How does knee position influence the hamstrings' contribution to hip extension?

• A. They contribute to strong hip extension when the knee is flexed
• B. They contribute more when the knee is extended
• C. Knee position has no effect on hip extension
• D. They cause internal rotation of the hip

Answer: (A)

When a muscle crosses more than one joint, its ability to produce force at one joint depends on the position of the other joint. The hamstrings cross both the hip and the knee, so knee angle changes how much they can contribute to extending the hip. With the knee flexed, the hamstrings are at a favorable length to shorten across the hip as you extend the hip, allowing a stronger hip-extension effort. If the knee is extended, the hamstrings are stretched across the knee and can’t shorten as effectively at the hip, so their contribution to hip extension is reduced. They are not primary internal rotators of the hip.