As training proximity to failure increases from 3-repetitions-in-reserve (RIR) to 0-RIR, perceived discomfort, exertion, and muscle soreness increase linearly, while perceived recovery and general feelings decrease.

When lifting weights closer to maximum effort, muscles recruit more powerful but fatigable fibers, which rapidly use up energy and build up waste chemicals like acid and phosphate. These chemicals disrupt muscle contraction, force the brain to reduce signals to the muscles to prevent damage, and trigger signals that the body interprets as discomfort, soreness, and exhaustion. The worse the muscle fatigue, the more intense these sensations become, and the longer it takes for the body to feel recovered.