The effect of tension in the quadriceps and hamstring muscle groups on measures of abduction and adduction of the leg at the knee joint was determined in this investigation. Male students (N = 20) at The Pennsylvania State University volunteered to serve as subjects. Measures of abduction and adduction were obtained on a specially constructed apparatus under standardized conditions, Electromyographic apparatus (EMG) was used to monitor muscular activity under four levels of contraction: assumed relaxation (no visual feedback), relaxation (visual feedback provided), weak contraction, and strong contraction. Results indicated that knee abduction and adduction were limited significantly when quadriceps and harnstriags were pretensed. Measures of abduction and adduction with the thigh muscles in a relaxed state were not significantly different from those measures obtained with the thigh muscles in an assumed relaxed state. These results indicate the influence of quadriceps and hamstring muscles as stabilizers of the knee joint.