What is the primary physiological reason for muscle soreness after intense exercise?

The primary physiological reason for muscle soreness after intense exercise is micro-tears in muscle fibers leading to inflammation. When muscles are subjected to intense activity, particularly when performing eccentric movements (where muscles lengthen under tension), small tears can occur in the muscle fibers. This process is a normal and necessary response to exercise, as it helps to stimulate muscle repair and growth.

As these micro-tears occur, they trigger an inflammatory response. The body sends various biochemical signals to the injured area, which leads to increased blood flow and the activation of the immune system to help repair the damage. This repair process contributes to the sensation of muscle soreness known as delayed onset muscle soreness (DOMS), which typically peaks 24 to 72 hours after exercise.

Contrarily, lactic acid accumulation can occur during intense exercise and may lead to temporary muscle fatigue, but it is not the primary cause of soreness after the fact, as lactic acid is generally cleared from the muscles relatively quickly after exercise. Excessive hydration does not contribute to muscle soreness and, in fact, proper hydration is important for muscle function and recovery. Similarly, while increased muscle temperature occurs during exercise, this is a normal physiological response, and does not directly relate to post-exercise muscle soreness.