With a knee-testing device we measured the valgus stiffness of the
medial musculoligamentous complex of the knee in five volunteers. Tests
were done with the muscles quiescent and with contraction of the sartorius
and vastus medialis. Contraction of those muscles substantially increased
the valgus stiffness. In eight subjects the voluntary reaction times were
measured. With either a visual or a tactile impulse for prophylactic muscle
contraction, the muscles acted too slowly to augment the stiffness of the
knee so as to prevent injury. In fifty-one volunteers we applied the
maximum torque to the leg, just short of eliciting pain, and showed that
even then the muscle contraction would occur too late to protect the knee
in typical sports situations.