Low-load (≤60% 1RM) and high-load (>60% 1RM) resistance training performed to muscular failure produce similar effects on type I and type II muscle fiber hypertrophy in the quadriceps of healthy adults, with confidence intervals for type I fibers ranging from -0.71 to 1.28 and for type II fibers from -0.28 to 0.88, indicating no statistically significant difference between the two training methods.

When lifting light or heavy weights until muscles can't move anymore, the body uses all its muscle fibers in both cases. Light weights start by using slow-twitch fibers, but as those get tired, the body calls in fast-twitch fibers to keep going. Heavy weights use all fibers right away. Either way, by the end of the set, every fiber is working hard, creating similar levels of strain and chemical buildup, which triggers the same amount of muscle growth in both fiber types.