An M-wave is the compound muscle action potential evoked by electrical stimulation of the motoneurons innervating a muscle.

• By normalising to M_max, which represents the maximal recruitment of motor units via peripheral stimulation, changes in excitability that may occur within the muscle or motor axons are controlled for. This ensures that any observed changes in the evoked response amplitude are more likely to reflect the neurophysiological mechanisms of interest (that we aim to estimate through interpretation of the evoked response) rather than changes in peripheral excitability.
• Expressing evoked responses as a percentage of M_max allows researchers to quantify the proportion of the motoneuron pool that is recruited by a specific stimulus. This provides a standardised metric (%M_max) that enhances comparability across different experimental conditions or subjects.

• For some muscles, it may be difficult to evoke maximal M-waves. This is because the motor nerves that need to be stimulated are too deep to be effectively reached, or the stimulation intensity needed to evoke a maximal M-wave is excessively high, causing intolerable pain for participants.
• It is not possible to determine with absolute certainty whether a peripheral nerve stimulation activates all axons of motoneurons and the muscle fibres they innervate (i.e., motor units). Even though it is possible to express the number of motor units recruited by a stimulus as a percentage of the M_max, this is valid only for the portion of muscle where the bipolar electrode is placed. It might not be possible to generalise it to the entire muscle.
• Even though expressing evoked responses as a percentage of M_max allows researchers to estimate the proportion of the motoneuron pool that is recruited by a specific stimulus, it is important to note that this is an approximation. The amplitude of the evoked response is influenced not only by the number of recruited motor units but also by factors such as the size of the muscle fibres recruited and the distance between the recording electrodes and the muscle fibres.

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