Which comparison correctly differentiates open kinetic chain from closed kinetic chain exercises, with a relevant example?

• A. Open chain: distal segment moves freely (leg extension); Closed chain: distal segment fixed (squat)
• B. Open chain: distal segment fixed; Closed chain: leg extension
• C. Open chain: upper body only; Closed chain: lower body only
• D. Open chain: for rehab; Closed chain: for sport

Answer: (A)

The crucial distinction is whether the far end of the limb is free to move or fixed to a surface during the exercise. In an open kinetic chain movement, the distal segment moves freely through space, often isolating movement around a single joint. A typical example is the leg extension, where the foot can move while the thigh stays relatively stabilized and the knee flexes or extends. In a closed kinetic chain movement, the distal segment is fixed to something (usually the ground), so multiple joints and muscles work together in a linked sequence as the body moves, such as in a squat where the feet stay planted and the hips, knees, and ankles move in concert.

This is why the given correct description matches: the open-chain example illustrates the distal segment moving freely, and the closed-chain example shows the distal segment fixed. The other options either reverse the concept, separate by body region rather than the chain nature, or describe usage rather than the mechanical setup.