Influence of a voluntary fatigue test on the contralateral homologous muscle in humans?

Evaluating peripheral neuromuscular function with brief movement-evoked pain

Movement-evoked pain is an understudied manifestation of musculoskeletal conditions that contributes to disability, yet little is known about how the neuromuscular system responds to movement-evoked pain. The present study examined whether movement-evoked pain impacts force production, electromyographic (EMG) muscle activity, and the rate of force development (RFD) during submaximal muscle contractions. Fifteen healthy adults (9 males; age = 30.3 ± 10.2 yr, range = 22-59 yr) performed submaximal isometric first finger abduction contractions without pain (baseline) and with movement-evoked pain induced by laser stimulation to the dorsum of the hand. Normalized force (% maximal voluntary contraction) and RFD decreased by 11% (P < 0.001) and 15% (P = 0.003), respectively, with movement-evoked pain, without any change in normalized peak EMG (P = 0.77). Early contractile RFD, force impulse, and corresponding EMG amplitude computed within time segments of 50, 100, 150, and 200 ms relative to the onset of movement were also unaffected by movement-evoked pain (P > 0.05). Our results demonstrate that movement-evoked pain impairs peak characteristics and not early measures of submaximal force production and RFD, without affecting EMG activity (peak and early). Possible explanations for the stability in EMG with reduced force include antagonist coactivation and a reorganization of motoneuronal activation strategy, which is discussed here.NEW & NOTEWORTHY We provide neurophysiological evidence to indicate that peak force and rate of force development are reduced by movement-evoked pain despite a lack of change in EMG and early rapid force development in the first dorsal interosseous muscle. Additional evidence suggests that these findings may coexist with a reorganization in motoneuronal activation strategy.

Immediate crossover fatigue after unilateral submaximal eccentric contractions of the knee flexors involves peripheral alterations and increased global perceived fatigue

After a unilateral muscle exercise, the performance of the non-exercised contralateral limb muscle can be also impaired. This crossover fatigue phenomenon is still debated in the literature and very few studies have investigated the influence of eccentric contractions. This study was designed to assess neuromuscular adaptations involved in the crossover fatigue of the non-exercised contralateral knee flexor muscles. Seventeen healthy young men performed a unilateral submaximal eccentric exercise of the right knee flexors until a 20% reduction in maximal voluntary isometric contraction torque was attained in the exercised limb. Before (PRE), immediately after exercise cessation (POST) and 24 hours later (POST24), neuromuscular function and perceived muscle soreness were measured in both the exercised limb and non-exercised limb. In addition, global perceived fatigue was assessed at each measurement time. At POST, significant reductions in maximal voluntary isometric contraction were observed in the exercised limb (-28.1%, p < 0.001) and in the non-exercised limb (-8.5%, p < 0.05), evidencing crossover fatigue. At POST, voluntary activation decreased in the exercised limb only (-6.0%, p < 0.001), while electrically evoked potentiated doublet torque was impaired in both the exercised limb and the non-exercised limb (-11.6%, p = 0.001). In addition, global perceived fatigue significantly increased at POST (p < 0.001). At POST24, all measured variables returned to PRE values, except for perceived muscle soreness scores exhibiting greater values than PRE (p < 0.05). A possible cumulative interaction between peripheral alterations and global perceived fatigue may account for the immediate crossover fatigue observed in the non-exercised limb.

Unilaterally Induced Quadriceps Fatigue during Sustained Submaximal Isometric Exercise Does Not Alter Contralateral Leg Extensor Performance

This study investigated the effects of fatiguing unilateral exercise on the ipsilateral, exercised, and contralateral, non-exercised limb's post-exercise performance in males and females. Ten males and ten females performed a fatiguing, unilateral isometric leg extension at 50% maximal voluntary isometric contraction (MVIC) force. Prior to and immediately after the fatiguing tasks, MVICs were performed for the exercised and non-exercised limb, with surface electromyographic (sEMG) and mechanomyography (sMMG) amplitude (AMP) and mean power frequency (MPF) recorded from each limb's vastus lateralis. There were no fatigue-induced, sex-dependent, differences in time to task failure (p = 0.265) or ipsilateral performance fatigability (p = 0.437). However, there was a limb by time interaction (p < 0.001) which indicated decreases in MVIC force of the ipsilateral, exercised (p < 0.001), but not the contralateral, non-exercised limb (p = 0.962). There were no sex-dependent, fatigue-induced differences in neurophysiological outcomes between the limbs (p > 0.05), but there was a fatigue-induced difference in sEMG MPF (p = 0.005). To summarize, there were no differences in fatigability between males and females. Moreover, there was insufficient evidence to support the presence of a general crossover effect following submaximal unilateral isometric exercise. However, independent of sex, the neurophysiological outcomes suggested that competing inputs from the nervous system may influence the performance of both limbs following unilateral fatigue.

Ipsilateral and contralateral responses following unimanual fatigue with and without illusionary mirror visual feedback

Illusionary mirror visual feedback alters interhemispheric communication and influences cross-limb interactions. Combining forceful unimanual contractions with the mirror illusion is a convenient way to provoke robust alterations within ipsilateral motor networks. It is unknown, however, if the mirror illusion affects cross-limb fatigability. We examine this concept by comparing the ipsilateral and contralateral handgrip force and electromyographic (EMG) responses following unimanual fatigue with and without illusionary mirror visual feedback. Participants underwent three experimental sessions (mirror, no-mirror, and control), performing a unimanual fatigue protocol with and without illusionary mirror visual feedback. Maximal handgrip force and EMG activity were measured before and after each session for both hands during maximal unimanual and bimanual contractions. The associated EMG activity from the inactive forearm during unimanual contraction was also examined. The novel findings demonstrate greater relative fatigability during bimanual versus unimanual contraction following unimanual fatigue (-31.8% vs. -23.4%, P < 0.01) and the mirror illusion attenuates this difference (-30.3% vs. -26.3%, P = 0.169). The results show no evidence for a cross-over effect of fatigue with (+0.62%, -2.72%) or without (+0.26%, -2.49%) the mirror illusion during unimanual or bimanual contraction. The mirror illusion resulted in significantly lower levels of associated EMG activity in the contralateral forearm. There were no sex differences for any of the measures of fatigability. These results demonstrate that the mirror illusion influences contraction-dependent fatigue during maximal handgrip contractions. Alterations in facilitatory and inhibitory transcallosal drive likely explain these findings.NEW & NOTEWORTHY Illusionary mirror visual feedback is a promising clinical tool for motor rehabilitation, yet many features of its influence on motor output are unknown. We show that maximal bimanual force output is compromised to a greater extent than unimanual force output following unimanual fatigue, yet illusionary mirror visual feedback attenuates this difference. The mirror illusion also reduces the unintended EMG activity of the inactive, contralateral forearm during unimanual contraction.

Sustained Isometric Wrist Flexion and Extension Maximal Voluntary Contractions on Corticospinal Excitability to Forearm Muscles during Low-Intensity Hand-Gripping

The wrist extensors demonstrate an earlier fatigue onset than the wrist flexors. However, it is currently unclear whether fatigue induces unique changes in muscle activity or corticospinal excitability between these muscle groups. The purpose of this study was to examine how sustained isometric wrist extension/flexion maximal voluntary contractions (MVCs) influence muscle activity and corticospinal excitability of the forearm. Corticospinal excitability to three wrist flexors and three wrist extensors were measured using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation. Responses were elicited while participants exerted 10% of their maximal handgrip force, before and after a sustained wrist flexion or extension MVC (performed on separate sessions). Post-fatigue measures were collected up to 10-min post-fatigue. Immediately post-fatigue, extensor muscle activity was significantly greater following the wrist flexion fatigue session, although corticospinal excitability (normalized to muscle activity) was greater on the wrist extension day. Responses were largely unchanged in the wrist flexors. However, for the flexor carpi ulnaris, normalized MEP amplitudes were significantly larger following wrist extension fatigue. These findings demonstrate that sustained isometric flexion/extension MVCs result in a complex reorganization of forearm muscle recruitment strategies during hand-gripping. Based on these findings, previously observed corticospinal behaviour following fatigue may not apply when the fatiguing task and measurement task are different.

Transcutaneous electrical nerve stimulation improves fatigue performance of the treated and contralateral knee extensors

PURPOSE

Transcutaneous electrical nerve stimulation (TENS) can reduce acute and chronic pain. Unilateral fatigue can produce discomfort in the affected limb and force and activation deficits in contralateral non-exercised muscles. TENS-induced local pain analgesia effects on non-local fatigue performance are unknown. Hence, the aim of the study was to determine if TENS-induced pain suppression would augment force output during a fatiguing protocol in the treated and contralateral muscles.

METHODS

Three experiments were integrated for this article. Following pre-tests, each experiment involved 20 min of TENS, sham, or a control condition on the dominant quadriceps. Then either the TENS-treated quadriceps (TENS_Treated) or the contralateral quadriceps (TENS_Contra) was tested. In a third experiment, the TENS and sham conditions involved two\; 100-s isometric maximal voluntary contractions (MVC) (30-s recovery) followed by testing of the contralateral quadriceps (TENS_Contra-Fatigue). Testing involved single knee extensors (KE) MVCs (pre- and post-test) and a post-test 30% MVC to task failure.

RESULTS

The TENS-treated study induced greater (p = 0.03; 11.0%) time to KE (treated leg) failure versus control. The TENS_Contra-Fatigue induced significant (p = 0.04; 11.7%) and near-significant (p = 0.1; 7.1%) greater time to contralateral KE failure versus sham and control, respectively. There was a 14.5% (p = 0.02) higher fatigue index with the TENS (36.2 ± 10.1%) versus sham (31.6 ± 10.6%) conditions in the second fatigue intervention set (treated leg). There was no significant post-fatigue KE fatigue interaction with the TENS_Contra.

CONCLUSIONS

Unilateral TENS application to the dominant KE prolonged time to failure in the treated and contralateral KE suggesting a global pain modulatory response.

Cross-limb dynamics of postural tremor due to limb loading to fatigue: neural overflow but not coupling

Many experiments have shown independence of the index finger dynamics under bilateral postural tremor protocols. Here we investigated in young adults the dynamics of bilateral multidirectional postural tremor and forearm muscle activity under the progressively fatiguing conditions supporting an external weight to the point of induced postural failure. When no loads were applied, tremor in the vertical (VT) and mediolateral (ML) directions was similar with prominent peaks within 2- to 4-Hz and 8- to 12-Hz bandwidths. Contrastingly tremor in the anterior-posterior (AP) direction was characterized by a single peak between 0 and 2 Hz. Although no tremor coupling occurred cross limbs, strong within-limb coupling was found between ML and VT directions when no loads were applied (coherence range: 0.77–0.85), implying that these oscillations are related and likely derived from mechanical sources. Applying an external load to the index finger(s) led to significant increases in the amplitude of VT tremor and EMG activity within that limb but also caused increases in tremor directions not aligned with the gravitational vector (AP and ML). Significant increases in VT and ML tremor and EMG activity in the contralateral (unloaded) limb were also found when a single index finger was loaded; however, this bilateral increase did not align with increases in interlimb coupling (coherence <0.21). The effects of fatigue caused by prolonged loading were widespread, affecting tremor and muscle activity in both limbs through a combination of neural and mechanical mechanisms. The single- and dual-limb loading to fatigue increased neural overflow but not tremor coupling between the index fingers.

NEW & NOTEWORTHY This study investigated bilateral multidirectional tremor under unloaded and loaded conditions. We found that tremor in the mediolateral and vertical directions within a limb were strongly coupled, a result not reported previously. Furthermore, when holding a weight to failure, tremor in all directions increased. Tremor also increased in the contralateral (unloaded) limb despite no interlimb coupling. This contralateral increase in tremor following loading a limb until fatigue is hypothesized to stem from motor-overflow effects.

Contralateral fatigue during severe-intensity single-leg exercise: influence of acute acetaminophen ingestion

Exhaustive single-leg exercise has been suggested to reduce time to task failure (T_lim) during subsequent exercise in the contralateral leg by exacerbating central fatigue development. We investigated the influence of acetaminophen (ACT), an analgesic that may blunt central fatigue development, on T_lim during single-leg exercise completed with and without prior fatiguing exercise of the contralateral leg. Fourteen recreationally active men performed single-leg severe-intensity knee-extensor exercise to T_lim on the left (Leg₁) and right (Leg₂) legs without prior contralateral fatigue and on Leg₂ immediately following Leg₁ (Leg_2-CONTRA). The tests were completed following ingestion of 1-g ACT or maltodextrin [placebo (PL)] capsules. Intramuscular phosphorus-containing metabolites and substrates and muscle activation were assessed using ³¹P-MRS and electromyography, respectively. T_lim was not different between Leg_1ACT and Leg_1PL conditions (402 ± 101 vs. 390 ± 106 s, P = 0.11). There was also no difference in T_lim between Leg_2ACT-CONTRA and Leg_2PL-CONTRA (324 ± 85 vs. 311 ± 92 s, P = 0.10), but T_lim was shorter in Leg_2ACT-CONTRA and Leg_2PL-CONTRA than in Leg_2CON (385 ± 104 s, both P < 0.05). There were no differences in intramuscular phosphorus-containing metabolites and substrates or muscle activation between Leg_1ACT and Leg_1PL and between Leg_2ACT-CONTRA and Leg_2PL-CONTRA (all P > 0.05). These findings suggest that levels of metabolic perturbation and muscle activation at T_lim are not different during single-leg severe-intensity knee-extensor exercise completed with or without prior fatiguing exercise of the contralateral leg. Despite contralateral fatigue, ACT ingestion did not alter neuromuscular responses, muscle metabolites, or exercise performance.

The effect of dominant first dorsal interosseous fatigue on the force production of a contralateral homologous and heterologous muscle

Crossover and nonlocal muscle fatigue (NLMF) has generally focused on large muscle groups. It is unclear if fatigue of a small muscle can result in NLMF of a larger muscle. The purpose of the present study was to examine the effect of small muscle (first dorsal interosseous; FDI) fatigue on the force and activation of contralateral homologous and larger heterologous muscles (biceps brachii; BB). Fifteen right-handed male subjects performed 3 pre-test index finger abduction or elbow flexion maximum voluntary isometric contractions (MVICs) on the nondominant side. Subsequently, they performed two 100-s index finger abduction MVICs on the dominant side (experimental (fatigue) group) or rested for 5 min (control group). Afterwards, a single MVIC and a 12-repetition MVIC fatiguing protocol were completed with index finger abduction or elbow flexion on the nondominant side. Force and electromyography (EMG) were measured from both sides. The force and EMG (median frequency; MDF) of nonexercised index finger abductors (IFA)/FDI and elbow flexors (EF)/BB significantly decreased after the fatiguing protocol. Compared with the control condition, the nonexercised IFA (12.5% and 5.7%) had significantly greater force and MDF fatigue indexes than the EF (5.2% and 1.7%). There were no significant force differences with the single MVIC test between conditions. The small muscle fatiguing protocol produced NLMF effects on both contralateral homologous and larger heterologous muscles, with the force decrements greater with the homologous muscle.

The effect of eccentric exercise and delayed onset muscle soreness on the homologous muscle of the contralateral limb

High intensity eccentric exercise induces muscle fiber damage and associated delayed-onset muscle soreness (DOMS) resulting in an impaired ability of the muscle to generate voluntary force. This study investigates the extent to which DOMS, induced by high intensity eccentric exercise, can affect the activation and performance of the non-exercised homologous muscle of the contralateral limb. Healthy volunteers performed maximal voluntary contractions of knee extension and sustained isometric knee extension at 50% of maximal force until task failure on both the ipsilateral exercised limb and the contralateral limb. Surface electromyography (EMG) was recorded from the ipsilateral and contralateral knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis). Maximal isometric knee extension force (13.7% reduction) and time to task failure (38.1% reduction) of the contralateral non-exercised leg decreased immediately after eccentric exercise, and persisted 24 h and 48 h later (p < 0.05). Moreover, the amplitude of muscle activity recorded from the contralateral knee extensor muscles was significantly lower during the post exercise maximal and submaximal contractions following high intensity eccentric exercise of the opposite limb (p < 0.05). Unilateral high intensity eccentric exercise of the quadriceps can contribute to reduced neuromuscular activity and physical work capacity of the non-exercised homologous muscle in the contralateral limb.

Effects of ipsilateral and contralateral fatigue and muscle blood flow occlusion on the complexity of knee-extensor torque output in humans

NEW FINDINGS

What is the central question of this study? We addressed the question "what role do central and peripheral fatigue mechanisms play in the fatigue-induced loss of isometric torque complexity?" What is the main finding and its importance? When the contralateral limb is fatigued, the complexity of isometric torque output is unaffected even if the blood flow to the contralateral limb is occluded, which suggests that neither central fatigue nor afferent feedback from ischaemic muscle influences the complexity of torque output in an otherwise fresh muscle.

ABSTRACT

Neuromuscular fatigue reduces the temporal structure, or complexity, of torque output during muscular contractions. To determine whether the fatigue-induced loss of torque complexity could be accounted for by central or peripheral factors, nine healthy participants performed four experimental trials involving intermittent isometric contractions of the knee extensors at 50% of the maximal voluntary contraction torque. These trials involved: (i) two bouts of contractions to failure using the right leg separated by 3 min recovery (IPS); (ii) the same protocol but with cuff occlusion during the 3 min recovery (IPS-OCC); (iii) contractions of the left leg to failure, followed 1 min later by contractions of the right leg to failure (CONT); and (iv) the same protocol but with cuff occlusion applied to the left leg throughout both the recovery period and right leg contractions (CONT-OCC). Supramaximal electrical stimulation during maximal voluntary contractions was used to determine the degree of central and peripheral fatigue, whilst complexity was determined using approximate entropy (ApEn) and detrended fluctuation analysis α exponent (DFA α). Neuromuscular fatigue was consistently associated with a loss of torque complexity in all conditions [e.g. IPS bout 1, ApEn from (mean ± SD) 0.46 ± 0.14 to 0.12 ± 0.06 (P < 0.001)]. In IPS-OCC, occlusion abolished the recovery from fatigue, and torque complexity remained at the values observed at task failure in the preceding bout (IPS-OCC bout 2, first minute 0.14 ± 0.03, P < 0.001). Prior contralateral contractions, with or without blood flow occlusion, had no effect on torque complexity.

The Assessment of Motor Fatigability in Persons With Multiple Sclerosis: A Systematic Review

Background. Persons with multiple sclerosis (PwMS) are often characterized by increased motor fatigability, which is a performance change on an objectively measured criterion after any type of voluntary muscle contractions. This review summarizes the existing literature to determine which protocols and outcome measures are best to detect or study motor fatigability and the underlying mechanisms in MS. Methods. Two electronic databases, PubMed and Web of Science, were searched for relevant articles published until August 2016 with a combination of multiple sclerosis, fatigability, muscle fatigue, and motor fatigue. Results. A total of 48 articles were retained for data extraction. A variety of fatigability protocols were reported; protocols showed differences in type (isometric vs concentric), duration (15 to 180 s), and number of contractions (fixed or until exhaustion). Also, 12 articles reported motor fatigability during functional movements, predominantly assessed by changes in walking speed; 11 studies evaluated the mechanisms underlying motor fatigability, using additional electrical nerve or transcranial magnetic stimulation. Three articles reported psychometrics of the outcomes. Conclusions. The disparity of protocols and outcome measures to study different aspects of motor fatigability in PwMS impedes direct comparison between data. Most protocols use maximal single-joint isometric contractions, with the advantage of high standardization. Because there is no head-to-head comparison of the different protocols and only limited information on psychometric properties of outcomes, there is currently no gold standard to assess motor fatigability. The disability level, disease phenotype, and studied limb may influence the assessment of motor fatigability in PwMS.

Evidence of nonlocal muscle fatigue in male youth

Evidence for nonlocal muscle fatigue (NLMF) has been inconsistent in adults, with no studies investigating youth. The objective was to examine NLMF in youth. Forty-two young males (age, 10-13 years) were tested for maximal voluntary isometric contraction (MVIC) force of the ipsilateral and contralateral knee extensors at 90° and 120° knee flexion, elbow flexors at 90°, handgrip, knee extensor isokinetic torque (300°·s^-1 analyzed at 90° and 120° knee flexion), as well as a unilateral countermovement jump (CMJ) and Y Balance test (YBT). Isokinetic fatigue group (n = 15) had unilateral fatigue induced with 10 sets of 20 repetitions of maximal isokinetic knee extensor contractions at 300°·s^-1. Isometric fatigue group (n = 15) used 10 repetitions of 6-s knee extensor MVIC whereas the control group (n = 12) were not fatigued. There was no significant difference in the response to the isometric- or isokinetic-fatigue intervention protocols. Main time effects indicated that NLMF was evident with the contralateral knee extensor MVIC at 90° (p = 0.008; 8.9%), knee extensor isokinetic torque at 90° (p < 0.001; 11.4%), and 120° (p = 0.05; 5.4%), CMJ (p = 0.02; 11.5%), handgrip (p = 0.06; 4.5%), elbow flexors (p < 0.001; 7.7%), and YBT (p = 0.001; 5.6%). Ipsilateral NLMF deficits occurred with handgrip (p < 0.001; 7.3%), elbow flexors MVICs (p < 0.001; 10.7%), CMJ (p = 0.02; 12.2%), and YBT (p = 0.002; 3.8%). NLMF with similar relative fatigue-induced deficits of fatigued and nonfatigued limbs suggest that youth fatigue is highly dependent upon the extent of activation or inhibition of the nervous system. Coaches of young athletes might consider developing technical motor skills before fatiguing exercise components, which might hinder the proficiency of their performance.

Maximal intermittent contractions of the first dorsal interosseous inhibits voluntary activation of the contralateral homologous muscle

The aim of this study was to investigate how maximal intermittent contractions for a hand muscle influence cortical and reflex activity, as well as the ability to voluntarily activate, the homologous muscle in the opposite limb. Twelve healthy subjects (age: 24 ± 3 years, all right hand dominant) performed maximal contractions of the dominant limb first dorsal interosseous (FDI), and activity of the contralateral FDI was examined in a series of experiments. Index finger abduction force, FDI EMG, motor evoked potentials and heteronomous reflexes were obtained from the contralateral limb during brief non-fatiguing contractions. The same measures, as well as the ability to voluntarily activate the contralateral FDI, were then assessed in an extended intermittent contraction protocol that elicited fatigue. Brief contractions under non-fatigued conditions increased index finger abduction force, FDI EMG, and motor evoked potential amplitude of the contralateral limb. However, when intermittent maximal contractions were continued until fatigue, there was an inability to produce maximal force with the contralateral limb (~30%) which was coupled to a decrease in the level of voluntary activation (~20%). These declines were present without changes in reflex activity, and regardless of whether cortical or motor point stimulation was used to assess voluntary activation. It is concluded that performing maximal intermittent contractions with a single limb causes an inability of the CNS to maximally drive the homologous muscle of the contralateral limb. This was, in part, mediated by mechanisms that involve the motor cortex ipsilateral to the contracting limb.

Bilateral Knee Extensor Fatigue Modulates Force and Responsiveness of the Corticospinal Pathway in the Non-fatigued, Dominant Elbow Flexors

Exercise-induced fatigue affects muscle performance and modulates corticospinal excitability in non-exercised muscles. The purpose of this study was to investigate the effect of bilateral knee extensor fatigue on dominant elbow flexor (EF) maximal voluntary force production and corticospinal excitability. Transcranial magnetic, transmastoid electrical and brachial plexus electrical stimulation (BPES) were used to investigate corticospinal, spinal, and muscle excitability of the dominant EF before and after a bilateral knee extensor fatiguing protocol or time matched rest period (control). For both sessions three stimuli were delivered every 1.5 s during the three pre-test time points and during the 1st, 3rd, 6th, 9th and 12th post-test 5 s EF isometric maximal voluntary contractions (MVC). In both conditions, overall, EF MVC force (p < 0.001) decreased progressively from repetition #1 to #12 during the post-test MVC protocol. EF MVC force (p < 0.001, ES = 0.9, Δ10.3%) decrements were more pronounced in the knee extensor fatigue intervention condition. In addition, there were no significant differences between conditions for biceps brachii electromyographic (EMG) activity (p = 0.43), motor evoked potentials (MEPs) amplitude (p = 0.908) or MEP silent period (SP; p = 0.776). However, the fatigue condition exhibited a lower MEP/cervicomedullary MEP (CMEP) ratio (p = 0.042, ES = 2.5, Δ25%) and a trend toward higher CMEP values (p = 0.08, ES = 0.5, Δ20.4%). These findings suggest that bilateral knee extensor fatigue can impair performance and modulate corticospinal excitability of the EF.

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.

Acute bouts of upper and lower body static and dynamic stretching increase non-local joint range of motion

PURPOSE

There are conflicts in the literature concerning the crossover or non-local effects of stretching. The objective of this study was to evaluate whether static (SS) and dynamic (DS) stretching of the shoulders would affect hip flexor range of motion (ROM) and performance and reciprocally whether SS and DS of the lower body would affect shoulder extension ROM and performance.

METHODS

A randomized crossover study design examined the acute effects of upper and lower body SS and DS on lower and upper body performance measures, respectively. Experimental sessions included upper and lower body control tests, upper body (shoulder horizontal abduction) SS and lower body (hip abduction) SS, upper body (shoulder horizontal abduction and adduction) DS and lower body DS (hip abduction and adduction). Passive static and dynamic ROM (hip flexion, shoulder extension), leg flexor and elbow flexor maximal voluntary contraction isometric force, fatigue endurance and electromyography were measured.

RESULTS

There were significant shoulder ROM increases following lower body SS (P < 0.010, ∆% = 8.2%) and DS (P < 0.019, ∆% = 9%). There was a significant hip flexor ROM (P < 0.016, ∆% = 5.2%) increase following upper body SS. There were no significant main effects or interactions for dynamic ROM or muscle force and activation variables.

CONCLUSION

The lack of stretch-induced force and fatigue changes suggests that rather than a mechanical or neural drive mechanism, an enhanced stretch tolerance was likely the significant factor in the improved ROM.

Elbow flexor fatigue modulates central excitability of the knee extensors

The present study investigated the effects of exercise-induced elbow flexor fatigue on voluntary force output, electromyographic (EMG) activity and motoneurone excitability of the nonexercised knee extensor muscles. Eleven participants attended 3 testing sessions: (i) control, (ii) unilateral fatiguing elbow flexion and (iii) bilateral fatiguing elbow flexion (BiFlex). The nonfatigued knee extensor muscles were assessed with thoracic motor evoked potentials (TMEPs), maximal compound muscle action potential (Mmax), knee extensor maximal voluntary contractions (MVCs), and normalized EMG activity before and at 30 s, 3 min, and 5 min postexercise. BiFlex showed significantly lower (Δ = -18%, p = 0.03) vastus lateralis (VL) normalized EMG activity compared with the control session whereas knee extension MVC force did not show any statistical difference between the 3 conditions (p = 0.12). The TMEP·Mmax(-1) ratio measured at the VL showed a significantly higher value (Δ = +46%, p = 0.003) following BiFlex compared with the control condition at 30 s postexercise. The results suggest that the lower VL normalized EMG following BiFlex might have been due to a reduction in supraspinal motor output because spinal motoneuronal responses demonstrated substantially higher value (30 s postexercise) and peripheral excitability (compound muscle action potential) showed no change following BiFelex than control condition.

Fatigue-related firing of muscle nociceptors reduces voluntary activation of ipsilateral but not contralateral lower limb muscles

During fatiguing upper limb exercise, maintained firing of group III/IV muscle afferents can limit voluntary drive to muscles within the same limb. It is not known if this effect occurs in the lower limb. We investigated the effects of group III/IV muscle afferent firing from fatigued ipsilateral and contralateral extensor muscles and ipsilateral flexor muscles of the knee on voluntary activation of the knee extensors. In three experiments, we examined voluntary activation of the knee extensors by measuring changes in superimposed twitches evoked by femoral nerve stimulation. Subjects attended on 2 days for each experiment. On one day a sphygmomanometer cuff occluded blood flow of the fatigued muscles to maintain firing of group III/IV muscle afferents. After a 2-min extensor contraction (experiment 1; n = 9), mean voluntary activation was lower with than without maintained ischemia (47 ± 19% vs. 87 ± 8%, respectively; P < 0.001). After a 2-min knee flexor maximal voluntary contraction (MVC) (experiment 2; n = 8), mean voluntary activation was also lower with than without ischemia (59 ± 21% vs. 79 ± 9%; P < 0.01). After the contralateral (left) MVC (experiment 3; n = 8), mean voluntary activation of the right leg was similar with or without ischemia (92 ± 6% vs. 93 ± 4%; P = 0.65). After fatiguing exercise, activity in group III/IV muscle afferents reduces voluntary activation of the fatigued muscle and nonfatigued antagonist muscles in the same leg. However, group III/IV muscle afferents from the fatigued left leg had no effect on the unfatigued right leg. This suggests that any "crossover" of central fatigue in the lower limbs is not mediated by group III/IV muscle afferents.

Increased bilateral interactions in middle-aged subjects

A hallmark of the age-related neural reorganization is that old versus young adults execute typical motor tasks by a more diffuse neural activation pattern including stronger ipsilateral activation during unilateral tasks. Whether such changes in neural activation are present already at middle age and affect bimanual interactions is unknown. We compared the amount of associated activity, i.e., muscle activity and force produced by the non-task hand and motor evoked potentials (MEPs) produced by magnetic brain stimulation between young (mean 24 years, n = 10) and middle-aged (mean 50 years, n = 10) subjects during brief unilateral (seven levels of % maximal voluntary contractions, MVCs) and bilateral contractions (4 × 7 levels of % MVC combinations), and during a 120-s-long MVC of sustained unilateral index finger abduction. During the force production, the excitability of the ipsilateral (iM1) or contralateral primary motor cortex (cM1) was assessed. The associated activity in the "resting" hand was ~2-fold higher in middle-aged (28% of MVC) versus young adults (11% of MVC) during brief unilateral MVCs. After controlling for the background muscle activity, MEPs in iM1 were similar in the two groups during brief unilateral contractions. Only at low (bilateral) forces, MEPs evoked in cM1 were 30% higher in the middle-aged versus young adults. At the start of the sustained contraction, the associated activity was higher in the middle-aged versus young subjects and increased progressively in both groups (30 versus 15% MVC at 120 s, respectively). MEPs were greater at the start of the sustained contraction in middle-aged subjects but increased further during the contraction only in young adults. Under these experimental conditions, the data provide evidence for the reorganization of neural control of unilateral force production as early as age 50. Future studies will determine if the altered neural control of such inter-manual interactions are of functional significance.

Instability resistance training across the exercise continuum

Context:

Instability resistance training (IRT; unstable surfaces and devices to strengthen the core or trunk muscles) is popular in fitness training facilities.

Objective:

To examine contradictory IRT recommendations for health enthusiasts and rehabilitation.

Data Sources:

A literature search was performed using MEDLINE, SPORT Discus, ScienceDirect, Web of Science, and Google Scholar databases from 1990 to 2012.

Study Selection:

Databases were searched using key terms, including “balance,” “stability,” “instability,” “resistance training,” “core,” “trunk,” and “functional performance.” Additionally, relevant articles were extracted from reference lists.

Data Extraction:

To be included, research questions addressed the effect of balance or IRT on performance, healthy and active participants, and physiologic or performance outcome measures and had to be published in English in a peer-reviewed journal.

Results:

There is a dichotomy of opinions on the effectiveness and application of instability devices and conditions for health and performance training. Balance training without resistance has been shown to improve not only balance but functional performance as well. IRT studies document similar training adaptations as stable resistance training programs with recreationally active individuals. Similar progressions with lower resistance may improve balance and stability, increase core activation, and improve motor control.

Conclusion:

IRT is highly recommended for youth, elderly, recreationally active individuals, and highly trained enthusiasts.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Time course of the cross-over effect of fatigue on the contralateral muscle after unilateral exercise

We investigated the cross-over effect of muscle fatigue and its time course on the non-exercising contralateral limb (NEL) after unilateral fatiguing contractions of the ipsilateral exercising limb (EL). For this purpose, 15 males performed two bouts of 100-second maximal isometric knee extensions with the exercising limb, and neuromuscular function of both the EL and NEL was assessed before (PRE), after a first fatiguing exercise (MID) and after a second fatiguing exercise (POST). Maximal voluntary isometric torque production declined in the EL after the first bout of exercise (-9.6%; p<0.001) while in the NEL, the decrease occurred after the second bout of exercise (-10.6%; p<0.001). At MID, torque decline of the EL was strictly associated to an alteration of the mechanical twitch properties evoked by neurostimulation of the femoral nerve (i.e., peak twitch torque, maximal rate of twitch development). According to these markers, we suggest that peripheral fatigue occurred. At POST, after the second bout of exercise, the voluntary activation level of the knee extensor muscles was altered from PRE (-9.1%; p<0.001), indicating an overall central failure in both the EL and NEL. These findings indicate that two bouts of unilateral fatiguing exercise were needed to induce a cross-over effect of muscle fatigue on the non-exercising contralateral limb. Differential adjustments of the motor pathway (peripheral fatigue vs. central fatigue) might contribute to the respective torque decline in the EL and the NEL. Given that our unilateral fatiguing exercise induced immediate maximal torque reduction in the EL and postponed the loss of torque production in the NEL, it is also concluded that the time course of muscle fatigue differed between limbs.

Peripheral fatigue limits endurance exercise via a sensory feedback-mediated reduction in spinal motoneuronal output

This study sought to determine whether afferent feedback associated with peripheral muscle fatigue inhibits central motor drive (CMD) and thereby limits endurance exercise performance. On two separate days, eight men performed constant-load, single-leg knee extensor exercise to exhaustion (85% of peak power) with each leg (Leg1 and Leg2). On another day, the performance test was repeated with one leg (Leg1) and consecutively (within 10 s) with the other/contralateral leg (Leg2-post). Exercise-induced quadriceps fatigue was assessed by reductions in potentiated quadriceps twitch-force from pre- to postexercise (ΔQtw,pot) in response to supramaximal magnetic femoral nerve stimulation. The output from spinal motoneurons, estimated from quadriceps electromyography (iEMG), was used to reflect changes in CMD. Rating of perceived exertion (RPE) was recorded during exercise. Time to exhaustion (∼9.3 min) and exercise-induced ΔQtw,pot (∼51%) were similar in Leg1 and Leg2 (P > 0.5). In the consecutive leg trial, endurance performance of the first leg was similar to that observed during the initial trial (∼9.3 min; P = 0.8); however, time to exhaustion of the consecutively exercising contralateral leg (Leg2-post) was shorter than the initial Leg2 trial (4.7 ± 0.6 vs. 9.2 ± 0.4 min; P < 0.01). Additionally, ΔQtw,pot following Leg2-post was less than Leg2 (33 ± 3 vs 52 ± 3%; P < 0.01). Although the slope of iEMG was similar during Leg2 and Leg2-post, end-exercise iEMG following Leg2-post was 26% lower compared with Leg2 (P < 0.05). Despite a similar rate of rise, RPE was consistently ∼28% higher throughout Leg2-post vs. Leg2 (P < 0.05). In conclusion, this study provides evidence that peripheral fatigue and associated afferent feedback limits the development of peripheral fatigue and compromises endurance exercise performance by inhibiting CMD.

The neurophysiology of central and peripheral fatigue during sub-maximal lower limb isometric contractions

Fatigue has been defined as an exercise-induced decline in force generation capacity because of changes at both the peripheral and central levels. Movement is preceded and accompanied by brain activities related to the preparation and execution of movement (movement related cortical potentials, MRCP), which have been correlated with the perception of effort (RPE). We combined force measurements, surface electromyography (sEMG), peripheral electrical stimulation (maximal twitch, MT) and MRCP analysis to further our understanding of the neural correlates of peripheral and central changes during a fatiguing task involving the lower limbs. Eighteen healthy volunteers performed 4 blocks of isometric knee extensions at 40% of the maximal voluntary contraction (MVC) for a total of 240 2-s contractions. At the baseline and after each block, we measured RPE, MT and MVC. We simultaneously recorded the force of the knee extensor muscles, root mean square (RMS) of the sEMG of the vastus lateralis muscle, and electroencephalography (EEG) from 64 channels. The MRCPs were extracted from the EEG recordings and averaged in the early (Block 1-2) and late (Block 3-4) blocks. Two cohorts were obtained by cluster analysis based on the RPE (i.e., perception of effort) and MT (i.e., peripheral fatigue). We observed a significant decline in both the MVC (-13%) and RMS (-25%) of the sEMG signal over the course of the task; thus, muscle fatigue had occurred in all of the participants regardless of the cohort. The MRCP amplitude was larger in the fatigued than the non-fatigued MT cohort in the supplementary and premotor areas, whereas the MRCP amplitude was larger in the fatigued than the non-fatigued RPE cohort in the aforementioned areas, and also in the primary motor and prefrontal cortices (PFC). The increase in the positive activity of the PFC, along with the perception of effort, represents a novel result, suggesting that it is modulated more by the perception of effort than peripheral fatigue.

Serotonin spillover onto the axon initial segment of motoneurons induces central fatigue by inhibiting action potential initiation

Motor fatigue induced by physical activity is an everyday experience characterized by a decreased capacity to generate motor force. Factors in both muscles and the central nervous system are involved. The central component of fatigue modulates the ability of motoneurons to activate muscle adequately independently of the muscle physiology. Indirect evidence indicates that central fatigue is caused by serotonin (5-HT), but the cellular mechanisms are unknown. In a slice preparation from the spinal cord of the adult turtle, we found that prolonged stimulation of the raphe-spinal pathway--as during motor exercise--activated 5-HT1A receptors that decreased motoneuronal excitability. Electrophysiological tests combined with pharmacology showed that focal activation of 5-HT1A receptors at the axon initial segment (AIS), but not on other motoneuronal compartments, inhibited the action potential initiation by modulating a Na(+) current. Immunohistochemical staining against 5-HT revealed a high-density innervation of 5-HT terminals on the somatodendritic membrane and a complete absence on the AIS. This observation raised the hypothesis that a 5-HT spillover activates receptors at this latter compartment. We tested it by measuring the level of extracellular 5-HT with cyclic voltammetry and found that prolonged stimulations of the raphe-spinal pathway increased the level of 5-HT to a concentration sufficient to activate 5-HT1A receptors. Together our results demonstrate that prolonged release of 5-HT during motor activity spills over from its release sites to the AIS of motoneurons. Here, activated 5-HT1A receptors inhibit firing and, thereby, muscle contraction. Hence, this is a cellular mechanism for central fatigue.

Effects of fatigue on synergies in a hierarchical system

We investigated the effect of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of one of the hands on performance in MVC and accurate cyclic force production tasks in right-handed subjects. Based on earlier studies, we hypothesized that fatigue would produce an increase in the indices of force-stabilizing synergies in both hands as well as between the hands in two-hand tasks. Synergies were defined as co-varied adjustments of commands to fingers (modes) across cycles that stabilized total force. Fatigue caused a significant reduction in the MVC of the exercised as well as the non-exercised hand. Indices of finger enslaving (lack of individuation) increased with fatigue in both hands, although the increase was significant in the exercised hand only. In contrast to the significant effects of fatigue on MVC forces performed by the non-exercised hand, there were no comparable transfer effects on the root mean square errors during accurate force production. During one-hand tasks, both hands showed high indices of force-stabilizing synergies. These indices were larger in the left hand. Fatigue led to a general increase in synergy indices. Exercise by the left hand had stronger effects on synergy indices seen in both hands. Exercise by the right hand showed ipsilateral effects only. Smaller effects of fatigue were observed on accuracy of performance of the force-down segments of the force cycles compared to the force-up segments. For the bimanual tasks, synergies were analyzed at two hierarchical levels, two-hand (four-finger) and within-a-hand (two-finger). An increase in the synergy index with fatigue was observed at the lower (two-finger) level of the hierarchy only. We interpret the lack of effects of fatigue at the upper (two-hand) level as a consequence of a trade-off between synergies at different levels of the hierarchy. The differences between the hands are discussed within the dynamic dominance hypothesis.

Load type influences motor unit recruitment in biceps brachii during a sustained contraction

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

Voluntary activation and cortical activity during a sustained maximal contraction: an fMRI study

Motor fatigue is an exercise-induced reduction in the force-generating capacity. The underlying mechanisms can be separated into factors residing in the periphery or in the central nervous system. We designed an experiment in which we investigated central processes underlying motor fatigue by means of magnetic resonance imaging in combination with the twitch interpolation technique. Subjects performed a sustained maximal abduction (2 min) with the right index finger. Brain activation was recorded with an MR scanner, together with index finger abduction force, EMG of several hand muscles and interpolated twitches. Mean activity per volume was calculated for the primary motor cortex and the secondary motor areas (supplementary motor, premotor, and cingulate areas) as well as mean force and mean rectified EMG amplitude. Results showed a progressive decline in maximal index finger abduction force and EMG of the target muscles combined with an increase in brain activity in the contralateral primary motor cortex and secondary motor areas. Analysis of the twitches superimposed on the sustained contraction revealed that during the contraction the voluntary drive decreased significantly. In conclusion, our data showed that despite an increase in brain activity the voluntary activation decreased. This suggests that, although the CNS increased its input to the relevant motor areas, this increase was insufficient to overcome fatigue-related changes in the voluntary drive.

Contralateral muscle activity and fatigue in the human first dorsal interosseous muscle

During effortful unilateral contractions, muscle activation is not limited to the target muscles but activity is also observed in contralateral muscles. The amount of this associated activity is depressed in a fatigued muscle, even after correction for fatigue-related changes in maximal force. In the present experiments, we aimed to compare fatigue-related changes in associated activity vs. parameters that are used as markers for changes in central nervous system (CNS) excitability. Subjects performed brief maximal voluntary contractions (MVCs) with the index finger in abduction direction before and after fatiguing protocols. We followed changes in MVCs, associated activity, motor-evoked potentials (MEP; transcranial magnetic stimulation), maximal compound muscle potentials (M waves), and superimposed twitches (double pulse) for 20 min after the fatiguing protocols. During the fatiguing protocols, associated activity increased in contralateral muscles, whereas afterwards the associated force was reduced in the fatigued muscle. This force reduction was significantly larger than the decline in MVC. However, associated activity (force and electromyography) remained depressed for only 5–10 min, whereas the MVCs stayed depressed for over 20 min. These decreases were accompanied by a reduction in MEP, MVC electromyography activity, and voluntary activation in the fatigued muscle. According to these latter markers, the decrease in CNS motor excitability lasted much longer than the depression in associated activity. Differential effects of fatigue on (associated) submaximal vs. maximal contractions might contribute to these differences in postfatigue behavior. However, we cannot exclude differences in processes that are specific to either voluntary or to associated contractions.

A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions

Magnetic and electrical stimulation at different levels of the neuraxis show that supraspinal and spinal factors limit force production in maximal isometric efforts ("central fatigue"). In sustained maximal contractions, motoneurons become less responsive to synaptic input and descending drive becomes suboptimal. Exercise-induced activity in group III and IV muscle afferents acts supraspinally to limit motor cortical output but does not alter motor cortical responses to transcranial magnetic stimulation. "Central" and "peripheral" fatigue develop more slowly during submaximal exercise. In sustained submaximal contractions, central fatigue occurs in brief maximal efforts even with a weak ongoing contraction (<15% maximum). The presence of central fatigue when much of the available motor pathway is not engaged suggests that afferent inputs contribute to reduce voluntary activation. Small-diameter muscle afferents are likely to be activated by local activity even in sustained weak contractions. During such contractions, it is difficult to measure central fatigue, which is best demonstrated in maximal efforts. To show central fatigue in submaximal contractions, changes in motor unit firing and force output need to be characterized simultaneously. Increasing central drive recruits new motor units, but the way this occurs is likely to depend on properties of the motoneurons and the inputs they receive in the task. It is unclear whether such factors impair force production for a set level of descending drive and thus represent central fatigue. The best indication that central fatigue is important during submaximal tasks is the disproportionate increase in subjects' perceived effort when maintaining a low target force.

Central fatigue of the first dorsal interosseous muscle during low-force and high-force sustained submaximal contractions

The aim of this study was to compare the extent of central fatigue in the first dorsal interosseous (FDI) muscle of healthy adults in low, moderate and high-force submaximal contractions. Nine healthy adults completed four experimental sessions where index finger abduction force was recorded during voluntary contractions and in response to brief trains (five pulses at 100 Hz) of electrical stimulation. The ability to maximally activate FDI under volition, or voluntary activation, and its change with sustained activity (central fatigue) was assessed using the twitch interpolation technique. The fatigue tasks consisted of continuous isometric index finger abduction contractions held until exhaustion at four target force levels: 30%, 45%, 60% and 75% of the maximal voluntary contraction. The main finding was the presence of central fatigue for the 30% task, but not for the three other fatigue tasks. The extent of central fatigue was also associated with changes in a measure reflecting the status of peripheral structures/mechanisms. It appears that central fatigue contributed to task failure for the lowest force fatigue task (30%), but not for the other (higher) contraction intensities.

MR compatible strain gauge based force transducer

In order to evaluate brain activation during motor tasks accurately one must also measure output parameters such as muscle force or muscle activity. Especially in clinical situations where the force output can be compromised by changes at different levels of the motor system, it is essential to standardize the task or force level. We have therefore developed a magnetic resonance compatible force transducer that is capable of recording index finger abduction force and to display the produced force in real-time. This transducer is based on strain-gauges techniques and designed to measure both small and large forces accurately (range 0.7-60N) as well as fast force fluctuations. Experiments showed that the MR environment did not affect the force measurements or vice versa. Although, this transducer is developed for measuring index finger forces, detailed schematic diagrams are provided such that the transducer can easily be adapted for measuring forces of other muscle groups.

Central fatigue explains sex differences in muscle fatigue and contralateral cross-over effects of maximal contractions

A sustained voluntary contraction increases central fatigue and produces a 'cross-over' of fatigue during a subsequent contraction of the contralateral limb. These studies compared the magnitude of these changes for men and women. Force and electromyographic responses from dominant (study 1; n = 8 men, 8 women) or non-dominant (study 2; n = 7 men, 8 women) leg extensors to nerve stimulation were recorded at rest and during brief maximal voluntary contractions (MVCs), before and after 100-s sustained MVCs performed with the dominant leg. For the dominant leg, force was reduced more for men (by approximately 24%) than women (by approximately 16%, P < 0.05) after the sustained contractions. Similarly, voluntary activation during these contractions was reduced more for men (by approximately 22%) than women (by approximately 9%, P < 0.05). Conversely, resting twitches changed similarly for both sexes (P > 0.05). For the non-dominant leg, men experienced a reduction in force (by approximately 13%, P < 0.001) and had greater deficits in activation than women ( approximately 9% vs approximately 3%, P < 0.05), after sustained contractions of the dominant leg. Therefore, sustained MVCs produce greater central fatigue and a more pronounced 'cross-over' of effects to the contralateral limb for men compared to women. These findings demonstrate distinct differences between sexes in the way the nervous system adapts to changes associated with fatigue.

Exhaustive stretch-shortening cycle exercise: no contralateral effects on muscle activity in maximal motor performances

Minor cross-over effects of unilateral muscle fatigue have been reported after isometric exercises. The present study re-examined this possibility after an exhaustive stretch-shortening cycle (SSC)-type exercise. Twenty-five subjects performed on a sledge apparatus a unilateral exhaustive rebound exercise involving mostly the triceps surae muscle group. Ipsilateral vs contralateral fatigue effects were compared in uni- and bilateral tests that included a maximal isometric voluntary contraction (MVC) and a series of 10 maximal drop jumps (DJ). These tests were carried out just before and after (POST) the exhaustive SSC exercise, and were repeated 2 days later (D2), at the expected time of major inflammation and pain. The exercised (fatigued) leg analysis revealed significant declines in MVC and DJ performances at POST and D2, the latter ones being associated with significant decreases in voluntary muscle activity. In contrast, no significant change was found for the non-fatigued leg. These results do not support the existence of cross-over effects after exhaustive SSC exercise, at least when tested in maximal static and dynamic unilateral motor tasks.

The effect of sustained low-intensity contractions on supraspinal fatigue in human elbow flexor muscles

Subjects quickly fatigue when they perform maximal voluntary contractions (MVCs). Much of the loss of force is from processes within muscle (peripheral fatigue) but some occurs because voluntary activation of the muscle declines (central fatigue). The role of central fatigue during submaximal contractions is not clear. This study investigated whether central fatigue developed during prolonged low-force voluntary contractions. Subjects (n=9) held isometric elbow flexions of 15% MVC for 43 min. Voluntary activation was measured during brief MVCs every 3 min. During each MVC, transcranial magnetic stimulation (TMS) was followed by stimulation of either brachial plexus or the motor nerve of biceps brachii. After nerve stimulation, a resting twitch was also evoked before subjects resumed the 15% MVC. Perceived effort, elbow flexion torque and surface EMG from biceps, brachioradialis and triceps were recorded. TMS was also given during the sustained 15% MVC. During the sustained contraction, perceived effort rose from approximately 2 to approximately 8 (out of 10) while ongoing biceps EMG increased from 6.9+/-2.1% to 20.0+/-7.8% of initial maximum. Torque in the brief MVCs and the resting twitch fell to 58.6+/-14.5 and 58.2+/-13.2% of control values, respectively. EMG in the MVCs also fell to 62.2+/-15.3% of initial maximum, and twitches evoked by nerve stimulation and TMS grew progressively. Voluntary activation calculated from these twitches fell from approximately 98% to 71.9+/-38.9 and 76.9+/-18.3%, respectively. The silent period following TMS lengthened both in the brief MVCs (by approximately 40 ms) and in the sustained target contraction (by approximately 18 ms). After the end of the sustained contraction, the silent period recovered immediately, voluntary activation and voluntary EMG recovered over several minutes while MVC torque only returned to approximately 85% baseline. The resting twitch showed no recovery. Thus, as well as fatigue in the muscle, the prolonged low-force contraction produced progressive central fatigue, and some of this impairment of the subjects' ability to drive the muscle maximally was due to suboptimal output from the motor cortex. Although caused by a low-force contraction, both the peripheral and central fatigue impaired the production of maximal voluntary force. While central fatigue can only be demonstrated during MVCs, it may have contributed to the disproportionate increase in perceived effort reported during the prolonged low-force contraction.

Decreases in motor unit firing rate during sustained maximal-effort contractions in young and older adults

Previous studies have suggested that older adults may be more resistant to muscular fatigue than young adults. We sought to determine whether motor unit firing rate might be a factor that determines the response to fatiguing exercise in young and older subjects. Motor unit recordings and muscular forces were obtained from the tibialis anterior (TA) muscle of 11 young and 8 older individuals. Maximal voluntary force was first measured during maximal-effort dorsiflexion contractions. Each subject then performed a series of 15 maximal isometric contractions, with each contraction lasting 30s. A 10-s rest period separated the fatiguing contractions. As a result of the fatiguing exercise, both subject groups demonstrated a significant loss in maximal force. The force decline was less in the older adults (20.4%) than in the young adults (33.8%). As expected, prior to muscle fatigue, maximal firing rates in the TA muscle were greater in the young (28.1+/-5.8 imp/s) than in the older adults (22.3+/-4.8 imp/s). The decrease in motor unit firing rate with fatigue was also greater in the young adults (34.9%), than in the older adults (22.0%). These results suggest that the greater fatigue-resistance exhibited by older individuals might be explained by the fact that the decline in motor unit firing rate during fatigue is greater in young persons than it is in older adults.

Central fatigue in continuous and intermittent contractions of triceps brachii

Conflicting results have been found across studies concerning the effect of rest periods on the development of central fatigue during prolonged muscle activity. Thus, the aim of the present study was to assess differences in the development of central fatigue between continuous and intermittent elbow extension fatigue tasks in the same subjects. Force and electromyographic data were collected on eight healthy volunteers. The ability to maximally activate the triceps brachii muscle was assessed by delivering trains of electrical stimulation during maximal voluntary efforts. This was done before, during, and after three fatigue tasks involving a maximal contraction in elbow extension. One short-duration ( approximately 55-s) and two long-duration (3-min) fatigue tasks were performed by all subjects on separate sessions. One 3-min task was intermittent (5-s rests every 30 s) and the other was continuous. The main findings were that the development and extent of central fatigue were task-dependent, with a greater decrease in the ability to maximally activate triceps brachii observed for the 3-min continuous task. Also, the voluntary activation (VA) ratio was found to be a more sensitive index of central fatigue than the central activation ratio (CAR). These results suggest that, when assessing central fatigue in patients, conclusions may vary depending on the continuous/intermittent nature of the task performed and the estimate of voluntary activation used.