Muscular endurance

Muscular endurance is the ability to keep producing force over repeated contractions or a sustained hold: how many press-ups you can do, how long you can hang from a bar, how well a muscle resists fatigue rather than how much force it can produce once.

It is distinct from maximal strength and from cardiovascular endurance, though it borrows from both. A stronger muscle uses a lower percentage of its maximum for any given task, so getting stronger improves endurance at that task even without training endurance directly.

Locally it depends on the muscle's own machinery: capillary density, mitochondrial content, buffering capacity against the metabolic conditions that accumulate during hard work, and fibre type composition.

Training it means higher repetitions with shorter rests, which sounds simple and has an important caveat. Sets have to be taken close enough to failure to matter, because a comfortable set of 20 trains very little, and light easy sets are the most common way people convince themselves they are training endurance while doing almost nothing.

The specificity is high. Endurance in one movement transfers poorly to another, and endurance at a given load transfers poorly to a different load, which is why testing it against a standard is only meaningful if you trained something similar.

For most people it is a by-product rather than a target. Ordinary hypertrophy training in the 8 to 15 rep range develops plenty of it, and dedicating a block to it makes sense mainly for sports, occupational tests and specific goals.

In practice, get stronger first, since that raises the ceiling everything else sits under, and add higher rep work near the end of sessions where the fatigue costs least.