National Academy of Sports Medicine (NASM) Group Fitness Instructor Practice Exam

What physiological reaction is primarily responsible for the "burn" felt during intense exercise?

1. Reduction of oxygen supply

2. Lactic acid accumulation

3. Dehydration of muscle fibers

4. Increased production of ATP

The correct answer is: Lactic acid accumulation

The sensation of "burn" experienced during intense exercise is primarily due to lactic acid accumulation in the muscles. When you engage in high-intensity activities, your muscles require a significant amount of energy quickly. In such anaerobic conditions, the body relies on carbohydrate metabolism, which leads to the conversion of glucose into energy (ATP). As a byproduct of this metabolic process, lactic acid is produced. During intense physical activity, if the energy demand exceeds the oxygen supply available for aerobic metabolism, the body switches to anaerobic metabolism, resulting in increased lactic acid levels. This buildup of lactic acid in the muscles can lead to an accumulation of hydrogen ions, which contributes to the decrease in pH in the muscle cells, creating the burning sensation. This physiological response is indicative of high-intensity muscular exertion and serves as a temporary measure to assist the body in meeting its energy needs when oxygen availability cannot keep up. Thus, lactic acid accumulation is the key factor behind the "burn" felt during such exertion.