During resistance exercise performed to volitional failure, heavy loads (80% 1RM) elicit higher peak electromyography amplitude per repetition compared to light loads (30% 1RM), but the total motor unit activation over the entire set is not greater with heavy loads than with light loads.

When muscles are worked until they can't keep going, buildup of waste products in the muscle forces the nervous system to recruit more muscle fibers to keep producing force. Heavy weights make each muscle fiber fire harder at first, but you can only do a few reps. Light weights make each fiber fire less hard, but you do many more reps, so eventually the nervous system has to call on just as many or more fibers to keep going. In both cases, the same total number of muscle fibers end up firing over the whole set.