Mechanisms of fatigue differ after low- and high-force fatiguing contractions in men and women

The magnitude of failure in voluntary drive after fatiguing contractions of different intensities in men and women is not known. The purpose of this study was to compare the time to task failure and voluntary activation of men and women for a sustained isometric contraction performed at a low and high intensity with the elbow flexor muscles. Nine men and nine women sustained an isometric contraction at 20% and 80% of maximal voluntary contraction (MVC) force until task failure during separate sessions. The men had a shorter time to failure than women for the 20% but not the 80% MVC task. Voluntary activation was reduced to similar levels for the men and women at the end of the fatiguing contractions but was reduced less after the 80% MVC task than the 20% MVC contraction. Twitch amplitude was reduced similarly at task failure for both sexes and to similar levels at termination of the 20% and 80% MVC tasks. The rate of change in mean arterial pressure was the main predictor of time to failure for the low-force sustained contraction. These results suggest that women experienced greater muscle perfusion, less peripheral fatigue, and a longer time to task failure than men during the low-force fatiguing contraction. However, the low-force task induced greater central fatigue than the high-force contraction for both men and women. Thus, low-force, long-duration fatiguing contractions can be used in rehabilitation to induce significant fatigue within the central nervous system and potentially greater neural adaptations in men and women.     

Nonlinear cortical modulation of muscle fatigue: a functional MRI study

Muscle fatigue has been studied for over a century, but almost no data are available to indicate how the brain perceives fatigue and modulates its signals to the fatiguing muscle. In this study, brain activation was measured by functional magnetic resonance imaging (fMRI) during a sustained (2-min) maximal-effort handgrip contraction while handgrip force and finger muscle electromyographic (EMG) data were recorded simultaneously by a magnetic resonance environment-adapted force-EMG measurement system. The results showed decoupled progresses in brain and muscle activities when muscle was fatigued and correlated behaviors among the cortical areas being analyzed. While handgrip force and EMG signals declined in parallel during the course of muscle fatigue, fMRI-measured brain activities first substantially increased and then decreased. This similar signal modulation occurred not only in the primary sensorimotor areas but also in the secondary and association cortices (supplementary motor, prefrontal, and cingulate areas). The nonlinear changes of brain signal may reflect an early adjustment to strengthen the descending command for force-loss compensation and subsequent inhibition by sensory feedback as fatigue became more severe. The close association in the activation pattern in many cortical regions may reflect integrated processing of information in the brain.     

Immobilization-induced increase in fatigue resistance is not explained by changes in the muscle metaboreflex

Immobilization has been reported to enhance fatigability, which is paradoxical in light of the metabolic and molecular alterations that occur in atrophied muscles. We examined whether the immobilization-induced enhancement in fatigability was associated with attenuation in the muscle metaboreflex response. Ten subjects were examined after 3 weeks of hand-forearm immobilization. The time to task failure of a handgrip contraction (20% intensity) was determined along with heart rate (HR) and mean arterial pressure (MAP) at rest, during the task and during a 2-min postexercise muscle ischemia (PEMI) test that continues to stimulate the metaboreflex. Immobilization decreased strength by 25% (P<0.01) and increased the time to task failure by 21% (P=0.03). However, no changes were observed for the HR and MAP responses to the exercise task or during PEMI (P>0.05). These findings indicate that the augmentation of time to task failure with immobilization is not associated with changes in the pressor or metaboreflex responses.     

Diminished central activation during maximal voluntary contraction in chronic fatigue syndrome.

OBJECTIVE: We have investigated whether central activation failure (CAF) is increased during local muscle fatigue in chronic fatigue syndrome (CFS). METHODS: Fourteen female CFS patients and 14 age-matched healthy female controls made a 2 min sustained maximal voluntary contraction (MVC) of the biceps brachii muscle. Before, during, and after sustained MVC, electrical endplate stimulation was applied. Force and 5 channel surface EMG (sEMG) were registered. RESULTS: Although force responses upon stimulation during rest did not differ between patients and controls, MVC was significantly lower in patients. Already at the beginning of sustained MVC, CFS patients showed significantly larger CAF than controls (36.5+/-17.0% and 12.9+/-13.3%, respectively). For all individual patients mean CAF over the first 45 s was higher than 30%, while it was below 30% for all controls. Less peripheral fatigue in patients was demonstrated by the changes in muscle fibre conduction velocity and the differences between force responses before and after contraction. CONCLUSIONS: Central activation is diminished in CFS patients. Possible causes include changed perception, impaired concentration, reduced effort and physiologically defined changes, e.g. in the corticospinal excitability or the concentration of neurotransmitters. As a consequence, demands on the muscle are lower, resulting in less peripheral fatigue. SIGNIFICANCE: CFS patients show reduced central activation during MVC. The underlying pathophysiological processes remain still to be determined.     

Effect of shoulder angle on the activation pattern of the elbow extensors during a submaximal isometric fatiguing contraction

The aim of this study was to examine the effect of shoulder angle on the electromyographic (EMG) activation pattern of the elbow extensors during a fatiguing contraction. Ten young men (23.5 ± 1.7) were tested on two occasions with the elbow angle at 90° and the shoulder at either 0° or 90° of flexion. EMG was recorded by fine wire electrodes inserted into the lateral, medial, and long heads of the triceps brachii and the anconeus. An EMG‐torque relationship was determined prior to a sustained isometric contraction at 20% of maximum voluntary contraction (MVC) until target failure. Endurance time was shorter, and postfatigue MVC torque was lower at 90° (40.4 ± 12.7 Nm) versus 0° (47.9 ± 14.7 Nm) of flexion. EMG activity of the long head during the final 10% of the fatiguing contraction was significantly greater at 90° versus 0° with no effect of shoulder angle on any other muscle portions. The findings suggest that measures from one muscle portion of the elbow extensors are not representative of the whole group, and the relative activation of the two‐joint long head was changed depending on shoulder angle during a fatigue task. Muscle Nerve, 2010     

Reliability of measurements of muscle strength and voluntary activation using twitch interpolation

We investigated the reproducibility of measurements of maximal voluntary torque and maximal voluntary activation using twitch interpolation. On 5 days, each of 5 subjects performed 10 maximal voluntary isometric contractions of their elbow flexors. Single supramaximal stimuli were delivered over biceps brachii at the measured peak torque during each effort, and in the relaxed muscle 5 s later. A voluntary activation score was calculated from the size of twitches evoked by the stimuli (resolution < 0.15 Nm). Although all subjects were able to drive the stimulated elbow flexor muscles maximally in some trials, they did not do so in 75% of all contractions. Maximal voluntary torques did not vary significantly within a subject between sessions. There were consistent differences in the level of maximal voluntary activation between subjects (P < 0.01), but no differences in voluntary activation within an individual across days in 4 of 5 subjects. Failure to drive the stimulated elbow flexor muscles maximally was not associated with inadvertent co-contraction of the antagonist muscles. © 1995 John Wiley & Sons, Inc.     

Voluntary rate of torque development is impaired after a voluntary versus tetanic conditioning contraction

Introduction: Both voluntary and evoked conditioning contractions will potentiate muscle twitch contractile properties. The response of a voluntary contraction to each condition type is not well understood but it may be a more functional model than evoked twitch potentiation. Methods: Baseline measurements from tibialis anterior included: maximal isometric twitch torque and rate of torque development (RTD); maximal evoked 50‐Hz torque; and maximal voluntary ballistic RTD. Potentiation was induced by a 10‐s voluntary or tetanic contraction (∽78% MVC), followed by 2 twitches and 2 ballistic contractions. Results: Twitch properties (torque and RTD) were potentiated equally after each conditioning contraction. Ballistic RTD was greater post‐tetanus (390.2 ± 59.3 Nm/s) than post‐voluntary (356.4 ± 69.1 Nm/s), but both were reduced from baseline (422.0 ± 88.9 Nm/s). Conclusions: Twitch potentiation was similar between conditioning contraction types, but ballistic RTD was lower after post‐tetanus than post‐voluntary. The results indicate central inhibition or fatigue concurrent with peripheral potentiation. Muscle Nerve 49 : 218–224, 2014     

Assessment of the reliability of central and peripheral fatigue after sustained maximal voluntary contraction of the quadriceps muscle

The aim of the present study was to further confirm the validity of measurements for characterizing neuromuscular alterations by establishing their reliability both before and after fatigue. Thirteen men (28 +/- 5 years) volunteered to participate in two separate identical sessions requiring the performance of a sustained maximal voluntary contraction (MVC) with the quadriceps muscle for 2 min. MVC and transcutaneous electrical stimulations were used before and immediately after the fatiguing contraction to investigate the reliability of MVC torque, central activation, and peripheral variables (M-wave properties, peak twitch, peak doublet) within and between sessions. Based on previous and present results, we advise the use of (1) voluntary activation level with potentiated doublet as a reference to describe central fatigue, (2) electromyographic activity of vastus lateralis muscle as a surrogate for quadriceps for both voluntary and evoked contraction, and (3) potentiated peak doublet amplitude to investigate contractile fatigue. These findings can be useful in the choice of the parameters describing central and peripheral fatigue of the quadriceps muscle in future studies.     

Age‐related fatigue resistance in the knee extensor muscles is specific to contraction mode

The question of whether skeletal muscle fatigue is preserved or enhanced in older adults is a point of controversy. Disparate findings may be attributed to differences in subject population and study protocols, including contraction mode. The purpose of this study was to test the hypotheses that healthy older (65–80 years of age, 8 males and 8 females) adults who were matched to young adults (21–35 years of age; 8 males and 8 females) with similar physical activity levels would: (1) fatigue less during isometric knee extensor (KE) contractions, but (2) would show similar fatigue during dynamic KE contractions performed at 120° s^?1. Fatigue was induced with 4 minutes of intermittent, isometric, or dynamic maximal voluntary contractions, performed on separate days. Electrically stimulated contractions were used to evaluate central activation during both fatigue protocols. Older subjects maintained a higher percentage of baseline maximum voluntary contraction (MVC) torque than young subjects during isometric contractions (mean ± SE: 71 ± 3% and 57 ± 3%, respectively, P < 0.01). In contrast, there was no difference between age groups in torque maintenance during dynamic contractions (43 ± 3% and 44 ± 3%, respectively, P = 0.86). For both groups, changes in electrically stimulated and voluntary contractions followed similar trends, suggesting that central activation did not play a role in the age‐related differences in fatigue. Fatigue during the isometric protocol was associated with fatigue during the dynamic protocol in the young group only (r = 0.62, P = 0.01), suggesting that distinct mechanisms influence fatigue during isometric and dynamic contractions in older adults. Muscle Nerve 39: 692–702, 2009     

Neuromuscular fatigue of the knee extensors during repeated maximal intensity intermittent‐sprints on a cycle ergometer

Introduction: We studied the time course of neuromuscular fatigue during maximal intensity intermittent‐sprint cycling. Methods: Eight participants completed 10, 10‐s sprints interspersed with 180 s of recovery. The power outputs were recorded for each sprint. Knee extensor maximum voluntary contraction (MVC) force, voluntary activation, and evoked contractile properties were recorded presprint, postsprint 5, and postsprint 10. Results: Total work over the 10 sprints decreased significantly (P < 0.05) and could be described by 2 linear relationships from sprints 1–5 compared with sprints 6–10. Participants had significantly (P < 0.05) lower MVC and twitch forces postsprint 5 compared with presprint. MVC, voluntary activation, and twitch force were decreased (P < 0.05) postsprint 10 compared with postsprint 5. Conclusions: The maximal intermittent sprints induced neuromuscular fatigue. Neuromuscular fatigue in the first 5 sprints was mainly peripheral, whereas in the last 5 sprints it was both peripheral and central. Muscle Nerve 51: 569–579, 2015     

Redetermination of the optimal stimulation intensity modifies resting H‐reflex recovery after a sustained moderate‐intensity muscle contraction

This study aimed to determine whether the time‐course of maximal resting H‐reflex amplitude (H_max) recovery after a prolonged moderate‐intensity muscle contraction differs according to the optimal stimulation intensity used (predetermined vs. readjusted). Thirteen males performed a sustained isometric plantar flexion at 40% of their maximal voluntary contraction torque output until exhaustion. H_max of the soleus muscle was recorded before and 2, 6, 10, and 14 min after the end of the contraction, then normalized by the respective maximal M‐wave to form the H_max/M_max ratio. During recovery, pre‐ and redetermined optimal stimulation intensities (mini‐recruitment curve drawn before each recovery measurement) were applied randomly to measure H_max. When using redetermined stimulation intensities, normalized H‐reflex values were systematically greater (+11, +16, +15, and +15% after 2‐, 6‐, 10‐, and 14‐min recovery periods, respectively) than those obtained with the predetermined intensity. Keeping the stimulation intensity constant to evoke H_max after a sustained muscle contraction can underestimate the H‐reflex facilitation occurring after exhaustive exercise. It is therefore more appropriate to redefine the optimal stimulation intensity to evoke H_max (using mini‐recruitment curves) when the purpose is to analyze spinal modulation during the recovery phase. Muscle Nerve, 2010     

Quadriceps fatigue caused by catchlike-inducing trains is not altered in old age

The relative loss of peak force from electrical stimulation protocols has provided inconsistent results when used to compare muscle fatigability between young and old adults. In addition to the effect of task on these comparisons, age-related alterations in the development and relaxation of force are possible factors that have not been considered. The purposes of this study were to compare the fatigability of the quadriceps of young (26.7 +/- 1.0 years) and old men (78.3 +/- 1.3 years), as assessed by changes in peak force, force time integral (FTI), and half-relaxation time (HRT), during intermittent electrical stimulation protocols, and to determine whether manipulation of the activation frequency affected the comparisons. Fatigue was caused by constant-frequency (CF), and catchlike-inducing (CI) train protocols, both of which consisted of intermittent trains (6 pulses on: 650 ms off) of stimulation. After each protocol, the force-generating capacity of the fatigued muscle was assessed with three trains of stimuli: a CF train, a CI train and a 1-s 50-HZ train. There was no effect of age on the loss of peak force or the development of low-frequency fatigue induced by either protocol. Conversely, irrespective of the protocol, the FTI was better maintained by approximately 9% in the old than young men. Because peak force did not differ between groups during fatigue, it is likely that the FTI was preserved by the exacerbated slowing of HRT in the quadriceps of the old men. The results confirm an apparent paradox between muscle fatigue and stimulation with CI trains: a single CI train produces greater force than a CF train in a fatigued muscle, but there is greater fatigue induced by repetitive CI than CF train stimulation. Old age did not affect this fatigue paradox.     

Inhibition of maximal voluntary contraction force by experimental muscle pain: a centrally mediated mechanism

Muscle weakness frequently accompanies conditions with musculoskeletal pain. It is not clear if this attenuation of force is due to peripheral or central processes. The effect of experimental muscle pain on maximal voluntary contraction torque and peripheral contractile properties was therefore assessed. Experimental muscle pain reduced the torque produced by isometric knee extension, but the contractile properties assessed by twitch interpolation were not affected. This indicates that force inhibition by muscle pain is centrally mediated. This has clinical implications for rehabilitation and training of patients with musculoskeletal pain.     

Recovery of submaximal upper limb force production is correlated with better arm position control and motor impairment early after a stroke

Objective

This study determined whether recovery of upper limb position control using submaximal force production correlates with an improvement in functional arm impairment during early recovery from stroke.

Methods

Ten consecutive inpatients were recruited from a stroke unit. Each patient was in early recovery (<8 weeks post-lesion) from their first ever stroke. Evaluations of submaximal continuous force production and position control, maximal force production at the shoulder and a clinical outcome measure of motor impairment (Fugl-Meyer score; FM) were performed 20 days post-stroke as a baseline and then once a week for the following four weeks.

Results

Submaximal force production and its modulation during a position-holding task improved in early recovery after stroke, whereas maximal force production did not. Better modulation of submaximal force production enabled improved arm position control which was significantly correlated to the changes in FM score of motor impairment during recovery.

Conclusions

This study demonstrated that improvement in submaximal force modulation can operate as a mechanism enabling better motor behaviour such as arm position control during early recovery from a stroke.

Significance

Future rehabilitation strategies may benefit from adding submaximal force development and modulation to early interventions after stroke.     

Limitation of physical performance in a muscle fatiguing handgrip exercise is mediated by thalamo-insular activity

In this study, we investigated central/supraspinal processes mediating cessation of a muscle fatiguing exercise. Fifteen male subjects performed 39 intermittent, isometric handgrip contractions (13 s on, 5-6 s off) with the dominant right hand while brain activation was assessed by means of functional magnetic resonance imaging (fMRI). An adaptive, partly stochastic protocol was designed such that in approximately 50% of the contraction trials the required force could not be held until the end of the trial (task failure trial). Trials performed in compliance with the force requirements (succeeded trial) were compared with task failure trials concerning neural activity during a small time window before task failure occurred. The data revealed significantly increased activation contralaterally in both the mid/anterior insular cortex and the thalamus during the investigated time window in the case of subsequent task failure. In accordance with other studies investigating sensations that alert the organism to urgent homeostatic imbalance such as air hunger, hunger for food, and pain, we assume that an increased thalamo-insular activation in the context of a fatigue-induced handgrip exercise could reflect increased homeostatic disturbance in the exercising muscle and may be of essential importance by mediating task failure to maintain the integrity of the organism.     

Group III/IV locomotor muscle afferents alter motor cortical and corticospinal excitability and promote central fatigue during cycling exercise

Objective

To investigate the influence of group III/IV muscle afferents on the development of central fatigue and corticospinal excitability during exercise.

Incomplete recovery of voluntary isometric force after fatigue is not affected by old age

The 60-min recovery profiles of voluntary and electrically stimulated force, contractile speed, surface electromyography, muscle activation via twitch interpolation, and muscle compound action potentials (M-waves) in the elbow flexors of seven young men (24 +/- 2 years) and seven men over 80 years of age (84 +/- 2 years) were compared following intermittent (3 s on, 2 s off) contractions at 60% of each subject's maximum voluntary contraction (MVC) force. There was no age-related difference between groups in the average time to fatigue or the rate of voluntary force loss; both groups lost 40% of their force within approximately 5 min. Despite a rapid increase to approximately 83% of the prefatigue MVC by the third minute of recovery for both groups, MVC force did not return to the prefatigue value within 60 min (94 +/- 4% young, 91 +/- 3% old). These results suggest that the incomplete recovery of voluntary force was likely due to a peripheral limitation in the muscle at the level of excitation--contraction coupling, and was not affected by age. Delayed recovery of voluntary force and a greater degree of low-frequency fatigue in the old men were not observed and there were no age-related impairments in any parameter normalized to the prefatigue value during fatigue or recovery. We suggest that the specific fatigue task may be more important to recovery than proposed alterations in the aged neuromuscular system when normalization and matching of the fatigue task criteria occurs.