Figure 1. Experimental set-up, schematic of the protocol and raw traces of twitches and evoked potentials from a representative subject.

A, diagram of the experimental set-up. Throughout the study, the transcranial magnetic stimulation (TMS; T) coil was held stable at the optimal position on the motor cortex by an experimenter. Motor nerve stimulations (MNS; M) were elicited at the brachial plexus and responses were measured from the biceps brachii and triceps brachii. An isometric myograph was used to measure mechanical evoked twitches. B, experimental protocol for sessions in which intrathecal fentanyl (FENT) was administered and neurophysiological assessment of the elbow flexor muscle was carried out during no leg exercise, non-fatiguing exercise and fatiguing exercise sustained to task failure, and immediately post-failure. Subjects repeated ‘no leg exercise’ and ‘non-fatiguing exercise’ trials following the administration of intrathecal fentanyl. C, raw traces of twitch forces and electromyographic (EMG) evoked responses to TMS and MNS from a single subject, representative of group data during control (CTRL) and FENT sessions, respectively. Background voluntary force during contractions has been offset to allow for direct comparisons. Data are taken from the elbow flexion contractions set at the start and failure of exercise in both CTRL and FENT sessions. The data show an increase in superimposed twitch, no change in M_max and a reduction in motor evoked potentials during CTRL. None of these variables were altered during FENT. M_max, maximum compound muscle action potential; MVC, maximal voluntary contraction.