A bi-articular muscle crosses how many joints?

• A. One joint
• B. Two joints
• C. Three joints
• D. Four joints

Answer: (B)

Bi-articular means a muscle crosses two joints. That allows the muscle to influence movement at both joints it spans and can help transfer energy between them. For example, the gastrocnemius crosses the knee and ankle, so it can contribute to knee flexion and ankle plantarflexion; the hamstrings cross the hip and knee; the rectus femoris crosses the hip and knee. In sprinting, such two-joint muscles help coordinate rapid leg motion and can aid propulsion by coordinating action at two joints simultaneously. So the idea is that a bi-articular muscle crosses two joints.