Average traces of elbow flexion force from one subject following a single motor nerve stimulus or transcranial magnetic stimulus to the motor cortex (TMS) at varying contraction strengths (a, 100 % MVC; b, 90 % MVC; c, 75 % MVC; d, 50 % MVC; e, 25 % MVC), with and without fatigue. Twitches to motor nerve stimulation were also recorded with the muscle at rest (dashed lines). Traces have been offset to allow comparison of the twitches. MVC, maximal voluntary contraction.