Why the Hamstrings Influence Both the Hip and the Knee

A Muscle Group That Crosses Two Joints

Unlike many muscles that act on a single joint, the hamstrings are biarticular — they cross and act on both the hip (extending it) and the knee (flexing it) simultaneously, meaning their effective length and tension at any given moment depends on the combined position of two joints, not one.

Why This Matters for Injury Risk

This biarticular arrangement places the hamstrings under their greatest combined length and tension during the late swing phase of sprinting — simultaneous hip flexion and knee extension — which is precisely the phase of the running gait cycle most strongly associated with hamstring strain injury (Opar, Williams & Shield, 2012). A muscle that must simultaneously manage tension demands at two separate joints has a narrower margin for error than a single-joint muscle would.

Why Rehabilitation Trains Both Joints Together

Because of this dual-joint role, effective hamstring rehabilitation typically includes exercises that load the muscle across both hip and knee positions simultaneously — rather than isolating knee flexion alone — to better replicate the lengthened, high-tension state in which hamstring strain injuries actually occur.