There is no nonvolitional method of assessing quadriceps strength which both supramaximally activates the muscle and is acceptable to subjects. In 10 normal subjects and 10 patients with suspected muscle weakness we used magnetic stimulation of the femoral nerve to elicit an isometric twitch and measured twitch tension (TwQ), surface electromyogram in addition to the maximum voluntary contraction force (MVC). Supramaximality was achieved in all subjects at a mean of 83% of maximum stimulator output. When supramaximal, TwQ was reproducible (mean coefficient of variation 3.6%, range 0.7-10.9) and correlated well with MVC (r2 = 0.83, P<0.001). In 7 normal subjects we measured TwQ before and after a fatiguing protocol; after 20 min TwQ was a mean of 55% (range 29-77%) of baseline and remained substantially reduced at 90 min. Magnetic femoral nerve stimulation is a painless, supramaximal method of assessing quadriceps strength and fatigue which is likely to be of value in clinical and physiological studies.