Muscle failure is defined as “the point during a resistance exercise set when the muscles can no longer produce sufficient force to control a given load.” It’s when you try to do one more repetition and you fail to complete it with proper form. Many people believe that you should train every set to failure to maximize strength and hypertrophy (muscle growth). But what does the research say?
There haven’t been many studies on this topic. Some that have been done have found a short-term benefit to training to failure for increasing strength. One of the better designed studies, had participants either train to failure or nonfailure (they performed half the reps of the failure group but twice the number of sets) while keeping volume and intensity constant. Similar strength gains were found for the bench press (a 23% increase for both), parallel squat (22% for failure; 23% for nonfailure), power output of the arm (27% for failure; 28% for nonfailure), muscle power for leg extensor muscles (26% for failure; 29% for nonfailure) and maximum number of repetitions on the parallel squat (66% for failure; 69% for nonfailure). The failure group outperformed the nonfailure group on the maximum number of bench press reps though. On the flip side, the nonfailure group had higher levels of resting testosterone and lower levels of cortisol (a stress hormone). The higher levels of cortisol in the failure group could be indicative of overtraining.
Given the risk of overtraining and the small to nil benefits to training to failure, it’s best to stop a rep before failure. A review on the topic concluded that “training to failure might provide the extra stimulus needed for advanced lifters to break through plateaus when incorporated periodically…however, training to failure should not be practiced repeatedly over long periods due to the potential for decreases in growth-promoting hormones and increases in overuse injuries.”
Izquierdo, Mikel, et al. “Differential effects of strength training leading to failure versus not to failure on hormonal responses, strength, and muscle power gains.” Journal of Applied Physiology 100.5 (2006): 1647-1656.
Willardson, Jeffrey M. “The application of training to failure in periodized multiple-set resistance exercise programs.” The Journal of Strength & Conditioning Research 21.2 (2007): 628-631.