Describe the basic contraction types and their metabolic costs.

• A. Concentric shortens muscle; Eccentric lengthens under load; Isometric holds; metabolic costs vary, with concentric typically more energy-intensive per work performed.
• B. Eccentric shortens; Concentric lengthens; Isometric length changes.
• C. Isometric shortens; Eccentric lengthens; Concentric changes.
• D. All contractions have the same metabolic cost.

Answer: (A)

The main idea here is understanding how muscles behave during three basic contraction types and how their energy costs compare. A concentric contraction is when the muscle shortens while generating force to produce movement. An eccentric contraction is when the muscle lengthens while resisting a load, controlling the movement as it yields. An isometric contraction occurs when the muscle produces force without changing length, holding a position.

Metabolic cost varies with how the muscle is used. In general, concentric contractions are more energy-intensive per unit of work than eccentric contractions because active shortening requires ongoing cross-bridge cycling and calcium cycling that consume ATP. Eccentric contractions can generate higher forces with less ATP expenditure, since much of the load is absorbed by the stretching muscle-tendon system. Isometric work depends on the force level and duration, so its energy cost is not fixed but related to how hard and how long you hold the contraction.

So the option that describes the three contraction types accurately and notes that metabolic costs vary, with concentric typically more energy-intensive per work performed, aligns with what we observe in muscle physiology. The other descriptions mix up which contraction shortens or lengthens, or claim equal costs across contractions, which aren’t correct.