Correct Response: D. When a muscle exerts force with no change in muscle length or in the angle of a joint to which it connects, it remains static and is in isometric contraction. Pushing against a wall, holding a heavy object above one's head, or staying in the up position of a push-up are examples of isometric exercises. A movement that lengthens a muscle due to extension of a joint (A) is an eccentric action, and a movement that increases tension on a muscle as it shortens due to joint flexion (C) is a concentric action. When movement is generated in opposition to gravity (B), the muscle action would be either eccentric or concentric, not isometric.
