How is flexibility defined?

Flexibility is defined as the range of movement at a joint, which refers to how far and in what directions a joint can move. This is important for overall functional fitness and is influenced by factors like muscle elasticity, joint structure, and the nervous system's ability to facilitate movement. Improved flexibility can enhance athletic performance, prevent injuries, and aid in recovery.

The other options describe different aspects of fitness and physical capability but do not pertain to the definition of flexibility. For instance, muscle mass is related to strength and body composition, while the ability to endure high-intensity workouts and sustained exercise relates more to cardiovascular endurance and muscular endurance, respectively. Hence, the correct understanding of flexibility centers on joint range of motion.