What is the main difference between concentric and eccentric contraction?

The principal distinction between concentric and eccentric contractions lies in the behavior of the muscle fibers during these types of movements. Concentric contraction occurs when a muscle shortens as it generates force, enabling it to overcome resistance. Classic examples include the upward phase of a bicep curl, where the biceps muscle shortens to lift a weight.

On the other hand, eccentric contraction happens when a muscle lengthens while still generating force, often to control the movement against resistance. During a bicep curl, the lowering phase where the weight is gradually let down exemplifies this, as the biceps are controlling the descent and therefore lengthening under tension.

Understanding this fundamental difference is crucial for effectively designing training programs, as both types of contractions play distinct roles in muscle development and functional movements. The other options introduce misconceptions about the mechanisms of these muscle actions, such as suggesting external weights are required for eccentric contractions or linking concentric contractions solely to flexibility.