Which fitness component is assessed by how long a muscle can sustain a contraction?

The fitness component that is assessed by how long a muscle can sustain a contraction is muscular endurance. This aspect of fitness refers to the ability of a muscle or group of muscles to perform repeated contractions over time without experiencing fatigue. In practical terms, when evaluating muscular endurance, individuals may perform exercises such as push-ups or sit-ups to see how many repetitions they can complete before their muscles can no longer continue effectively.

In contrast, muscular strength focuses on the maximum amount of force that a muscle can exert in a single effort, while cardiovascular endurance is related to the overall efficiency of the cardiovascular system and its ability to provide oxygen to muscles during sustained physical activity. Flexibility assesses the range of motion available at a joint and does not relate to the muscle's ability to sustain a contraction. Thus, muscular endurance is specifically tied to the duration of muscle contraction and is the correct choice in this scenario.