1. The force produced by isometric contractions of the quadriceps muscle have been studied during maximal voluntary contractions and when a substantial part of the muscle was electrically stimulated via surface electrodes. 2. In normal children and adults, the force of a maximal voluntary contraction of the quadriceps was proportional to body weight. 3. The function of the quadriceps has been described in terms of the force/frequency curve, speed of relaxation and the rate of loss of force during 18 s stimulation at 30 Hz and 100 Hz. 4. The functional characteristics of adductor pollicis when stimulated via the ulnar nerve were essentially similar to those of the quadriceps. 5. Studies of the function of these two muscles are complementary since quadriceps femoris is amenable to needle biopsy investigations of its structure and chemistry whereas adductor pollicis is more suitable for electrophysiological studies.