Final answer:
During a concentric contraction, the muscle shortens as it exceeds the load, while in an eccentric contraction, the muscle lengthens and tension diminishes but is less than the load. Both are types of isotonic contractions, where muscle tension remains constant as length changes to move a load, distinct from isometric contractions, where length stays the same.
Step-by-step explanation:
Understanding Muscle Contractions
In a concentric contraction, the muscle indeed shortens as it generates tension that exceeds the resistance, or load. This is seen when the biceps brachii contracts while lifting a hand weight upwards. During this process, muscle fibers' sarcomeres shorten, and myosin heads pull the actin, resulting in the forearm moving toward the body as the angle of the elbow joint decreases.
In contrast, an eccentric contraction occurs when a muscle lengthens under tension which is less than the load. This typically happens during the controlled lowering of a hand weight, as experienced when the biceps brachii releases tension and the angle of the elbow joint increases. During this phase, less nervous system stimulation decreases the frequency of cross-bridge formation. Eccentric contractions play a vital role in movements and maintaining balance of the body.
It's important to note that these are types of isotonic contractions, where the tension remains relatively constant as the muscle changes length to move a load. This is distinct from isometric contractions, where the muscle generates tension without a change in muscle length, usually because the load is too heavy to move.