**5. Results from simulations**

potential, which depends on the EPSPs and IPSPs the MN is receiving. Figure 14 shows F-wave recordings from the SO muscle (in response to supramaximal stimulation to the PTN) obtained

**Figure 14.** Nine superimposed EMG signals from the SO muscle showing stimulus artifacts, M-waves (MMAX) and Fwaves obtained in response to supramaximal stimulation (rectangular pulses with 0.2ms duration) delivered to the PTN of a resting subject (unpublished data). Surface stimulating electrodes were positioned with the cathode (2cm2) on the popliteal fossa and the anode (8cm2) on the patella. The stimulus intensity used to elicit F-waves was above that necessary to elicit MMAX. The same recordings are shown in **a** and **b**, with different amplitude gains (note the cali‐

As described in section 4.1, when a supramaximal stimulus is delivered to the nerve of a relaxed muscle, an M-wave is observed in the EMG with short latency and no H-reflex is observed due to the collision (see Figure 8) between antidromic and orthodromic spikes (there could be Fwaves, but they are not our focus here). However, if the subject maintains a steady voluntary contraction, and the same supramaximal stimulus is delivered to the peripheral nerve, a reflex response appears at a latency equal to the H-reflex. This reflex response, frequently referred to as a V-wave (associated with a voluntary drive), is an electrophysiological variant of the H-

The rationale behind the genesis of this response is that the descending drive activates a subset of MNs in the spinal cord making their axons conduct action potentials orthodromically. These action potentials collide with the antidromic volley generated at the electrical stimulation site by the supramaximal stimulus applied to the peripheral mixed nerve. Thus, this subset of MNs (recruited by the descending command) will be susceptible to be activated by the reflex afferent volley generated by the supramaximal electrical stimulus. Hence, the V-wave amplitude roughly reflects the number of spinal MNs being activated by the volitional drive, as well as the excitability associated with the stretch reflex pathway (previously discussed in section 4.1).

This electrophysiological measure has been used in several human neurophysiology studies, for instance: (1) neuronal plasticity associated with resistance training in healthy subjects [52]; (2) short-term effects of neuromuscular electrical stimulation [55]; (3) multiple sclerosis [56].

reflex and is used to measure the level of efferent drive [52-54].

in a subject at rest.

74 Electrodiagnosis in New Frontiers of Clinical Research

bration bars).

**4.4. The V-wave**
