Altered leg muscle activity in volleyball players with functional ankle instability during a sideward lateral cutting movement

Objectives

To study the activation patterns of tibialis anterior, peroneus longus and gastrocnemius lateralis muscles during a lateral shuffle maneuver in volleyball players with functional instability of the ankle joint.

Design

Observational case-control study.

Setting

Research laboratory.

Participants

Sixteen players with functional instability and 18 matched controls.

Main outcome measures

RMS values of tibialis anterior, peroneus longus and gastrocnemius lateralis muscles for the 50 ms before initial ground contact, timing of onset of muscle activity and linear envelopes for the period of ground contact were calculated.

Results

Onset values showed similar patterns of activation for both groups. In healthy subjects, gastrocnemius lateralis activated earlier, followed by peroneus longus and tibialis anterior. In the unstable subjects, gastrocnemius lateralis and peroneus longus activated at the same time, followed by tibialis anterior. Unstable subjects also presented lower peroneus longus activity during the 50 ms before initial ground impact, a lower peroneus longus peak magnitude and a higher gastrocnemius lateralis peak magnitude.

Conclusions

Volleyball players with ankle functional instability showed decreased peroneus longus activity before ground impact that may predispose them to repetitive sprains and explain their “giving way” sensation, since peroneus longus is the main ankle evertor and an important stabilizer against sudden and excessive inversion.     

Effects of regular Tai Chi practice and jogging on neuromuscular reaction during lateral postural control in older people

This study examined the effects of regular Tai Chi practice and jogging on the neuromuscular activity of the trunk, hip, and ankle joint muscles of older people during lateral postural perturbation. A total of 42 older people participated in the study and formed the Tai Chi, jogging, and sedentary control groups. Electromyography signals were collected from the peroneus longus, anterior tibialis, gluteus medius, and erector spinae during unpredictable mediolateral perturbation. The Tai Chi group exhibited significantly faster latencies of the tibialis anterior and erector spinae than the control group. The jogging group showed a significantly shorter neuromuscular reaction time of the erector spinae than the control group. No significant difference was observed between the Tai Chi and jogging groups. Long-term regular Tai Chi practice enhanced the neuromuscular reaction of the erector spinae and tibialis anterior to lateral perturbation and will help timely posture correction when lateral postural distributions occur.     

Foot posture is associated with morphometry of the peroneus longus muscle,tibialis anterior tendon,and Achilles tendon

The aim of this study was to investigate the association between foot type and the morphometry of selected muscles and tendons of the lower limb. Sixty‐one healthy participants (31 male, 30 female; aged 27.1 ± 8.8 years) underwent gray‐scale musculoskeletal ultrasound examination to determine the anterior‐posterior (AP) thickness of tibialis anterior, tibialis posterior, and peroneus longus muscles and tendons as well as the Achilles tendon. Foot type was classified based on arch height and footprint measurements. Potentially confounding variables (height, weight, hip and waist circumference, rearfoot and ankle joint range of motion, and levels of physical activity) were also measured. Multiple linear regression models were used to determine the association between foot type with muscle and tendon morphometry accounting for potentially confounding variables. Foot type was significantly and independently associated with AP thickness of the tibialis anterior tendon, peroneus longus muscle, and Achilles tendon, accounting for approximately 7% to 16% of the variation. Flat‐arched feet were associated with a thicker tibialis anterior tendon, a thicker peroneus longus muscle, and a thinner Achilles tendon. Foot type is associated with morphometry of tendons that control sagittal plane motion of the rearfoot; and the peroneus longus muscle that controls frontal plane motion of the rearfoot. These findings may be related to differences in tendon loading during gait.     

Eccentric reinforcement of the ankle evertor muscles after lateral ankle sprain

To compare two methods of ankle evertor muscle reinforcement after ankle sprain: concentric vs eccentric reinforcement. Eighteen subjects practising sport with first episode of ankle sprain. The first randomized group underwent conventional physical therapy including concentric reinforcement of the evertor ankle muscles [concentric group (CG)]. The second randomized group underwent eccentric reinforcement [eccentric group (EG)]. At the end of the physical therapy, the strength of the evertor muscles was tested using an isokinetic dynamometer. The measurements were peak torques in the concentric and eccentric modes; ankle strength deficits, expressed as percentages of the healthy ankle values recorded in the concentric and eccentric modes; ratios between concentric/eccentric values. After the concentric reinforcement, in the CG group, there is both significant concentric strength deficit and an eccentric strength deficit on the injured side in comparison with the healthy side. After the eccentric reinforcement in the EG group, the muscle strength was significantly greater during concentric movements. Eccentric rehabilitation therefore restored the strength of the injured evertor muscles. These results show the value of this method, especially as the weakness of these muscles after sprains is one of the main risk factors contributing to instability and the recurrence of sprains.     

Arthrogenic muscle response to a simulated ankle joint effusion

BACKGROUND: Arthrogenic muscle inhibition (AMI) is a continuing reflex reaction of the musculature surrounding a joint after distension or damage to the structures of that joint. This phenomenon has been well documented after knee joint injury and has been generalised to occur at other joints of the human body, yet minimal research has been conducted in this regard. The response of the muscles crossing the ankle/foot complex after ankle injury and effusion is not well understood. AMI may occur after an ankle sprain contributing to residual dysfunction. OBJECTIVE: To determine if AMI is present in the soleus, peroneus longus, and tibialis anterior musculature after a simulated ankle joint effusion. METHODS: Eight neurologically sound volunteers (mean (SD) age 23 (4) years, height 171 (6) cm, mass 73 (10) kg) participated. Maximum H-reflex and maximum M-wave measurements were collected using surface electromyography after delivery of a percutaneous stimulus to the sciatic nerve before its bifurcation into the common peroneal and posterior tibial nerves. RESULTS: The H-reflex and M-wave measurements in all muscles increased (p< or =0.05) after the simulated ankle joint effusion. CONCLUSIONS: Simulated ankle joint effusion results in facilitation of the soleus, peroneus longus, and tibialis anterior motoneurone pools. This may occur to stabilise the foot/ankle complex in order to maintain posture and/or locomotion.     

Neuromuscular control of the ankle during pre-landing in athletes with chronic ankle instability: Insights from statistical parametric mapping and muscle co-contraction analysis

ObjectiveThe present study aimed to compare the neuromuscular control of the muscles around the ankle between athletes with CAI and without history of any ankle sprain (Non-CAI) by using statistic parametric mapping (SPM) and co-contraction analyses.DesignCross-sectional study; Setting: Laboratory; Participants: 40 athletes (20 CAI, 20 Non-CAI) were pair-matched for age and gender.Main outcome measuresNeuromuscular control was examined using surface electromyography (EMG) amplitude and muscle co-contraction 200 ms before foot-contact with the ground during a jump-landing task.ResultsThe EMG amplitude of tibialis anterior, peroneus longus, and gastrocnemius medialis were analyzed using statistic parametric mapping. The CAI group exhibited decreased EMG amplitude of peroneus longus during preparation for foot-contact. There were no significant co-contraction differences between groups.ConclusionsOur findings demonstrate that SPM combined with the co-contraction provides a comprehensive EMG analysis to detect the differences of neuromuscular control between athletes with and without chronic ankle instability. Additionally, this finding indicates that CAI contributed to altered neuromuscular control during the pre-landing phase, which may contribute to re-injury mechanisms.     

Deficits in eccentric versus concentric torque in children with spastic cerebral palsy

PURPOSE: The purposes of this study were to determine whether ambulatory children with spastic cerebral palsy (CP) had abnormal isokinetic eccentric peak torque values at the knee and ankle, and to gain further insights on the influence of spasticity on voluntary force production in this population. METHODS: Twenty-four children with spastic CP (mean = 11.1+/-2.6 yr) and twenty children of comparable age with no neuromotor pathology (mean = 10.3+/-2.6 yr) participated in an isokinetic testing protocol on a Biodex dynamometer that measured eccentric and concentric peak torques of the knee extensors, knee flexors, ankle dorsiflexors, and ankle plantarflexors. Angular velocity of the eccentric trials was 30 degrees x s(-1) and of the concentric trials was 30 degrees x s(-1), 60 degrees x s(-1), and 120 degrees x s(-1). Peak torque values were normalized by body weight and compared across groups by using ANOVA procedures. Eccentric to concentric (E/C) peak torque ratios at 30 degrees x s(-1) were computed for each muscle and compared across groups. The torque values in CP were also expressed as a percent of the mean normalized value of the comparison group and compared across conditions using repeated measures ANOVA (P < 0.05). RESULTS: Children with CP demonstrated decreased eccentric and concentric peak torques for all muscle groups tested. The relative deficit in eccentric torque was less than the concentric torque and the decrement in concentric torque across speeds was greater in CP for all muscle groups except the ankle dorsiflexors. The E/C ratios for the knee extensors and flexors were also greater in CP. CONCLUSIONS: Children with CP have diminished eccentric as well as concentric peak torques at the knee and ankle. The influence of spasticity on voluntary force production can be inferred from the bias toward greater eccentric torque and the greater decrement in concentric torque across speeds in children with spastic CP.     

Intramuscular pressure and torque during isometric, concentric and eccentric muscular activity

Intramuscular pressures, electromyography (EMG) and torque generation during isometric, concentric and eccentric maximal isokinetic muscle activity were recorded in 10 healthy volunteers. Pressure and EMG activity were continuously and simultaneously measured side by side in the tibialis anterior and soleus muscles. Ankle joint torque and position were monitored continuously by an isokinetic dynamometer during plantar flexion and dorsiflexion of the foot. The increased force generation during eccentric muscular activity, compared with other muscular activity, was not accompanied by higher intramuscular pressure. Thus, this study demonstrated that eccentric muscular activity generated higher torque values for each increment of intramuscular pressure. Intramuscular pressures during antagonistic co-activation were significantly higher in the tibialis anterior muscle (42–46% of maximal agonistic activity) compared with the soleus muscle (12–29% of maximal agonistic activity) and was largely due to active recruitment of muscle fibers. In summary, eccentric muscular activity creates higher torque values with no additional increase of the intramuscular pressure compared with concentric and isometric muscular activity.     

Relationship between strength qualities and short track speed skating performance in young athletes

This study analyzed the relationships between isometric as well as concentric maximum voluntary contraction (MVC) strength of the leg muscles and the times as well as speeds over different distances in 17 young short track speed skaters. Isometric as well as concentric single‐joint MVC strength and multi‐joint MVC strength in a stable (without skates) and unstable (with skates) condition were tested. Furthermore, time during maximum skating performances on ice was measured. Results indicate that maximum torques during eversion and dorsal flexion have a significant influence on skating speed. Concentric MVC strength of the knee extensors was higher correlated with times as well as speeds over the different distances than isometric MVC strength. Multi‐joint MVC testing revealed that the force loss between measurements without and with skates amounts to 25%, while biceps femoris and soleus showed decreased muscle activity and peroneus longus, tibialis anterior, as well as rectus femoris exhibited increased muscle activity. The results of this study depict evidence that the skating times and speeds are primarily influenced by concentric MVC strength of the leg extensors. To be able to transfer the strength onto ice in an optimal way, it is necessary to stabilize the knee and ankle joints.     

Effect of isokinetic training on strength,functionality and proprioception in athletes with functional ankle instability

The purpose of this study was to investigate the effects of isokinetic exercise on strength, joint position sense and functionality in recreational athletes with functional ankle instability (FAI). Strength, proprioception and balance of 24 recreational athletes with unilateral FAI were evaluated by using isokinetic muscle strength measurement, ankle joint position sense and one leg standing test. The functional ability was evaluated using five different tests. These were; single limb hopping course (SLHC), one legged and triple legged hop for distance (OLHD–TLHD), and six and cross six meter hop for time (SMHT–CSMHT). Isokinetic peak torque of the ankle invertor and evertor muscles were assessed eccentrically and concentrically at test speeds of 120°/s. Isokinetic exercise protocol was carried out at an angular velocity of 120°/s. The exercise session was repeated three times a week and lasted after 6 weeks. At baseline, concentric invertor strength was found to be significantly lower in the functionally unstable ankles compared to the opposite healthy ankles (p < 0.001). This difference was not present after executing the 6 weeks exercise sessions (p > 0.05). Ankle joint position sense in the injured ankles declined significantly from 2.35 ± 1.16 to 1.33 ± 0.62° for 10° of inversion angle (p < 0.001) and from 3.10 ± 2.16 to 2.19 ± 0.98° for 20° of inversion angle (p < 0.05) following the isokinetic exercise. One leg standing test score decreased significantly from 15.17 ± 8.50 to 11.79 ± 7.81 in the injured ankles (p < 0.001). Following the isokinetic exercise protocol, all of the worsened functional test scores in the injured ankles as compared to the opposite healthy ankles displayed a significant improvement (p < 0.01 for OLHD and CSMHT, p < 0.001 for SLHC, TLHD, and SMHT). These results substantiate the deficits of strength, proprioception, balance and functionality in recreational athletes with FAI. The isokinetic exercise program used in this study had a positive effect on these parameters.     

Strength gains following different combined concentric and eccentric exercise regimens

BACKGROUND: Loss of muscle strength and cross-sectional area is a well-recognized consequence of spaceflight. Existing countermeasures have not been fully effective in preventing muscle weakness and atrophy in microgravity. Resistance exercise programs that consist of both eccentric and concentric actions have resulted in strength and muscle mass gains in ground-based studies. HYPOTHESES: 1) A concentric/eccentric combination exercise regimen (with a bias of either concentric or eccentric exercise) will result in a greater strength gain than concentric exercise alone; and 2) an eccentrically biased regimen will result in the greatest strength gain of all. METHODS: The 31 subjects were randomly assigned to one of three isokinetic exercise groups (CON-ECC: 75% concentric and 25% eccentric; ECC-CON: 75% eccentric and 25% concentric; CON: 100% concentric); each subject trained the right leg 3 d per week for 5 wk. Pre- and post-training isokinetic concentric/ eccentric strength tests and DEXA scans assessed changes in muscle strength and/or mass. RESULTS: All three groups showed an increase in eccentric muscle strength with the CON group showing the smallest gain (10.1%). Significantly larger gains were noted in the two combination groups (19.5%, 18.1%; p < 0.042), with the largest gains in eccentric strength. No significant change was noted in muscle mass. CONCLUSIONS: A resistance exercise protocol which includes eccentric as well as concentric exercise, particularly when the eccentric exercise is emphasized, appears to result in greater strength gains than concentric exercise alone. Findings suggest eccentric exercise may be an important component of the in-flight resistance exercise protocol for long-duration spaceflight.     

The effect of ankle disk training on muscle reaction time in subjects with a history of ankle sprain

The purpose of this study was to determine whether 8 weeks of ankle disk training alters ankle muscle onset latency of patients with a history of lateral ankle sprain. The training was completed by eight minimally symptomatic subjects with a history of nonrehabilitated, unilateral, inversion ankle sprain sustained between 6 and 16 months before entry into the study. Ankle inversion perturbations monitored by fine-wire electromyography were performed in four lower extremity muscles (anterior tibialis, posterior tibialis, peroneus longus, and flexor digitorum longus) of all subjects on both the injured (experimental) and noninjured (control) legs. Testing was performed at study entry and after 8 weeks of ankle disk training on the previously injured ankle. Results revealed a statistically significant decrease in the anterior tibialis onset latency in both the experimental (67.6 +/- 20.3 to 51.7 +/- 17.6) and control (65.5 +/- 9.8 to 53.8 +/- 23.7) ankles after the training period. These findings indicate that muscle onset latency decreases in specific ankle muscle groups after ankle disk training in previously injured ankles. Both the experimental and control ankles demonstrated a significant change, which raises the question as to whether a proprioceptive cross-training effect occurred.     

Ankle tenography: a key to unexplained symptomatology. Part I: Normal tenographic anatomy

Fifty cadaver ankles were examined with ankle tenography. The normal tenographic appearance of the peroneus longus and brevis, posterior tibial, flexor digitorum longus, flexor hallucis longus, anterior tibialis, extensor hallucis longus, and extensor digitorum longus tendons and sheaths are described and illustrated for clinical reference. The baseline measurements and demonstration of the normal radiographic appearance of these tendons should assist in the evaluation of hindfoot foot disability and ankle pain.     

The effects of an ankle destabilization device on muscular activity while walking

Chronic Ankle Instability sprain causes are unclear and many factors or mechanisms may contribute to recurrence of this injury. The aim of the study was to investigate how an ankle destabilization device affects the EMG patterns of the ankle muscles during ankle stabilization against inversion. The left foot was equipped with a mechanical device mounted under the heel of the shoe. This mechanical device induces subtalar joint destabilization necessitating the control of ankle muscles. Surface electrodes were placed over the tibialis anterior, the peroneus longus, the peroneus brevis, the gastrocnemius lateral, and the gastrocnemius medial. Nine healthy subjects (mean age 37+/-12 yr; mean mass 68+/-17 kg; mean height 1.73+/-0.7 m) were instructed to walk normally along a tape fixed on the floor. The ankle destabilization device altered the walking pattern of all subjects. More specifically, the walking pattern is disturbed resulting in higher amplitude of the EMG activity of the peroneal muscles and the Tibialis Anterior and anticipatory reactions in the peroneal muscles. The results suggest that the ankle destabilization device could be beneficial for rehabilitation programs especially during the training of walking. Using this material may help to a specific reinforcement of muscles involved in anti-inversion ankle movement.     

Metabolic reaction after concentric and eccentric endurance-exercise of the knee and ankle

PURPOSE: Power training plays an essential part in many sport disciplines. The importance of eccentric power training remains a matter of controversial discussion. The objective of this study was therefore to investigate the difference in metabolic reaction between eccentric and concentric stress in comparable work. METHODS: Sixty-four men between 22 and 60 yr of age performed maximum isokinetic 1-min endurance tests of the knee and ankle each in concentric (180 degrees.s-1) and eccentric (60 degrees.s-1) modes with comparable total area of contraction-time curve (NS). Higher strength values (mean peak torque, P < 0.01), lower fatigue (fatigue index, P < 0.001), lower increase in lactate (P < 0.01), and lower ammonia production (P < 0.01) were found in eccentric than in concentric exercise, independent of the joint. The eccentric form of stress showed lower decrease and thus age-dependence in maximum strength and in fatigue than the concentric form. RESULTS: The results permit the conclusion that eccentric exercise leads to less fatigue and lower lactate and ammonia reaction than concentric exercise in comparable work levels. Variable visco-elastic properties of the muscle fibers themselves with additive passive strength in eccentric mode is considered as the cause. CONCLUSIONS: It remains uncertain whether the lower metabolic stress might be useful during the training process. A greater scope of training and increased number of training stimuli might be applied in primarily eccentric forms of exercise.     

The significance of peroneus tertius muscle in ankle injuries: a prospective study

BACKGROUND: The peroneus tertius is absent in 5% to 17% of the human white population. The function of the peroneus tertius is eversion and dorsiflexion of the foot. These 2 strength parameters have been identified as important parameters in the development of ankle ligament injuries. HYPOTHESIS: Subjects without the peroneus tertius are at higher risk for ankle ligament injuries. STUDY DESIGN: Cohort study (Prevalence); Level of evidence, 1. METHODS: One hundred physical education students were palpated for the presence of peroneus tertius before the start of their education; they were also isokinetically measured for eversion and dorsiflexion strength. All ankle injuries were registered during 2 years, and exposure to sport was recorded. RESULTS: The peroneus tertius was absent in 37 of the 200 subjects (18.5%). In the group with peroneus tertius (n = 163), 21 ankle sprains (12.9%) were observed during the 2-year follow-up. In the group without peroneus tertius (n = 37), 7 ankle sprains (18.9%) were noted. Statistical analysis (Cox regression) showed no significant difference between both groups in the incidence of ankle sprains (P = .335). Comparisons between the group of subjects with peroneus tertius and the group without peroneus tertius concerning the concentric and eccentric isokinetic eversion and the concentric dorsiflexion strength did not attain significance at any tested velocity (P > .05). CONCLUSION: This study shows that subjects without peroneus tertius are not at higher risk for an ankle ligament injury. In addition, subjects without peroneus tertius do not exhibit decreased eversion or dorsiflexion strength. CLINICAL RELEVANCE: This study is the first to clarify the exact contribution of the peroneus tertius within the muscular system around the ankle. As a result, the clinical importance of the peroneus tertius concerning prevention and treatment of ankle ligament injuries is low.     

向心/离心肌肉疲劳对踝关节本体感觉及平衡能力的影响

目的:探讨向心/离心两种不同工作形式引发的肌肉疲劳对踝关节本体感觉及人体站立平衡能力的影响。方法:选取健康男性大学生受试者40名,随机分为向心疲劳组和离心疲劳组,每组20名,分别进行向心/离心工作形式下的疲劳干预(连续踝关节背屈及跖屈运动),并于干预前后分别进行本体感觉测试及站立平衡测试,对目标角度(背屈10°、跖屈25°位)的绝对差值和身体摆动指数进行统计分析。结果:与干预前相比,疲劳干预后两组受试者目标角度的绝对差值增加(P<0.05),身体摆动指数增加(P<0.05);同时,离心疲劳干预组变化更为显著(P<0.05)。结论:踝关节周围肌肉疲劳引发其关节本体感觉及人体平衡控制能力下降,且在离心收缩引发的肌肉疲劳中更为明显。     

Landing in netball: effects of taping and bracing the ankle

OBJECTIVES: To investigate the effect of bracing and taping on selected electromyographic, kinematic, and kinetic variables when landing from a jump. METHODS: Fifteen netball players performed a jump, so as to land on their dominant limb on a force plate. Electromyographic activity was recorded from the gastrocnemius, tibialis anterior, and peroneus longus muscles. Subjects were also filmed and measures of rearfoot motion were derived. RESULTS: Significantly less electromyographic activity (p<0.007) was observed from the gastrocnemius and peroneus longus muscle groups when subjects were braced. No other significant electromyographical findings were observed. Peak vertical ground reaction force and time to peak for vertical ground reaction force were not affected by bracing and taping, nor were the rearfoot and Achilles tendon angles at foot strike. CONCLUSIONS: The effect of bracing and taping on the selected biomechanics variables associated with landing was specifically limited to a reduction in muscle action, particularly for the braced condition. Netball players can be confident that the biomechanics of their landing patterns will not be altered whether they choose to wear a brace or tape their ankle joints.     

Altered neuromuscular control and ankle joint kinematics during walking in subjects with functional instability of the ankle joint

BACKGROUND: The ankle joint requires very precise neuromuscular control during the transition from terminal swing to the early stance phase of the gait cycle. Altered ankle joint arthrokinematics and muscular activity have been cited as potential factors that may lead to an inversion sprain during the aforementioned time periods. However, to date, no study has investigated patterns of muscle activity and 3D joint kinematics simultaneously in a group of subjects with functional instability compared with a noninjured control group during these phases of the gait cycle. PURPOSE: To compare the patterns of lower limb 3D joint kinematics and electromyographic activity during treadmill walking in a group of subjects with functional instability with those observed in a control group. STUDY DESIGN: Controlled laboratory study. METHODS: Three-dimensional angular velocities and displacements of the hip, knee, and ankle joints, as well as surface electromyography of the rectus femoris, peroneus longus, tibialis anterior, and soleus muscles, were recorded simultaneously while subjects walked on a treadmill at a velocity of 4 km/h. RESULTS: Before heel strike, subjects with functional instability exhibited a decrease in vertical foot-floor clearance (12.62 vs 22.84 mm; P < .05), as well as exhibiting a more inverted position of the ankle joint before, at, and immediately after heel strike (1.69 degrees , 2.10 degrees , and -0.09 degrees vs -1.43 degrees , -1.43 degrees , and -2.78 degrees , respectively [minus value = eversion]; P < .05) compared with controls. Subjects with functional instability were also observed to have an increase in peroneus longus integral electromyography during the post-heel strike time period (107.91%.millisecond vs 64.53%.millisecond; P < .01). CONCLUSION: The altered kinematics observed in this study could explain the reason subjects with functional instability experience repeated episodes of ankle inversion injury in situations with only slight or no external provocation. It is hypothesized that the observed increase in peroneus longus activity may be the result of a change in preprogrammed feed-forward motor control.     

Decreased EMG median frequency during a second bout of eccentric contractions

PURPOSE: Others have reported preferential recruitment of fast motor units in muscles during performance of eccentric contractions and there is evidence that fast muscle fibers are more susceptible to eccentric contraction-induced injury. We tested the hypothesis that during a second bout of maximal eccentric contractions 1 wk after the first, there would be a reduction in the electromyographic (EMG) median frequency (MF) with minimal change in the EMG root-mean-square (RMS), indicating greater reliance on slower motor units. This could provide an explanation for the enhanced resistance to eccentric contraction-induced injury after a single bout of eccentric exercise. METHODS: Human subjects performed 50 maximal voluntary eccentric (N = 10) or concentric (N = 10) contractions of the anterior crural muscles on two occasions separated by 1 wk. To determine whether MF changes during the second bout could be a consequence of injury to fibers in fast motor units, the anterior crural muscles of mice were electrically stimulated to perform 50 maximal eccentric (N = 10) or concentric (N = 9) contractions on two occasions separated by 1 wk. In both the humans and mice, torque production and tibialis anterior muscle RMS and MF were measured during the two exercise bouts. RESULTS: In human tibialis anterior muscle, MF was 30% lower (P < 0.01) during the second eccentric bout although RMS was the same. In the mice, RMS and MF were unchanged at any time after the first eccentric bout despite torque deficits similar to those observed in the humans. CONCLUSIONS: The data indicate that with repetition of maximal voluntary eccentric contractions, there is an increased activation of slow motor units and a concomitant decrease in activation of fast units.