Resistance training with light loads (30% of one-repetition maximum) to muscular failure results in greater improvements in muscular endurance at that specific load compared to heavy-load resistance training, indicating that adaptations are specific to the load and repetition range used.

When you lift a light weight until you can't do another rep, your muscles first use the slow-tired fibers that are good for endurance. As those fibers get tired from repeated use, your body starts recruiting the faster-twitch fibers that usually handle heavy lifting. This forces all your muscle fibers to work hard for a long time, which builds up metabolic byproducts like lactic acid and lowers the energy supply in the muscle. This sustained stress signals the muscle to adapt by improving its ability to handle fatigue, making it better at doing many reps with that same light weight — even if you never lifted heavy.