THE EFFECT OF LENGTHENING OF THE FEMUR ON THE EXTENSORS OF THE KNEE -AN ELECTROMYOGRAPHIC STUDY

Surface electromyography of the quadriceps femoris muscle was performe d in seven patients in order to determine the effect of extensive leng thening of the femur on the muscles that extend the knee. Electromyogr ams were made during sustained (thirty-second) isometric extension of the knee, at an angle of flexion of the knee of 30 degrees, while a lo ad equal to 15 per cent of body weight was applied to the leg just pro ximal to the ankle. The average circumference of the thigh was smaller , the motor-unit recruitment of the muscles was slower, and the fatiga bility was greater on the involved side compared with the uninvolved s ide. The vastus medialis exhibited greater fatigability and slower mot or-unit recruitment than the rectus femoris or the vastus lateralis. T he atrophy index of the involved muscles correlated well with the musc le fatigability, the preoperative limb-length discrepancy, the percent age of lengthening, and the motor-unit recruitment. Analysis also reve aled a correlation between muscle fatigability and the percentage of l engthening of the bone and between muscle fatigability and the preoper ative limb-length discrepancy. The percentage of lengthening correlate d with the extent of motor-unit recruitment. The results of this study suggest that the amount of damage to neuromuscular tissue varies acco rding to the extent of the lengthening of the femur. Of all of the kne e extensors, the vastus medialis was affected the most.