Flexibility in physical fitness relates to which of the following?

Flexibility in physical fitness specifically refers to the degree to which muscles can stretch through their range of motion. This involves the ability of joints and muscles to move freely and easily, allowing for a greater range of motion in physical activities. Flexibility is essential for overall bodily function and can contribute to improved performance in various sports and activities, as well as help in reducing the risk of injuries.

In contrast, the other options pertain to different aspects of physical fitness. Lifting weights effectively relates to strength training and muscular endurance, while the heart's ability to pump blood relates to cardiovascular fitness. The balance of muscle mass to fat in the body examines body composition. Understanding these distinctions helps clarify the specific role flexibility plays within the broader context of fitness.