Effect of delayed-onset muscle soreness on muscle recovery after a fatiguing isometric contraction

An increase to above-baseline levels of electromyography (EMG) mean power spectral frequency (MPF) has been observed previously during muscle recovery following fatiguing contractions and has been explained by membrane hyperpolarization due to increased activation of the Na⁺–K⁺ pump. It is hypothesized that this membrane mechanism is impaired by muscle fiber damage following eccentric exercise. Thus, the aim of the study was to investigate surface EMG signal characteristics during recovery from fatigue after eccentric exercise. Ten healthy subjects performed sustained isometric knee extensions at 40% of the maximal torque (MVC) until task failure before, immediately after and 24 and 48 h after eccentric exercise. Bipolar surface EMG signals were recorded from six locations over the quadriceps during the sustained isometric contraction and during 3-s long contractions at 40% MVC separated by 1-min intervals for 15 min (recovery). Before the eccentric exercise, MPF of EMG signals increased to values above baseline during recovery from the fatiguing isometric contraction ( P <0.001), whereas immediately after and 24 and 48 h after the eccentric task, MPF was lower than baseline during the entire recovery period ( P <0.01). In conclusion, delayed-onset muscle soreness abolished the supranormal increase in EMG MPF following recovery from fatigue.     

Discharge characteristics of motor units and the surface EMG during fatiguing isometric contractions at submaximal tensions

The RMS amplitude of the surface electromyogram (EMG) and the frequency of discharge of motor units was examined throughout the duration of isometric contractions of the adductor pollicis muscles sustained to fatigue at tensions of 25, 40, and 55% of the maximum voluntary strength (MVC) of eight male subjects during fatiguing isometric contractions. The maximum strength of the muscle and the EMG above the adductor pollicis muscles was also assessed during 3 s of voluntary and electrically induced isometric contractions interposed at 25, 50, 75, and 100% of the duration of the fatiguing contractions. At the point of fatigue from submaximal isometric contractions, the RMS amplitude of the surface EMG was highest for contractions at 55 as compared to 40 and 25% MVC. The lower RMS amplitude of the EMG during contractions at lower as compared to higher tensions at the point of fatigue was paralleled by a lower discharge frequency of the alpha motor neurons in the fatigued muscle during contractions at 25% as compared to 40 and 70% MVC. The reduction in discharge frequency was probably of a sufficient order of magnitude to account for the lower amplitude of the EMG at the end of fatiguing isometric contractions at lower tensions.     

Effect of high-intensity intermittent cycling sprints on neuromuscular activity

High-intensity intermittent sprints induce changes in metabolic and mechanical parameters. However, very few data are available about electrical manifestations of muscle fatigue following such sprints. In this study, quadriceps electromyographic (EMG) responses to repeated all-out exercise bouts of short duration were assessed from maximal voluntary isometric contractions (MVC) performed before and after sprints. Twelve men performed ten 6-s maximal cycling sprints, separated by 30-s rest. The MVC were performed pre-sprints ( pre), post-sprints ( post), and 5 min post-sprints ( post5). Values of root-mean-square (RMS) and median frequency (MF) of vastus lateralis (VL) and vastus medialis (VM) were recorded during each MVC. During sprints, PPO decreased significantly in sprints 8, 9, and 10, compared to sprint 1 (- 8 %, - 10 %, and - 11 %, respectively, p < 0.05). Significant decrements were found in MVC post (- 13 %, p < 0.05) and MVC post5 (- 10.5 %, p < 0.05) compared to MVC pre. The RMS value of VL muscle increased significantly after sprints (RMS pre vs. RMS post: + 15 %, p < 0.05). Values of MF decreased significantly in both VL and VM after sprints. In conclusion, our results indicate that the increase in quadriceps EMG amplitude following high-intensity intermittent short sprints was not sufficient to maintain the required force output. The concomitant decrease in frequency components would suggest a modification in the pattern of muscle fiber recruitment, and a decrease in conduction velocity of active fibers.     

最大随意收缩诱发肌肉疲劳的表面肌电信号变化

目的 研究最大随意收缩（MVC）诱发肌肉疲劳的表面肌电（sEMG）信号的变化规律。方法 10名青年志愿者完成肱二头肌非疲劳性持重负荷试验和疲劳性最大随意收缩试验。采用线性和非线性信号处理方法,分析MVC下降到80％、70％、60％和50％时刻的sEMG信号特征变化并与非疲劳性活动时相同肌肉活动水平的sEMG信号进行比较。结果 疲劳状态下的平均肌电值（AEMG）和确定性线段百分比（％DET）显著高于非疲劳状态,而平均功率谱（MPF）和Lempel—Ziv复杂度[C（n）]显著低于非疲劳状态。随着肌肉疲劳的发展,MPF和C（n）显著递减,而AEMG和％DET无显著变化。AEMG、MPF和C（n）在疲劳状态和负荷水平间存在显著的交互作用。结论最大随意收缩诱发肱二头肌疲劳过程中,sEMG信号的MPF和C（n）单调递减;疲劳和非疲劳状态的sEMG信号特征存在显著差异。     

Evolution of electromyographic signal, running economy, and perceived exertion during different prolonged exercises

The purpose of this study was to compare the electromyographic (EMG) signal of the vastus lateralis muscle obtained during a run section of a triathlon and at the end of a prolonged run performed at the same running velocity. Seven subjects were studied on three occasions: a 2 h 15 min triathlon (30 min swimming, 60 min cycling, and 45 min treadmill running at 75% of the maximal aerobic speed), a 2 h 15 min run, where the last 45 min (Prolonged Run, PR) were run at the same speed as the Triathlon Run (TR) on a motorized treadmill, and a 45 min Isolated Run (IR) performed at the same TR and PR velocity. The three experimental trials were randomised. Oxygen uptake (VO2), heart rate (HR), and EMG data were recorded during the three run sections. The results confirm a greater VO2 and HR during PR compared with IR (P<0.01) and TR (P<0.05). Also the VO2 values obtained during TR were significantly greater compared to IR (P < 0.05). EMG signal, obtained from the vastus lateralis muscle during 4 sec of isometric contraction at 35 % of maximal voluntary contraction (MVC), showed that after PR the mean power frequency (MPF) shifted significantly to lower frequencies (P<0.01) compared with MPF recorded before the prolonged run. Moreover, the signal amplitude (RMS) was increased significantly after PR in comparison to pre-trial (P < 0.01). Similar results were obtained for the TR at P < 0.05. The integrated EMG flow, QIEMG (iEMG/burst duration), recorded during all run sections, was significantly increased near the end of PR (i.e. 2 h 10 min of running) compared with QiEMG recorded after 1 h 30 min of running. No significant increase in QiEMG was observed with TR and IR situations. The results suggest that a long exercise bout of running led to a greater increase in muscle fatigue compared with a triathlon or an isolated run performed at the same running speed. In addition it is suggested that the rating of perceived exertion recorded during isometric contractions is a good indice to approach the level of fatigue during prolonged exercises.     

EMG power spectrum analysis of first dorsal interosseous muscle in pianists

PURPOSE: Long-term piano training may induce potential biochemical and structural adaptations in hand intrinsic muscles or alter the motor strategy in the nervous systems. METHODS: This study investigated, by electromyography (EMG) and strain gauge, whether this pedagogical training is aerobic or anaerobic by nature. Changes in EMG power spectrum during incremental isometric muscle contractions before and after a sustained, fatiguing muscular performance were studied in the first dorsal interosseous muscle (FDI) of 13 female pianists and 15 sedentary controls. The maximal voluntary contraction (MVC) force of the FDI and the time needed to induce fatigue were also measured. RESULTS: During fatigue, the median frequency (MF) shifted toward the low frequencies in both groups. The MF of 50% and 25%MVC subsequent to fatigue were significantly lower than those before fatigue in the control group (104.0 +/- 45.5 Hz vs 116.7 +/- 41.4 Hz at 50%, P < 0.05; 114.7 +/- 43.4 Hz vs 123.3 +/- 46.7 Hz at 25%, P < 0.05). There were no significant MF changes within the pianists before and after fatigue. The root mean squares (RMS) of 50%, 25%, and 10%MVC of control subjects after fatigue were significantly higher than those before fatigue, but not in pianists. There was no difference in MF and RMS between the two groups at any level of %MVC before or after fatigue, but the pianists tended to show higher MF at each level of %MVC. Although the MVC was similar in both groups, the pianists, however, needed a much longer time to induce fatigue than controls (14.3 +/- 5.8 min vs 5.8 +/- 3.3 min, P < 0.005). CONCLUSIONS: These observations are discussed in terms of the training-induced metabolic adaptations, the changes in the strategy of motor unit recruitment, and the possibility of differences in muscle fiber composition. This study indicates that piano training should not be classified as power training, rather as endurance training. Such findings may also provide information about fine motor training for athletes and other professionals.     

电极放置位置对表面肌电信号特征的影响

目的:探讨不同电极放置位置对表面肌电(sEMG)信号特征的影响。方法:健康男性大学生14名,分别采用50%、70%和90%最大随意收缩力量(MVC)强度静态屈肘运动诱发肱二头肌疲劳,同步采集肱二头肌上三个不同电极位置上的sEMG信号,采用线性和非线性信号分析方法,计算线性指标平均肌电值(AEMG)和平均功率频率(MPF)以及非线性指标C(n)和DET%。结果:电极位置只对sEMG的四个测试指标的均值产生影响,对其变化斜率无明显影响,而四个指标的变化斜率中,除AEMG变化斜率对负荷强度无明显区分度外,其余三个指标对负荷强度均有良好区分度。结论:采用MPF、C(n)和DET%变化斜率作为评价局部肌肉疲劳的客观指标,既可以忽略电极放置位置对sEMG的影响,又对不同负荷强度有良好区分度。     

Mechanomyography and electromyography during and after fatiguing shoulder eccentric contractions in males and females

The aim of this study was to investigate changes in mechanomyographic (MMG) and the surface electromyographic (EMG) signals during and after fatiguing shoulder eccentric contractions in a group consisting of 12 males and 12 females. Exerted force, MMG, EMG, pain and rate of perceived exertion were assessed before, during and after repeated high-intensity eccentric exercises. Bouts of eccentric contractions caused a decrease in the exerted force for males (P<0.05) and an increase in the rate of perceived exertion and pain for both genders (P<0.05). During eccentric exercise, the root mean square (RMS) values of the MMG signal increased (P<0.05). The mean power frequency (MPF) values of the EMG signal decreased at the end of each eccentric bout for both genders (P<0.05); the decrease was higher for females compared with males (P<0.05). Immediately after eccentric exercise in static abduction of the upper limbs, the MMG RMS and MPF values increased (P<0.05). The present study showed that (1) neuromuscular changes associated with pain and changes in muscle stiffness and (2) changes in motor units strategy during fatigue development in shoulder muscle are reflected in the MMG and EMG signals.     

Acute fatigue-induced changes in muscle mechanical properties and neuromuscular activity in elite handball players following a handball match

The purpose of the present study was to determine the acute fatigue development in muscle mechanical properties and neuromuscular activity in response to handball match play. Male elite handball players ( n = 10) were tested before and after a simulated handball match for maximal isometric strength [maximal voluntary contraction (MVC)] and rate of force development (RFD) with synchronous electromyography (EMG) recording, while maximal vertical jump parameters were assessed using force plate analysis. Quadriceps and hamstrings MVC and RFD decreased significantly post-match (∼10%, P <0.05 and ∼16–21%, P <0.05, respectively). During quadriceps, MVC mean EMG amplitude [mean average voltage (MAV)] decreased for the vastus lateralis (VL) and rectus femoris (RF) (21–42%, P ≤0.05), while MAV also decreased in the antagonist biceps femoris (BF) muscle (48–55%, P <0.01). During hamstring MVC, MAV was reduced in BF (31%, P <0.01). Maximum EMG amplitude during quadriceps MVC was reduced for the VL (28%, P <0.01) and the RF (5%, P <0.05). During hamstring MVC, maximum EMG was reduced for BF (21%, P <0.01). Post-match maximal jump height was reduced (5.2%, P <0.01), as was also work (6.8%, P <0.01), velocity of center of mass (2.4–4.0%, P <0.01) and RFD (∼30%, P <0.05). In conclusion, maximal (MVC) and rapid muscle force characteristics (RFD, impulse) were acutely affected concurrently with marked reductions in muscle EMG following handball match play, which may potentially lead to impaired functional performance.     

表面肌电信号指标评价低负荷等长收缩诱发屈肘肌疲劳的敏感性和稳定性分析

目的:分析表面肌电(sEMG)信号指标评定低负荷等长收缩诱发运动性肌肉疲劳的敏感性和稳定性,为疲劳评价提供参考。方法:以15名男性青年志愿者为研究对象,先测试受试者屈肘关节最大自主收缩力矩(Maximal Voluntary Contration,MVC),令受试者右臂腕部拉住可在肘关节处产生20%MVC的重物,记录右侧肱桡肌和肱二头肌sEMG,至不能坚持负荷时结束实验。计算时域指标均方根振幅(RMS),基于傅里叶变换计算中值频率(MF)和平均功率频率(MPF),基于小波包分析计算中值频率(MDF)和平均功率频率(MNF),基于非线性分析计算Lempel-Ziv复杂度C(n)。评定上述指标在低负荷等长收缩诱发屈肘肌疲劳方面的敏感性和稳定性。结果:肱二头肌与肱桡肌sEMG的RMS随负荷实验持续时间延长表现出逐渐增加的趋势,而MF、MPF、MDF、MNF、C(n)则表现出逐渐减小的趋势。MF、MPF、MDF、MNF、C(n)与负荷运动持续时间之间具有显著负相关,RMS与负荷运动持续时间之间具有显著的正相关。肱二头肌与肱桡肌RMS反映疲劳的敏感性和稳定性优于其他指标。结论:由于低负荷收缩诱发疲劳引起募集运动单位数量增加等因素作用,RMS表现出较显著的增加趋势,是评定由低负荷等长收缩诱发屈肘肌疲劳的较好指标。     

Why does knee extensor muscles torque decrease after eccentric-type exercise?

AIM: The purpose of this study was to re-examine central and peripheral origins of neuromuscular fatigue after a highly strenuous eccentric exercise of the knee extensor muscles (KE) using both voluntary/evoked contractions and electromyographic recordings (EMG). METHODS: Before, and 30 min after 15 min of intermittent one-logged downhill running, maximal percutaneous electrical stimulations (single twitch, 0.5 s tetanus at 20 Hz and 80 Hz) were applied to the femoral nerve of 10 male subjects. Electrically evoked superimposed twitches were delivered during isometric maximal voluntary contraction (MVC) to determine maximal voluntary activation (%VA). Vastus lateralis (VL), vastus medialis (VM) and biceps femoris (BF) EMG were recorded during MVC and quantified using the root mean square (RMS) value. M-wave characteristics were also determined. RESULTS: KE MVC and %VA decreased significantly with fatigue (-19.6+/-6.1%; P<0.001 and -7.8+/-6.6%; P<0.01, respectively). Peak tetanus tension at 20 and 80 Hz (P20 and P80, respectively) declined (P<0.001), concurrently with a decrement of the P20 x P80(-1) ratio (-37.3+/-16.6%; P<0.001). Antagonist muscle coactivation, RMS to M-wave peak-to-peak amplitude and MVC x P80(-1) ratios were unchanged after the fatiguing exercise. CONCLUSIONS: The results reveal that part of the large loss in MVC may have a central origin but most of the MVC decrement is due to the presence of low-frequency fatigue while possible contractile failure cannot be excluded.     

Effects of muscle damage induced by eccentric exercise on muscle fatigue

PURPOSE: The present study was designed to determine to what extent muscle damage induced by repetitive eccentric exercise with maximal voluntary effort (ECC) affects the time course of central and peripheral fatigue during sustained maximal voluntary contraction (MVC). METHODS: Ten healthy male volunteers were asked to perform brief (control MVC) and sustained MVC (fatigue test of 60 s in duration) with elbow flexion before and 2 and 4 d after ECC. Transcranial magnetic stimulation (TMS) was applied to the motor cortex to determine changes in voluntary activation (VA), the size of the motor evoked potential (MEP), and length of electromyographic (EMG) silencing. The ratio of the root mean square value for the surface EMG of the biceps brachii and exerted force within 50 ms before TMS was also calculated (RMS/F). RESULTS: In two subjects, no significant changes in MVC and muscle soreness were seen after ECC so that their data was excluded from further analysis. Control MVC and muscle soreness was significantly decreased and increased, respectively, 2 and 4 d after ECC compared with that before ECC (P < 0.001). During the fatigue test, VA, which was determined by a phasic increase in the twitch force after TMS, significantly decreased 2 and 4 d after ECC compared with that beforehand (P < 0.01). In addition, the RMS/F was significantly increased 2 and 4 d after ECC (P < 0.001). Although the degree of facilitation of the MEP was significantly increased (P < 0.05), the length of EMG silencing was less affected by ECC. CONCLUSIONS: Muscle damage and/or muscle soreness induced by repetitive eccentric exercise with maximal effort may be a strong modifier of central and peripheral fatigue during sustained MVC.     

Changes in neuromuscular performance in voluntary and reflex contraction during strength training in man

Eleven male subjects went through heavy resistance strength training 3 times a week for 16 weeks. The training program consisted mainly of dynamic exercise for the knee extensor muscles with loads of 80%-120% of one maximum repetition in the squat lift. The investigation was undertaken to examine effects of strength training on neuromuscular performance both in voluntary and reflex contractions with special interest in their possible relationships. In addition to a large (P less than 0.001) increase in maximal isometric force and a more (P less than 0.05) economical activation of the knee extensor muscles, significant (P less than 0.05) improvements were noted in isometric force-time parameters, which were related (P less than 0.05) to the increase in the fast-twitch/slow-twitch (FT/ST) muscle fiber area ratio. No changes were observed in reflex time components, but the relative change in reflex electromechanical delay (EMD) was related (P less than 0.05) to the relative change in the FT/ST area ratio. A significant (P less than 0.05) decrease in the peak-to-peak amplitude of the reflex electromyogram (EMG), was noted during the training, and a decrease (P less than 0.05) in reflex EMG/force ratio was related (P less than 0.02) to the change in maximal integrated electromyogram (iEMG)/force ratio of the voluntary contraction. The decrease of reflex EMG may indicate a change in sensitivity of the muscle spindle. The interrelationship between the changes in EMG/force ratios of the reflex and voluntary contractions suggests that the mechanical response of individual muscle fibers of the respective motor units has improved.     

Submaximal motor unit firing rates after 8 wk of isometric resistance training

PURPOSE: The purpose of this study was to test the hypothesis that average motor unit firing rates change in parallel with the contractile properties of vastus lateralis following 8 wk of isometric resistance training. METHODS: The firing rates from more than 400 motor units of vastus lateralis were obtained during voluntary isometric contractions of 50% MVC, before and again after training in male subjects (N = 10) and their untrained controls (N = 10). Single motor unit spike trains were recorded with tungsten microelectrodes. RESULTS: Training resulted in a 36% (P < 0.05) increase in MVC. We also found significant increases (P < 0.05) in maximal twitch amplitude (+17%), time to peak tension (+9%) and the maximal instantaneous rate of contraction (+20%) in the trained leg of the experimental group. Neither the maximal integrated EMG nor the rate of increase of integrated EMG was different after training. There were no significant changes in any of these measures from the untrained leg or the control group. Average firing rates were not different after training despite the increase in twitch contractile speed. CONCLUSION: These findings suggest that the control properties of the nervous system are not altered despite sizable changes in the contractile properties of muscle following 8 wk of resistance training.     

Acute fatigue impairs neuromuscular activity of anterior cruciate ligament‐agonist muscles in female team handball players

In sports, like team handball, fatigue has been associated with an increased risk of anterior cruciate ligament (ACL) injury. While effects of fatigue on muscle function are commonly assessed during maximal isometric voluntary contraction (MVC), such measurements may not relate to the muscle function during match play. The purpose of this study was to investigate the effect of muscle fatigue induced by a simulated handball match on neuromuscular strategy during a functional sidecutting movement, associated with the incidence of ACL injury. Fourteen female team handball players were tested for neuromuscular activity [electromyography (EMG)] during a sidecutting maneuver on a force plate, pre and post a simulated handball match. MVC was obtained during maximal isometric quadriceps and hamstring contraction. The simulated handball match consisted of exercises mimicking handball match activity. Whereas the simulated handball match induced a decrease in MVC strength for both the quadriceps and hamstring muscles (P<0.05), a selective decrease in hamstring neuromuscular activity was seen during sidecutting (P<0.05). This study shows impaired ACL‐agonist muscle (i.e. hamstring) activity during sidecutting in response to acute fatigue induced by handball match play. Thus, screening procedures should involve functional movements to reveal specific fatigue‐induced deficits in ACL‐agonist muscle activation during high‐risk phases of match play.     

The clinical significance of electromyography normalisation techniques in subjects with anterior cruciate ligament injury during treadmill walking

This study investigated the clinical interpretation of three electromyographic (EMG) normalisation techniques to detect neuromuscular alterations in patients diagnosed with anterior cruciate ligament knee injury during treadmill walking. The EMG signal was normalised using the mean value during the gait cycles (MEA), the maximum value during the gait cycles (MAX), and a maximum voluntary isometric contraction (MVC) test in 16 male and female subjects. The MAX method detected an increase in total muscle activity in the injured limb rectus femoris (11.6%; P=0.02) while the MVC method detected decreased injured limb gastrocnemius medialis (GM) overall muscle activity (34.4%; P=0.02). The MAX method identified decreased GM activity in three portions of the gait cycle. This study indicates the importance of choosing the appropriate normalisation technique since its choice will change outcome measures and subsequent clinical interpretation.     

Effects of rate of force development on EMG amplitude and frequency

The purpose of this study was to compare the amplitude and frequency of the gastrocnemius EMG during ramp and ballistic contractions in highly trained sprint athletes. Sixteen female sprinters performed ramp and ballistic isometric contractions on a Biodex dynamometer. RMS and median frequency of the gastrocnemius EMG signals were obtained at the following torque levels: 25 +/- 5 %, 50 +/- 5 %, 75 +/- 5 %, 100 % MVC. The average rate of force development (RFD), was 610.2 +/- 123.1 N . m/s and 212.3 +/- 155.6 N . m/s for the ballistic and ramp contractions, respectively. In the ramp contractions the EMG amplitude increased as a function of torque. In the ballistic contractions the EMG amplitude decreased from 25 % to 100 % MVC. The highest RFD of 889.45 N . m/s was generated in ballistic contractions by a muscular activation pattern with high EMG amplitude (475.7 microV) and low frequency (116.7 Hz) at 25 % MVC. The findings suggest that the CNS utilizes different muscular activation patterns to modulate RFD in ramp and ballistic contractions. In ramp contractions the EMG amplitude increased linearly with force. In ballistic contractions a high RFD is generated with a muscular activation pattern consisting of high amplitude and low frequency at the start of the contraction.     

Quadriceps concentric and eccentric exercise 1: Changes in contractile and electrical activity following eccentric and concentric exercise

The purpose of this study was to determine whether or not losses of strength or endurance following eccentric and concentric exercise are associated with reduced excitation. The effects of eccentric and concentric work on maximal voluntary isometric contraction (MVC) and surface electromyogram (EMG) of the quadriceps were studied in 10 healthy male subjects following bench-stepping for 20 min with a constant leading leg. Prior to stepping and at 0, 0.25, 0.50, 0.75, 1, 3, 24 and 48 h afterwards the subjects performed a 30 s leg extension MVC with each leg during which the isometric force and the root mean square voltage of the EMG were recorded. In the eccentrically exercised muscles (ECC), MVC0-3 (force during the first 3 s of contraction) fell immediately after the bench-stepping exercise to 88+/-2% (mean+/-SE) of the pre-exercise value and remained significantly lower than the concentrically exercised muscles (p<0.05). The muscle weakness in the ECC could not be attributed to central fatigue as surface EMG amplitude at MVC0-3 increased during the recovery period. Muscle weakness after eccentric exercise appears to be due to contractile failure, which is not associated with a reduction in excitation as assessed by surface EMG. Muscular fatigue over 30 s did not change in the two muscle groups after exercise (p = 0.79), indicating that the ECC were weaker but not more fatiguable after exercise.     

Neural activation after maximal isometric contractions at different muscle lengths

PURPOSE: To investigate i) whether neural activation dependence on muscle length is preserved with neuromuscular fatigue and ii) whether fatigue induced by a maximal isometric exercise is muscle length dependent. METHODS: Twelve male subjects performed two fatiguing quadriceps muscle exercises: FS is the fatigue carried out at short muscle length (S) (S = 40 degrees of knee flexion) and FL is the fatigue at long muscle length (L) (L = 100 degrees). Before and after each fatiguing exercise (i.e., three maximal isometric contractions maintained until 80, 60, and 40% of the initial maximal torque, respectively), activation level (AL, assessed by means of twitch interpolation technique), EMG activity (RMS), and peak doublet torque (Pd) were measured at the two lengths (S and L). RESULTS: First, AL was greater (P < 0.05) at L compared with S before and after both exercises. Second, despite a similar decrease in maximal voluntary torque (approximately 21% of the initial value) after the two exercises, AL and RMS were significantly reduced after FS (P < 0.05) but remained unchanged after FL, whereas the Pd decrease was more pronounced after FL than FS (P < 0.05). Nevertheless, after a given fatiguing exercise (i.e., FS or FL), AL, RMS, and Pd changes were similar at both postexercise test lengths (S and L). CONCLUSION: These results clearly demonstrate that i) the neural activation dependence on quadriceps muscle length is maintained with fatigue, and ii) neuromuscular fatigue after maximal isometric contractions is dependent on the muscle length at which the exercise is performed: short length preferentially induces neural activation impairment, whereas long length leads to higher contractile failure.     

Electromechanical delay during knee extensor contractions.

The purpose of this study was to investigate the magnitude of electromechanical delay (EMD) and its possible dependence on muscle type, type of contraction, fatigue, level of force, initial muscle length, and muscle contraction velocity. This was achieved using an experiment that measured voluntary knee extensor torques and surface EMG activity for a variety of different contractile conditions in seven male subjects. EMD values were obtained using a cross-correlation technique in three experimental KIN-COM dynamometer conditions of vastus medialis, rectus femoris, and vastus lateralis. In the first condition, a series of 10 repetitive submaximal (50% and 70% MVC) isometric knee extensor contractions were performed at knee angles of 90 degrees and 130 degrees extension. In the second condition, 10 maximal isokinetic knee extensor contractions were performed during passive shortening and lengthening. As such, the dynamometer was used to passively move the knee joint at 30 degrees.s-1 and 60 degrees.s-1. Both during lengthening and shortening, the contractions occurred at an angular position of 110 degrees. In the last condition, a repetitive submaximal isometric knee extensor fatigue test was performed for 100 s (150 contractions). At 10, 40, and 90 s during the time course of this fatigue test, a series of 10 contractions were recorded. To avoid a phase lag, which is introduced with one-way filtering, the EMG was processed with a bidirectional low-pass filter application. A significant main effect in EMD for the factor level of force was found. The EMD values obtained at a force level of 50% MVC were longer than at 70% MVC (107 vs 98 ms).(ABSTRACT TRUNCATED AT 250 WORDS)