**4. Discussion**

**Figure 5.** Relative changes in VL activation at week 8 at three knee joint-angles in SL (black bars), LX (white bars) and

SL 25 12.7±2.3 13.9±2.0\* 13.2±1.8 12.9±1.6

LL 25 14.0±1.3 15.3±1.6\*\* 15.6±1.1\*\* 15.3±0.9\*\*

LX 25 12.2±2.7 14.2±1.9\*\* 13.0±2.0\*\* 12.4±1.8

**Table 1.** Changes in Vastus Lateralis muscle width at each measurement site throughout training and detraining. \*

Significantly different to baseline (P<0.05) \*\* Significantly different to baseline (P<0.01)

**Week 8 (cm)**

50 12.8±2.7 14.2±2.5\*\* 13.6±1.9 13.3±1.9 75 9.6±2.2 11.0±1.9\* 10.1±1.2 9.6±1.1

50 13.8±1.6 15.6±2.0\*\* 15.8±1.5\*\* 15.4±1.3\*\* 75 9.4±1.8 11.4±1.4\*\* 11.5±1.2\*\* 11.1±1.3\*\*

50 12.1±2.6 14.3±2.3\*\* 13.1±2.0\*\* 12.3±2.0 75 8.2±2.2 10.6±1.5\*\* 9.6±1.6\*\* 8.8±1.2\*

**Week 10 (cm)**

**Week 12 (cm)**

LL (grey bars). \* Significantly different to baseline. # Significantly different to SL group.

**Baseline (cm)**

**Group % Femur Length**

164 Electrodiagnosis in New Frontiers of Clinical Research

Resistance training presents a medium through which muscular function can be enhanced. In order to devise an appropriate and effective resistance training program tailored with functional and structural enhancement objectives, it is necessary to understand the responses to both an acute bout of exercise, and the adaptations to exercise over a prolonged period of training. An important aspect for muscular performance is the degree to which the muscle can be activated. Previous work using isometric contractions of the knee extensors, has demon‐ strated that the magnitude of maximal muscle activation is dependent on the joint-angle (and thus muscle-length) used during exercise, even when external torque produced is maintained at a similar level at the different joint angles [13]. This earlier study showed that activation of the quadriceps is significantly greater at 90o knee flexion compared to both 30o and 60o , despite isometric MVC torque being significantly less than 60o and identical to 30o . In the current study, unlike previous research, our participants exercised dynamically over a range-of-motion that was predominantly over shorter muscle lengths (0-50o ), longer muscle lengths (40-90o ) or over both short and long muscle lengths (0-90o ) during exercise using absolute and relative loading patterns. During absolute loading, weight lifted increased in a graded manner, and was reflected by significantly increased muscle activation between each absolute load in the training groups. This result was a more easily predicted outcome and reflects one of the fundamental properties of the neuromuscular system, i.e. the size principle [22], where a greater number of motor units are recruited in order to meet the increasing demands of force production. When exercising over longer muscle lengths (LL) and the complete ROM (LX), muscle activation was significantly greater during absolute and relative loading compared to shorter muscle lengths (SL). So why would a muscle exhibit greater activation whilst moving the same external weight but at different muscle lengths? By moving through a range of muscle lengths or joint-angles, the moment arm of the in-series elastic component (i.e. the tendon) also changes. As the amount of force needed to lift an external load (F) is F = f × d, where f is the internal force produced by the muscle and d is the length of the moment arm, when d is greater f will be smaller and vice versa, and therefore when the external force produced is the same but the moment arm (d) is smaller, the contribution from internal muscle force production increases. An example of this experienced in daily living is the increased difficulty in rising from a low seat position compared to a higher seated position. It has been demonstrated previously that when the joint-angle in the knee extensors is at 90o flexion (such as the end of LL and LX group ROM), the moment arm is considerably shorter [16] than when at 50o (the end of SL ROM). Therefore when exercising at 90o , internally the muscle must produce a greater amount of contractile force to overcome the external weight than that required at 50o knee angle. Again due to the overloading principle of training response, a larger number of motor units will have needed to be recruited to match the force demands at the longer muscle lengths, reflected by the increase in RMS-EMG activity of the VL muscle. In support of this hypothesis, Kubo et al. [21] trained the knee extensors isometrically at either 50o or 100o of knee flexion. Based on their MVC and EMG recordings, they estimated that the internal force on the quadriceps muscles was 2.3 times greater at longer muscle lengths (i.e. 100o ) than at shorter lengths. A further variable that must be considered is the influence of changing muscle lengths on the force-length relationship of muscle (for review see [23]). In short, when one alters the length of a muscle, the basic contractile units of individual muscle fibres, known as sarcomeres, also change length. The ability of sarcomeres (and thus muscle) to exert force is determined mainly by actin and myosin filaments interaction and cross-bridge formation. As sarcomere (or muscle) lengths increase, cross-bridges number and force is increased up to an optimal length. Beyond this length (i.e. with further lengthening), decreases cross-bridges formation and force are seen (NB. The caveat here is lies with contractile speed, and preceding type and

degree of muscle contraction [24]. If longer muscle lengths are less optimal for force production and cross-bridge formation than shorter muscle lengths, then greater motor unit recruitment will be necessary to overcome the external resistance. Therefore the two factors likely for greater activation in LL and LX compared to SL may be due to the greater internal mechanical stress on the muscle because of a shorter moment arm, and/ or the length of the muscle reducing cross-bridge formation and force production per sarcomere, all other things (contraction type,

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167

**Figure 7.** Vastus Lateralis muscle efficiency (i.e. activation per unit of torque) at week 8 at three joint-angles in SL (black bars), LL (white bars) and LX (grey bars). \* Significantly different to SL (P<0.05). N.B. there were no between

In the current study, oxygen consumption (VO2) was shown to be significantly greater at 80%1RM compared to 40% 1RM, and also significantly greater in both the LL and LX ROMs compared to SL ROM at 80% 1RM loading. VO2 is used in exercise physiology to provide valuable information, such as an indicator of energy expenditure. Oxygen is 'consumed' by the working muscle during oxidative phosphorylation in order to produce, maintain and/ or replenish the energy used during the many different processes involved with muscular contraction. Therefore if a particular form of exercise requires the use of greater volumes of oxygen, this indicates that the system is working harder in order to meet the demands. It has been demonstrated previously in humans that oxygen consumption increases with work

speed and history) being equal.

group differences at baseline.

degree of muscle contraction [24]. If longer muscle lengths are less optimal for force production and cross-bridge formation than shorter muscle lengths, then greater motor unit recruitment will be necessary to overcome the external resistance. Therefore the two factors likely for greater activation in LL and LX compared to SL may be due to the greater internal mechanical stress on the muscle because of a shorter moment arm, and/ or the length of the muscle reducing cross-bridge formation and force production per sarcomere, all other things (contraction type, speed and history) being equal.

be activated. Previous work using isometric contractions of the knee extensors, has demon‐ strated that the magnitude of maximal muscle activation is dependent on the joint-angle (and thus muscle-length) used during exercise, even when external torque produced is maintained at a similar level at the different joint angles [13]. This earlier study showed that activation of

unlike previous research, our participants exercised dynamically over a range-of-motion that

patterns. During absolute loading, weight lifted increased in a graded manner, and was reflected by significantly increased muscle activation between each absolute load in the training groups. This result was a more easily predicted outcome and reflects one of the fundamental properties of the neuromuscular system, i.e. the size principle [22], where a greater number of motor units are recruited in order to meet the increasing demands of force production. When exercising over longer muscle lengths (LL) and the complete ROM (LX), muscle activation was significantly greater during absolute and relative loading compared to shorter muscle lengths (SL). So why would a muscle exhibit greater activation whilst moving the same external weight but at different muscle lengths? By moving through a range of muscle lengths or joint-angles, the moment arm of the in-series elastic component (i.e. the tendon) also changes. As the amount of force needed to lift an external load (F) is F = f × d, where f is the internal force produced by the muscle and d is the length of the moment arm, when d is greater f will be smaller and vice versa, and therefore when the external force produced is the same but the moment arm (d) is smaller, the contribution from internal muscle force production increases. An example of this experienced in daily living is the increased difficulty in rising from a low seat position compared to a higher seated position. It has been demonstrated

LL and LX group ROM), the moment arm is considerably shorter [16] than when at 50o (the

amount of contractile force to overcome the external weight than that required at 50o knee angle. Again due to the overloading principle of training response, a larger number of motor units will have needed to be recruited to match the force demands at the longer muscle lengths, reflected by the increase in RMS-EMG activity of the VL muscle. In support of this hypothesis,

Based on their MVC and EMG recordings, they estimated that the internal force on the

lengths. A further variable that must be considered is the influence of changing muscle lengths on the force-length relationship of muscle (for review see [23]). In short, when one alters the length of a muscle, the basic contractile units of individual muscle fibres, known as sarcomeres, also change length. The ability of sarcomeres (and thus muscle) to exert force is determined mainly by actin and myosin filaments interaction and cross-bridge formation. As sarcomere (or muscle) lengths increase, cross-bridges number and force is increased up to an optimal length. Beyond this length (i.e. with further lengthening), decreases cross-bridges formation and force are seen (NB. The caveat here is lies with contractile speed, and preceding type and

knee flexion compared to both 30o

and identical to 30o

), longer muscle lengths (40-90o

) during exercise using absolute and relative loading

and 60o

flexion (such as the end of

or 100o of knee flexion.

) than at shorter

, internally the muscle must produce a greater

. In the current study,

, despite

) or over

the quadriceps is significantly greater at 90o

166 Electrodiagnosis in New Frontiers of Clinical Research

both short and long muscle lengths (0-90o

isometric MVC torque being significantly less than 60o

was predominantly over shorter muscle lengths (0-50o

previously that when the joint-angle in the knee extensors is at 90o

Kubo et al. [21] trained the knee extensors isometrically at either 50o

quadriceps muscles was 2.3 times greater at longer muscle lengths (i.e. 100o

end of SL ROM). Therefore when exercising at 90o

**Figure 7.** Vastus Lateralis muscle efficiency (i.e. activation per unit of torque) at week 8 at three joint-angles in SL (black bars), LL (white bars) and LX (grey bars). \* Significantly different to SL (P<0.05). N.B. there were no between group differences at baseline.

In the current study, oxygen consumption (VO2) was shown to be significantly greater at 80%1RM compared to 40% 1RM, and also significantly greater in both the LL and LX ROMs compared to SL ROM at 80% 1RM loading. VO2 is used in exercise physiology to provide valuable information, such as an indicator of energy expenditure. Oxygen is 'consumed' by the working muscle during oxidative phosphorylation in order to produce, maintain and/ or replenish the energy used during the many different processes involved with muscular contraction. Therefore if a particular form of exercise requires the use of greater volumes of oxygen, this indicates that the system is working harder in order to meet the demands. It has been demonstrated previously in humans that oxygen consumption increases with work intensity during constant isometric loading of the knee extensors [13, 14], and this is reflected by the increased VO2 at 80% 1RM compared to 40% 1RM, where although performing the same ROMs, participants were exerting greater force, requiring more energy to supply muscular contraction. The relative VO2 levels are much lower than normally encountered during aerobic exercise for example, due to the shorter duration of exercise bouts and greater contribution to energy supply from anaerobic sources such as ATP-PCr system and glycolysis. Of more interest in the present study was the fact that both LL and LX ROMs had significantly greater VO2 compared to SL at 80% 1RM. Previous research using near-infrared spectroscopy has demonstrated that during isometric exercise of the knee extensors, VL muscle VO2 is signifi‐ cantly increased at longer muscle lengths (60o and 90o ) compared to shorter muscle lengths (30o ). This was even despite the fact that MVC torque relative to the maximum torque capacity (MTC) tended to be greater (~85% of MTC) at 30o compared to both 60o and 90o (~75% MTC). A subsequent study by Kooistra et al. [14] demonstrated that knee extensor muscle activation and VO2 were significantly less, and time to VO2max significantly longer at the same relative torque levels at 30o compared to 60o and 90o . An additional indication of the increased stress at longer muscle lengths was the observation that at 80% 1RM, although covering almost half the ROM of LX, LL group showed a trend (though not statistically significant) to consuming greater volumes of oxygen. This suggests that the energetic cost of constantly working at longer muscle lengths is at least just as, if not more demanding than, alternating between longer and shorter muscle lengths even when over a relatively large ROM. With significantly higher heart rates in both LL and LX groups (and also greater blood pressure in LX) compared to SL during exercise, the results also suggest that the cardiovascular system was also under greater stress at longer muscle lengths. Taken into consideration with both the aforementioned differences between the LL and LX groups compared to SL with regards muscle activation and oxygen consumption, it appears that performing exercise over predominantly longer muscle lengths (or incorporating longer muscle lengths into a full ROM) present a more potent stress to both neuromuscular and cardiovascular systems than performing exercise over mainly shorter muscle lengths. So what factors are present that would require greater oxygen consumption at longer muscle lengths? First of all, as mentioned previously, there are a number of processes that occur in order for a muscle to contract and produce force. One such process has been termed excitation-contraction coupling, where an action potential induces the release of calcium ions (Ca2+) from the muscle membrane (sarcoplasmic reticulum) and these ions interact with the thin filaments of a sarcomere, allowing muscle contraction to occur. Ca2+ ions are then transported back into the sarcoplasmic reticulum for storage, allowing the muscle to relax. These processes are ATP-dependent (i.e. energy consuming) and as energy is consumed during activation, the amount of energy is measured as heat [25]. Therefore if greater activation of the muscle is occurring at longer muscle lengths, the possibility exists that the energy cost of this activation is also greater, and that this mechanism requires greater oxygen consumption to supply the energy. In addition, potentiation is force enhancement following muscle contrac‐ tion, and is dependent on the contractile history. Place et al.[26] showed that following fatiguing contractions in the quadriceps muscles at either shorter (35o ) or longer (75o ) muscle lengths, peak twitch potentiation and doublet force were significantly greater at shorter muscle lengths, which may also allow for a reduction in energy cost as activation may be reduced.

Secondly, we have already discussed the likelihood that due to the internal architecture of muscles and tendons, that the length of the moment arm will dictate that greater muscle force will have to be produced at longer muscle lengths compared to shorter muscle lengths. Production of the additional force through recruitment of more motor units would mean that more of the contractile machinery would be used and be consuming energy, as muscular contraction from cross-bridge cycling also requires ATP [27, 28]. Therefore the additional oxygen consumption observed at longer muscle lengths may be the result of both the energetic requirements of muscle activation and the increased energetic requirements of force produc‐ tion. This hypothesis is consistent with the fact that endurance performance is significantly reduced with time to fatigue at longer muscle lengths compared to shorter muscle length, regardless of of the intensities of loading and circulatory conditions [15, 26, 29]. Consistent with an increased oxygen demand, would be an increase in heart rate which was observed

How Deep Should You Squat to Maximise a Holistic Training Response?...

When exercise is performed on a regular basis, the above acute responses to a bout of exercise will eventually result in long-term adaptations (i.e. repeated bout effect), which will allow the body to complete the same exercise bout as before but with relatively less disturbance to homeostasis. During the resistance training program, the three groups performed exercise over the same range-of-motion as during the acute bouts (i.e. SL, LL and LX), with the only differences being the degree of loading. SL and LX exercised at 80% 1RM, whereas LL exercised

muscle forces to be as similar as possible during resistance training between SL and LL. Following 8 weeks of resistance training, absolute changes in muscle activation did not increase

longer lengths) during an isometric MVC. There have been conflicting reports throughout the literature concerning the possible increase in agonist activation following resistance training, as there have been studies published that have reported significant changes [3, 6-12], whereas some have not [30-33]. However, comparing longitudinal changes in agonist EMG both within and between studies can prove difficult due to methodological differences [34]. In one lengthspecific resistance training study, Thepaut-Mathieu et al. [19] reported an increase in iEMGforce relationships at the specific joint angles used during training. These findings were also supported by Kubo et al. [21] who found that iEMG of the quadriceps (rectus femoris, vastus lateralis and vastus medialis) increased significantly in groups that trained at either shorter or longer muscle lengths, with no differences between the groups at any of the joint-angles tested. In the current study there were also no significant differences in maximal activation levels between groups and muscle lengths. However, one of the main findings from the current study was the significant relative increases in activation at all muscle lengths in LX, at longer muscle lengths in LL, and significant decreases in activation at longer muscle lengths in SL. This is further evidence of the muscle length (or joint-angle) specificity phenomenon following resistance training. Whereas a previous study [21] found that relative quadriceps iEMG

training at shorter muscle lengths, our results show a decrease in activation at longer muscle lengths occurred following training at shorter muscle lengths. Interestingly from the study of Kubo et al. [21] was the fact that although iEMG increased within the range of ~25-45% over

; shorter lengths, 70o

) and the internal

more optimal lengths, 90o

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169

) following 12 weeks of isometric resistance

at 55% 1RM, where this was to allow the length of muscle excursion (50o

significantly at any of the angles tested (50o

increased at all measured knee angles (40-110o

between the groups.

Secondly, we have already discussed the likelihood that due to the internal architecture of muscles and tendons, that the length of the moment arm will dictate that greater muscle force will have to be produced at longer muscle lengths compared to shorter muscle lengths. Production of the additional force through recruitment of more motor units would mean that more of the contractile machinery would be used and be consuming energy, as muscular contraction from cross-bridge cycling also requires ATP [27, 28]. Therefore the additional oxygen consumption observed at longer muscle lengths may be the result of both the energetic requirements of muscle activation and the increased energetic requirements of force produc‐ tion. This hypothesis is consistent with the fact that endurance performance is significantly reduced with time to fatigue at longer muscle lengths compared to shorter muscle length, regardless of of the intensities of loading and circulatory conditions [15, 26, 29]. Consistent with an increased oxygen demand, would be an increase in heart rate which was observed between the groups.

intensity during constant isometric loading of the knee extensors [13, 14], and this is reflected by the increased VO2 at 80% 1RM compared to 40% 1RM, where although performing the same ROMs, participants were exerting greater force, requiring more energy to supply muscular contraction. The relative VO2 levels are much lower than normally encountered during aerobic exercise for example, due to the shorter duration of exercise bouts and greater contribution to energy supply from anaerobic sources such as ATP-PCr system and glycolysis. Of more interest in the present study was the fact that both LL and LX ROMs had significantly greater VO2 compared to SL at 80% 1RM. Previous research using near-infrared spectroscopy has demonstrated that during isometric exercise of the knee extensors, VL muscle VO2 is signifi‐

and 90o

compared to both 60o

). This was even despite the fact that MVC torque relative to the maximum torque capacity

A subsequent study by Kooistra et al. [14] demonstrated that knee extensor muscle activation and VO2 were significantly less, and time to VO2max significantly longer at the same relative

at longer muscle lengths was the observation that at 80% 1RM, although covering almost half the ROM of LX, LL group showed a trend (though not statistically significant) to consuming greater volumes of oxygen. This suggests that the energetic cost of constantly working at longer muscle lengths is at least just as, if not more demanding than, alternating between longer and shorter muscle lengths even when over a relatively large ROM. With significantly higher heart rates in both LL and LX groups (and also greater blood pressure in LX) compared to SL during exercise, the results also suggest that the cardiovascular system was also under greater stress at longer muscle lengths. Taken into consideration with both the aforementioned differences between the LL and LX groups compared to SL with regards muscle activation and oxygen consumption, it appears that performing exercise over predominantly longer muscle lengths (or incorporating longer muscle lengths into a full ROM) present a more potent stress to both neuromuscular and cardiovascular systems than performing exercise over mainly shorter muscle lengths. So what factors are present that would require greater oxygen consumption at longer muscle lengths? First of all, as mentioned previously, there are a number of processes that occur in order for a muscle to contract and produce force. One such process has been termed excitation-contraction coupling, where an action potential induces the release of calcium ions (Ca2+) from the muscle membrane (sarcoplasmic reticulum) and these ions interact with the thin filaments of a sarcomere, allowing muscle contraction to occur. Ca2+ ions are then transported back into the sarcoplasmic reticulum for storage, allowing the muscle to relax. These processes are ATP-dependent (i.e. energy consuming) and as energy is consumed during activation, the amount of energy is measured as heat [25]. Therefore if greater activation of the muscle is occurring at longer muscle lengths, the possibility exists that the energy cost of this activation is also greater, and that this mechanism requires greater oxygen consumption to supply the energy. In addition, potentiation is force enhancement following muscle contrac‐ tion, and is dependent on the contractile history. Place et al.[26] showed that following

and 90o

fatiguing contractions in the quadriceps muscles at either shorter (35o

lengths, peak twitch potentiation and doublet force were significantly greater at shorter muscle lengths, which may also allow for a reduction in energy cost as activation may be reduced.

) compared to shorter muscle lengths

. An additional indication of the increased stress

and 90o

) or longer (75o

) muscle

(~75% MTC).

cantly increased at longer muscle lengths (60o

torque levels at 30o compared to 60o

168 Electrodiagnosis in New Frontiers of Clinical Research

(MTC) tended to be greater (~85% of MTC) at 30o

(30o

When exercise is performed on a regular basis, the above acute responses to a bout of exercise will eventually result in long-term adaptations (i.e. repeated bout effect), which will allow the body to complete the same exercise bout as before but with relatively less disturbance to homeostasis. During the resistance training program, the three groups performed exercise over the same range-of-motion as during the acute bouts (i.e. SL, LL and LX), with the only differences being the degree of loading. SL and LX exercised at 80% 1RM, whereas LL exercised at 55% 1RM, where this was to allow the length of muscle excursion (50o ) and the internal muscle forces to be as similar as possible during resistance training between SL and LL. Following 8 weeks of resistance training, absolute changes in muscle activation did not increase significantly at any of the angles tested (50o ; shorter lengths, 70o more optimal lengths, 90o longer lengths) during an isometric MVC. There have been conflicting reports throughout the literature concerning the possible increase in agonist activation following resistance training, as there have been studies published that have reported significant changes [3, 6-12], whereas some have not [30-33]. However, comparing longitudinal changes in agonist EMG both within and between studies can prove difficult due to methodological differences [34]. In one lengthspecific resistance training study, Thepaut-Mathieu et al. [19] reported an increase in iEMGforce relationships at the specific joint angles used during training. These findings were also supported by Kubo et al. [21] who found that iEMG of the quadriceps (rectus femoris, vastus lateralis and vastus medialis) increased significantly in groups that trained at either shorter or longer muscle lengths, with no differences between the groups at any of the joint-angles tested. In the current study there were also no significant differences in maximal activation levels between groups and muscle lengths. However, one of the main findings from the current study was the significant relative increases in activation at all muscle lengths in LX, at longer muscle lengths in LL, and significant decreases in activation at longer muscle lengths in SL. This is further evidence of the muscle length (or joint-angle) specificity phenomenon following resistance training. Whereas a previous study [21] found that relative quadriceps iEMG increased at all measured knee angles (40-110o ) following 12 weeks of isometric resistance training at shorter muscle lengths, our results show a decrease in activation at longer muscle lengths occurred following training at shorter muscle lengths. Interestingly from the study of Kubo et al. [21] was the fact that although iEMG increased within the range of ~25-45% over all testing angles (40-110o ) following training at shorter muscle lengths, MVC only significantly increased between 40-80o in this group. Previous work from our laboratory has shown that MVC torque did not change significantly at longer muscle lengths following a period of resistance training at shorter muscle lengths [35], and results from the current investigation show that this could be in part be mediated by a reduction in maximal activation at these lengths. Further evidence of muscle-length specificity was the fact that only LX group, who covered an entire ROM, actually demonstrated a significant relative increase in activation at each angle tested, and also that LL only showed significant relative increases in activation at longer muscle lengths (lengths where the majority of training would have taken place). In order to allow us to describe the impact of changes in activation on strength changes, we have shown that there was significantly greater muscule efficiency (EMG per unit of torque) at longer muscle lengths (i.e. in LL and LX) compared to SL, following the 8-week training program. Changes in torque generating capacity are not accounted for solely, or at times at all by increased muscle activation. Changes in muscle architecture, morphology and/ or muscle specific tension are just a few of the many other factors that can impact a muscle's ability to produce force following resistance training as well as neural adaptations (for review see [34]). However in this case, there appears to be a relationship between the increased activation of the VL muscle and the changes in torque production following resistance training in LL and LX at longer muscle lengths.

factor mRNA [38]. The LL and LX groups when performing exercise at longer muscle lengths would have experienced a larger degree of muscle stretch compared to SL, and would have also been simultaneously producing force. In addition, because LX group worked at an intensity of 80% 1RM, peak force generation would also have been greater in this group. This is supported by the mean relative increase in VL muscle width being greatest in this group at all measurement sites, although due to the variation between subjects, this was not statistically significant. It is encouraging that despite the greater absolute force generations in LX compared to LL, the LL group (who remained at longer muscle lengths throughout each training session, and therefore muscle stretch would probably have persisted compared to LX group who worked between shorter and longer muscle lengths), these two groups exhibited similar muscle hypertrophy responses. What is more, yet another encouraging aspect of LL training was the fact that at week 10, VL muscle widths were significantly greater in LL and LX at all measurement sites compared to baseline, whereas the SL group had returned to baseline values. Following a further two weeks of detraining, LX group muscle widths only remained significantly greater than baseline values at 75% femur length, whereas LL group retained post-training increments in muscle width at all measurement sites for the entirety of the detraining period. Therefore not only does training at longer muscle lengths possibly confer more beneficial adaptations following training, but it also appears to allow retention of these adaptations for a longer period of time. This is a positive finding from the current study, in that following any periods on illness, injury or tapering that occur to the individual, longerterm retention of the benefits of the preceding resistance exercise will minimise the impact of

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171

Performing resistance training over predominantly longer muscle lengths compared to shorter muscle lengths produces stepwise degrees of acute muscular, energetic and cardiovascular responses, which then culminate to differential magnitudes of chronic training as well as detraining adaptations. As a progression to the earlier research evidence from isometric exercise in terms of both acute [13, 14] and chronic [21] muscle length-specific training, the current study is the first to systematically show that dynamic exercise at longer muscle lengths also results in greater activation and oxygen consumption. The nature of the acute responses suggests that the muscle is more physiologically stressed at longer muscle lengths. Following a prolonged period of resistance training (i.e. an accumulation of training bouts), we show that long-length trained muscle exhibits relatively greater muscle activation, neuromuscular efficiency and hypertrophy compared with its short-length trained counterpart. Similarly with detraining, long-length training was associated with a greater retention of improvements in muscle characteristics. It is likely that in this case also, the more beneficial size increments in particular, were the result of greater physiological stress, a result of the combined effects of smaller moment arm and enhanced muscle stretch. These findings have implications for

athletic, elderly, or post-operative populations to name but a few end users.

such deleterious events.

**5. Conclusion**

As indicated above, one of the other factors influencing changes in torque or force production following resistance exercise is muscle morphology, such as size. There is a strong positive relationship between the size of a muscle and the force it is able to exert [1]. In the current study, all of the three training groups increased the size of the VL muscle at proximal (25%), central (50%) and distal (75% of femur length) measurement sites at week 8. However in the SL group, the muscle size increment was more significant centrally rather than at proximal and distal sites of the VL, whereas both LL and LX had fairly equal distribution of size increment along the length of the muscle. Firstly, this information suggests that the resistance training program was effective in increasing muscle size, which is a well established charac‐ teristic of resistance training. Secondly, the results also suggest that the ROM involved during resistance training (i.e. the muscle lengths used) may produce region specific variations in muscle growth. Our laboratory has provided more conclusive evidence that muscle size increments at distal regions are enhanced to a greater degree immediately following resistance training at longer muscle lengths [35], however in the current study this was only apparent following two weeks of detraining, although these were still present following a total of four weeks detraining. The region specific variation in muscle size has been previously documented throughout literature (e.g. [31]), and is probably due to the unique way in which forces are transmitted along the length of a muscle when exercised at different lengths. Forces in muscles are transmitted both serially and in parallel [36], and when training at longer muscle lengths, there may be a more pronounced parallel transmission of force at distal regions of the muscle, providing a stimulus for growth in this location. In terms of muscle growth, force production and muscle stretch are potent stimulators of muscle protein synthesis, with a combination of both having an additive effect [37]. In vitro experiments have shown that when muscle cells are stretched to longer lengths, there is a marked increase in protein synthesis and growth factor mRNA [38]. The LL and LX groups when performing exercise at longer muscle lengths would have experienced a larger degree of muscle stretch compared to SL, and would have also been simultaneously producing force. In addition, because LX group worked at an intensity of 80% 1RM, peak force generation would also have been greater in this group. This is supported by the mean relative increase in VL muscle width being greatest in this group at all measurement sites, although due to the variation between subjects, this was not statistically significant. It is encouraging that despite the greater absolute force generations in LX compared to LL, the LL group (who remained at longer muscle lengths throughout each training session, and therefore muscle stretch would probably have persisted compared to LX group who worked between shorter and longer muscle lengths), these two groups exhibited similar muscle hypertrophy responses. What is more, yet another encouraging aspect of LL training was the fact that at week 10, VL muscle widths were significantly greater in LL and LX at all measurement sites compared to baseline, whereas the SL group had returned to baseline values. Following a further two weeks of detraining, LX group muscle widths only remained significantly greater than baseline values at 75% femur length, whereas LL group retained post-training increments in muscle width at all measurement sites for the entirety of the detraining period. Therefore not only does training at longer muscle lengths possibly confer more beneficial adaptations following training, but it also appears to allow retention of these adaptations for a longer period of time. This is a positive finding from the current study, in that following any periods on illness, injury or tapering that occur to the individual, longerterm retention of the benefits of the preceding resistance exercise will minimise the impact of such deleterious events.
