Recovery pattern of motor reflex after a single bout of neuromuscular electrical stimulation session

We aimed at determining the recovery pattern of neural properties of soleus muscle after a single bout of neuromuscular electrical stimulation (NMES) session. Thirteen subjects performed an NMES exercise (75 Hz, 40 contractions, 6.25 s per contraction). Maximal voluntary contraction (MVC), H‐reflex at rest and during voluntary contraction fixed at 60% of MVC (respectively, H_max and H_sup) and volitional (V) wave were measured before and during the recovery period following this exercise [i.e., immediately after, 2 h (H2), 2 days (D2) and 7 days (D7)]. MVC exhibited an immediate and a delayed declines at 2 days (respectively, ?29.8±4.6%, P<0.001; ?13.0±3.4%, P<0.05). Likewise, V/M_sup was decreased immediately and 2 days after NMES session (respectively, ?43.3±11.6%, P<0.05; 35.3±6.6%, P<0.05). The delayed decrements in MVC and V‐wave occurred concomitantly with muscle soreness peak (P<0.001). It could be concluded that motor command alterations after an NMES resistance session contributed to the immediate and also to the delayed decreases in MVC without affecting resting and active H‐reflex excitability. These results suggested that spinal circuitry function of larger motoneurons was inhibited by NMES (as indicated by the depressed V‐wave responses) contrary to the smaller one (indicated by the unchanged H‐reflex responses).     

Spinal modulations accompany peripheral fatigue during prolonged tennis playing

To examine the time course of alteration in neural process (spinal loop properties) during prolonged tennis playing, 12 competitive players performed a series of neuromuscular tests every 30 min during a 3‐h match protocol. Muscle activation (twitch interpolation) and normalized EMG activity were assessed during maximal voluntary contraction (MVC) of plantar flexors. Spinal reflexes and M‐waves were evoked at rest (i.e., H_max and M_max, respectively) and during MVC (i.e., H_sup, V‐wave, M_sup, respectively). MVC torque declined significantly (P<0.001) across the match protocol, due to decrease (P<0.001) in muscle activation and in normalized EMG activity. The impairment in MVC was significantly correlated (r=0.77; P<0.05) with the decline in muscle activation. H_max/M_max (P<0.001), H_sup/M_sup (P<0.01) and V/M_sup (P<0.05) ratios were depressed with fatigue and decreased by ～80%, 46% and 61% at the end of exercise, respectively. Simultaneously, peak twitch torque and M‐wave amplitude were significantly (P<0.01) altered with exercise, suggesting peripheral alterations. During prolonged tennis playing, the compromised voluntary strength capacity is linked to a reduced neural input to the working muscles. This central activation deficit partly results from a modulation in spinal loop properties.     

Modulation of the Hoffmann reflex in soleus and medial gastrocnemius during stair ascent and descent in young and older adults

BackgroundThe Hoffmann (H) reflex can provide relevant information on spinal control of leg muscles during locomotor tasks in young and older adults.Research questionIs the H reflex in the leg muscles differently modulated during stair gait in young and older adults?MethodThe H reflex in soleus (SOL) and medial gastrocnemius (MG) (normalized to the maximal M-wave amplitude obtained during upright standing; M_max) was recorded in 19 young and 18 older adults during upright standing, and stair ascent and descent of a 3-step staircase.ResultsH-reflex amplitude during upright standing was greater in young than older adults for SOL (48% vs. 26% M_max; p = 0.001) and MG (23% vs. 14% M_max; p = 0.02). When data were averaged across groups during stair ascent, H-reflex amplitude in SOL increased from 15% M_max at the beginning of the stance phase to 29% M_max at mid-stance, then decreased to be 4% M_max in the swing phase. During stair descent, H-reflex amplitude was maximal (20% M_max) at the beginning of the stance phase, decreased to 5% M_max at the end of stance, and increased to 11% M_max in the swing phase. Similar adjustments were observed for the H reflex in MG for both ascent and descent. H-reflex modulation during gait cycle (relative to upright standing) is less pronounced in older adults (p < 0.05). However, no difference was observed between subgroups of young and older adults matched for H-reflex amplitude in upright standing. In both groups, H-reflex modulation was not associated with changes in background electromyographic activity.SignificanceThis study indicates that the H reflex is modulated within the stair gait cycle during ascent and descent. Although its magnitude was slightly reduced, the overall modulation of the H reflex is not affected in healthy older adults.     

Alteration in neuromuscular function of the plantar flexors following caffeine ingestion

The aim of this study was to compare the neuromuscular function of the plantar flexors following caffeine or placebo administration. Thirteen subjects (25 ± 3 years) ingested caffeine or placebo in a randomized, controlled, counterbalanced, double‐blind crossover design. Neuromuscular tests were performed before and 1 h after caffeine or placebo intake. During neuromuscular testing, rate of torque development, isometric maximum voluntary torque, and neural drive to the muscles were measured. Triceps surae muscle activation was assessed by normalized root mean square of the EMG signal during the initial phase of contraction (0–100 ms, 100–200 ms) and maximal voluntary contraction (MVC). Furthermore, evoked spinal reflex responses of the soleus muscle (H‐reflex evoked at rest and during MVC, V‐wave) and peak twitch torques were evaluated. The isometric maximum voluntary torque and evoked potentials were not different. However, we found a significant difference between groups for rate of torque development in the time intervals 0–100 ms [41.1 N·m/s (95% CI: 8.3–73.9 N·m/s, P = 0.016)] and 100–200 ms [32.8 N·m/s (95% CI: 2.8–62.8 N·m/s, P = 0.034)]. These changes were accompanied by enhanced neural drive to the plantar flexors. Data suggest that caffeine solely increased explosive voluntary strength of the triceps surae because of enhanced neural activation at the onset of contraction whereas MVC strength was not affected.     

Isometric knee extensor fatigue following a Wingate test: peripheral and central mechanisms

Central and peripheral fatigue have been explored during and after running or cycling exercises. However, the fatigue mechanisms associated with a short maximal cycling exercise (30 s Wingate test) have not been investigated. In this study, 10 volunteer subjects performed several isometric voluntary contractions using the leg muscle extensors before and after two bouts of cycling at 25% of maximal power output and two bouts of Wingate tests. Transcranial magnetic stimulation (TMS) and electrical motor nerve stimulation (NM) were applied at rest and during the voluntary contractions. Maximal voluntary contraction (MVC), voluntary activation (VA), twitch amplitude evoked by electrical nerve stimulation, M wave and motor potential evoked by TMS (MEP) were recorded. MVC, VA and twitch amplitude evoked at rest by NM decreased significantly after the first and second Wingate tests, indicating central and peripheral fatigue. MVC and VA, but not the twitch amplitude evoked by NM, recovered before the second Wingate test. These results suggest that the Wingate test results in a decrease in MVC associated with peripheral and central fatigue. While the peripheral fatigue is associated with an intramuscular impairment, the central fatigue seems to be the main reason for the Wingate test‐induced impairment of MVC.     

Unilateral elbow flexion fatigue modulates corticospinal responsiveness in non‐fatigued contralateral biceps brachii

Exercise‐induced fatigue can change motor performance in non‐exercised muscles. The objective was to investigate unilateral elbow flexion (EF) fatigue effects on the maximal voluntary force (MVC) and corticospinal excitability of contralateral non‐exercised biceps brachii (BB). Transcranial magnetic, transmastoid electrical, and brachial plexus electrical stimulation were used to elicit motor evoked potentials (MEP), cervicomedullary motor evoked potentials (CMEP), and compound muscle action potentials in the contralateral non‐exercised BB of 12 participants before and after (i) two bouts of 100‐s unilateral EF (fatigue) or (ii) control. Three stimuli were evoked every 1.5 s during a series of 6‐s isometric EF at 100%, 50%, and 5% of MVC. The non‐exercised EF MVC force, electromyographic activity, and voluntary activation were not significantly different between fatigue and control. Non‐exercised BB MEP and CMEP amplitudes during 100% MVCs demonstrated significantly higher (P = 0.03) and lower values (P = 0.01), respectively, after fatigue compared with control. There was no difference between the two conditions for MEP and CMEP amplitudes during 50% and 5% MVCs. Unilateral exercise‐induced EF fatigue did not lead to cross‐over central fatigue to the contralateral homologous muscle but enhanced the supraspinal responsiveness (MEP/CMEP) of the neural circuitries supplying central commands to non‐exercised muscles at higher contraction intensity.     

Sustained submaximal contraction yields biphasic modulation of soleus Post‐activation depression in healthy humans

The amplitude of the H‐reflex during the development and progression of fatigue reflects a complex interplay between central and peripheral factors. The purpose of this study is to characterize H‐reflex homosynaptic post‐activation depression (PAD) in an online fashion during a sustained submaximal fatigue task. The task required a high motor output in order to increase the likelihood of creating partial muscle ischemia with accumulation of fatigue metabolites, an important potential inhibitory influence upon the H‐reflex during the progression of fatigue. Eleven subjects without neurologic impairment maintained volitional, isometric plantar flexion at 60% of maximal voluntary contraction until exhaustion. A paired‐pulse stimulus (2 Hz) was delivered to the tibial nerve to elicit paired H‐reflexes before, during, and after the fatigue protocol. The normalized amplitude of the second H‐reflex (depression ratio) served as an estimate of PAD. Depression ratio increased during the first half of the fatigue protocol (P < 0.001), indicating a diminution of PAD, and then returned as exhaustion approached. The biphasic behavior of homosynaptic H‐reflex depression during fatigue to exhaustion suggests a role for metabolic mediators of post‐activation depression during fatigue.     

Effects of muscle activation on shear between human soleus and gastrocnemius muscles

Lateral connections between muscles provide pathways for myofascial force transmission. To elucidate whether these pathways have functional roles in vivo, we examined whether activation could alter the shear between the soleus (SOL) and lateral gastrocnemius (LG) muscles. We hypothesized that selective activation of LG would decrease the stretch‐induced shear between LG and SOL. Eleven volunteers underwent a series of knee joint manipulations where plantar flexion force, LG, and SOL muscle fascicle lengths and relative displacement of aponeuroses between the muscles were obtained. Data during a passive full range of motion were recorded, followed by 20° knee extension stretches in both passive conditions and with selective electrical stimulation of LG. During active stretch, plantar flexion force was 22% greater (P < 0.05) and relative displacement of aponeuroses was smaller than during passive stretch (P < 0.05). Soleus fascicle length changes did not differ between passive and active stretches but LG fascicles stretched less in the active than passive condition when the stretch began at angles of 70° and 90° of knee flexion (P < 0.05). The activity‐induced decrease in the relative displacement of SOL and LG suggests stronger (stiffer) connectivity between the two muscles, at least at flexed knee joint angles, which may serve to facilitate myofascial force transmission.     

Task failure during sustained low-intensity contraction is not associated with a critical amount of central fatigue

Fatigue-related mechanisms induced by low-intensity prolonged contraction in lower limb muscles are currently unknown. This study investigated central fatigue kinetics in the knee extensors during a low-intensity sustained isometric contraction. Eleven subjects sustained a 10% maximal voluntary contraction (MVC) until task failure (TF) with neuromuscular evaluation every 3 minutes. Testing encompassed transcranial magnetic stimulation to evaluate maximal voluntary activation (VA_TMS), motor evoked potential (MEP), and silent period (SP), and peripheral nerve stimulation to assess M-wave. Rating of perceived exertion (RPE) was also recorded. MVC progressively decreased up to 50% of the time to TF (ie, 50%_TTF) and then plateaued, reaching ~50% at TF (P < .001). VA_TMS progressively decreased up to 90%_TTF and then plateaued, the decrease reaching ~20% at TF (P < .001). SP was lengthened early (ie, from 20%_TTF) during the exercise and then plateaued (P < .01). No changes were reported for MEP evoked during MVC (P = .87), while MEP evoked during submaximal contractions decreased early (ie, from 20%_TTF) during the exercise and then plateaued (P < .01). RPE increased linearly during the exercise to be almost maximal at TF. M-waves were not altered (P = .88). These findings confirm that TF is due to the subjects reaching their maximal perceived effort rather than any particular central event or neuromuscular limitations since MVC at TF was far from 10% of its original value. It is suggested that strategies minimizing RPE (eg, motivational self-talk) should be employed to enhance endurance performance.     

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.     

Fitness and its association with fatigue in persons with multiple sclerosis

This study compared physical fitness components between fatigued and non‐fatigued persons with MS and examined those components as correlates of fatigue. Sixty‐two ambulatory persons with MS completed the Modified Fatigue Impact Scale (MFIS) and underwent assessments of cardiorespiratory capacity, lower extremity muscle strength (i.e., peak torque and asymmetry), body composition, and static balance over two different sessions 7 days apart. Participants were allocated into fatigue groups based on MFIS scores (non‐fatigued group (i.e., MFIS ≤38), n = 26; and fatigued group (MFIS >38), n = 36). The fatigued group had significantly (P < 0.05) lower cardiorespiratory capacity (VO_2peak) and muscular strength (i.e., knee flexion peak torque) than the non‐fatigued group. VO_2peak and knee extension peak torque were the two physical fitness components significantly correlated with fatigue scores in the fatigued group (P < 0.05), and follow‐up stepwise linear regression revealed that VO_2peak was a significant predictor of fatigue scores (R² = 0.13). Discriminant function analysis further identified VO_2peak as a significant (P < 0.05) correlate of fatigue status. This model explained 21% of variance in group status (i.e., fatigued vs non‐fatigued) and correctly classified approximately 76% of cases into fatigue status groups. The improvement of cardiorespiratory capacity should be considered in rehabilitation programs for persons with MS, especially those presenting with elevated fatigue.     

Muscle excitability during sustained maximal voluntary contractions by a separate analysis of the M‐wave phases

This study was designed to examine separately the changes in the first and second phases of the muscle compound action potential (M‐wave) during and after a sustained 3‐minutes maximal voluntary contraction (MVC). M‐waves were evoked by supramaximal single shocks to the femoral nerve given at 10‐seconds intervals throughout a sustained isometric 3‐minutes MVC and also during six brief MVCs performed throughout a 30‐minutes recovery period. The amplitude, duration, and area of the M‐wave first and second phases, together with muscle conduction velocity and force, were measured. During the 3‐minutes MVC, the amplitude of the first phase increased progressively for the first minute (33%‐43%, P<.01) and remained stable thereafter, whereas the second phase initially increased for 25‐35 seconds (30%‐50%, P<.01), but subsequently decreased significantly before stabilizing. During the recovery period, the amplitude of the M‐wave first phase showed a decreasing trend, returning to pre‐fatigue values (P>.01) within 5‐10 minutes, while the second phase increased progressively and remained higher than control (7%‐20%, P<.01) after the 30‐minutes recovery time. Maximal cross‐correlations between the time course of the first phase amplitude and those of conduction velocity and force (0.9‐0.93) occurred for a lag of 0 seconds, whereas maximal cross‐correlations corresponding to the second‐phase amplitude (0.6‐0.7) occurred for a 50‐seconds time lag. The present findings indicate that the potentiation of the first phase results from impaired muscle membrane excitability. The peak‐to‐peak amplitude and second‐phase amplitude are not valid indicators of muscle excitability as they might be critically affected by muscle architectural features.     

Central and peripheral fatigue in knee and elbow extensor muscles after a long‐distance cross‐country ski race

Although elbow extensors (EE) have a great role in cross‐country skiing (XC) propulsion, previous studies on neuromuscular fatigue in long‐distance XC have investigated only knee extensor (KE) muscles. In order to investigate the origin and effects of fatigue induced by long‐distance XC race, 16 well‐trained XC skiers were tested before and after a 56‐km classical technique race. Maximal voluntary isometric contraction (MVC) and rate of force development (RFD) were measured for both KE and EE. Furthermore, electrically evoked double twitch during MVC and at rest were measured. MVC decreased more in KE (?13%) than in EE (?6%, P = 0.016), whereas the peak RFD decreased only in EE (?26%, P = 0.02) but not in KE. The two muscles showed similar decrease in voluntary activation (KE ?5.0%, EE ?4.8%, P = 0.61) and of double twitch amplitude (KE ?5%, EE ?6%, P = 0.44). A long‐distance XC race differently affected the neuromuscular function of lower and upper limbs muscles. Specifically, although the strength loss was greater for lower limbs, the capacity to produce force in short time was more affected in the upper limbs. Nevertheless, both KE and EE showed central and peripheral fatigue, suggesting that the origins of the strength impairments were multifactorial for the two muscles.     

Effects of eccentric versus concentric contractions of the biceps brachii on intracortical inhibition and facilitation

Differences in the neural mechanisms underpinning eccentric (ECC) and concentric (CON) contractions exist; however, the acute effects of fatiguing muscle contractions on intracortical and corticospinal excitability are not well understood. Therefore, we compared maximal ECC and CON contractions of the right biceps brachii (BB) muscle for changes in corticospinal excitability, short‐ (SICI) and long‐interval intracortical inhibition (LICI) and intracortical facilitation (ICF) up to 1 hour post‐exercise. Fourteen right‐handed adults (11 M/3F; 26.8 ± 2.9 year) undertook a single session of 3 sets of 10 maximal ECC or CON contractions (180‐second rest between sets) on an isokinetic dynamometer (40°/s) separated by 1 week, in a randomized crossover study. Maximum voluntary isometric contraction torque (MVIC), maximal muscle compound waves (M_MAX), and motor‐evoked potentials elicited through transcranial magnetic stimulation (TMS) were recorded via surface electromyography from the right BB. MVIC decreased (P < 0.001) immediately after ECC and CON contractions similarly, but the decrease was sustained at 1 hour post‐ECC contractions only. M_MAX was reduced immediately (P = 0.014) and 1 hour post‐exercise (P = 0.019) only for ECC contractions. SICI and ICF increased immediately after ECC and CON contractions (P < 0.001), but LICI increased only after ECC contractions (P < 0.001), and these increases remained at 1 hour post‐ECC contractions only. These findings suggest that ECC contractions induced a longer‐lasting neuromodulatory effect on intracortical inhibition and facilitation, which could indicate a central compensatory response to peripheral 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.     

Non‐uniform displacement and strain between the soleus and gastrocnemius subtendons of rat Achilles tendon

Achilles tendon (AT ) comprises of 3 subtendons arising from the soleus (SOL ) and the lateral (LG ) and medial (MG ) heads of the gastrocnemius muscle. While recent human studies show differential displacement within AT , these displacements have not been attributed to specific subtendons. We tested the hypothesis that the SOL and LG subtendons show differential displacement and strain during various combinations of SOL , LG , and MG excitations. Movement of knots, sutured onto SOL and LG subtendons of 12 Wistar rats, was videotaped, while the muscles were stimulated intramuscularly and ankle torque was assessed. When SOL only was stimulated, the plantar flexion torque was the smallest among the different conditions (P  < .001). In this condition, from passive to active state, the displacement (0.57 vs 0.47 mm, P  = .002) and strain (8.4% vs 2.4%, P  < .001) in the SOL subtendon were greater than in LG subtendon. When LG only was stimulated, a higher ankle torque was measured as compared to SOL stimulation (P  < .001); the displacement was similar in both subtendons (~0.6 mm), while the strain was greater in LG than in SOL (4.7% vs 1.7%, P  < .001). When all 3 muscles were stimulated simultaneously, ankle torque was highest and the displacement (0.79 vs 0.74 mm, P  = .002) and strain (7.7% vs 4.4%, P  = .003) were greater in SOL than in LG . These data show that the different subtendons of AT can experience relative displacement and differential strains. Together with anatomical dissections, the results revealed that such uniformities may be due to a lower stiffness of SOL subtendon compared to LG .     

Concentric versus eccentric cycling at equal power output or effort perception: Neuromuscular alterations and muscle pain

This study aimed to compare neuromuscular alterations and perceptions of effort and muscle pain induced by concentric and eccentric cycling performed at the same power output or effort perception. Fifteen participants completed three 30-min sessions: one in concentric at 60% peak power output (CON) and two in eccentric, at the same power output (ECC_POWER) or same perceived effort (ECC_EFFORT). Muscle pain, perception of effort, oxygen uptake as well as rectus femoris and vastus lateralis electromyographic activities were collected when pedaling. The knee extensors maximal voluntary contraction (MVC) torque, the torque evoked by double stimulations at 100 Hz and 10 Hz (Dt100; Dt10), and the voluntary activation level (VAL) were evaluated before and after exercise. Power output was higher in ECC_EFFORT than CON (89.1 ± 23.3% peak power). Muscle pain and effort perception were greater in CON than ECC_POWER (p < 0.03) while muscle pain was similar in CON and ECC_EFFORT (p > 0.43). MVC torque, Dt100, and VAL dropped in all conditions (p < 0.04). MVC torque (p < 0.001) and the Dt10/ Dt100 ratio declined further in ECC_EFFORT (p < 0.001). Eccentric cycling perceived as difficult as concentric cycling caused similar muscle pain but more MVC torque decrease. A given power output induced lower perceptions of pain and effort in eccentric than in concentric yet similar MVC torque decline. While neural impairments were similar in all conditions, eccentric cycling seemed to alter excitation-contraction coupling. Clinicians should thus be cautious when setting eccentric cycling intensity based on effort perception.     

Chronic exertional compartment syndrome: muscle changes with isometric exercise

Chronic exertional compartment syndrome (CECS) is a well-documented cause of lower leg pain in active individuals. The pathophysiology is unclear, although it is generally believed to be associated with increased intramuscular pressure, but there is very little information about muscle function in relation to the onset of pain. PURPOSE: To investigate strength, fatigue, and recovery of the anterior tibial muscles in CECS patients and healthy subjects during an isometric exercise protocol. METHODS: Twenty patients and 22 control subjects (mean age 27.6 yr and 33.0 yr, respectively) performed a 20-min isometric exercise protocol consisting of intermittent maximal voluntary contractions (MVC). Central fatigue was evaluated by comparing changes in electrically stimulated (2 s at 50 Hz) and voluntary contraction force before and during the exercise, and then throughout 10 min of recovery. Muscle size was measured by ultrasonography. Pain and cardiovascular parameters were also examined. RESULTS: The absolute MVC forces were similar, but MVC:body mass of the patients was lower (P < 0.05) as was the ratio of MVC to muscle cross-sectional area (P < 0.01). The extent of central and peripheral fatigue was similar in the two groups. The patients reported significantly higher levels of pain during exercise (P < 0.05 at 4 min) and after the first minute of recovery (P < 0.001). An 8% increase in muscle size after exercise was observed for both groups. There were no differences in the cardiovascular responses of the two groups. CONCLUSIONS: CECS patients were somewhat weaker than normal but fatigued at a similar rate during isometric exercise. Patients reported higher pain than controls despite comparable changes in muscle size, suggesting that abnormally tight fascia are not the main cause of CECS symptoms.     

Reproducible voluntary muscle performance during constant work rate dynamic leg exercise

During constant intensity treadmill or cycle exercise, progressive muscle fatigue is not readily quantified and endurance time is poorly reproducible. However, integration of dynamic knee extension (DKE) exercise with serial measurement of maximal voluntary contraction (MVC) force of knee extensor muscles permits close tracking of leg fatigue. We studied reproducibility of four performance indices: MVC force of rested muscle (MVC(rest)) rate of MVC force fall, time to exhaustion, and percentage of MVC(rest) (%MVC(rest)) at exhaustion in 11 healthy women (22+/-1 yrs) during identical constant work rate 1-leg DKE (1 Hz) on 2 separate days at sea level (30 m). Means+/-SD for the two test days, and the correlations (r), standard estimate errors and coefficients of variation (CV%) between days were, respectively: a) MVC(rest)(N), 524+/-99 vs 517+/-111, 0.91, 43.0, 4.9%; b) MVC force fall (N x min(-1)), -10.77+/-9.3 vs -11.79+/-12.1, 0.94, 3.6, 26.5 %; c) Time to exhaustion (min), 22.6+/-12 vs 23.9+/-14, 0.98, 2.7, 7.5 %; and d) %MVC(rest) at exhaustion, 65+/-13 vs 62+/-14, 0.85, 7.8, 5.6%. There were no statistically significant mean differences between the two test days for any of the performance measures. To demonstrate the potential benefits of evaluating multiple effects of an experimental intervention, nine of the women were again tested within 24hr of arriving at 4,300 m altitude using the identical force, velocity, power output, and energy requirement during constant work rate dynamic leg exercise. Low variability of each performance index enhanced the ability to describe the effects of acute altitude exposure on voluntary muscle function.     

Central fatigue affects plantar flexor strength after prolonged running

The primary aim of this study was to examine central fatigue of the plantar flexor muscle group after prolonged running using the twitch interpolation technique. Eight healthy, habitually active male subjects ran on a motorized treadmill for 2 h at a speed corresponding to 75% of peak oxygen uptake (VO(2peak)). Maximal voluntary isometric contraction (MVC) strength as well as the electrically induced twitch produced during MVC [interpolated twitch (IT)] and at rest [resting twitch (RT)] were measured before and after running. The level of activation (LOA) during each MVC was calculated as LOA (%)=100(1-IT/RT). Both MVC and LOA decreased (17+/-16% and 19+/-15%, respectively, P<0.05) after running, whereas RT did not change. The decrease in MVC was correlated with the decrease in LOA (r=0.87, P<0.05). The results demonstrate that after 2 h of treadmill running at an intensity of 75% of VO(2peak), there was a reduction in maximal voluntary plantar flexor muscle strength that was mainly related to central fatigue.