Which phrase best defines acute onset muscle soreness?

Acute onset muscle soreness is best defined as transient soreness that develops during or after exercise. This type of soreness often occurs as a result of physical activity, especially when the body endures a new or intense form of exercise that it is not accustomed to. The soreness is typically characterized by a feeling of stiffness and discomfort, but it is usually short-lived. It often appears during or shortly after the activity, reflecting the immediate impact of exertion on the muscles.

The other options do not accurately capture the essence of acute onset muscle soreness. For instance, progressive pain that develops over several days aligns more with delayed onset muscle soreness (DOMS), which occurs after a period of recovery following strenuous activity. Pain that occurs immediately after exercise does address the timing but does not encompass the transient nature of the soreness experienced. Pain that only occurs after heavy lifting is too specific and excludes the fact that acute soreness can arise from various types of physical activities, not just heavy lifting. Thus, the definition provided in the correct answer captures the essence of acute onset muscle soreness effectively.