What describes the ability of the muscular system to work efficiently over time against a light fixed load?

Muscular endurance refers to the capacity of a muscle or group of muscles to sustain repeated contractions or to continue exerting force against a fixed load over an extended period without fatigue. This quality is especially important in activities that require maintaining a certain level of strength for a long duration, such as running, cycling, or rowing with a lighter weight.

In contrast, muscular strength is primarily about the maximum amount of force a muscle can produce in a single effort, which doesn't focus on endurance but on intensity. Cardiovascular endurance relates to the efficiency of the heart and lungs in supplying oxygen to the body during sustained physical activity, while flexibility pertains to the range of motion of joints and muscles. These other aspects do not specifically address the efficiency of working against a light fixed load over time, making muscular endurance the correct choice in this context.