Resistance-trained young men complete significantly more repetitions with light-load (30% 1RM) resistance exercise to failure than with heavy-load (80% 1RM) resistance exercise to failure, and this difference is due to the requirement of greater total time under tension to achieve equivalent fatigue with light-load conditions.

When lifting light weights to failure, muscles contract many more times over a longer period, which depletes energy stores and builds up metabolic byproducts. This stress reduces the muscle's ability to release calcium, weakening contractions over time. The muscle keeps working despite weakening because the nervous system keeps recruiting more fibers to compensate, until total fatigue finally occurs after many more repetitions than with heavy weights.