ACL injury has been associated with increased instability of the knee in both in vitro and in vivo studies using passively applied forces. This study assessed tibiofemoral rotation after ACL injury during the functional activity of treadmill ambulation using a triaxial electrogoniometer. Isometric and isokinetic peak torques of the quadriceps and hamstrings were obtained using the Cybex II isokinetic dynamometer. Significant increases in tibiofemoral rotation were noted with increased running speed in the injured and normal knees. The degree of rotation in the injured limb did not significantly exceed that of the normal limb. Isokinetic strength of the quadriceps and hamstrings correlates significantly with the observed extent of rotation.