What causes the stiffness and heaviness of muscles during acute soreness?

The stiffness and heaviness of muscles during acute soreness, often experienced after intense exercise, is primarily linked to the accumulation of lactic acid. During strenuous physical activity, especially when performed at high intensity or when the muscles are not accustomed to the workload, the body can produce lactic acid faster than it can be cleared away. This buildup occurs due to the anaerobic metabolism of glucose, where the insufficient oxygen supply leads to the conversion of pyruvate into lactic acid.

Lactic acid itself can lead to an increase in the acidity of the muscle tissue, which may contribute to the sensation of soreness and discomfort. It's important to recognize that while lactic acid is often associated with immediate muscle fatigue and a burning sensation during intense exercise, it does not reside in the muscles long-term. The muscle stiffness and heaviness experienced after exercise can also be attributed to micro-tears in muscle fibers, metabolic waste products, and inflammation, but in the context of acute soreness following intense activity, lactic acid accumulation is a primary factor.

In summary, the heaviness and stiffness of muscles that a person feels during acute soreness is largely a direct result of lactic acid buildup during and immediately after intense exercise, making it the most accurate answer.