Isotonic (concentric and eccentric) and Isometric.

The idea

Muscle contraction is classified by whether the muscle changes length and whether a joint moves. In isotonic contraction the muscle changes length and a joint moves. Isotonic contraction has two phases: concentric, where the muscle shortens under tension to produce movement, as when the biceps brachii shortens to lift a dumbbell during elbow flexion; and eccentric, where the muscle lengthens under tension while controlling movement, as when the biceps brachii lengthens to lower the dumbbell during elbow extension. In isometric contraction the muscle develops tension but its length stays the same and no joint movement occurs, as when holding a plank or pushing against an immovable object. The same muscle can work concentrically in one phase and eccentrically in the next, so the classification depends on what the muscle is doing at that moment.

What earns the marks

• Defines isotonic contraction as contraction in which the muscle changes length and a joint moves.
• Defines concentric contraction as the muscle shortening under tension to produce movement, for example the biceps brachii during elbow flexion.
• Defines eccentric contraction as the muscle lengthening under tension while controlling movement, for example the biceps brachii during elbow extension.
• Defines isometric contraction as the muscle developing tension without changing length and without joint movement, for example holding a plank.
• Applies the correct term to a described action, such as labelling the lowering phase of a biceps curl as eccentric.
• Recognises that the same muscle can act concentrically and eccentrically in different phases of one movement.

Common mistakes and how to put them right

• Believing that eccentric contraction means the muscle is relaxed.

  Put it right: The muscle lengthens while still under tension and controlling the movement.

• Assuming isometric contraction involves no muscle activity.

  Put it right: Tension is developed, but muscle length stays the same and no joint movement occurs.

• Calling any shortening of a muscle concentric regardless of context.

  Put it right: Concentric contraction requires the muscle to shorten under tension to produce movement.

• Treating a muscle as permanently concentric or eccentric.

  Put it right: The same muscle can act concentrically in one phase and eccentrically in the next.

Exam tips

• Identify whether the stem describes a joint moving or a position being held before selecting a contraction type.
• For a described action, decide whether the muscle is shortening or lengthening under tension, then match that to concentric or eccentric.
• Check that the option you choose accounts for both muscle length change and joint movement, since these distinguish the contraction types.
• Use a familiar example such as a biceps curl or a plank to test each option against what the muscle is actually doing.