Final answer:
The decline in physical performance during early adulthood is primarily attributed to a decline in muscle mass. This loss of muscle mass reduces strength and physical performance, particularly noticeable during middle adulthood, and can subsequently decrease seniors' levels of physical activity.
Step-by-step explanation:
During early adulthood, the factor that contributes to the decline in physical performance is a) Increased metabolism b) Decline in muscle mass c) Enhanced cardiovascular health d) Improved flexibility.
As individuals age, especially in middle adulthood, there is a loss of muscle mass that can lead to a decrease in strength and physical performance. This typically begins to become noticeable during the middle adulthood years, which range from the 40s to the 60s. The decline in physical abilities, including muscle strength, may impede seniors from engaging in sufficient physical activity, causing a feedback loop that results in further muscle loss and reduced metabolism.