**3. Normalization of EMG signals**

To be able to compare EMG activity in the same muscle on different days or in different individuals or to compare EMG activity between muscles, the EMG must be normalized [4, 17, 18]. Normalization of EMG signals is usually performed by dividing the EMG signals during a task by a reference EMG value obtained from the same muscle. By normalizing to a reference EMG value collected using the same electrode configuration, factors that affect the EMG signals during the task and the reference contraction are the same. Therefore, one can validly obtain a relative measure of the activation compared to the reference value.

The common consensus is that a "good" reference value to which to normalize EMG signals should have high repeatability, especially in the same subject in the same session, and be meaningful. By choosing a reference value repeatable within an individual, one can compare the levels obtained from any task to that reference value. The choice of reference value should allow comparisons between individuals and between muscles. To be able to do so, the reference value should have similar meaning between individuals and between muscles. The choice of normalization method is critical in the interpretation of the EMG signals as it will influence the amplitude and pattern of the EMG signals [8]. Unfortunately, there is no consensus as to a single "best" method for normalization of EMG data [8, 18] and a variety of methods have been used to obtain normalization reference values:


### **3.1. Maximum (peak) activation levels during maximum contractions**

#### *3.1.1. Maximal voluntary isometric contractions*

Computational Intelligence in Electromyography Analysis – 176 A Perspective on Current Applications and Future Challenges

**2. Raw EMG signals (without normalization)** 

critical and EMG normalization is not required.

the mean above baseline levels.

**3. Normalization of EMG signals** 

and examples of its uses will be provided.

comparisons such as:

appropriate [8]. In this chapter, we will outline when the presentation of raw EMG is acceptable and when normalization is essential as well as the various methods used to normalize EMG signals. A discussion of the advantages and disadvantages of each method

As indicated in the introduction, there are many factors that influence the EMG signal. However, it is generally accepted that within a data collection session and within an individual where no changes have been made to the configuration of the EMG set-up (electrode placement, amplification, filtering etc), under constant temperature and humidity conditions and within a short period of time, the raw EMG can be used for limited

1. the analysis of the frequency content of the EMG signal. In this type of analysis, the power spectrum of the EMG signal can be obtained by applying a Fast Fourier Transform to the EMG signal. The power density function of the EMG provides a distribution of the signal power as a function of frequency. Changes in the shape of the power density function of the EMG is usually analysed and shifts in the power density to lower frequencies is associated with fatigue. Since the shape of the power spectra is what is important, the

amplitude of the EMG signal is not critical and EMG normalization is not required. 2. the decomposition of the EMG into wavelets for an analysis of motor unit firing patterns, or cross talk between muscles. In this analysis, the EMG signal is decomposed into small wavelets (small waveforms). The wavelets are then used to identify and characterize motor unit action potentials by compressing and/or rescaling the wavelets and identifying them in the EMG signal. Again, the amplitude of the EMG signal is not

3. the time of the initiation of muscle activation. This type of analysis does not require EMG normalization as the time of activation is usually identified from the raw signal e.g. when the raw EMG signal amplitude reaches 2 [10] or 3 [11] standard deviations of

4. amplitude comparisons of signals from a given muscle between short term interventions/movements within an individual in the same session under the same experimental conditions without changes to the EMG electrode set-up [12] e.g. when comparing the EMG signal between different interventions/movements in a given muscle in each individual [13-16]. Because the absolute amplitude of the signal is meaningless, one cannot evaluate the level of activity in the muscle, but only that it is more or less active in one intervention/movement compared to the other. Therefore,

comparison of muscle activity levels between muscles or individuals is not valid.

To be able to compare EMG activity in the same muscle on different days or in different individuals or to compare EMG activity between muscles, the EMG must be normalized [4, The most common method of normalizing EMG signals from a given muscle uses to the EMG recorded from the same muscle during a maximal voluntary isometric contraction (MVIC) as the reference value [19-23]. The process of normalization using MVICs is that a reference test (usually a manual muscle test) is identified which produces a maximum contraction in the muscle of interest. Based on the repeatability between tests measures, it is recommended that at least 3 repetitions of the test be performed separated by at least 2 minutes to reduce any fatigue effects [12]. The EMG signals are then processed either by high-pass filtering, rectifying and smoothing or by calculating the root mean square of the signal. The maximum value obtained [12] from the processed signals during all repetitions of the test is then used as the reference value for normalizing the EMG signals, processed in the same way, from the muscle of interest. This allows the assessment of the level of activity of the muscle of interest during the task under investigation compared to the maximal neural activation capacity of the muscle [24-26].

This method sounds simple enough. However, when trying to implement it, investigators are faced with an important question: *What test should be used to produce maximum neural activation in a given muscle?* The choice of MVIC should reflect the maximal neural activation capacity of the given muscle [27]. Unfortunately, there is no consensus as to which test produces maximal activation in all individuals in any given muscle. Table 1 provides some examples of different tests that have been used for the same muscle in different studies. Note the number of different reference tests used for each muscle indicating the lack of consensus as to what test generates maximum activity in any given muscle.

Normalization of EMG Signals: To Normalize or Not to Normalize and What to Normalize to? 179

• knee extension, knee flexed 60°, hip flexed 90° (sitting) [44, 45]

• knee extension, knee flexed 90°, hip flexed 90° (sitting) [43] rectus femoris • knee extension, knee flexed 90°, hip flexed 80° to 90° (sitting) [35, 36, 38, 43] • knee extension, knee flexed 60°, hip flexed 90° (sitting) [44-46]

• knee flexion, knee flexed 90°, hip flexed 90° (sitting) [38]

• ankle plantar flexion, ankle -15°, knee flexed 30° [44]

• ankle plantar flexion, ankle -15°, knee flexed 30° [44]

tibialis anterior • ankle dorsi flexion, ankle, knee and hip in neutral position (supine) [45]

**Table 1.** Examples of MVIC tests used to generate maximum activity levels in various muscles

Although the repeatability of the EMG recorded during MVICs within individuals on the same day has been questioned [34], the majority of studies indicate that the reliability of MVICs within individuals on the same day is high [42, 48, 49]. High repeatability requires proper guidance of the subjects to perform the tests identically with each repetition, familiarity of the subjects with the production of maximum effort and the avoidance of

Because the test that will yield maximal activation in any given muscle is not known, many studies report EMG levels during various tasks that are >100% MVIC particularly during rapid, forceful contractions [18] or eccentric contractions [50]. For example, Jobe et al. [51] reported EMG signals from serratus anterior and triceps brachii during the acceleration phase of the over arm throw to be 226% and 212% respectively of the EMG from maximal manual muscle tests which were not described. Reported normalized EMG signals >100% indicate that the normalization test used to generate the MVIC is not accurately revealing the maximum muscle activation capacity. If maximum activity in each muscle is not

• ankle plantar flexion (supine) [33] soleus • ankle plantar flexion, mid ankle position (prone) [38]

(quadruped position) [45, 46]

(quadruped position) [46]

• knee flexion, knee flexed 90°, hands clasped behind head (prone) [37]

• ankle plantar flexion, mid ankle position (standing unilateral – body weight)

• ankle plantar flexion, ankle, knee and hip in neutral position (prone) [45]

• ankle plantar flexion, ankle, knee and hip in neutral position (prone) [38, 45]

• ankle plantar flexion, mid ankle position (standing unilateral – body weight)

• ankle plantar flexion, ankle in neutral position; knee and hip flexed 90°

• ankle dorsi flexion, ankle in neutral position; knee and hip flexed 90°

Muscles investigated Manual muscle test

vastus medialis • knee extension, knee flexed 60° (sitting) [44, 45]

[47]

[47]

lateral hamstring (biceps femoris) long

gastrocnemius lateralis

gastrocnemius medialis

fatigue.

head

vastus lateralis • knee extension, knee flexed 90°, hip flexed 90° (sitting) [38, 43]

• knee extension, knee flexed 45° (sitting) [37]

• knee flexion, knee flexed 60° (sitting) [44, 46] • knee flexion, knee flexed 60° (prone) [45]



Computational Intelligence in Electromyography Analysis – 178 A Perspective on Current Applications and Future Challenges

upper trapezius • shoulder shrug [28, 29]

examples of different tests that have been used for the same muscle in different studies. Note the number of different reference tests used for each muscle indicating the lack of

• shoulder abduction in scapular plane at 90° abduction [31, 32]

• shoulder abduction at 90°, elbow flexed to 90° (seated) [34]

infraspinatus • shoulder external rotation, arm at side, elbow flexed to 90° (seated) [28, 31, 34]

subscapularis • shoulder internal rotation, arm at side, elbow flexed to 90° (seated) [28, 34]

latissimus dorsi • shoulder depression with resistance or adduction and internal rotation, arm at

internal oblique • trunk flexion and lateral flexion, hips and knees flexed to 90°, feet supported, trunk in full flexion and rotated contra-laterally (supine) [35]

trunk in full flexion and rotated ipsi-laterally (supine) [36]

gluteus medius • hip abduction at 10° abduction, leg fully extended (side lying) contra-lateral

• hip abduction at 25° abduction, leg fully extended (side lying) [42]

• back extension, hip flexed 30° (seated) [39]

knee and hip flexed 30° [40]

knee and hip flexed 30° [40, 41]

the coronal plane and internally rotated [29] • shoulder extension (prone lying) [35, 36] serratus anterior • scapular protraction, shoulder abducted to 90°-100° (seated) [28]

• combined shoulder elevation/arm flexion/abduction in the scapular plane at

• shoulder external rotation and abduction, shoulder abducted to 20°, elbow

• shoulder external rotation, shoulder abducted to 45°, elbow flexed to 90°, no

• shoulder internal rotation, shoulder abducted to 45°, elbow flexed to 90°, no

• shoulder extension and internal rotation with arm straight, abducted to 30° in

• scapular protraction, elbow flexed to 45°, shoulder abducted to 75° and

• trunk flexion, hips and knees flexed to 90°, feet supported, trunk in full flexion

• trunk flexion, legs bent at 45°,and secured, trunk position not mentioned

• trunk flexion and lateral flexion, hips and knees flexed to 90°, feet supported,

• hip abduction at 10° abduction, leg fully extended (side lying) contra-lateral

consensus as to what test generates maximum activity in any given muscle.

Muscles investigated Manual muscle test

90° abduction [30]

• lumbar extension [33]

shoulder flexion [29]

shoulder flexion [29]

internally rotated to 45° [29]

side (seated) [28]

(supine) [35, 36]

gluteus maximus • hip extension, hip flexed 45° (prone) [38]

(supine) [37]

upper rectus

abdominis

supraspinatus • shoulder abduction at 90°, internal rotation (seated) [28]

flexed to 90°, no shoulder flexion [29]

**Table 1.** Examples of MVIC tests used to generate maximum activity levels in various muscles

Although the repeatability of the EMG recorded during MVICs within individuals on the same day has been questioned [34], the majority of studies indicate that the reliability of MVICs within individuals on the same day is high [42, 48, 49]. High repeatability requires proper guidance of the subjects to perform the tests identically with each repetition, familiarity of the subjects with the production of maximum effort and the avoidance of fatigue.

Because the test that will yield maximal activation in any given muscle is not known, many studies report EMG levels during various tasks that are >100% MVIC particularly during rapid, forceful contractions [18] or eccentric contractions [50]. For example, Jobe et al. [51] reported EMG signals from serratus anterior and triceps brachii during the acceleration phase of the over arm throw to be 226% and 212% respectively of the EMG from maximal manual muscle tests which were not described. Reported normalized EMG signals >100% indicate that the normalization test used to generate the MVIC is not accurately revealing the maximum muscle activation capacity. If maximum activity in each muscle is not obtained during the normalization contractions, a systematic error will be introduced which leads to an over estimation of activation levels [30]. This could lead to an incorrect interpretation of the intensity of the muscle activity required to perform a given task. In addition, if the activity in all muscles is not being referenced to the same activity level, e.g. maximum capacity, comparison of activity levels between muscles is not valid.

Normalization of EMG Signals: To Normalize or Not to Normalize and What to Normalize to? 181

resistance

resistance

and pre-rotated at ± 30º

elbow at 90º flexion

elbow at 90º flexion

4. shoulder elevation 5. flexion arm horizontal

horizontal

horizontal

horizontal

horizontal

humeral rotation angle 1. abduction at 0º; -45º 2. abduction at 0º; 0º 3. abduction at 0º; +45º 4. abduction at 45º; -45º 5. abduction at 45º; 0º 6. abduction at 45º; +45º 7. abduction at 90º; -45º 8. abduction at 90º; 0º 9. abduction at 90º; +45º 10.external rotation at 0º; -45º 11.external rotation at 45º; -45º 12.external rotation at 90º; -45º 13.external rotation at 90º; 0º 14.external rotation at 90º; +45º 15.internal rotation at 0º; 0º 16.internal rotation at 90º; -45º 17.internal rotation at 90º; 0º

the muscles investigated

5. hanging over the edge of the test table supine and flexing upward against resistance 6. hanging over the edge of the test table on side and lateral bending upward against

7. clockwise and anticlockwise trunk twist at 0º

1. internal rotation shoulder at 0º abduction,

2. external rotation shoulder at 0º abduction,

6. flexion hand 25 cm above and 25 cm right of

7. flexion hand 25 cm above and 25 cm left of

8. flexion hand 25 cm below and 25 cm right of

9. flexion hand 25 cm below and 25 cm left of

Coded: Activity at shoulder abduction angle;

3. abduction shoulder at 0º abduction

1. resisted bent-knee sit-up (feet restrained trunk at 30º hands behind head). 2. standing pelvis fixed flexing forward 3. standing pelvis fixed lateral bend 4. hanging over the edge of the test table in a prone posture and extending upward against

Study Muscles investigated MVIC test Isometric tests that produce maximum EMG in

1,2,6,7 1,2,5,6,7 1,3,5,6,7 2,3,6,7 3,4,7 4

5,6,7,8 2,5,6,7 5,6,7,8 2,3,4,6,7 1,2,5,6,8 3,5,6,7 2,3,5,6,7 1,4,5,9

7-9,12-14 10-12 16,17 1-9 7 12 16,17 15

McGill (1991)

Nieminen et al (1993) [61]

Kelly et al 1996 [54]

rectus abdominis external oblique internal oblique latissimus dorsi upper erector spinae (T9) lower erector spinae (L3)

supraspinatus infraspinatus upper trapezius middle trapezius lower trapezius anterior deltoid middle deltoid pectoralis major

supraspinatus infraspinatus subscapularis anterior deltoid middle deltoid posterior deltoid latissimus dorsi pectoralis major

[59]

The problem of not eliciting maximum capacity in each muscle tested would be avoided if standard tests that reliably elicit maximum activation levels were identified [52]. A number of studies have attempted to identify voluntary isometric tests that produce maximum activation levels in various muscles. These studies have shown that multiple tests can produce maximum recording from any given muscle [52-56] and that no specific test produces maximum recording from a given muscle in all individuals tested [27, 53, 54, 56- 63]. These findings indicate that the use of single MVIC test to identify maximum activity in a given muscle is not valid and that sets of tests are required in order to ensure maximum activity in a given muscle is recorded from all subjects. Table 2 summarizes the sets of MVIC tests that have been shown to produce maximum activity in face, trunk, shoulder and leg muscles.

Provided that maximum neural activation is achieved in all muscles and individuals tested, using MVICs is a highly reliable method to normalize EMG data and can be used to compare activity between muscles, between tasks and between individuals. To achieve the maximum neural activation in all muscles and individuals, sets of MVIC tests that produce maximum activation in each muscle need to be identified. The highest value recorded for each muscle from at least 3 attempts at these MVIC tests should be used as the normalization value to ensure that the recorded values reflect maximum neural activation levels.



Computational Intelligence in Electromyography Analysis – 180 A Perspective on Current Applications and Future Challenges

muscles.

levels.

O'Dwyer et al (1981) [56]

levator labii superiori zygomaticus major buccinator risorius

orbicularis oris superioris orbicularis oris inferioris depressor anguli oris depressor labii inferioris

intrinsic tongue muscles anterior genioglossus styloglossus/hyoglossus

digastric (anterior belly) internal (medial) pterygoid

mentalis

geniohyoid mylohyoid

temporalis

obtained during the normalization contractions, a systematic error will be introduced which leads to an over estimation of activation levels [30]. This could lead to an incorrect interpretation of the intensity of the muscle activity required to perform a given task. In addition, if the activity in all muscles is not being referenced to the same activity level, e.g.

The problem of not eliciting maximum capacity in each muscle tested would be avoided if standard tests that reliably elicit maximum activation levels were identified [52]. A number of studies have attempted to identify voluntary isometric tests that produce maximum activation levels in various muscles. These studies have shown that multiple tests can produce maximum recording from any given muscle [52-56] and that no specific test produces maximum recording from a given muscle in all individuals tested [27, 53, 54, 56- 63]. These findings indicate that the use of single MVIC test to identify maximum activity in a given muscle is not valid and that sets of tests are required in order to ensure maximum activity in a given muscle is recorded from all subjects. Table 2 summarizes the sets of MVIC tests that have been shown to produce maximum activity in face, trunk, shoulder and leg

Provided that maximum neural activation is achieved in all muscles and individuals tested, using MVICs is a highly reliable method to normalize EMG data and can be used to compare activity between muscles, between tasks and between individuals. To achieve the maximum neural activation in all muscles and individuals, sets of MVIC tests that produce maximum activation in each muscle need to be identified. The highest value recorded for each muscle from at least 3 attempts at these MVIC tests should be used as the normalization value to ensure that the recorded values reflect maximum neural activation

Study Muscles investigated MVIC test Isometric tests that produce maximum EMG in

1. unilateral snarl 2. broad laugh

3. puff out cheeks, mouth closed 4. broad smile, mouth closed

7. depress comers of mouth 8. depress lower lip, jaw closed

10.curl sides of tongue up 11.saliva swallow

12.gentle tongue protrusion 13.lower jaw against resistance 14.intercuspal bite on hard object

15.clench jaw.

Maximum EMG from each muscle across all tests

the muscles investigated

5. compress upper lip against upper incisors 6. compress lower lip against lower incisors

9. raise and evert lower lip while wrinkling chin

maximum capacity, comparison of activity levels between muscles is not valid.


Computational Intelligence in Electromyography Analysis –

182 A Perspective on Current Applications and Future Challenges


Normalization of EMG Signals: To Normalize or Not to Normalize and What to Normalize to? 183

8. IN-45-0 9. IN-45-45 10.IN-45-90 11.IN-90-0 12.IN-90-45 13.IN-90-90

1. upper trunk flexion 2. lower trunk flexion 3. upper trunk twisting 4. lower trunk twisting 5. upper trunk bending 6. lower trunk bending 7. upper trunk extension 8. lower trunk extension

9. shoulder rotation and adduction

1. knee extension at 45º knee flexion in sitting 2. combined knee extension + hip flexion at 45º

3. knee extension at 15º knee flexion in supine

4. knee flexion at 15º knee flexion in supine

5. knee flexion at 55º knee flexion in sitting 6. knee flexion at 55º knee flexion in prone

8. unilateral plantar-flexion in standing

7. plantar-flexion at neutral ankle, knee and hip

10.abdominal hollowing

knee flexion in sitting

position

position

position

in supine position

the muscles investigated

Study Muscles investigated MVIC test Isometric tests that produce maximum EMG in

1,2,6 1,2,3,4 1,3,4,5,6,10 1,3,5,6 2,3,5,6 3,4,5,9 7,8,9 7,8

2,4,5,6,7,8 4,5,6,7,8 1,2,3,7,8 1,2,3,7,8 1,2,3 4,5,6

4,5,6

**Table 2.** Examples of studies that have identified tests that produce maximum recordings from given muscles and recommend the use of multiple tests to make sure maximum activation is produced by all

*3.1.2. The maximum activation obtained during the task under investigation performed at* 

To reduce the possibility of obtaining normalized EMG levels during a task greater than 100%, investigators have used the EMG obtained during the task under investigation performed at maximum effort as the normalization value. For example, maximum EMG recorded during isometric shoulder abduction has been used to normalize the EMG during submaximal abduction [65], maximum crunch exercise for submaximal crunch exercise [66], maximum sprinting for normalizing the EMG during walking [44, 67] and maximum sprint

This method of normalizing EMG data produces high reliability between trials [44, 67] and greatly reduces the possibility of obtaining EMG levels during the task of interest greater

Vera-Garcia et al (2010) [64]

Rutherford et al (2011) [58]

individuals tested.

*maximum effort* 

upper rectus abdominis lower rectus abdominis lateral external oblique medial external oblique internal oblique latissimus dorsi (T9) erector spinae (T9) erector spinae (L5)

lateral gastrocnemius medial gastrocnemius vastus lateralis vastus medialis rectus femoris lateral hamstrings (biceps femoris) medial hamstrings (semitendinosus)

cycling for normalizing the EMG during cycling [38].


Computational Intelligence in Electromyography Analysis – 182 A Perspective on Current Applications and Future Challenges

> upper trapezius middle trapezius lower trapezius serratus anterior

tibialis anterior lateral gastrocnemius medial gastrocnemius

supraspinatus infraspinatus subscapularis lower subscapularis upper trapezius middle trapezius lower trapezius serratus anterior latissimus dorsi rhomboid major teres major anterior deltoid middle deltoid posterior deltoid pectoralis major (clavicular head)

anterior deltoid middle deltoid pectoralis major (clavicular head) pectoralis major (sternal head)

soleus vastus lateralis vastus medialis rectus femoris lateral hamstrings (biceps femoris) medial hamstrings (semitendinosus)

Ekstrom et al (2005) [27]

Hsu et al (2006) [45]

Boettcher et al 2008 [53] and Ginn et al 2011 [57]

Chopp et al (2010) [52]

Study Muscles investigated MVIC test Isometric tests that produce maximum EMG in

1,2,3,4,5,7 5,6,7 1,2,3,5,7,8 1,2,3

Maximum EMG from each muscle across all tests

Maximum EMG from each muscle across all 5 tests provides >95% chance of eliciting maximum for all muscles

> 1,4-6,10 2-6 7-12

> > 7,8,10

the muscles investigated

1. shoulder flexion at 125º with scapula

6. shoulder horizontally abducted and

7. arm raised above the head in line with the

8. shoulder externally rotated at 90º abduction

1. entire leg flexion and extension, seated with backrest reclined 45º, hip flexed 110º, knee

1. shoulder extension seated with the arm at 30º abduction, elbow fully extended, and thumb toward the body; arm extended as resistance

3. shoulder internal rotation in 90º abduction 4. shoulder flexion at 125º with scapula

5. shoulder horizontal adduction at 90º flexion

1. Coded: force direction – shoulder exion angle – horizontal abduction angle

applied over the distal forearm. 2. shoulder abduction at 90º with internal

rotation

resistance

2. UP-45-0 3. UP-45-45 4. UP-45-90 5. UP-90-0 6. UP-90-45 7. UP-90-90

2. knee flexion and extension, seated with backrest vertical, knee flexed 60º

2. shoulder abducted to 125º scapular plane 3. shoulder abducted to 90º with the neck side bent, rotated to the opposite side, and extended 4. scapula elevated with the neck side bent, rotated to the opposite side, and extended 5. shoulder horizontally abducted and

resistance

externally rotated

internally rotated

lower trapezius muscle

flexed 60º, ankle neutral.

**Table 2.** Examples of studies that have identified tests that produce maximum recordings from given muscles and recommend the use of multiple tests to make sure maximum activation is produced by all individuals tested.
