Increasing proximity to muscular failure is associated with graded worsening of perceptual responses, including discomfort, exertion, muscle soreness, and reduced recovery, with the most negative responses occurring at momentary muscular failure.

As muscles work harder and closer to their limit, they produce more waste chemicals like acid and phosphate, which build up inside the muscle fibers. This buildup disrupts the muscle's ability to contract properly and triggers signals that tell the brain the body is under extreme stress. The brain responds by reducing the signals sent to the muscles to prevent damage, which makes the muscles feel weaker and more tired. The more these processes happen, the more uncomfortable, sore, and drained a person feels — with the worst feelings happening exactly when the muscle can no longer complete another repetition.