Isolation exerciseSingle-joint exercise

An isolation exercise involves movement at just one joint and focuses the work on a single muscle — a biceps curl, a leg extension, or a lateral raise, for instance.

It matters because isolation work lets you add targeted volume to a specific muscle, bring up a lagging area, or train a muscle the big compounds miss, with less overall fatigue than a heavy compound.

In practice, isolation movements are typically used as accessories after the main compound lifts, often in higher rep ranges with strict form.