Definition of the method

This method includes:

• Several electrical and magnetic stimulation techniques are used to evoke compound muscle action potentials to study neurophysiological mechanisms (e.g., H-reflexes, F-waves, and motor-evoked potentials from transcranial magnetic stimulation of the motor cortex). The amplitudes of these evoked responses are often used to estimate the excitability status of the motoneuron pool, as these techniques recruit motoneurons via various neural circuits. However, changes in the amplitude of these evoked responses can result from both central (e.g., at the cortical level) and/or peripheral physiological processes (i.e., within the muscle). To remove the possible influence of peripheral mechanisms when interpreting these evoked responses, the amplitude from evoked responses should be normalised to the amplitude of the maximal compound muscle action potential (M_max). The M_max is a short-latency evoked response where all muscle fibres are presumably recruited via peripheral electrical stimulation of motor axons.

  Let’s assume that we aim to estimate changes in corticospinal excitability by recording motor-evoked potentials (MEPs) before and after an intervention. Before the intervention, we record an MEP with amplitude A_pre and an M_max with amplitude B_pre. After the intervention, we record an MEP with amplitude A_post and an M_max with amplitude B_post. We then observe that both A_post and B_post are smaller than their respective pre-intervention values. However, when comparing normalised values (i.e., A_pre/B_pre vs A_post/B_post), we find no differences between pre- and post-intervention. This suggests that the reduction in MEP amplitude was driven by a decrease in peripheral excitability rather than a change in the central mechanisms we aimed to study. Had we not normalised MEPs to M_max, we might have mistakenly concluded that corticospinal excitability decreased following the intervention.

  As the M_max represents the maximal synchronised activation of motor units recorded by surface EMG electrodes, this normalisation method also allows researchers to estimate the proportion of a motor unit pool recruited by a stimulus (presented as a % M_max). For example, eliciting a soleus H-reflex amplitude that is 10% of the soleus M_max suggests that 10% of the soleus motoneurons are being recruited from the entire soleus motoneuron pool.