Figure 3. Voluntary EMG, and torque and EMG responses to motor cortical stimulation during the sustained 15% maximal voluntary contraction (MVC) and in brief recovery 15% MVCs.

All data are shown as mean ± s.e.m for the group of 9 subjects. Two breaks in each data set show periods when subjects pulled with the hand strap rather than the wrist strap and no stimuli were given. The end of the sustained 15% MVC is indicated by the vertical broken lines. A, voluntary EMG recorded from biceps brachii (○) and brachioradialis (•). Rms EMG recorded from both elbow flexor muscles increased throughout the sustained 15% MVC and recovered towards initial values during brief 15% MVCs performed in the recovery period. EMG is normalized to the maximum recorded during the brief control MVCs. B, motor evoked potentials (MEPs) elicited from biceps brachii (○), brachioradialis (•) and triceps brachii (▵) by stimulation of the motor cortex. The areas of the MEPs are expressed relative to the maximal M-wave (M_max) recorded under the same conditions. In biceps and brachioradialis, an initial fall in the MEP area (probably due to potentiation of the muscle by the first brief MVC), was followed by a gradual increase throughout the 15% MVC. The MEP in triceps remained small throughout the experiment. C, torque response to motor cortical stimulation (superimposed twitch). The superimposed twitch increased during the sustained effort. D, change in the duration of the silent period following motor cortical stimulation in biceps brachii (○) and brachioradialis (•). The difference in the duration of the silent period from the mean measured during brief control contractions is shown. The silent period lengthened during the sustained contraction and recovered quickly.