When resistance training is performed to or near muscular failure, repetition tempos spanning a wide range of durations produce comparable levels of muscle hypertrophy.

When you push your muscles to their limit, the buildup of metabolic byproducts like lactic acid and hydrogen ions triggers signals that tell muscle cells to grow, no matter how fast or slow you move. At the same time, the muscle fibers are stretched and pulled for long enough that the physical strain itself also activates growth signals. Together, these two forces — chemical stress and physical tension — turn on the same molecular switch inside muscle cells that tells them to make more protein and get bigger.