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

The Stretch-Shortening Cycle

Neuromuscular fatigue has traditionally been examined using isolated forms of either isometric, concentric or eccentric actions. However, none of these actions are naturally occurring in human (or animal) ground locomotion. The basic muscle function is defined as the stretch-shortening cycle (SSC), where the preactivated muscle is first stretched (eccentric action) and then followed by the shortening (concentric) action. As the SSC taxes the skeletal muscles very strongly mechanically, its influence on the reflex activation becomes apparent and very different from the isolated forms of muscle actions mentioned above. The ground contact phases of running, jumping and hopping etc. are examples of the SSC for leg extensor muscles; similar phases can also be found for the upper-body activities. Consequently, it is normal and expected that the fatigue phenomena should be explored during SSC activities. The fatigue responses of repeated SSC actions are very versatile and complex because the fatigue does not depend only on the metabolic loading, which is reportedly different among muscle actions. The complexity of SSC fatigue is well reflected by the recovery patterns of many neuromechanical parameters. The basic pattern of SSC fatigue response (e.g. when using the complete exhaustion model of hopping or jumping) is the bimodality showing an immediate reduction in performance during exercise, quick recovery within 1-2 hours, followed by a secondary reduction, which may often show the lowest values on the second day post-exercise when the symptoms of muscle soreness/damage are also greatest. The full recovery may take 4-8 days depending on the parameter and on the severity of exercise. Each subject may have their own time-dependent bimodality curve. Based on the reviewed literature, it is recommended that the fatigue protocol is 'completely' exhaustive to reduce the important influence of inter-subject variability in the fatigue responses. The bimodality concept is especially apparent for stretch reflex responses, measured either in passive or active conditions. Interestingly, the reflex responses follow parallel changes with some of the pure mechanical parameters, such as yielding of the braking force during an initial ground contact of running or hopping. The mechanism of SSC fatigue and especially the bimodal response of performance deterioration and its recovery are often difficult to explain. The immediate post-exercise reduction in most of the measured parameters and their partial recovery 1-2 hours post-exercise can be explained primarily to be due to metabolic fatigue induced by exercise. The secondary reduction in these parameters takes place when the muscle soreness is highest. The literature gives several suggestions including the possible structural damage of not only the extrafusal muscle fibres, but also the intrafusal ones. Temporary changes in structural proteins and muscle-tendon interaction may be related to the fatigue-induced force reduction. Neural adjustments in the supraspinal level could naturally be operative, although many studies quoted in this article emphasise more the influences of exhaustive SSC fatigue on the fusimotor-muscle spindle system. It is, however, still puzzling why the functional recovery lasts several days after the disappearance of muscle soreness. Unfortunately, this and many other possible mechanisms need more thorough testing in animal models provided that the SSC actions can be truly performed as they appear in normal human locomotion.

Contralateral muscle fatigue in human quadriceps muscle: evidence for a centrally mediated fatigue response and cross-over effect

The purpose of this investigation was to examine the effects of voluntary muscular fatigue in one lower limb and determine whether a 'cross-over' of fatigue is evident in the contralateral limb. Twenty-eight subjects (13 males and 15 females) performed a series of voluntary and evoked isometric contractions of both the dominant (exercised) and non-dominant (non-exercised) leg extensor muscles, prior to and after a fatigue protocol consisting of a 100-s sustained maximal isometric contraction (MVC) performed by the dominant limb only. Force values and surface electromyography (EMG) from the vastus lateralis muscle were obtained allowing for the determination of twitch and compound action potential (M-wave) values. Maximal twitch tension and peak-to-peak amplitude were significantly decreased after the fatigue test in the dominant limb, as was maximal voluntary force (approximately 65 N reduction), EMG activity (approximately 0.1 mV decrease) and voluntary activation (approximately 17% decline). However, no significant changes were observed in the non-dominant limb with respect to twitch and M-wave properties nor in MVC force. The voluntary activation of the non-dominant limb decreased significantly by 8.7% after the fatigue test, which was performed only on the dominant limb. The results of the present study suggest that the decrease in force production in the exercised limb was primarily related to peripheral fatigue mechanisms, with central fatigue making a lesser contribution. Centrally mediated mechanisms appear to be the sole contributor to fatigue in the non-exercised limb suggesting an anticipatory fatigue response and a 'cross-over' of central fatigue between the exercised and non-exercised contralateral limb.

The effects of unilateral muscle fatigue on bilateral physiological tremor

The aim of this study was to examine the post-exercise effects of fatiguing the wrist extensor muscles of a single arm on postural tremor and muscle activity in both arms. Previous research has shown that, for neurologically normal subjects, the tremor seen within a single limb segment is uncorrelated to that seen contralaterally. However it has been speculated that some bilateral relation does exist, and that the nature of the relation may only become evident under conditions where the neuromuscular system is perturbed. To further investigate this potential bilateral relation, seven healthy subjects were required to adopt a bilateral postural pointing position after exercise-induced fatigue of the wrist extensor muscles of a single arm. Tremor from the forearm, hand and finger segments of each arm, surface EMG activity from extensor digitorum (ED) of each arm, and blood lactate data were collected prior to and after the exercise intervention. The main result was that fatiguing the distal muscles of one arm resulted in a bilateral increase in both the physiological tremor and ED activity. The change in tremor was confined to the index finger with no change in the tremor for the hand or forearm segments of either arm. While three peaks were seen in the frequency profile of the finger tremor, the effects of fatigue were confined to an increase in the peak power of the neurally generated 8-12 Hz tremor component. The contralateral increase in muscle activity was also reflected by a change in the frequency profile of the EMG output, with an increase in the peak power of both muscles following exercise of the wrist extensors of a single arm. The bilateral increases in physiological tremor and EMG activity of ED were only observed during the bilateral pointing task, with no changes in tremor or EMG activity seen for the non-exercised limb during the unilateral exercise protocol. The specificity of the resultant increases in the neurally generated 8-12 Hz component of finger tremor amplitude and EMG activity, coupled with the lack of any changes in tremor for the more proximal arm segments, indicate that these bilateral effects were mediated by an increase in the central neural drive to both limbs. Together this set of results challenges the general assumption of bilateral independence of tremor production, and further illustrate the task dependent nature of exercise-induced fatigue.

Comparison of brain activation after sustained non-fatiguing and fatiguing muscle contraction: a positron emission tomography study

The concept of fatigue refers to a class of acute effects that can impair motor performance, and not to a single mechanism. A great deal is known about the peripheral mechanisms underlying the process of fatigue, but our knowledge of the roles of the central structures in that process is still very limited. During fatigue, it has been shown that peripheral apparatus is capable of generating adequate force while central structures become insufficient/sub-optimal in driving them. This is known as central fatigue, and it can vary between muscles and different tasks. Fatigue induced by submaximal isometric contraction may have a greater central component than fatigue induced by prolonged maximal efforts. We studied the changes in regional cerebral blood flow (rCBF) of brain structures after sustained isometric muscle contractions of different submaximal force levels and of different durations, and compared them with the conditions observed when the sustained muscle contraction becomes fatiguing. Changes in cortical activity, as indicated by changes in rCBF, were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) rest condition; (2) contraction of the m. biceps brachii at 30% of MVC, sustained for 60 s; (3) contraction at 30% of MVC, sustained for 120 s, and; (4) contraction at 50% of MVC, sustained for 120 s. The level of rCBF in the activated cortical areas gradually increased with the level and duration of muscle contraction. The fatiguing condition was associated with predominantly contralateral activation of the primary motor (MI) and the primary and secondary somatosensory areas (SI and SII), the somatosensory association area (SAA), and the temporal areas AA and AI. The supplementary motor area (SMA) and the cingula were activated bilaterally. The results show increased cortical activation, confirming that increased effort aimed at maintaining force in muscle fatigue is associated with increased activation of cortical neurons. At the same time, the activation spread to several cortical areas and probably reflects changes in both excitatory and inhibitory cortical circuits. It is suggested that further studies aimed at controlling afferent input from the muscle during fatigue may allow a more precise examination of the roles of each particular region involved in the processing of muscle fatigue.

The effect of a contralateral contraction on maximal voluntary activation and central fatigue in elbow flexor muscles

A long-duration, submaximal contraction of a hand muscle increases central fatigue during a subsequent contraction in the other hand. However, this 'cross-over' of central fatigue between limbs is small and the location within the central nervous system at which this effect occurs is unknown. We investigated this 'cross-over' by measurement of the force and EMG responses to transcranial magnetic stimulation of the motor cortex (TMS). To produce central fatigue, we used sustained maximal voluntary contractions (MVCs). In the first study, subjects (n=10) performed four 1-min sustained MVCs of the elbow flexors, alternating between the left and right arms (two MVCs per arm). The sustained MVCs were performed consecutively with no rest periods. In the second study, the same subjects made two sustained 1-min MVCs with the same arm with a 1-min rest between efforts. During each sustained MVC, a series of TMS and brachial plexus stimuli were delivered. Surface EMG was recorded from biceps brachii and brachioradialis muscles bilaterally. Voluntary activation was estimated during each MVC using measurement of the force increments to TMS. On average during each sustained MVC, voluntary activation declined by 7-12% (absolute change, P<0.001) and voluntary force declined by 35-45% MVC (P<0.001), whereas the cortical motor-evoked potential increased (P<0.001) and the subsequent silent period lengthened (P<0.001). The average voluntary activation and voluntary force were similar during two sustained MVCs performed by the same arm, when separated by 1 min of rest. However, when the 1-min rest interval was replaced with a sustained contraction performed by the other arm, the average voluntary activation was 2.9% worse in the second contraction (absolute change, P<0.05), while it did not alter voluntary force production or the EMG responses to TMS. Therefore, in maximal exercise of 4 min duration, the 'cross-over' of central fatigue between limbs is small in the elbow flexors and has a minor functional effect. Our data suggest that voluntary drive from the motor cortex is slightly less able to drive the muscle maximally after a fatiguing voluntary contraction on the contralateral side.

Neuromuscular fatigue of elbow flexor muscles of dominant and non-dominant arms in healthy humans

The purpose of this study was to assess differences in fatigue-related changes in variables related to structures within the neuromuscular system, between the dominant and non-dominant elbow flexor muscles of right-handed individuals. Two experimental sessions were performed on the right arm and one on the left arm. For each session, maximum voluntary torque, level of voluntary activation, M-wave amplitude, twitch/train or twitch/doublet torque ratio and EMG median frequency were obtained before and up to 20 min after a sustained maximum isometric fatigue task. Our main results were: 1) reproducible fatigue-induced changes in all variables of interest between the two sessions performed with the right arm, 2) significantly greater failure in voluntary activation and neuromuscular propagation with sustained activity for the non-dominant compared with dominant side, and 3) no effect of dominance on MVC torque, endurance time, and fatigue-induced changes in EMG median frequency and elicited torques. These results suggest that the preferential use of elbow flexor muscles with the dominant arm leads to more fatigue resistance in certain structures/mechanisms of the neuromuscular system, but not in others.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

The effect of a fatiguing exercise by the index finger on single- and multi-finger force production tasks

We studied the effects of fatigue, induced by a 60-s maximal isometric force production with the index finger, on multi-finger coordination and force production by the other fingers of the hand. Finger forces were measured during single- and multi-finger maximal voluntary force production (MVC) at two sites, the middle of the distal or the middle of the proximal phalanges. Two fatiguing exercises involving force production by the index finger were used, one at the distal phalanx and the other at the proximal phalanx. The MVC of the index finger dropped by about 33% when it was produced at the site involved in the fatiguing exercise. In addition, large transfer effects of fatigue were observed across sites of force application and across fingers. Force deficit increased under fatigue, especially due to a drop in the recruitment of the index finger. Under fatigue, the index finger was less enslaved during force production by other fingers. During multi-finger tasks, the percentage of total force produced by the index finger was significantly reduced after the fatiguing exercise. The principle of minimization of secondary moments was violated under fatigue. We suggest that the most impaired (fatigued) finger shows less interaction with other fingers or, in other words, is being progressively removed from the multi-finger synergy. Some of the observed changes in finger coordination suggest effects of fatigue at a central (neural) level.

Bilateral Interactions During Contractions of Intrinsic Hand Muscles

During demanding voluntary contractions (e.g., high force or fatigue), activation is not restricted to the target muscle but extends to other ipsilateral muscles; even contralateral muscles become activated. The contralateral “irradiation” of activity was measured in five subjects during submaximal and maximal voluntary contractions (MVCs) of the first dorsal interosseous (FDI) (index finger abduction) and during unfatigued and fatigued conditions. All subjects were tested five times with at least one week between tests. Unilateral MVCs were associated with a substantial amount of contralateral FDI activation [mean = 7.9 ± 6.7% (SD) MVC prior to fatigue]. The amount of such contralateral irradiation was significantly different between different individuals and was positively correlated between dominant and nondominant hands. During fatigue tests, the contractile activity of the contralateral “nontarget” index finger showed progressive increase (force, electromyogram) as was measured during both the submaximal task and interspersed MVCs of the target finger. In addition, a superimposed saw-tooth pattern of intermittently waxing and waning contractions commonly appeared contralaterally. The expression of contralateral irradiation force was itself fatigue-sensitive: less irradiation was seen in a recently fatigued muscle than was seen before the fatigue test. These fatigue effects could not be explained as having been caused by changes in muscle properties. Possible anatomical sites of contralateral irradiation are briefly discussed.