Surface electromyography of the quadriceps femoris muscle was performed in seven patients in order to determine the effect of extensive lengthening of the femur on the muscles that extend the knee. Electromyograms were made during sustained (thirty-second) isometric extension of the knee, at an angle of flexion of the knee of 30 degrees, while a load equal to 15 per cent of body weight was applied to the leg just proximal to the ankle. The average circumference of the thigh was smaller, the motor-unit recruitment of the muscles was slower, and the fatigability was greater on the involved side compared with the uninvolved side. The vastus medialis exhibited greater fatigability and slower motor-unit recruitment than the rectus femoris or the vastus lateralis. The atrophy index of the involved muscles correlated well with the muscle fatigability, the preoperative limb-length discrepancy, the percentage of lengthening, and the motor-unit recruitment. Analysis also revealed a correlation between muscle fatigability and the percentage of lengthening of the bone and between muscle fatigability and the preoperative limb-length discrepancy. The percentage of lengthening correlated with the extent of motor-unit recruitment. The results of this study suggest that the amount of damage to neuromuscular tissue varies according to the extent of the lengthening of the femur. Of all of the knee extensors, the vastus medialis was affected the most.