Which aspect of fitness is directly attributable to how effectively your muscles function?

The aspect of fitness that is directly attributable to how effectively your muscles function is muscular strength. Muscular strength refers to the maximum amount of force that a muscle or group of muscles can exert in a single effort. This makes it a fundamental measure of the effectiveness and capacity of your muscles in terms of their performance and function.

While flexibility pertains to the range of motion in a joint or group of joints, cardiorespiratory fitness relates to the efficiency of the heart, lungs, and vascular system in delivering oxygen to working muscles during physical activity. Muscular endurance is the ability of a muscle or group of muscles to sustain repeated contractions or to continue applying force against a fixed object. Although each of these aspects contributes to overall fitness, it is muscular strength that most directly reflects how effectively your muscles generate force and perform tasks.