Explanation:
Skeletal muscle recruits motor neurons based on the level of force required. The muscle fibers of a motor unit respond to a threshold stimulus and the higher the stimulus, the more motor neurons are recruited. In the case of resistance training, higher resistance causes the recruitment of more motor neurons. Fast-glycolytic (Type IIb/x) fibers are recruited to move a heavy weight and are fueled by glycolytic respiration. Slow-oxidative (Type I) fibers are recruited at all times and are fueled by aerobic respiration. Type I fibers are in use through every muscle contraction and during light intensity exercise like jogging. Fast-oxidative (Type IIa) fibers are used between glycolytic and aerobic respiration, leaning more toward aerobic respiration for energy. These fibers are used in high-intensity interval training programs. When Type IIa fibers are recruited, Type I fibers are also recruited. When Type IIb/x fibers are recruited, so are both Type IIa and Type I fibers.
