Sex comparisons of non-local muscle fatigue in human elbow flexors and knee extensors.

OBJECTIVES: To examine non-local muscle fatigue (NLMF) in both contralateral homologous and non-related heterogonous muscles for both sexes. METHODS: Ten men and nine women participated in this study. After the familiarization visit, subjects completed four separate randomly sequenced experimental visits, during which the fatiguing interventions (six sets of 30-second maximal isometric contractions) were performed on either their right elbow flexors or knee extensors. Before (Pre-) and after (Post-) the fatiguing interventions, the isometric strength and the corresponding surface electromyographic (EMG) amplitude were measured for the non-exercised left elbow flexors or knee extensors. RESULTS: For the non-exercised elbow flexors, the isometric strength decreased for both sexes (sex combined mean±SE: Pre vs. Post=339.67±18.02 N vs. 314.41±16.37 N; p⟨0.001). For the non-exercised knee extensors, there is a time × sex interaction (p=0.025), showing a decreased isometric knee extension strength for men (Pre vs. Post =845.02±66.26 N vs. 817.39±67.64 N; p=0.019), but not for women. CONCLUSIONS: The presence of NMLF can be affected by factors such as sex and muscle being tested. Women are less likely to demonstrate NLMF in lower body muscle groups.

Mechanomyographic responses for the biceps brachii are associated with failure times during isometric force tasks.

In order to characterize the physiological adjustments within the neuromuscular system that contribute to task failure, this study examined the surface mechanomyographic (MMG) response during maximal and submaximal isometric force tasks of the elbow flexors sustained to failure. The time and frequency components of the MMG signal have shown to be influenced by motor unit activation patterns as well as tetanus. Therefore, it was hypothesized that the rate of change for the MMG response would associate with failure times and would be reduced to a similar degree between the two tasks. The isometric force tasks were performed by the dominant elbow flexors of twenty healthy males (age: 25 ± 4 years) and MMG was collected from the biceps brachii. Regression analyses were used to model the relationships between the rates of change for MMG versus failure times. There were high levels of interindividual variability in the response patterns, yet the models demonstrated significant negative associations between the rate of change for the MMG responses and failure times during both tasks (R2  = 0.41-0.72, P < 0.05). Similarly, the mean MMG amplitude and frequency values were reduced to comparable levels at the failure point of the two tasks. The results of this study demonstrated that force failure is associated with the rate of diminution in the properties of the muscle force twitch.

Adaptations in antagonist co-activation: Role in the repeated-bout effect.

Eccentric exercise results in an adaptation which attenuates muscle damage from subsequent exercise-termed the "repeated-bout effect (RBE)." PURPOSE: Study examined antagonist co-activation and motor-unit recruitment strategy, assessed via dEMG, concomitant to the RBE. METHODS: Nine participants performed 5 sub-maximal isometric trapezoid (ramp-up, hold, ramp-down) contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Surface EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between mean firing rate (MFR) of each motor-unit and its recruitment threshold (RT) was examined using linear regression. Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. Surface EMG of the biceps and triceps were collected during eccentric exercise. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. RESULTS: Changes in MVC (-32±14% vs -25±10%; p = 0.034), ROM (-11% vs 6%; p = 0.01), and DOMS (31.0±19mm vs 19±12mm; p = 0.015) were attenuated following the second bout of exercise. Triceps EMG was reduced (16.8±9.5% vs. 12.6±7.2%; p = 0.03) during the second bout of eccentric exercise. The slope (-0.60±0.13 vs -0.70±0.18; p = 0.029) and y-intercept (46.5±8.3 vs 53.3±8.8; p = 0.020) of the MFR vs. RT relationship was altered during contractions at 80% of MVC prior to the second bout of eccentric exercise. No changes were observed at 50% of MVC. CONCLUSION: A reduction in antagonist co-activation during the second bout of eccentric exercise suggests less total force was required to move an identical external load. This finding is supported by the increased negative slope coefficient and an increased y-intercept of the linear relationship between RT and MFR.

Noninvasive Assessment of Skeletal Muscle Myosin Heavy Chain Expression in Trained and Untrained Men.

Fry, AC, Housh, TJ, Cramer, JB, Weir, JP, Beck, TW, Schilling, BK, Miller, JD, and Nicoll, JX. Noninvasive assessment of skeletal muscle myosin heavy chain expression in trained and untrained men. J Strength Cond Res 31(9): 2355-2362, 2017-Numerous conditions and types of physical activity (e.g., exercise, aging, and muscle-related diseases) can influence muscle fiber types and the proteins expressed. To date, muscle fibers can only be characterized by actually obtaining a tissue sample using the invasive muscle biopsy procedure. Mechanomyography (MMG) is the assessment of the vibration properties of contracting skeletal muscle and has been proposed as a possible noninvasive method for muscle fiber analysis. Therefore, the purpose of this project was to examine the feasibility of using MMG and muscle performance measures to noninvasively assess muscle fiber characteristics. Fifteen men (5 endurance-trained, 5 weight-trained, and 5 sedentary) provided muscle samples from their vastus lateralis muscle. These samples were analyzed for relative myosin heavy chain (MHC) protein expression, which is highly correlated with % muscle fiber type areas. Additionally, each subject performed several muscle performance tests, and MMG of the quadriceps was assessed during a knee extension exercise. Multiple regression was used to develop prediction equations for determining relative muscle content of MHC types I, IIa, and IIx. A combination of MMG and knee extension performance variables estimated types I, IIa, and IIx MHCs with approximately 80% accuracy. Although preliminary, these data suggest that muscle performance tests in addition to MMG assessments during a simple muscle performance task (knee extension) can be used to estimate muscle fiber type composition in a healthy male population. Such methods could ultimately be used to noninvasively monitor muscle health and fitness.

Unilateral fatiguing exercise and its effect on ipsilateral and contralateral resting mechanomyographic mean frequency between aerobic populations.

The purpose of this investigation was to establish a better understanding of contralateral training and its effects between homologous muscles following unilateral fatiguing aerobic exercise during variable resting postural positions, and to determine if any observable disparities could be attributed to the differences between the training ages of the participants. Furthermore, we hypothesized that we would observe a contralateral cross-over effect for both groups, with the novice trained group having the higher mechanomyographic mean frequency values in both limbs, across all resting postural positions. Twenty healthy male subjects exercised on an upright cycle ergometer, using only their dominate limb, for 30 min at 60% of their VO2 peak. Resting electromyographic and mechanomyographic signals were measured prior to and following fatiguing aerobic exercise. We found that there were resting mechanomyographic mean frequency differences of approximately 1.9 ± 0.8% and 0.9 ± 0.7%; 9.1 ± 0.3% and 10.2 ± 3.7%; 2 ± 1.8% and 3 ± 1.4%; and 0.9 ± 0.6% and 0.2 ± 1.3% between the novice and advanced trained groups (for the upright sitting position with legs extended 180°; upright sitting position with legs bent 90°; lying supine position with legs extended 180°; and lying supine with legs bent 90°, respectively), from the dominant and nondominant limbs, respectively. We have concluded that despite the relative matching of exercise intensity between groups, acute responses to contralateral training become less accentuated as one progresses in training age. Additionally, our results lend support to the notion that there are multiple, overlapping neural and mechanical mechanisms concurrently contributing to the contralateral cross-over effects observed across the postexercise resting time course.

The time course of short-term hypertrophy in the absence of eccentric muscle damage.

BACKGROUND: It has been proposed that the increase in skeletal muscle mass observed during the initial weeks of initiating a resistance training program is concomitant with eccentric muscle damage and edema. PURPOSE: We examined the time course of muscle hypertrophy during 4 weeks of concentric-only resistance training. METHODS: Thirteen untrained men performed unilateral concentric-only dumbbell curls and shoulder presses twice per week for 4 weeks. Sets of 8-12 repetitions were performed to failure, and training loads were increased during each session. Subjects consumed 500 ml of whole milk during training. Assessments of soreness, lean mass, echo intensity, muscle thickness, relaxed and flexed arm circumference, and isokinetic strength were performed every 72 or 96 h. RESULTS: Soreness, echo intensity, relaxed circumference, and peak torque data did not significantly change. Significant increases in lean mass, muscle thickness, and flexed circumference were observed within seven training sessions. Lean mass was elevated at tests #7 (+109.3 g, p = .002) and #8 (+116.1 g, p = .035), with eight different subjects showing changes above the minimal difference of 139.1 g. Muscle thickness was elevated at tests #6 (+0.23 cm, p = .004), #7 (+0.31 cm, p < .001), and #8 (+0.27 cm, p < .001), with ten subjects exceeding the minimal difference of 0.24 cm. There were no changes for the control arm. CONCLUSION: In individuals beginning a resistance training program, small but detectable increases in hypertrophy may occur in the absence of eccentric muscle damage within seven training sessions.

Low-load resistance training with low relative pressure produces muscular changes similar to high-load resistance training.

INTRODUCTION: This study compares the acute and chronic response of high-load resistance training (HL) to low-load resistance training with low blood flow restriction (LL-BFR) pressure. METHODS: Participants completed elbow flexion with either HL or LL-BFR or nonexercise. In the chronic study, participants in the HL and LL-BFR groups were trained for 8 weeks to determine differences in muscle size and strength. The acute study examined the changes in pretesting/posttesting (Pre/Post) torque, muscle swelling, and blood lactate. RESULTS: In the chronic study, similar changes in muscle size and strength were observed for both HL and LL-BFR. In the acute study, Pre/Post changes in the torque, muscle swelling, and blood lactate were similar between HL and LL-BFR. DISCUSSION: Our findings indicate that pressure as low as 50% arterial occlusion can produce similar changes in muscle mass and strength compared with traditional HL. Muscle Nerve 56: E126-E133, 2017.

Appendicular lean mass and site-specific muscle loss in the extremities correlate with dynamic strength.

BACKGROUND: Previous studies have reported the non-homogenous loss of muscle mass (site-specific muscle loss) with ageing, but this relationship to the loss of strength is not totally understood. PURPOSE: To investigate the relationship between maximal dynamic strength and site-specific muscle mass of the thigh and upper arm. METHODS: Thirty-five recreationally active men were separated into young-aged (YG, 20-39 years, n = 12), middle-aged (MG, 40-59 years, n = 13) and old-aged groups (OG, 60-75 years, n = 10). One-repetition maximum strength (1-RM; leg press, chest press, knee flexion, lat pull-down, and knee extension), muscle thickness (MTH, anterior and posterior thigh and upper arm) and appendicular lean mass (aLM) index were obtained from participants. Site-specific thigh MTH ratio was determined by dividing anterior thigh MTH (50% of thigh length) by posterior thigh MTH (50% of thigh length). RESULTS: aLM index was not significantly different between age groups, but a significantly smaller site-specific MTH thigh ratio was found in the OG. Collapsed across age groups both site-specific thigh MTH ratio and aLM index were significantly correlated with leg press, knee extension and knee flexion1-RM strength (r = 0·390-0·699), but not with knee extension: knee flexion (KE:KF) 1-RM strength ratios (r = 0·037-0·081). Separated by age groups only aLM index was correlated with KE:KF 1-RM ratio for the OG (r = 0·780). CONCLUSIONS: Site-specific thigh MTH ratio may be an important assessment tool in older individuals as it is different among age groups and is significantly related to dynamic maximal strength. However, maximal dynamic strength ratios appear to be less sensitive to differences in site-specific MTH ratios.

Intensity-dependent EMG response for the biceps brachii during sustained maximal and submaximal isometric contractions.

PURPOSE: There have been recent attempts to characterize the mechanisms associated with fatigue-induced task failure. We compared the time to failure and the corresponding changes in the surface electromyogram (EMG) during sustained maximal and submaximal isometric force tasks. METHODS: EMG activity was measured from the biceps brachii of 18 male participants as they sustained either a maximal or submaximal (60 % MVC) isometric contraction of the dominant elbow flexors until force could not be maintained above 55 % MVC. RESULTS: Intensity-dependent patterns of change were observed for EMG amplitude and mean power frequency (MNF) between the two force tasks. Interestingly, the only significant predictor of failure time was the rate of change in EMG MNF during the submaximal task (r (2) = 0.304). In addition, EMG amplitude at submaximal failure was significantly lower (p < 0.05) than the values obtained during MVC. CONCLUSIONS: The patterns of EMG response emphasize the basis of neuromuscular fatigue and task dependency. Additionally, our data suggest that the EMG MNF should be used when monitoring the progression of local muscle fatigue.

Examination of a neural cross-over effect using resting mechanomyographic mean frequency from the vastus lateralis muscle in different resting positions following aerobic exercise.

PURPOSE: To evaluate the potential neural cross-over effect between the vastus lateralis muscles in different postural resting positions. METHODS: Subjects exercised on an upright cycle ergometer, using only their dominate leg, for 2 min at 30 % VO2 peak. Following this warm-up, subjects then cycled (still using only their dominant leg) for 30 min at 60 % VO2 peak. After the aerobic phase, subjects cooled down (again, using only their dominant leg) for 2 min at 30 % VO2 peak. Resting mechanomyography mean frequency was measured prior to and following aerobic exercise. RESULTS: There was an approximate 6.3 ± 6.8 and a 10 ± 5.1 % increase (upright sitting position with the subject's knee joint angle fixed at 180°); an approximate 7 ± 6.6 and a 16.1 ± 6.5 % increase (upright sitting position with the subject's knee joint angle fixed at 90°); an approximate 0.5 ± 6.8 and 3.7 ± 5.6 % increase (lying supine position with the subject's knee joint angle fixed at 180°); and an approximately 2 ± 8.3 and 2.5 ± 8.6 % increase (lying supine position with the subject's knee joint angle fixed at 90°) in normalized mechanomyography mean frequency after aerobic exercise for the dominant and non-dominate vastus lateralis muscles, respectfully. CONCLUSION: There appears to be a statistically significant neural cross-over effect for the vastus lateralis muscle, during three of the four postural resting positions, with the non-dominant vastus lateralis muscle having a greater increase in mechanomyography mean frequency.

Differential Effects of Unilateral Concentric Vs. Eccentric Exercise on the Dominant and Nondominant Forearm Flexors.

The purpose of this study was to compare the electromyographic (EMG) intensity patterns after unilateral concentric vs. eccentric exercise in the dominant (DOM) and nondominant (NONDOM) forearm flexors. Twenty-six men (mean ± SD: age, 24.0 ± 3.7 years) volunteered to perform a maximal isometric muscle action of the DOM and NONDOM forearm flexors before (PRE) and immediately after (POST) a series of maximal concentric isokinetic or maximal eccentric isokinetic muscle actions of the DOM forearm flexors. The concentric isokinetic and eccentric isokinetic muscle actions were performed on separate days that were randomly ordered. However, in both cases, the subjects performed 6 sets of 10 maximal muscle actions. A bipolar surface EMG signal was detected from the biceps brachii of the DOM and NONDOM limbs during the PRE and POST isometric muscle actions. The signals were then analyzed with a wavelet analysis, and the resulting intensity patterns were classified with a paired pattern classification procedure. The results indicated that the EMG intensity patterns could be correctly classified into their respective PRE vs. POST categories with an accuracy rate that was significantly better than random (20 of 26 patterns = 76.9% accuracy) but only for the DOM limb following the eccentric muscle actions. All other classifications were not significantly better than random. These findings indicated that eccentric exercise had a significant influence on the muscle activation pattern for the forearm flexors. It is possible that the muscle damage resulting from eccentric exercise affects muscle spindle or golgi tendon organ or both activity, thereby altering the muscle activation pattern.

Influence of prolonged static stretching on motor unit firing properties.

INTRODUCTION: The purpose of this study was to examine the influence of a stretching intervention on motor control strategy of the biceps brachii muscle. METHODS: Ten men performed twelve 100-s passive static stretches of the biceps brachii. Before and after the intervention, isometric strength was tested during maximal voluntary contractions (MVCs) of the elbow flexors. Subjects also performed trapezoid isometric contractions at 30% and 70% of MVC. Surface electromyographic signals from the submaximal contractions were decomposed into individual motor unit action potential trains. Linear regression analysis was used to examine the relationship between motor unit mean firing rate and recruitment threshold. RESULTS: The stretching intervention caused significant decreases in y-intercepts of the linear regression lines. In addition, linear slopes at both intensities remained unchanged. CONCLUSIONS: Despite reduced motor unit firing rates following the stretches, the motor control scheme remained unchanged.

Effects of age on arterial stiffness and central blood pressure after an acute bout of resistance exercise.

PURPOSE: To investigate the influence of age on arterial stiffness and blood pressure after performing a resistance exercise bout. METHODS: Recreationally active men were separated into young (YG, n = 12, 26.5 ± 3.3 years), middle (MG, n = 14, 49.4 ± 5.7 years), and old (OG, n = 10, 67.4 ± 6.3 years)-aged groups. In a randomized cross-over design, participants performed control and exercise conditions with at least 3 days separating conditions. The exercise condition consisted of leg press, chest press, knee flexion, lat pulldown and knee extension at ~65% one-repetition maximum for three sets of 10 repetitions. Brachial and central blood pressures, augmented pressure, augmentation index, central and peripheral pulse wave velocities were measured prior to each condition and starting at 5 min post-exercise. RESULTS: Brachial systolic blood pressure (SBP) significantly increased similarly after exercise for all age groups (YG, 8 ± 8 mmHg; MG, 5 ± 5 mmHg; OG, 5 ± 6 mmHg; p < 0.05). However, central SBP did not significantly increase for any age group after exercise. Augmentation index significantly increased after exercise only in the YG (11 ± 8%, p < 0.05). Central pulse wave velocity did not significantly increase in any age group after exercise when compared to the control condition. CONCLUSIONS: When performing a whole body moderate resistance exercise bout, acute changes in arterial stiffness and blood pressure appear to be minimally affected by age.

An Examination of Fatigue Index and Velocity-Related Force Loss for the Forearm Flexors.

The purpose of this study was to examine the relationship between the fatigue index from an isokinetic fatigue test and the velocity-related decrease in peak torque for the forearm flexors. After a familiarization session, 13 habitually active males (mean ± SD, age = 23.8 ± 3.1 years) reported to the laboratory to perform either 50 repeated, maximal, concentric isokinetic muscle actions of the dominant forearm flexors at a velocity of 180°·s(-1) or 6 separate sets of 3 maximal concentric isokinetic muscle actions at randomly ordered velocities of 30, 90, 150, 270, and 330°·s(-1). The correlation between the relative percent declines in peak torque during the 2 isokinetic tests was then examined. The results indicated an inverse relationship (r = -0.75, p < 0.01) between these 2 variables. That is, the subjects who demonstrated the greatest fatigue indexes (i.e., those who were most susceptible to fatigue) were generally the most resistant to a velocity-related torque loss. These findings support the possibility of using a multiple-velocity isokinetic test to estimate fiber type composition, just as fatigue-based tests have been used.

Prolonged passive static stretching-induced innervation zone shift in biceps brachii.

The purpose of this study was to examine the influence of a bout of repeated and prolonged passive static stretching on the innervation zone (IZ) location of the human biceps brachii muscle. Eleven men performed 12 sets of 100-s passive stretches on their biceps brachii. Before (Pre) and immediately after (Post) the stretching intervention, isometric strength was tested during the maximal voluntary contractions (MVCs) of the forearm flexors. The subjects also performed several separate isometric forearm flexion muscle actions at 30%, 50%, and 70% of their predetermined MVCs for examining the locations of the IZ at different contraction intensities. The IZ was identified through multi-channel surface electromyographic (EMG) recordings from a linear electrode array. The stretching intervention induced an average of 10% isometric strength loss for the forearm flexors (mean±SD: Pre-MVC vs. Post-MVC=332.12±59.40 N vs. 299.53±70.51 N; p<0.001). In addition, the average IZ shift was nearly 4.5 mm in average in the proximal direction. However, this shift was not specific to the contraction intensity. We believe that the IZ shift was caused by the elongation of the entire muscle-tendon unit in the proximal direction. Therefore, caution should be taken when using surface EMG technique to examine possible changes in the EMG variables after a stretching protocol, as these variables can be contaminated by the shift of the IZ.

EMG spectral differences among the quadriceps femoris during the stretch reflex.

INTRODUCTION: The purpose of this study was to examine the electromyographic (EMG) spectral characteristics of the quadriceps femoris muscles during tendon tap stretch reflexes. METHODS: Sixteen healthy subjects (mean ± SD age = 21.2 ± 2.8 years) performed tendon tap reflexes of the leg extensors as surface EMG signals were detected from the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles of the dominant thigh. All EMG signals were processed with a wavelet analysis, and the resulting spectra were decomposed with nonparametric spectral decomposition. RESULTS: The results showed that the spectra for the VL had significantly more high-frequency power than those for the RF and VM, with similar spectral shapes for the RF and VM. CONCLUSIONS: These findings could be due to differences in the width of the innervation zone, or the fiber type composition of the muscles, although the latter seems to be more likely.

Local muscle endurance is associated with fatigue-based changes in electromyographic spectral properties, but not with conduction velocity.

The purpose of this study was to examine the associations amongst muscle fiber action potential conduction velocity (CV), spectral characteristics of the surface electromyographic (EMG) signal, and endurance time during a sustained submaximal isometric muscle action. Eleven men (mean±SD age=23±4yrs) performed a sustained, submaximal isometric muscle action of the dominant forearm flexors at 60% of the maximum voluntary contraction (MVC) until the designated force level could no longer be maintained. Sixteen separate bipolar surface EMG signals were detected from the biceps brachii with a linear electrode array during this contraction. Two channels from this array were used to measure CV, and one of these two channels was used for further EMG signal processing. The channels that provided the highest signal quality were used for the CV measurements and further data analysis. A wavelet analysis was then used to analyze the bipolar EMG signal, and the resulting wavelet spectrum was decomposed with a nonparametric spectral decomposition procedure. The results showed that the time to exhaustion during the sustained contraction was not correlated with the rate of decrease in CV, but it was highly correlated with both the decrease in high-frequency spectral power (r=0.947) and the increase in low-frequency spectral power (r=0.960). These findings are particularly interesting, considering that the decrease in traditional EMG spectral variables (e.g., mean frequency or median frequency) with fatigue is generally attributed to reductions in CV. While this may indeed be true, the present results suggested that other factors (i.e., other than CV) that can affect the shape of the EMG frequency spectrum during fatigue are more important in determining the endurance capabilities of the muscle than is CV.

Acute effects of concentric vs. eccentric exercise on force steadiness and electromyographic responses of the forearm flexors.

The purpose of this study was to examine the acute effects of concentric (CON) vs. eccentric (ECC) exercise on isometric strength, force steadiness, and surface electromyographic (EMG) responses of the forearm flexors. Fifteen resistance-trained men (mean ± SD: age = 23.7 ± 3.5 years; height = 178.9 ± 4.7 cm; body weight = 86.2 ± 9.8 kg) performed 6 sets of 10 maximal CON isokinetic or ECC isokinetic muscle actions using the dominant forearm flexors on 2 separate experimental visits. Before and immediately after the exercise interventions, isometric strength testing and submaximal trapezoid isometric contractions were performed, with bipolar EMG signals detected from the biceps brachii. The coefficient of variation of the force output from the mid 8-second portion of each submaximal trapezoid isometric contraction was calculated to assess force steadiness. In addition, the EMG signal was selected from the same portion as the force signal. The results showed that both CON and ECC caused similar isometric strength losses, but ECC caused a greater loss of force steadiness than CON. In addition, EMG amplitude increased similarly after both exercise interventions, but the magnitude of the increase in EMG mean frequency after ECC tended to be smaller, when compared with that after CON. These findings suggested that, even for resistance-trained individuals that are more resistant to ECC exercise-induced muscle damage than untrained individuals, their ability to maintain a steady submaximal force can be impaired after a bout of ECC exercise.

Acute effects of dynamic exercises on the relationship between the motor unit firing rate and the recruitment threshold.

The aim of this study was to compare the acute effects of concentric versus eccentric exercise on motor control strategies. Fifteen men performed six sets of 10 repetitions of maximal concentric exercises or eccentric isokinetic exercises with their dominant elbow flexors on separate experimental visits. Before and after the exercise, maximal strength testing and submaximal trapezoid isometric contractions (40% of the maximal force) were performed. Both exercise conditions caused significant strength loss in the elbow flexors, but the loss was greater following the eccentric exercise (t=2.401, P=.031). The surface electromyographic signals obtained from the submaximal trapezoid isometric contractions were decomposed into individual motor unit action potential trains. For each submaximal trapezoid isometric contraction, the relationship between the average motor unit firing rate and the recruitment threshold was examined using linear regression analysis. In contrast to the concentric exercise, which did not cause significant changes in the mean linear slope coefficient and y-intercept of the linear regression line, the eccentric exercise resulted in a lower mean linear slope and an increased mean y-intercept, thereby indicating that increasing the firing rates of low-threshold motor units may be more important than recruiting high-threshold motor units to compensate for eccentric exercise-induced strength loss.