More Fatigue Does Not Make More Muscle

You’ve probably heard it said that fatiguing your muscles helps them grow by recruiting more muscle fibers. A new study questioned this concept, with interesting results.

Working to repeated muscular failure has long been a popular form of gym exercise, but its fame may not be warranted. Despite controversy over its effectiveness, the practice of extreme workouts in the gym may actually be increasing as of late. A study this month in the Journal of Strength and Conditioning Research shed more light on the usefulness of muscular fatigue.

The basic idea behind muscle exhaustion is that greater fatigue recruits more motor units. A motor unit is made up of nerves and muscle fibers that create movement in your body. The large muscles of your body each contain many motor units. However, when you engage a muscle, not all of its motor units are firing. To achieve the greatest strength or size changes from exercise, recruiting more motor units is the name of the game. The question of this study is whether or not fatiguing the muscles is the way to do that.

One way to study motor units is by using electromyography, or EMG. In this study, the researchers took EMG measurements on the vastus lateralis, the outermost muscle of the quadriceps, during a Bulgarian split squat. Participants did eight sets of split squats with 75% of their maximum force output, and each set was performed to exhaustion. A maximum force test was performed before the sets began, and then again in between each set. EMG readings were recorded throughout.

The findings of the study may surprise you. There was no association between motor recruitment and fatigue, and thus no association between fatigue and maximal strength or muscle building. In fact, there was no change in muscle recruitment at all, from the start of the average set to the end. There was also no change in recruitment as the sets progressed, despite the fact that force production declined. In fact, the only change in recruitment came at the end of the eighth set, and only in those participants with the highest performance levels. The researchers noted these changes did not seem to result from an increase in fatigue.

The researchers noted that one of the major take-away points from this study is the prescription of exercise based on percentages of 1RM. Using percentages might be the most common method that coaches use to dole out programs, but with such a huge variation in ability at the 75% of 1RM load used in this study, it may not be warranted. Even more significantly, the motor response to exercise was varied between the participants. With such a diverse group, a single percentage-based program like the one used could yield very different results.

What the researchers recommended instead is a repetition maximum approach. With this approach, a coach can simply prescribe a five rep max, ten rep max, and so on, rather than using a percentage of the one rep max. But even if you use this method, it seems that excessive fatigue doesn’t stimulate the muscular fibers any more in most people. If you’re looking for greater strength or size, this study suggests fatigue isn’t all that important of a factor.


1. Harrison T. Finn, et. al., “Muscle activation does not increase after a fatigue plateau is reached during eight-sets of resistance exercise in trained individuals,Journal of Strength and Conditioning Research Publish Ahead of Print, DOI: 10.1097/JSC.0000000000000226

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