An electromyographic study is presented of twenty-one patients with paralysis of the lower extremity due to poliomyelitis on whom thirty-nine muscle transplantations for quadriceps insufficiency were performed. Electromyograms and gait movies were filmed simultaneously on the same movie film by specially designed apparatus. The phasic activity of each muscle studied was plotted on polar coordinate graph paper, for both the patients with poliomyelitis and for six normal control subjects. Numerous phase conversions were evident in both the transplanted and non-transplanted muscles in the patients with poliomyelitis. Extension of the phasic activity of the gluteus maximus through most of the stance phase and conversion of hamstring activity from swing to stance phase were observed in non-transplanted muscles. Ten of fourteen hamstring transplants achieved stance-phase activity roughly comparable to that of the normal quadriceps femoris. Two of eleven sartorius transplants and four of the twelve tensor transplants achieved stance-phase activity. It was noted that the tensor and sartorius transplants frequently functioned while both the hip and knee were flexing. A preliminary report is made of sartorius and tensor transplants that were converted to one-joint muscles by detachment of their proximal ends from the anterosuperior iliac spine with reattachment in the region of the greater trochanter. Some of the factors that may have bearing on phase conversion are discussed.