1. After severe muscular contraction in man recovery of force is largely complete in a few minutes, but is not wholly so for many hours. The long‐lasting element of fatigue is found to occur primarily for low frequencies of stimulation (e.g. 20/sec), and is much less pronounced, or absent, at high frequencies (80/sec). The twitch force is an unreliable measure of the state of fatigue. 2. The long‐lasting element of fatigue is not due to depletion of high‐energy phosphate nor is it due to failure of electrical activity as recorded from surface electrodes. It is probably the result of an impairment of the process of excitation‐contraction coupling. Its practical importance for man could be significant as an explanation of the subjective feelings of weakness following exercise.