Triceps

Roughly two-thirds of arm size comes from the triceps, not the biceps — which makes an arm day that splits time evenly between curls and pushdowns a math problem before it's anything else, weighted toward the muscle doing less of the work. Fixing that ratio takes understanding where the stimulus actually needs to go: the triceps has three heads, and only one — the long head — crosses two joints at once, the elbow and the shoulder. That's what decides how to train it. Keep the arm pinned to the torso, the way a pushdown does, and the lateral and medial heads carry the set, since they only act at the elbow; take the arm overhead, as on a dumbbell skull crusher, and the long head stretches and finally gets pulled in fully, being the only one of the three that also crosses the shoulder. On a bench press, an overhead press, a dumbbell shoulder press, a dumbbell bench press, or dips, the triceps only steps in to finish the elbow extension at the very end of the range — never the prime mover there the way it is on an isolation exercise, so compounds alone, no matter how many sets, leave all three heads short of full work. Let the elbow float away from the torso and climb during a pushdown, and the movement quietly turns into a partial shoulder press that gives up the exact tension the cable was built to hold; on dips, keeping the torso upright rather than leaning forward is what keeps the load on the triceps instead of letting it drift onto the chest.