Which action best represents an isotonic contraction?

• A. Biceps curl
• B. Wall sit
• C. Plank hold
• D. Static hold in a squat position

Answer: (B)

Isotonic contractions involve the muscle changing length as it generates force to move a joint. In a biceps curl, the elbow joint angle changes as you lift the weight, and the biceps muscle shortens during the lifting phase (concentric contraction) and lengthens during the lowering phase (eccentric contraction). This length change with movement is what defines an isotonic contraction.

A wall sit keeps the knee and hip at fixed angles, so the quadriceps contract to hold you up but do not change length significantly—this is an isometric contraction. Plank holds and other static positions are also isometric for the same reason: they involve maintaining posture without joint movement.