EMG activity in the gluteus maximus, rectus femoris, biceps femoris, vastus lateralis, gastrocnemius, and anterior tibialis muscles was studied in six healthy male subjects. Each subject pedaled against changes in workload, pedaling rates, and different weight flywheels, with and without toe clips and from both sitting and standing positions. All the muscles tested were shown to be active at workloads greater than 300 kilograms per minute. In addition, the amplitude of the EMG signals increased with increasing workloads. No real differences in muscle activity timing were found between the light and heavy flywheels or between the competitive and noncompetitive cyclists. Standing while pedaling produced increased activity in the rectus femoris and vastus lateralis at high rpm and low workloads. J Orthop Sports Phys Ther 1981;2(4):163-170.