Time under tension is the cumulative time a muscle spends under load during a set. Calculated from tempo × reps.
Mechanism — Sustained mechanical tension drives hypertrophy. The same 10 reps with a 1-second tempo vs a 4-second tempo deliver very different stimulus despite identical rep count. Productive hypertrophy TUT is typically 30-60 seconds per set.
Use case — Manipulate tempo (e.g., 3 seconds down, 1 second up) to drive more stimulus from less weight. Especially useful when joints can't tolerate maximum loads.
Caveats — TUT isn't everything — load matters too. Don't sacrifice productive load for arbitrarily slow tempos. The sweet spot is balanced.