

#### *2.1.2. Results*

getting off the impingement raising the glenoid fossa [4]. However, not only these, scapulo‐ thoracic joint is also related to the "exertion of muscle power" on shoulder joint, so we have to consider keeping rehabilitation in mind. However, there remain some ambiguities regarding the function of scapular muscle. In previous studies, scapular muscles with high activation during specific exercises for scapular muscles [5, 6] have been reported. And it was reported that the combination of standard rehabilitation interventions and scapulothoracic joint con‐ trol training exercises were effective treatments for the shoulder joint dysfunction [7, 8]. But the change of shoulder muscle strength by only one scapular muscle exercise is not examined in exact detail, not by a number of exercises. In other words, the evidence is not enough to choose the best exercise from a number of exercises for each patient. Additionally, it is not revealed how the mechanism runs for scapular stability. Wherefore, which exercise is the best for the improvement of scapular movement for each patient remains a matter of research. And so, I suggest some studies in this section to understand the function of scapulothoracic joint.

From this, I expect upgrading your ability about the clinical rehabilitation.

**2.1. The changes of muscle strength on shoulder joint after strengthening of serratus** 

The muscle weakness is usually involved in the rehabilitation of patients with shoulder disease. So, we have to have the skill to improve this. The reason for muscle weakness on shoulder joint is not only due to glenohumeral joint like the weakness on deltoid and rotator cuff muscle but due to scapular instability with scapular muscle weakness. The patient with scapular instability usually have "winging scapula." The winging scapula was also caused by long thoracic nerve paralysis and the patients with this have weakness of shoulder muscle strengths. Serratus anterior innervated by long thoracic nerve is scapu‐ lar protractor muscle and it plays a prominent role in scapular stability in consort with trapezius. Clinically, the exercise for serratus anterior has an effect on improved strength on shoulder joint. But the study which investigated how the change in the muscle strength on shoulder joint by the strengthening for serratus anterior is modest in size. Wherefore, I examined the changes of muscle strength on shoulder joint after the exercise for serratus

Thirty‐six healthy young men who had no orthopedic and neurological abnormality partici‐ pated in this study. The subjects were separated into two groups randomly, one was the train‐ ing group where members were exercised for serratus anterior, and the other was the control group where members did not exercise. The mean anthropometric characteristics ± standard deviation (SD) of the training group were age, 22.1 ± 1.5 years; height, 172.2 ± 6.5 cm; and weight, 62.7 ± 8.7 kg. Those of the control group were age, 21.6 ± 1.5 years; height, 171.4 ± 5.6

**2. The study**

2 Clinical Physical Therapy

**anterior**

anterior.

*2.1.1. Materials and methods*

cm; and weight, 62.4 ± 6.3 kg.

The result is shown in **Table 1**. Representative value is as follows. The muscle strength of scapular protraction in the right side was 113 ± 14%, shoulder flexion 106 ± 9%, abduction 110 ± 10%, external rotation 104 ± 8%, and internal rotation 112 ± 12% in the training group. In the control group, scapular protraction 99 ± 4%, shoulder flexion 101 ± 6%, abduction 100 ± 5%, external rotation 101 ± 5%, and internal rotation 101 ± 6%. The same tendency was indicated in the left side on both groups.

In the training group, the muscle strengths of scapular protraction, shoulder flexion, shoulder abduction, and internal rotation after 1 week were significantly higher than the first time on both sides. The muscle strength of shoulder external rotation was not increased. On the other hand, the muscle strength in the control group was not changed after 1 week.

**Figure 1.** The measurement position of scapular protraction. The measurement position of scapular protraction was defined at supine and 90° shoulder flexion and 90° elbow flexion. From this position, the subjects thrusted their upper limb for vertical upward.

#### *2.1.3. Discussion*

In this study, the muscle strength of scapular protraction was higher after 1 week in the train‐ ing group. So, the exercise was effective for serratus anterior. And it was indicated that the exercise for serratus anterior can increase the shoulder muscle strength, but the direction is

**Figure 2.** The modified elbow pushup plus. The basic position of this exercise was on elbow, and femoral area was grounded. They moved their scapular to maximum scapular protraction position like getting their body away from the floor.

not all limited. We have to gain consciousness of this fact. In the rehabilitation of shoulder dis‐ ease patients, we usually assess the scapular position and scapular movement. If the therapist infers that abnormal scapular position and movement encumber the normal motion, then we will treat for the scapular abnormality. Additionally, the reason for the scapular abnormality is assessed in the impairment of functional level. We assess the scapular muscle strength, like manual muscle testing for scapula. In this assessment, even if the weakness of serratus ante‐ rior is found, it is not always effective for the chief complaint motion. From this study, serratus anterior must influence the motion which consists of shoulder flex, abduction, and internal rotation, but does not influence the motion which does not consist of their movements. So, we have to consider the task characteristically and calculate the weakness of serratus anterior


**Table 1.** The muscle strength after 1 week.

*2.1.3. Discussion*

4 Clinical Physical Therapy

limb for vertical upward.

In this study, the muscle strength of scapular protraction was higher after 1 week in the train‐ ing group. So, the exercise was effective for serratus anterior. And it was indicated that the exercise for serratus anterior can increase the shoulder muscle strength, but the direction is

**Figure 1.** The measurement position of scapular protraction. The measurement position of scapular protraction was defined at supine and 90° shoulder flexion and 90° elbow flexion. From this position, the subjects thrusted their upper for improved motion. As a matter of fact, the relationship between scapular stability and each scapular muscle strength remains unclear, and the therapist often confuses oneself. Therapists usually believe that one scapular muscle can contribute to scapular stability of all motion. However, this is not true. As previously noticed, therapists need to consider the movement direction of the task and make a decision about the muscle's influence to the task. This deci‐ sion is important.

The reason that the exercise for serratus anterior led to an increase in the muscle strength of shoulder flexion, abduction, and internal rotation will relate to interlocking between humerus and scapula. Shoulder joint is constituted with glenohumeral joint and scapu‐ lothoracic joint and so on. Every shoulder motion involves scapular movement. Shoulder flexion includes scapular upward rotation and posterior tilt, shoulder abduction includes scapular upward rotation, and internal rotation includes scapular protraction (**Figure 3**). The function of serratus anterior is scapular upward rotation, protraction, and posterior tilt, so the scapular movements of shoulder flexion, abduction, and internal rotation, where muscle strengths were increased in this study, agree with the functions of serratus anterior. Wherefore, I conclude that the increasing of serratus anterior muscle strength conduced the increasing of these shoulder muscle strengths. On the other hand, shoulder external rotation includes scapular retraction, which does not agree with the function of serratus anterior. Therefore, I suspect that the muscle strength of shoulder external rotation did not increase in this study.

In this study, it is revealed that the muscle strength at scapulothoracic joint influences the muscle strength at shoulder complex, but not all movement directions. The muscle strength of the only shoulder movement direction having the scapular movement agrees with the function of strengthened muscle which will be increased. From this reflection, if the scapular retractor, like middle trapezius, was strengthened, the muscle strength of shoulder external

**Figure 3.** Scapular movement of each shoulder motion. Shoulder flexion includes scapular upward rotation and posterior tilt, shoulder abduction includes scapular upward rotation, and internal rotation includes scapular protraction. These scapular movements agree in the functions of serratus anterior.

rotation will be increased because external rotation at 0° abduction is associated with scapular retraction.

#### **2.2. The changes of muscle strength on shoulder joint after strengthening of scapular retractor muscles**

In a previous study, the muscle strengths of shoulder flexion, abduction, and internal rotation were increased after exercise for serratus anterior. And it was thought that the reason for this change must be related to the scapular movement during each motion. Contrary to previous study, if the exercise for scapular retractor was done, the muscle strength of external rotation will be increased. So, I examined the change of muscle strength on shoulder joint after exercise for scapular retractor, mainly on the middle trapezius.

#### *2.2.1. Materials and methods*

for improved motion. As a matter of fact, the relationship between scapular stability and each scapular muscle strength remains unclear, and the therapist often confuses oneself. Therapists usually believe that one scapular muscle can contribute to scapular stability of all motion. However, this is not true. As previously noticed, therapists need to consider the movement direction of the task and make a decision about the muscle's influence to the task. This deci‐

The reason that the exercise for serratus anterior led to an increase in the muscle strength of shoulder flexion, abduction, and internal rotation will relate to interlocking between humerus and scapula. Shoulder joint is constituted with glenohumeral joint and scapu‐ lothoracic joint and so on. Every shoulder motion involves scapular movement. Shoulder flexion includes scapular upward rotation and posterior tilt, shoulder abduction includes scapular upward rotation, and internal rotation includes scapular protraction (**Figure 3**). The function of serratus anterior is scapular upward rotation, protraction, and posterior tilt, so the scapular movements of shoulder flexion, abduction, and internal rotation, where muscle strengths were increased in this study, agree with the functions of serratus anterior. Wherefore, I conclude that the increasing of serratus anterior muscle strength conduced the increasing of these shoulder muscle strengths. On the other hand, shoulder external rotation includes scapular retraction, which does not agree with the function of serratus anterior. Therefore, I suspect that the muscle strength of shoulder external rotation did not increase

In this study, it is revealed that the muscle strength at scapulothoracic joint influences the muscle strength at shoulder complex, but not all movement directions. The muscle strength of the only shoulder movement direction having the scapular movement agrees with the function of strengthened muscle which will be increased. From this reflection, if the scapular retractor, like middle trapezius, was strengthened, the muscle strength of shoulder external

**Figure 3.** Scapular movement of each shoulder motion. Shoulder flexion includes scapular upward rotation and posterior tilt, shoulder abduction includes scapular upward rotation, and internal rotation includes scapular protraction. These

scapular movements agree in the functions of serratus anterior.

sion is important.

6 Clinical Physical Therapy

in this study.

Thirty healthy men participated in this study. These subjects were divided into two groups, training group and control group, randomly. The mean anthropometric characteristics of 15 subjects in the training group were age, 23.3 ± 2.8 years; height, 172.1 ± 5.8 cm; and weight, 65.7 ± 8.5 kg. Those of the control group were age, 22.5 ± 1.6 years; height, 170.7 ± 5.3 cm; and weight, 63.7 ± 11.5 kg. There were no significant differences between two groups regarding anthropometric characteristics.

The subjects in the training group and control group were measured for muscle strength on the first day. Then, in the training group, they performed exercise every day for a week. The exercise was scapular retraction exercise at 0° shoulder abduction. In the control group, they spent without the exercise. After 1 week, they were measured for the muscle strength in com‐ mon for the first time.

The muscle strengths on scapulothoracic retraction, shoulder flexion, abduction, external rota‐ tion, and internal rotation were measured. The measurement of muscle strength on shoulder flexion, abduction, external rotation, and internal rotation was the same as above‐referenced previous study. The measurement of muscle strength on scapular retraction was on prone. The subject performed scapular retraction at 90° shoulder abduction and 90° elbow flexion (**Figure 4**). At that time, the muscle strength was measured by resisting their movement at the point of 3 cm inside from the axilla using handheld‐dynamometer (MicroFET2, Hoggan Health Industries, Inc., USA).

The exercise was scapular retraction at 0° shoulder abduction consulted from the report by Andrews and Wilk [11] (**Figure 5**). The subjects performed scapular retraction with shoulder internal rotation for preventing to strengthen external rotator muscles by external rotation accompanied by scapular retraction. And they returned to neutral position. This process was defined one time. The subjects in the training group performed this exercise in three sets: one set was 20 times, each day for 1 week.

Each muscle strength for the first time was normalized as 100% and the muscle strengths after 1 week were computed as percentage. The muscle strengths between the first time and after 1 week were compared for each group using paired *t*‐test.

**Figure 4.** The measurement position of scapular retraction. The subject performed scapular retraction at 90° shoulder abduction and 90° elbow flexion. At that time, the muscle strength was measured by resisting their movement at the point of 3 cm inside from the axilla using handheld‐dynamometer.

**Figure 5.** Scapular retraction exercise. The subjects performed scapular retraction with shoulder internal rotation for preventing to strengthen external rotator muscles by external rotation accompanied by scapular retraction. And they returned to neutral position.

#### *2.2.2. Results*

The results are shown in **Table 2**. The muscle strength of scapular retraction in the right side was 106 ± 8%, shoulder flexion 101 ± 4%, abduction 103 ± 6%, external rotation 105 ± 6%, and inter‐ nal rotation 103 ± 7% in the training group. In the control group, scapular retraction 101 ± 9%, shoulder flexion 100 ± 6%, abduction 101 ± 5%, external rotation 101 ± 5%, and internal rotation 100 ± 6%. The same tendency was indicated in the left side on both groups.

In the training group, the muscle strengths of scapular retraction and shoulder external rotation after 1 week were significantly higher than the first time on both sides. The muscle strengths of shoulder flexion, abduction, and internal rotation were not increased. On the other hand, the muscle strength in control group was not changed after 1 week.

#### *2.2.3. Discussion*

**Figure 4.** The measurement position of scapular retraction. The subject performed scapular retraction at 90° shoulder abduction and 90° elbow flexion. At that time, the muscle strength was measured by resisting their movement at the

**Figure 5.** Scapular retraction exercise. The subjects performed scapular retraction with shoulder internal rotation for preventing to strengthen external rotator muscles by external rotation accompanied by scapular retraction. And they

point of 3 cm inside from the axilla using handheld‐dynamometer.

returned to neutral position.

8 Clinical Physical Therapy

In this study, the exercise for middle trapezius led to an increase in the muscle strength of scapular retraction and shoulder external rotation. From this result, the hypothesis was proved to be correct. The reason regarding increasing the muscle strength of shoulder exter‐ nal rotation will be the same as the previous study of exercise for serratus anterior. Shoulder external rotation includes scapular retraction. The function of middle trapezius is scapular retraction. This homology must be the reason about increasing the muscle strength of shoul‐ der external rotation.

Considering these two studies, the muscle strength of serratus anterior must have a part of mus‐ cle strength of shoulder flexion, abduction, and internal rotation, and that of middle trapezius


**Table 2.** The results about the changing muscle strength.

must have a part of shoulder external rotation. In fact, the muscle strength of shoulder complex will combine the muscle strength of glenohumeral joint and scapulothoracic joint. Therefore, muscle weakness occurred either on glenohumeral joint or on scapulothoracic joint, and the muscle strength of shoulder complex will decrease (**Figure 6**). Winging scapula by long thoracic nerve palsy decreases the shoulder muscle strength, and this case is an easy‐to‐follow typical example. It is necessary to consider the function of not only glenohumeral joint but also scapu‐ lothoracic joint during the measurement of muscle strength. And we have to make a hypothesis about specific scapular muscle weakness when shoulder muscle strength is decreased.

#### **2.3. The activities of scapular muscles during isometric contraction**

I indicated the characteristic about scapular muscle strengthening in the previous paragraphs. From this, it was shown that scapular muscle strength relates with the muscle strength of shoulder complex. During daily living and sports situation, there are many opportunities requiring powerful force, for example, transfer the burden and have a frying pan. In contact sports, they require continuing their performance resisting external force. In these cases, which scapular muscle is important? I thought that it is necessary to investigate the scapular muscle activity during shoulder isometric contraction to clear the question. From previous studies, to have all scapular muscle activities during shoulder isometric contraction is hard to imagine and I predicted the characteristic muscle‐active pattern. Therefore, I investigated each scapular muscle activity during isometric shoulder abduction, internal rotation, and external rotation.

#### *2.3.1. Materials and methods*

Seventeen healthy men participated in this study. The mean anthropometric characteristics were age, 26.0 ± 3.6 years; height, 172.9 ± 4.3 cm; and weight, 65.8 ± 9.1 kg.

**Figure 6.** Muscle weakness on shoulder complex. Muscle weakness occurred either on glenohumeral joint or on scapulo‐ thoracic joint.

Muscle activity was measured during isometric shoulder abduction, internal rotation, and external rotation in the sitting position. Surface electromyography (EMG; MQ‐8, Kissei Comtec, Japan) was used to collect raw EMG data during the task. EMG was reported in 3 s and the signal was recorded at a sampling rate of 1000 Hz. The measurement posi‐ tion of shoulder abduction was configured at shoulder 45° abduction and internal rotation and external rotation were at shoulder 0° abduction (**Figure 7**). The external load was set at three weights in order to evaluate the change with different loads using a handheld‐ dynamometer (Mobie, SAKAI Medical Co., Japan). The external load during abduction and internal rotation was set at 5, 10, and 15% of each subject's body weight and external rotation was set at 3, 5, and 10%. The target muscles at scapulothoracic joint of abduction task were the serratus anterior, upper, and lower trapezius. Those of internal and exter‐ nal rotation were serratus anterior and middle trapezius. The EMG electrode position was set according to previous studies by Ekstrom et al. and Delagi and Perotto [12, 13]. The integrated EMG (IEMG) was obtained for 3 s in the stable wave during each task and was counted per second. The IEMG per second was normalized by the IEMG at each starting position without an external load. Relative IEMGs were used for statistical comparison of each load in the same muscle. The Friedman test was used to determine whether a load condition had a statistically significant effect on relative IEMG for each muscle. Post hoc analysis was then applied for specific comparisons among the three loads and determined for individual effect differences.

#### *2.3.2. Results*

must have a part of shoulder external rotation. In fact, the muscle strength of shoulder complex will combine the muscle strength of glenohumeral joint and scapulothoracic joint. Therefore, muscle weakness occurred either on glenohumeral joint or on scapulothoracic joint, and the muscle strength of shoulder complex will decrease (**Figure 6**). Winging scapula by long thoracic nerve palsy decreases the shoulder muscle strength, and this case is an easy‐to‐follow typical example. It is necessary to consider the function of not only glenohumeral joint but also scapu‐ lothoracic joint during the measurement of muscle strength. And we have to make a hypothesis

I indicated the characteristic about scapular muscle strengthening in the previous paragraphs. From this, it was shown that scapular muscle strength relates with the muscle strength of shoulder complex. During daily living and sports situation, there are many opportunities requiring powerful force, for example, transfer the burden and have a frying pan. In contact sports, they require continuing their performance resisting external force. In these cases, which scapular muscle is important? I thought that it is necessary to investigate the scapular muscle activity during shoulder isometric contraction to clear the question. From previous studies, to have all scapular muscle activities during shoulder isometric contraction is hard to imagine and I predicted the characteristic muscle‐active pattern. Therefore, I investigated each scapular muscle activity during isometric shoulder abduction, internal rotation, and external rotation.

Seventeen healthy men participated in this study. The mean anthropometric characteristics

**Figure 6.** Muscle weakness on shoulder complex. Muscle weakness occurred either on glenohumeral joint or on scapulo‐

were age, 26.0 ± 3.6 years; height, 172.9 ± 4.3 cm; and weight, 65.8 ± 9.1 kg.

about specific scapular muscle weakness when shoulder muscle strength is decreased.

**2.3. The activities of scapular muscles during isometric contraction**

*2.3.1. Materials and methods*

10 Clinical Physical Therapy

thoracic joint.

The results of raw waveform are shown in **Figure 8**. Typical raw wave was shown on internal and external rotation. During isometric shoulder internal rotation, the muscle activity of ser‐ ratus anterior was high and middle trapezius was slightly high. In an opposite way, during isometric shoulder external rotation, the muscle activity of middle trapezius was high and serratus anterior was slightly high. During isometric shoulder abduction, all muscle activities, serratus anterior, upper trapezius, and lower trapezius, were high.

**Figure 7.** Measurement position regarding shoulder isometric contraction. The measurement position of shoulder abduction was configured at shoulder 45° abduction and internal rotation and external rotation were at shoulder 0° abduction. The external load was set at three weights using a handheld‐dynamometer.

**Figure 8.** The results of raw waveform during isometric contraction. During isometric shoulder internal rotation, the muscle activity of serratus anterior was high and middle trapezius was slight. In an opposite way, during isometric shoulder external rotation, the muscle activity of middle trapezius was high and serratus anterior was slight. During isometric shoulder abduction, all muscle activities, serratus anterior, upper trapezius, and lower trapezius were high.

Next, relative IEMG was focused (**Table 3**). The relative IEMGs of all muscles were signifi‐ cantly increased at 15% compared to 5 and 10% during abduction and internal rotation. In a similar way, the relative IEMGs of all muscles were significantly increased at 10% compared to 3 and 5% during external rotation. Basically, all muscle activities increased with increasing external load during all trials.

#### *2.3.3. Discussion*

From the result of internal and external rotation, it was shown that the amount of muscle activi‐ ties was unequal during isometric contraction on all scapular muscles. Some muscles have high activities, others have low. But the relative EMGs of all muscle were increased with increasing external load, so these muscles which have low activities were also necessary during isometric contraction. I inferred which muscle had high activity from this and previous studies. If the muscle has the same function with the scapular movement being comprised within the shoul‐ der motion, the muscle must have high activity. In particular, internal rotation includes scapular protraction, which is the function of serratus anterior, and external rotation includes scapular retraction, which is the function of middle trapezius. It is just the same as the result of the stud‐ ies about strengthening scapular muscles. About isometric shoulder abduction task, serratus anterior, upper trapezius, and lower trapezius have relation to scapular upward rotation, so all



Next, relative IEMG was focused (**Table 3**). The relative IEMGs of all muscles were signifi‐ cantly increased at 15% compared to 5 and 10% during abduction and internal rotation. In a similar way, the relative IEMGs of all muscles were significantly increased at 10% compared to 3 and 5% during external rotation. Basically, all muscle activities increased with increasing

**Figure 8.** The results of raw waveform during isometric contraction. During isometric shoulder internal rotation, the muscle activity of serratus anterior was high and middle trapezius was slight. In an opposite way, during isometric shoulder external rotation, the muscle activity of middle trapezius was high and serratus anterior was slight. During isometric shoulder abduction, all muscle activities, serratus anterior, upper trapezius, and lower trapezius were high.

From the result of internal and external rotation, it was shown that the amount of muscle activi‐ ties was unequal during isometric contraction on all scapular muscles. Some muscles have high activities, others have low. But the relative EMGs of all muscle were increased with increasing external load, so these muscles which have low activities were also necessary during isometric contraction. I inferred which muscle had high activity from this and previous studies. If the muscle has the same function with the scapular movement being comprised within the shoul‐ der motion, the muscle must have high activity. In particular, internal rotation includes scapular protraction, which is the function of serratus anterior, and external rotation includes scapular retraction, which is the function of middle trapezius. It is just the same as the result of the stud‐ ies about strengthening scapular muscles. About isometric shoulder abduction task, serratus anterior, upper trapezius, and lower trapezius have relation to scapular upward rotation, so all

external load during all trials.

*2.3.3. Discussion*

12 Clinical Physical Therapy


**Table 3.** The results of relative IEMG.

muscles might have high activities. As can be seen, it suggested that not all scapular muscles have high activity during isometric contraction and the changes in scapular muscle activity by movement direction are analogous to the case of isotonic contraction. So, for example, when we improve the ability of the endurance to the collision in athletes, we have to consider which direction the force is added and predict which muscle is actively higher. When this is done, the good rehabilitation program will be provided to the patients.

Additionally, considering the result of unequal scapular muscle activities, we can infer the mechanism about scapula stability. Generically, the theory that the scapula was sta‐ bilized by "fixation to thorax" has been heard. If something is to fix the scapula to the thorax, the scapula needs to be pulled by the same tension from side to side and up and down. Co‐contraction with scapular muscle is needed. Scapula does not fix the thorax if the only unilateral scapular muscle has high activity. So, from this study, the scapula is stabilized not by the fixation to thorax, but by individualized scapular muscle activity to counteract the external load. Individualized scapular muscle activity must contribute to maintain the scapular position. In other words, these scapular muscles which have high activities have the function of resisting external load as the agonist at glenohumeral joint. These muscles may be called the agonist at scapulothoracic joint. Scapular muscles may be classified the agonist and the antagonist like other joint muscles. From this study, it is predicted that the agonist at scapulothoracic joint has the function of countering the external load and the antagonist at scapulothoracic joint has the function of adjusting the scapular position.

#### **2.4. The changes of muscle activity on shoulder joint during maximum isometric contraction by the difference of muscle‐weakness region**

As above, it was suggested that scapular muscle reacts individually and has the function of counteracting the external load. In clinical situation, therapists usually evaluate the scapular stability using the assessment of whether or not scapular downward rotation occurred dur‐ ing shoulder abduction manual muscle testing. And therapists evaluate whether the muscle strength of shoulder abduction is increased with manual fixation against scapular downward rotation (**Figure 9**). From this assessment, therapists calculate the policy about scapular stabil‐ ity. Considering previous studies, it is predicted that we can use this evaluation at all direc‐ tions as shoulder abduction. And if the patients have the scapula instability, the scapula will be swaying during the test. Wherein, I verified whether or not this phenomenon occurs in the patients with shoulder disease. Additionally, I measured the scapular muscle activities using electromyography during the muscle testing.

#### *2.4.1. Materials and methods*

Two patients with shoulder disease participated in this study. One had muscle weakness mainly at glenohumeral joint (Patient A) and the other had muscle weakness mainly at scapulo‐ thoracic joint (Patient B). Additionally, a healthy man without shoulder disease was measured and compared to the patients (Subject C). Patient A was affected with dislocated shoulder and dislocation was reduced. The muscle strength is shown in **Figure 10**. On manual muscle testing,

muscles might have high activities. As can be seen, it suggested that not all scapular muscles have high activity during isometric contraction and the changes in scapular muscle activity by movement direction are analogous to the case of isotonic contraction. So, for example, when we improve the ability of the endurance to the collision in athletes, we have to consider which direction the force is added and predict which muscle is actively higher. When this is done, the

Additionally, considering the result of unequal scapular muscle activities, we can infer the mechanism about scapula stability. Generically, the theory that the scapula was sta‐ bilized by "fixation to thorax" has been heard. If something is to fix the scapula to the thorax, the scapula needs to be pulled by the same tension from side to side and up and down. Co‐contraction with scapular muscle is needed. Scapula does not fix the thorax if the only unilateral scapular muscle has high activity. So, from this study, the scapula is stabilized not by the fixation to thorax, but by individualized scapular muscle activity to counteract the external load. Individualized scapular muscle activity must contribute to maintain the scapular position. In other words, these scapular muscles which have high activities have the function of resisting external load as the agonist at glenohumeral joint. These muscles may be called the agonist at scapulothoracic joint. Scapular muscles may be classified the agonist and the antagonist like other joint muscles. From this study, it is predicted that the agonist at scapulothoracic joint has the function of countering the external load and the antagonist at scapulothoracic joint has the function of adjusting the

**2.4. The changes of muscle activity on shoulder joint during maximum isometric** 

As above, it was suggested that scapular muscle reacts individually and has the function of counteracting the external load. In clinical situation, therapists usually evaluate the scapular stability using the assessment of whether or not scapular downward rotation occurred dur‐ ing shoulder abduction manual muscle testing. And therapists evaluate whether the muscle strength of shoulder abduction is increased with manual fixation against scapular downward rotation (**Figure 9**). From this assessment, therapists calculate the policy about scapular stabil‐ ity. Considering previous studies, it is predicted that we can use this evaluation at all direc‐ tions as shoulder abduction. And if the patients have the scapula instability, the scapula will be swaying during the test. Wherein, I verified whether or not this phenomenon occurs in the patients with shoulder disease. Additionally, I measured the scapular muscle activities using

Two patients with shoulder disease participated in this study. One had muscle weakness mainly at glenohumeral joint (Patient A) and the other had muscle weakness mainly at scapulo‐ thoracic joint (Patient B). Additionally, a healthy man without shoulder disease was measured and compared to the patients (Subject C). Patient A was affected with dislocated shoulder and dislocation was reduced. The muscle strength is shown in **Figure 10**. On manual muscle testing,

good rehabilitation program will be provided to the patients.

**contraction by the difference of muscle‐weakness region**

electromyography during the muscle testing.

*2.4.1. Materials and methods*

scapular position.

14 Clinical Physical Therapy

**Figure 9.** Evaluation about the change of muscle strength by scapular manual fixation. Therapists evaluate whether the muscle strength of shoulder abduction is increased with manual fixation against scapular downward rotation.

scapular protraction and protraction/downward rotation were indicated as level 4, but shoulder external rotation and internal rotation were indicated as level 3. Patient A had muscle weakness mainly at glenohumeral joint. Patient B was affected with humeral head fracture. The muscle strength was that shoulder abduction and external rotation were indicated as level 3 and inter‐ nal rotation was level 4, but scapular retraction/downward rotation and protraction/upward


**Figure 10.** Muscle strength of two patients. Patient A had the muscle weakness at glenohumeral joint and Patient B had the weakness at scapulothoracic joint.

rotation were level 2. Patient B had the muscle weakness mainly at scapulothoracic joint. Two patients had no pain and had comfortable range of motion to measurement. The measurement task was set as three kinds of maximum isometric contraction: shoulder flexion at 90° shoulder flexion, shoulder internal rotation at 0° shoulder abduction, and shoulder external rotation at 0° shoulder abduction. The subjects were measured for the muscle activities and scapular move‐ ment during three tasks with external load to break their position slightly. The muscles that were predicted to have high activities during these tasks consulted from previous studies were measured. I assessed the change in the muscle activities when the external load was given.

#### *2.4.2. Results*

The result in Subject C, which was healthy man, was shown first. Please look at the raw wave (**Figure 11**). On shoulder flexion, not only just anterior deltoid but also serratus anterior, upper trapezius, middle trapezius, and lower trapezius had high activities that corresponded with the external load. In a way like that, infraspinatus, middle trapezius, and lower trapezius had high activities on external rotation, and pectoralis major and serratus anterior on internal rotation. In other words, the high muscle activities appeared not only at glenohumeral joint but also at scapulothoracic joint by external force.

Next, the result in Patient A, which had the muscle weakness at glenohumeral joint, was shown. On shoulder flexion, the muscle activity of anterior deltoid was increased. But serratus anterior, upper trapezius, middle trapezius, and lower trapezius were not increased like anterior deltoid.


**Figure 11.** Raw wave in each subject. The result in Patient A was that the muscle activity at glenohumeral joint was high corresponding to the external load but that at scapulothoracic joint was modest in size. In Patient B and Subject C, muscle activities at glenohumeral joint and scapulothoracic joint were increased together.

On external rotation, the muscle activity of infraspinatus was increased in a measure, but those of upper trapezius, middle trapezius, and lower trapezius which were increased in Subject C were not changed. On internal rotation, the muscle activity of pectoralis major was increased by external load and serratus anterior had little change. When taken together, the result was that the muscle activity at glenohumeral joint was high corresponding to the external load but that at scapulothoracic joint was modest in size. From the movie, which was filmed for the scapular stability at the same time, the scapular swaying did not appear by external load.

Lastly, the result in Patient B, which had weakness at scapulothoracic joint, was shown. On shoulder flexion, the muscle activities of anterior deltoid, upper trapezius, middle trapezius, and lower trapezius were increased by external load. On external rotation, not only infraspi‐ natus but also upper trapezius, middle trapezius, and lower trapezius had high activities. On internal rotation, pectoralis major and serratus anterior had high activities. On the whole, muscle activities at glenohumeral joint and scapulothoracic joint were increased together like Subject C. And the movies showed that the scapular movement occurred by external load, the task of flexion; scapular downward rotation, external rotation; scapular protraction, internal rotation; scapular retraction.

#### *2.4.3. Discussion*

rotation were level 2. Patient B had the muscle weakness mainly at scapulothoracic joint. Two patients had no pain and had comfortable range of motion to measurement. The measurement task was set as three kinds of maximum isometric contraction: shoulder flexion at 90° shoulder flexion, shoulder internal rotation at 0° shoulder abduction, and shoulder external rotation at 0° shoulder abduction. The subjects were measured for the muscle activities and scapular move‐ ment during three tasks with external load to break their position slightly. The muscles that were predicted to have high activities during these tasks consulted from previous studies were measured. I assessed the change in the muscle activities when the external load was given.

The result in Subject C, which was healthy man, was shown first. Please look at the raw wave (**Figure 11**). On shoulder flexion, not only just anterior deltoid but also serratus anterior, upper trapezius, middle trapezius, and lower trapezius had high activities that corresponded with the external load. In a way like that, infraspinatus, middle trapezius, and lower trapezius had high activities on external rotation, and pectoralis major and serratus anterior on internal rotation. In other words, the high muscle activities appeared not only at glenohumeral joint

Next, the result in Patient A, which had the muscle weakness at glenohumeral joint, was shown. On shoulder flexion, the muscle activity of anterior deltoid was increased. But serratus anterior, upper trapezius, middle trapezius, and lower trapezius were not increased like anterior deltoid.

**Figure 11.** Raw wave in each subject. The result in Patient A was that the muscle activity at glenohumeral joint was high corresponding to the external load but that at scapulothoracic joint was modest in size. In Patient B and Subject C, muscle

activities at glenohumeral joint and scapulothoracic joint were increased together.

*2.4.2. Results*

16 Clinical Physical Therapy

but also at scapulothoracic joint by external force.

As can be seen, there were different results between Patient A and Patient B. Now, let us think about these results with the inclusion of previous studies. First, I had indicated that the muscle weakness at scapulothoracic joint causes the muscle weakness on whole shoul‐ der joint. In Patient B, because he had the scapular muscle weakness and the scapular move‐ ment occurred by external load, the reason for the whole shoulder muscle weakness will be the weakness at scapulothoracic joint. By the electromyographic wave, scapular muscles had high activities and it is predicted that their activities appeared to resist the external force. So, it is thought that the external force to upper limb reached scapulothoracic joint through glenohumeral joint because the muscle strength at glenohumeral joint was main‐ tained for some level, and the scapular muscles had the activities to resist the external force. But the upper limb was moved causing scapular movement because the muscle strength at scapulothoracic joint was not enough (**Figure 6**). The reason is the weakness at scapulotho‐ racic joint.

On the other hand, in Patient A, it is predicted that the external load did not reach scapulotho‐ racic joint because he had muscle weakness at glenohumeral joint and humerus was moved by external load. So, scapular muscles did not have activities and scapular sway might not appear. The important factor is that the load did not reach scapulothoracic joint.

From this, we will be able to calculate which joints have weakness for improvement by observ‐ ing the scapular movement during muscle testing with every movement direction not only abduction (**Figure 12**). And it is important that this scapular movement during muscle testing is anticipated movement direction. If the scapular movement appears seriously, we have to pro‐ vide the exercise for scapular muscle. We can evaluate the responsible region by understanding which muscle should have high activities and analyzing the motion to see the swaying region.

**Figure 12.** Evaluation regarding scapular movement during muscle testing. We will be able to calculate which joints have weakness for improvement by observing the scapular movement during muscle testing with every movement direction. For example, during muscle testing of external rotation, if scapular protraction appears serious, the responsible region of shoulder muscle weakness will be at scapulothoracic joint.

#### **2.5. The study about the order of activity in scapular muscles**

In previous studies, I have shown that there is high activity of scapular muscle and little activ‐ ity of scapular muscle during shoulder isometric contraction, and these muscles are changed by movement direction. And the muscle strength of scapular muscle influences the muscle strength of whole shoulder joint. From these facts, I have described that we will classify the scapular muscles into the agonist and the antagonist, and the agonist at scapulothoracic joint will have the function of resisting the external load. I thought that if the agonist at scapulo‐ thoracic joint has the function of resisting the external load like the agonist at glenohumeral joint, the time of starting the muscle activity of the agonist at scapulothoracic joint will be the same as the agonist at glenohumeral joint. In other words, I thought that the hypothesis about the classification of scapular muscles will be able to be verified by examination in the timing of scapular muscle activity during shoulder motion. However, as the muscle activity of trans‐ versus abdominis muscle starts before the motion of upper limb, therapists sometimes regard that scapular stability by scapular muscle activity is established before humeral motion. In this case, scapular muscle activity must start before the muscle activity at glenohumeral joint. Which pattern appeared? So, I examined the order of muscle activities during shoulder motion.

#### *2.5.1. Materials and methods*

**2.5. The study about the order of activity in scapular muscles**

of shoulder muscle weakness will be at scapulothoracic joint.

18 Clinical Physical Therapy

In previous studies, I have shown that there is high activity of scapular muscle and little activ‐ ity of scapular muscle during shoulder isometric contraction, and these muscles are changed by movement direction. And the muscle strength of scapular muscle influences the muscle strength of whole shoulder joint. From these facts, I have described that we will classify the scapular muscles into the agonist and the antagonist, and the agonist at scapulothoracic joint will have the function of resisting the external load. I thought that if the agonist at scapulo‐ thoracic joint has the function of resisting the external load like the agonist at glenohumeral joint, the time of starting the muscle activity of the agonist at scapulothoracic joint will be the same as the agonist at glenohumeral joint. In other words, I thought that the hypothesis about the classification of scapular muscles will be able to be verified by examination in the timing of scapular muscle activity during shoulder motion. However, as the muscle activity of trans‐ versus abdominis muscle starts before the motion of upper limb, therapists sometimes regard that scapular stability by scapular muscle activity is established before humeral motion. In this case, scapular muscle activity must start before the muscle activity at glenohumeral joint. Which pattern appeared? So, I examined the order of muscle activities during shoulder motion.

**Figure 12.** Evaluation regarding scapular movement during muscle testing. We will be able to calculate which joints have weakness for improvement by observing the scapular movement during muscle testing with every movement direction. For example, during muscle testing of external rotation, if scapular protraction appears serious, the responsible region Seventeen healthy men participated in this study. The mean anthropometric characteristics of 15 subjects were age, 25.5 ± 2.2 years; height, 169.4 ± 5.6 cm; and weight, 62.8 ± 7.8 kg. The trials were isometric works against external load in shoulder internal and external rotation at 0° shoulder abduction. And two conditions were set that was and was not informed of the timing of obtaining external load. Then, we measured the time to start activities in the agonist at GHJ and the agonist and antagonist at STJ in these trials.

The subjects were measured in sitting position. A cord of 1.5‐kg plumb bob was tightly fas‐ tened at the end to the subject's end of the forearm. The cord was set on the level run on the same height‐adjusted device. The internal and external rotation directional external load was given to the subject by dropping the plumb bob using trochlear fundamental (**Figure 13**). Electrical switch was set to the subject's forearm and the starting movement was measured by this switch. To measure the muscle activities during the task, electrodes of electromyogra‐ phy were applied to pectoralis major, infraspinatus, serratus anterior, and middle trapezius. The task was tried in two conditions: one was to inform the timing of obtaining external load

**Figure 13.** The method of the measurement. A cord of 1.5‐kg plumb bob was tightly fastened on the end to the subject's end of the forearm. The cord was set on the level run on the same height‐adjusted device. The internal and external rotation directional external load was given to the subject by dropping the plumb bob using trochlear fundamental.

(informational task) and the other was uninformed (non‐informational task). The order of the tasks was random.

I calculated the time from starting the motion to starting each muscle activities. Starting the motion was judged by the electrical switch. The definition of starting muscle activity was decided by the time point that recorded amplitude over twice of maximum amplitude in 500 ms before the task. And I classified the measured muscle into the agonist at glenohumeral joint, the agonist at scapulothoracic joint, and the antagonist at scapulothoracic joint on each movement direction (**Figure 14**). First, in shoulder internal rotation at 0° shoulder abduction, the agonist at glenohumeral joint was the pectoralis major. The agonist at scapulothoracic joint was set as the serratus anterior because scapular protraction is by shoulder internal rotation. The antagonist at scapulothoracic joint was defined as the middle trapezius because it has the opposite function of the serratus anterior. The agonist at glenohumeral joint for the external rotation task at 0° shoulder abduction was set as infraspinatus. The middle trapezius was defined as the agonists at scapulothoracic joint because scapular retraction is achieved by external rotation. Serratus anterior was defined as the antagonist at scapulothoracic joint.

The reaction times were compared between the agonist at glenohumeral joint, the agonist at scapulothoracic joint, and the antagonist at scapulothoracic joint in each trial. From this, I examined the order of muscle activities.

#### *2.5.2. Results*

The result is shown in **Figure 15** in internal rotation task. Both informational task and non‐ informational task, the reaction time on serratus anterior which was defined as the agonist at scapulothoracic joint was the same as pectoralis major which was defined as the agonist at glenohumeral joint. However, the reaction time on middle trapezius which was set to the antagonist at scapulothoracic joint was late than on these two muscles significantly.

The result in external rotation was as follows. Both informational task and non‐informational task, the reaction time on middle trapezius which was the agonist at scapulothoracic joint was not different from that on infraspinatus which was the agonist at glenohumeral joint. But the reaction time on serratus anterior which was defined as the antagonist at scapulothoracic joint was late than on these two muscles significantly.

In all, similar results were obtained.


**Figure 14.** Classification about the agonist and the antagonist. I classified the measured muscle into the agonist at glenohumeral joint, the agonist at scapulothoracic joint, and the antagonist at scapulothoracic joint on each movement direction by scapular movement during the shoulder motion.

**Figure 15.** The reaction time of each muscle. The reaction time of the agonist at scapulothoracic joint was the same as that of the agonist at glenohumeral joint. However, the reaction time of the antagonist at scapulothoracic joint was significantly delayed than that of these two muscles.

#### *2.5.3. Discussion*

(informational task) and the other was uninformed (non‐informational task). The order of the

I calculated the time from starting the motion to starting each muscle activities. Starting the motion was judged by the electrical switch. The definition of starting muscle activity was decided by the time point that recorded amplitude over twice of maximum amplitude in 500 ms before the task. And I classified the measured muscle into the agonist at glenohumeral joint, the agonist at scapulothoracic joint, and the antagonist at scapulothoracic joint on each movement direction (**Figure 14**). First, in shoulder internal rotation at 0° shoulder abduction, the agonist at glenohumeral joint was the pectoralis major. The agonist at scapulothoracic joint was set as the serratus anterior because scapular protraction is by shoulder internal rotation. The antagonist at scapulothoracic joint was defined as the middle trapezius because it has the opposite function of the serratus anterior. The agonist at glenohumeral joint for the external rotation task at 0° shoulder abduction was set as infraspinatus. The middle trapezius was defined as the agonists at scapulothoracic joint because scapular retraction is achieved by external rotation. Serratus

The reaction times were compared between the agonist at glenohumeral joint, the agonist at scapulothoracic joint, and the antagonist at scapulothoracic joint in each trial. From this, I

The result is shown in **Figure 15** in internal rotation task. Both informational task and non‐ informational task, the reaction time on serratus anterior which was defined as the agonist at scapulothoracic joint was the same as pectoralis major which was defined as the agonist at glenohumeral joint. However, the reaction time on middle trapezius which was set to the

The result in external rotation was as follows. Both informational task and non‐informational task, the reaction time on middle trapezius which was the agonist at scapulothoracic joint was not different from that on infraspinatus which was the agonist at glenohumeral joint. But the reaction time on serratus anterior which was defined as the antagonist at scapulothoracic joint

**Figure 14.** Classification about the agonist and the antagonist. I classified the measured muscle into the agonist at glenohumeral joint, the agonist at scapulothoracic joint, and the antagonist at scapulothoracic joint on each movement

antagonist at scapulothoracic joint was late than on these two muscles significantly.

anterior was defined as the antagonist at scapulothoracic joint.

examined the order of muscle activities.

was late than on these two muscles significantly.

direction by scapular movement during the shoulder motion.

In all, similar results were obtained.

tasks was random.

20 Clinical Physical Therapy

*2.5.2. Results*

In all trials, the reaction time of the agonist at scapulothoracic joint was the same as that of the agonist at glenohumeral joint. However, the reaction time of the antagonist at scapulothoracic joint was significantly delayed than that of these two muscles.

The study about the relationship between agonist and antagonist was tested a lot for other joints. It is common knowledge that the training for agonist leads to bigger torque at the joint and be able to maintain the joint against stronger external load. On the other hand, regard‐ ing antagonist, the muscle activity was examined at the knee, elbow, and glenohumeral joint and the slight muscle activity comes in during isometric, isotonic, and isokinetic contraction [14–16]. And it was known that the muscle activity of antagonist increases on unstable condi‐ tion at the lower leg. From these facts, it is known that the antagonist has the function about the control of the joint movement caused by agonist and the contribution to the joint stability by co‐contraction with agonist [17, 18]. Additionally, Hase et al. reported the result of the measurement about muscle activity at wrist joint during catching of a falling ball dropped on the palm [19]. The maximal muscle activity of the antagonist appeared after that of the agonist slightly and the antagonist had persistent muscle activity before catching. It was concluded that the antagonist will work for the fixation of the joint. Compared to my studies with the previous studies about antagonist on other joints, it was founded that these are some similar points. In my previous studies, the exercise for the agonist at scapulothoracic joint led to an increase in the muscle strength of whole shoulder joint. And the study about the measurement of muscle activities during isometric contraction showed that the antagonist at scapulotho‐ racic joint had slight muscle activity. These results are the same as the other joint studies. Additionally, in this study, the reaction time of the antagonist at scapulothoracic joint was significantly delayed than that of the agonist at glenohumeral joint and scapulothoracic joint. This result has similarity in part as the previous study of wrist joint. By the way, although I did not indicate this report, the same result, the reaction time of the antagonist at scapulothoracic joint was significantly delayed than that of the agonist at glenohumeral joint and scapulotho‐ racic joint, was demonstrated in my study of the task about starting the movement for oneself. As just described, the studies about scapular muscles indicate very similar result to the studies regarding agonist and antagonist at other joints, so we must be able to consider the scapular muscles using the classification as agonist and antagonist. It is conceivable that the agonist at scapulothoracic joint has the function of resisting the external load and the antagonist works for controlling the scapular position.
