What biological change causes loss of strength and stamina as a person ages?

The loss of strength and stamina as a person ages is primarily attributed to the biological change involving the loss of muscle cells, which are often replaced by fat. As individuals age, there is a natural decline in muscle mass, a condition known as sarcopenia. This process reduces the overall strength and endurance of the body because muscle tissue is more metabolically active and contributes more to physical capability compared to fat tissue.

Muscle loss can lead to decreased functional ability, reduced metabolic rate, and greater difficulty in performing everyday activities. This shift from muscle to fat composition not only diminishes physical strength but also impacts coordination and balance, leading to increased risk of falls and injuries.

The other choices may factor into overall fitness and health but do not directly explain the specific biological change responsible for the decline in strength and stamina associated with aging.