Set

A set is a run of repetitions done back to back, followed by rest. Training is normally written as sets by reps — three sets of eight, say — and the rest between them is part of the prescription even when it goes unwritten.

It matters because weekly hard sets per muscle group is the most useful single measure of training volume, and volume is one of the primary drivers of muscle growth. Counting sets that were taken reasonably close to failure gives you a number you can actually steer by.

In practice, rest between sets should be long enough to let performance recover — often two to three minutes on heavy compound lifts, less on isolation work. Cutting rest short mostly reduces the quality of the following set rather than adding a useful training effect.