Resistance training at loads of 30% or greater of one-repetition maximum (1RM) induces a consistent level of muscle deoxygenation (HHb) at task failure, whereas training at 10% 1RM does not, indicating a metabolic threshold near 30% 1RM that may define the minimum load required to achieve sufficient metabolic perturbation during exhaustive exercise.

When lifting weights at 30% or more of your maximum strength, the muscle contracts so long and hard that it uses up oxygen faster than blood can bring more, causing oxygen levels to drop to a consistent low point by the time you can't do another rep. This happens because the squeezing of the muscle blocks blood flow, and the energy system switches to one that produces waste chemicals like acid and phosphate, which slow down the muscle's ability to contract. At lower weights, the muscle doesn't get stressed enough to block blood flow or build up these chemicals, so you can keep going without hitting that same oxygen low point.