The pattern of stimulation influences the amount of oscillatory work done by frog muscle.

1. A doublet is the interjection of an additional action potential at the beginning of a regular motoneuron discharge pattern. The present experiments were designed to estimate the magnitude of the effect of doublets on the capacity of isolated frog sartorius muscle to do work during oscillatory length changes. 2. For the work loop method, the muscle was subjected to sinusoidal length changes at 4 Hz. Work was calculated from the loop formed when force was plotted against length. Work done was positive when the muscle was shortening and was negative when the muscle was lengthening; net work was the difference. 3. Adding pulses at the start of the stimulus pulse train increased isometric force and markedly increased net work per cycle. Adding one pulse increased the net work by about 52%, but increased isometric force by only 24%. 4. Eighty per cent of the maximum net work was achieved by adding only three pulses at the start of a low-frequency pulse train, i.e. 80% of the work achieved with seventeen pulses could be achieved with only six pulses. 5. The maximum net work per stimulus pulse was achieved with a stimulus train that consisted of five pulses with two being additional initial pulses at 5 ms interpulse intervals, i.e. a triplet. 6. The results of the present study imply that doublet stimulation patterns may be important to reduce the fatigue that occurs during artificial neuromuscular stimulation of skeletal muscle.