Fast, explosive movements such as jumps, sprints and rapid changes of direction require a combination of strength and power for optimal performance. These movements are largely powered by fast twitch muscle fibers. Fast twitch fibers have the ability to renew energy at a very quick rate but also fatigue quickly.
A motor unit is a motor neuron and all of the muscle fibers that it innervates. High threshold motor units predominantly consist of fast twitch muscle fibers. It is these motor units that need to be trained in the weight room and on the practice field. Being able to recruit and utilize high threshold motor units will enable an athlete to be faster and more explosive.
How to effectively train high threshold motor units typically involves maximal strength training (i.e., >85% 1RM) or explosive power training where moderate loads are accelerated with maximal intended velocity. However, it has been hypothesized that training to muscular failure with low-moderate loads will also recruit high threshold motor units. The working theory is that as lower threshold motor units fatigue during a set, higher threshold motor units will be recruited to squeeze out the last few reps before muscular failure. Some practitioners believe that this training method will produce similar results as conventional strength and power training.
Recent research by Schoenfeld et al. (2014) published in the European Journal of Applied Physiology put this theory to the test. The researchers assessed surface -EMG activity of the quadriceps and biceps femoris during low load (30% 1RM) and high load (75% 1RM) sets in 10 resistance training males during leg press exercise. The sets were counter-balanced to control for order effect. 15 minutes of rest was provided between sets.
The results overwhelmingly refuted the hypothesis that low load training until failure recruits high threshold motor units similarly to higher load training. Peak EMG activity was significantly greater in the high load sets compared to low load (on average, 177.3 vs. 137.73, respectively). In addition, mean EMG activity was also significantly higher in high load versus low load training (on average, 63.7 vs. 41.63, respectively.
It appears that coaches and practitioners should stick with higher loads for stimulating high threshold motor units during resistance training. Maximally recruiting fast twitch muscle fibers will lead result in greater improvements in hypertrophy, strength and power. Training with light loads to failure will not accomplish this as effectively as higher load training.
Schoenfeld, B. J., Contreras, B., Willardson, J. M., Fontana, F., & Tiryaki-Sonmez, G. (2014). Muscle activation during low-versus high-load resistance training in well-trained men. European Journal of Applied Physiology, 114(12), 2491-2497.