I: Introduction
An unavoidable outcome of an extended, voluntary muscle contraction is called muscle fatigue. When we have maintained skeletal muscle movement, circumstances that assist in developing force, such as post-activation potentiation(PAP), are challenged with conditions that reduce force output. Eventually, however, a loss in force production capacity happens, and the strength or energy to sustain a given force drops. As muscle fatigue improves, adaptations in how the muscle is activated by the nervous system happen; specifically, there are fatigue-relevant changes in the motor unit depletion rate.
Marsden et al. developed a hypothesis regarding muscle fatigue called the “muscular wisdom hypothesis” that advanced that reducing the motor unit depletion rate during extended contractions reduced fatigue. The studies measured the mechanisms for reducing the depletion rate of motor units in permanently maintained maximal voluntary contractions (MVCs). There are other studies related to reducing the motor unit depletion rate during fatiguing MVCs to the decrease in the resting rate of the whole muscle. They recommended that, with fatigue, the reduction or slowing of the motor unit depletion rate would result in a fully activated tissue because of the decrease in fusion frequencies connected with the prolonged relaxation times. This review aims to provide an overview of the muscular wisdom hypothesis, generally called the muscle wisdom hypothesis, and to test its applicability for reducing muscle fatigue across several types of contraction that have been tested since its thought.
II: Muscular wisdom hypothesis
The muscular wisdom hypothesis derived from former work that asked to mimic the reduction in force evident during MVC (maximal voluntary contractions) using maximal electrical stimulation. Previous scientists found that no single stimulation frequency could generate the schema of force loss recognized during a 1-min MVC of adductor pollicis muscle. The primary assumption of the muscular wisdom hypothesis is that there is a similarity between the motor unit depletion rate and the muscle’s fatigue-relevant contractile attributes. The simultaneous slowing of the twitch resting of the whole muscle as the muscle fatigues have also been recognized in single motor units and muscle fibers. Slowing and reducing the domain of the twitch contraction with fatigue may not affect in the loss of peak tetanic force; however, the force-time integral is smaller. It is thought that sensory afferent nerves starting within the fatigued muscle cater a signal to the motoneuron pool to produce a lower depletion rate of the motor units. The decrease in depletion rate that becomes visible to match the obligations of the muscle was stated on fatigue. That is, the reduction of the resting time of the muscle by cooling the hand did not have a result on the motor unit depletion rate in the adductor pollicis muscle during maximal voluntary contractions.
Other scientists changed the contractile attributes of the human tibialis anterior muscle by shortening the muscle length. They established no changes in the motor unit depletion rates despite differences in contractile characteristics, irrespective of whether subjects performed maximal or submaximal contractions. Thus, managing the contractile attributes of the muscle alone in the loss of fatigue did not affect a connected change in the motor unit depletion rate.
This concept of the muscular wisdom hypothesis has become publicly accepted. However, it is significant to discuss that the situation under which the muscular wisdom hypothesis was expanded was focused on MVC and electrical stimulation protocols.
III: Submaximal contractions
Hypodermic fine wire electrodes have been used to examine the compilation of depletion rate in single motor units during extended submaximal fatiguing contractions. Typical motor unit behavior in the triceps brachii muscle that has been witnessed during sustained 20% maximal voluntary contractions shows that the activity of a single motor unit that was working from the beginning of the task showed a reduction in depletion rate, and additional motor units were recruited as the contraction followed. Overall, there is a difference in the response schema of motor units during extended submaximal fatiguing contractions. That is, some motor units improved in depletion rate, and others have shown a similar constant discharge rate, but most motor units that were active from the beginning of the depletion showed a decrease in depletion rate. This decrease was not brought by a slowing of the whole muscle contractile attributes. The precise question from the collected data is what disposed of some motor units to show again in depletion rate whereas most others showed a decline. The argument that a reduction in depletion rate would bypass activation failure does not seem permanent in submaximal contractions in which the depletion rates are as low as 10 to 25 Hz and unlikely to develop activation failure.
IV: Dynamic contractions and twitch potentiation:
There is reasonable information for the motor unit action throughout fatiguing dynamic contractions. And it was that the spindle input to the motoneuron pool helped in support of the depletion rate in dynamic contractions. To test this hypothesis, the vibration was utilized for 2 s every 10 s to the triceps brachii muscle throughout an isometric fatiguing 20% maximal voluntary contraction of the elbow extensor muscles. This stopped the reduction in the depletion rate in the majority of motor units. The muscular wisdom hypothesis attributes functional importance to the drop-in firing rate, reducing fatigue. If the theory is correct in all conditions, then the failure of the depletion rate to reduce should have followed in less “minimization” of fatigue.
Dynamic twitch contractions presented developments in work and average power “ranging from 25 to 50% after 5 Hz, 20 s conditioning stimuli” that were introduced throughout different states of muscle shortening in the mouse extensor digitorum longus muscle. Conversely, the isometric twitch force was potentiated only 14%. If the decrease in firing rate were offsetting the additional power developed by potentiation, then, during concentric forms, it could be guessed that the motor unit depletion rate could reduce to a higher range than an isometric contraction at a relative load and still sustain the force.
V: Conclusion
The idea of the muscular wisdom hypothesis has been practiced for almost 20 years in different experimentation with an intention to combine muscle activation with force output. As I read more about muscle fatigue and it associated with muscle wisdom, I learned that some candidate mechanisms are working either independently or in a compound that could describe the reduced muscle activation that happens with fatigue. Decreased excitability of the motoneurons, decreased excitatory influence from peripheral sources, increased inhibitory influences on motoneurons, and reduced excitation of motoneurons from supraspinal sources.
References:
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2. Marsden, C.D., Meadows, J.C.and Merton. P.A. (1983). “Muscular wisdom” that minimized fatigue during prolonged effort in man: peak rates of motoneuron discharge and slowing of discharge during fatigue. Adv. Neurol. 39: 169–211.
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