DefinitionOne-rep max is the heaviest weight you can lift once with good form for a given exercise.
What it means
Testing a true max is hard and carries some injury risk, so most people estimate it from a lighter set. The Epley formula, for example, is weight times one plus reps divided by 30.
Estimates are most accurate for sets of about 10 reps or fewer. Beyond that, the formulas drift.
During weight loss, tracking estimated 1RM on a few lifts is a simple check on muscle. If your strength holds while the scale drops, that is a good sign you are keeping lean mass.
Most people do not need to lift near their max to build or keep muscle. Sets of 6 to 15 reps taken close to fatigue work well for many adults. Never test a true max without a spotter or proper safety setup.
Why it matters
Strength trends are a practical way to watch for muscle loss while on a GLP-1.