Tri-set training increases internal training load by 24.3% compared to traditional training in resistance-trained men, primarily due to higher perceived exertion rather than increased time under tension, indicating that metabolic stress and fatigue accumulation contribute more to perceived intensity than mechanical work duration.

When muscles are worked back-to-back with almost no rest, they burn through energy fast and build up waste chemicals like lactic acid. This draws water into the muscle cells, making them swell. The swelling and chemical buildup activate nerves that send fatigue signals to the brain, making the workout feel much harder — even if the total lifting weight or time doesn't increase.