Some of you might have come across other articles or discussions regarding high-threshold motor units that wer rather technical and confusing. Perhaps the only thing you got out of the article was that
activating more of these units could result in greater muscle size and strength. But wait, what high-threshold motor units really are? Let's start from the basics and keep it simple!
OK, let's start with some basic neuromuscular anatomy. Each and every one of your muscle fibers is stimulated by motor neurons, which are part of your central nervous system. When the body needs to produce force (as in weightlifting), the motor neurons fire off an electrochemical signal to the muscle fibers, in order to activate them and produce the force required. A motor neuron and all the muscle fibers it stimulates are collectively called a "motor unit." Each muscle fiber type (type I, type IIa, and type IIb) has different motor neurons that fire at different rates and strengths. Type I motor units, or slow-twitch motor units, have low anaerobic power, contract slower, and have high levels of endurance. On the other hand, type II motor units, or fast-twitch motor units, have high-anaerobic power and contract very quickly, yet fatigue quickly. The type II motor units can be further subdivided into types IIa and lib, but for the purposes of your question, we'll focus on the type lib motor units, which are otherwise known as "high-threshold motor units."
The high-threshold motor units are responsible for explosive strength, power, and maximum muscle growth. However, they're the motor units that are hardest to activate. So how do you recruit the high-threshold motor units? By lifting weights greater than 70 percent of your one-rep max, with reps in the 3 to 5 range. In addition to this, train explosively by slowly lowering the weight during the eccentric phase of the lift and then exploding on the concentric phase. (Keep in mind that it takes more muscle fibers to move a weight in one second than it does to move it in two seconds.) In order for the body to produce this power, you'll have to activate more high-threshold motor units. The result: a greater potential for an increase in strength, power, and muscle growth.
However, it's not wise to only train within these parameters in an attempt to stimulate the type lib muscle fibers. Doing so would mean you'd be neglecting the type Ila and even type I muscle fibers. For maximum development, the entire spectrum of muscle fibers needs to be trained.