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 cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

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Biomechanics And Motor Control Of Human Movement

Movement, posture and equilibrium: interaction and coordination

1. Using two magnetic stimulators, we investigated the effect of a conditioning magnetic stimulus over the motor cortex of one hemisphere on the size of EMG responses evoked in the first dorsal interosseous (FDI) muscle by a magnetic test stimulus given over the opposite hemisphere. 2. A single conditioning shock to one hemisphere produced inhibition of the test response evoked from the opposite hemisphere when the conditioning-test interval was 5-6 ms or longer. We shall refer to this as interhemispheric inhibition. However, the minimum latency of inhibition observed using surface EMG responses may have underestimated the true interhemispheric conduction time. Single motor unit studies suggested values 4-7 ms longer than the minimum interval observed with surface EMG. 3. Interhemispheric inhibition was seen when the test muscle was active or relaxed. Increasing the intensity of the conditioning stimulus increased the duration of inhibition: increasing the intensity of the test stimulus reduced the depth of inhibition. 4. The conditioning coil had to be placed on the appropriate area of scalp for inhibition to occur. The effect of the conditioning stimulus was maximal when it was applied over the hand area of motor cortex, and decreased when the stimulus was moved medial or lateral to that point. 5. The inhibitory effect on the test stimulus probably occurred at the level of the cerebral cortex. In contrast to the inhibition of test responses evoked by magnetic test stimuli, test responses evoked in active FDI by a small anodal electric shock were not significantly inhibited by a contralateral magnetic conditioning stimulus. Similarly, H reflexes in relaxed forearm flexor muscles were unaffected by conditioning stimuli to the ipsilateral hemisphere. However, inhibition was observed if the experiment was repeated with the muscles active.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1992.sp019243

Interhemispheric inhibition of the human motor cortex.

Muscle fatigue encompasses a class of acute effects that impair motor performance. The mechanisms that can produce fatigue involve all elements of the motor system, from a failure of the formulation of the descending drive provided by suprasegmental centers to a reduction in the activity of the contractile proteins. We propose four themes that provide a basis for the systematic evaluation of the neural and neuromuscular fatigue mechanisms: 1) task dependency to identify the conditions that activate the various mechanisms; 2) force-fatigability relationship to explore the interaction between the mechanisms that results in a hyperbolic relationship between force and endurance time; 3) muscle wisdom to examine the association among a concurrent decline in force, relaxation rate, and motor neuron discharge that results in an optimization of force; and 4) sense of effort to determine the role of effort in the impairment of performance. On the basis of this perspective with an emphasis on neural mechanisms, we suggest a number of experiments to advance our understanding of the neurobiology of muscle fatigue.

Neurobiology of muscle fatigue.

Objective: To improve, with the aid of psychometric analysis, the Balance Evaluation Systems Test (BESTest), a tool designed to analyse several postural control systems that may contribute to poor functional balance in adults. Methods: Performance of the BESTest was examined in a convenience sample of 115 consecutive adult patients with diverse neurological diagnoses and disease severity, referred to rehabilitation for balance disorders. Factor (both explorative and confirmatory) and Rasch analysis were used to process the data in order to produce a new, reduced and coherent balance measurement tool. Results: Factor analysis selected 24 out of the 36 original BESTest items likely to represent the unidimensional construct of “dynamic balance”. Rasch analysis was then used to: (i) improve the rating categories, and (ii) delete 10 items (misfitting or showing local dependency). The model consisting of the remaining 14 tasks was verified with confirmatory factor analysis to meet the stringent requirements of modern measurement. Conclusion: The new 14-item scale (dubbed mini-BESTest) focuses on dynamic balance, can be conducted in 10–15 min, and contains items belonging evenly to 4 of the 6 sections from the original BESTest. Further studies are needed to confirm the usefulness of the mini-BESTest in clinical settings.

/pdf/using-psychometric-techniques-to-improve-the-balance-2jwd5s1bcl.pdf

Using psychometric techniques to improve the balance evaluation systems test: the mini-bestest

1. We have investigated the possibility that voluntary muscle lengthening contractions can be performed by selective recruitment of fast-twitch motor units, accompanied by derecruitment of slow-twitch motor units. 2. The behaviour of motor units in soleus, gastrocnemius lateralis and gastrocnemius medialis muscles was studied during (a) controlled isotonic plantar flexion against a constant load (shortening contraction, S), maintained plantar flexion, or dorsal flexion resisting the load and gradually yielding to it (lengthening contraction, L), (b) isometric increasing or decreasing plantar torque accomplished by graded contraction or relaxation of the triceps surae muscles, (c) isometric or isotonic ballistic contractions, and (d) periodic, quasi-sinusoidal isotonic contractions at different velocities. The above tasks were performed under visual control of foot position, without activation of antagonist muscles. The motor units discharging during foot rotation were grouped on the basis of the phase(s) during which they were active as S, S + L and L. The units were also characterized according to both the level of isometric ramp plantar torque at which they were first recruited and the amplitude of their action potential. 3. S units were never active during dorsal flexion; some of them were active during the sustained contraction between plantar and dorsal flexion. Most S + L units were active also during the maintenance phase and were slowly derecruited during lengthening; their behaviour during foot rotations was similar to that during isometric contractions or relaxations. L units were never active during either plantar or maintained flexion, but discharged during lengthening contraction in a given range of rotation velocities; the velocity of lengthening consistently influenced the firing frequency of these units. Such dependence on velocity was not observed in S + L units. 4. A correlation was found between the amplitude of the action potential and the threshold torque of recruitment among all the units. In addition, the amplitudes of both the action potential and the threshold torque were higher in the case of L units than in the case of S and S + L units. Most L units could be voluntarily recruited only in the case of ballistic isometric or isotonic contraction. 5. Occasionally, L units were directly activated by electrical stimulation of motor fibres and their conduction velocity was in the higher range for alpha-axons. In contrast, nerve stimulation could induce a reflex activation of S and S + L units.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective recruitment of high-threshold human motor units during voluntary isotonic lengthening of active muscles.

Upright stance and its reflex control were studied in parkinsonian patients and in age-matched normal subjects. They stood unperturbed on a force-measuring surface (static conditions), or were displaced by movement of a supporting platform (dynamic conditions). During quiet stance the following variables were analysed, with eyes open or closed: position of the centre of foot pressure (CFP), average sway area, length of sway path, amplitude and distribution of tonic leg muscle EMG activity. Perturbations of stance were induced by toe-up or toe-down rotations, and by backward or forward translations of the platform. Amplitude of short, medium and long-latency EMG responses to displacement were measured in the tibialis anterior (TA) and in the three heads of the triceps surae (TS) muscle. The perturbations were produced during both free and supported stance (holding onto a stable structure), under which condition normal subjects suppress medium and long-latency responses. Under static conditions, the only significant finding in parkinsonians was represented by a shift in the position of the CFP. This was correlated with the severity of the disease (Webster scale), the less affected patients being shifted backwards and the more affected patients forwards, with respect to normals. Under dynamic conditions, the reflex responses to perturbations of free stance were similar in both groups. Only the medium-latency burst of gastrocnemius lateralis and the long-latency burst TA evoked by TS stretch were larger in parkinsonians. The amplitude of these responses, as well as of all the others, was not related to the Webster score. Within the patients' group, a relationship between position of CFP and area of EMG burst was found for both medium and long-latency TA responses evoked by forward translation and toe-up rotation, respectively. Under supported conditions, the capability to suppress all medium and long-latency muscle responses to any perturbation was lost or impaired in the parkinsonians. The degree of impairment was unrelated to the position of the CFP, but was significantly related to the severity of the disease. The suppression to 40% (supported/nonsupported), of TA response to toe-down rotation is proposed as the point of separation between normals and parkinsonians. The forward projection of the CFP, occurring in the severe stages of the disease, and the increase in amplitude of some responses to perturbations of free stance might be a compensatory adaptation to the anomalous upright posture.(ABSTRACT TRUNCATED AT 400 WORDS)

Free and supported stance in Parkinson's disease. The effect of posture and 'postural set' on leg muscle responses to perturbation, and its relation to the severity of the disease.

Body sway variables (sway area and sway path) were recorded by a dynamometric platform in 13 young subjects, standing quiet with feet together, with eyes open (EO) and eyes closed (EC), prior to and following two types of physical exercise (treadmill walking and cycle ergometer pedalling). Each exercise was performed under both fatiguing (above anaerobic threshold) and non-fatiguing conditions. Following fatiguing treadmill exercise, we observed a significant increase in body sway with respect to pre-exercise values. The increase was present under both visual conditions, affected both sway area and sway path and lasted until about 15 min from the end of the exercise. The Romberg quotient (the ratio of EC/EO of sway area, or sway path) significantly increased after the fatiguing exercise with respect to the non-fatiguing exercise. The mean position of the centre of foot pressure (CFP) was unchanged after the exercise. Fatigue induced an increase in the median frequency of oscillation of the centre of foot pressure, independent of the amplitude of sway. Non-fatiguing treadmill exercise induced no significant changes in sway or in its frequency content. Following fatiguing cycle ergometer exercise, a negligible increase or a decrease (under eyes closed condition) in body sway were observed. Non-fatiguing cycling exercise induced no significant changes or a decrease in sway. Control experiments showed that simple repetition of successive stance trials (without intercalated exercise) was able by itself to induce a decrease in sway. By taking this effect into account, both types of cycling exercises revealed a mild capacity to increase sway. We concluded that body sway increased after strenuous physical exercise, but was little affected by exercise performed below the estimated anaerobic threshold. The effects of fatigue on sway were short-lasting and of moderate extent, and therefore were not liable to seriously threaten body equilibrium.

Fatigue effects on body balance

Background Recently, a new tool for assessing dynamic balance impairments has been presented: the 14-item Mini-BESTest.

Objective Aim of this study was to compare the psychometric performance of Mini-BESTest vs. Berg Balance Scale (BBS).

Methods Ninety-three subjects (mean age 66.2; 53 females) with balance deficits were recruited. Interrater (3 raters) and test-retest (1-3 days) reliability were calculated using intraclass correlation coefficients (ICCs). Responsiveness and minimal important change (MIC) were assessed (after 10 sessions of physical therapy) using both distribution-based and anchor-based methods (external criterion: a 15-point Global Rating of Change [GRC] scale).

Results At baseline, neither floor nor ceiling effects were found in either scale. After treatment, the maximum score was found in 12 patients (12.9%) with BBS and in 2 (2.1%) with Mini-BESTest. Test-retest reliability for total scores was significantly higher in Mini-BESTest (ICC=0.96) than in BBS (ICC=0.92), whereas interrater reliability was similar (ICC=0.98 vs. 0.97). The standard error of measurement (SEM) was 1.26 and minimum detectable change at the 95% confidence level (MDC95) 3.5 points for Mini-BESTest, whereas SEM was 2.18 and MDC95 6.2 points for BBS. In receiver operating characteristic (ROC) curves, the area under the curve was 0.92 for Mini-BESTest and 0.91 for BBS. The best Minimal Important Change (MIC) was 4 points for Mini-BESTest and 7 for BBS. After treatment, 38 patients at Mini-BESTest vs. only 23 at BBS (out of the 40 subjects who had a GRC ≥3.5) showed a score change equal or greater than MICs.

Conclusions The two scales behave similarly, but the Mini-BESTest scale seems to have a lesser ceiling effect, slightly higher reliability levels, and greater accuracy in classifying individual patients who show significant improvement in balance function.

/pdf/comparison-of-reliability-validity-and-responsiveness-of-the-2931gi3w3l.pdf

Antonio Nardone

Papers

Using psychometric techniques to improve the balance evaluation systems test: the mini-bestest

Selective recruitment of high-threshold human motor units during voluntary isotonic lengthening of active muscles.

Free and supported stance in Parkinson's disease. The effect of posture and 'postural set' on leg muscle responses to perturbation, and its relation to the severity of the disease.

Fatigue effects on body balance

Comparison of Reliability, Validity, and Responsiveness of the Mini-BESTest and Berg Balance Scale in Patients With Balance Disorders