Motor units
A motor unit is a single motor neuron plus all the muscle fibers to which it connects.
Some motor neurons only connect to a few muscle fibers, while other motor neurons connect
to hundreds of muscle fibers. All the muscle fibers randomly occur together in small bundles
in genetically predetermined proportions. These bundles are called motor units and are named
after the predominant type of fibers located in these motor units. If a motor unit is made up of
predominantly white, fast twitch tissue fibers, that motor unit is called a white, fast twitch motor
unit, and so on.
Motor unit Function
Generally speaking, white, fast twitch motor units are responsible for speed and strength,
and a person who has a predominance of these would be best suited for strength events. The
red, fast twitch motor units are responsible for sustaining a load over prolonged periods, and
a person with a predominance of these motor units would be best suited for events requiring
stamina, such as boxing, football, basketball, and so on. The red, slow twitch motor unit is
responsible for producing energy over long periods, and a person with a predominance of these
would be best suited for endurance events.
Motor unit Recruitment Methods
Immediate Recruitment
As a general rule, only the minimum number of motor units required to move a given
weight will contract in performing work. Pick up a very light object in your hand, bend your arm at
the elbow, and feel the contracted bicep with your free hand. The muscle will feel somewhat soft
because only the red, slow twitch motor units are contracting. The assistance of the remaining
motor units is not required. If the resistance is slightly greater, the red, fast twitch motor units
will assist the red, slow twitch motor units. Once again, for example, pick up a heavier object
and feel the contracted bicep. It will feel harder than it felt when lifting the lighter object because
more motor units are working. If the resistance is greater yet, the white, fast twitch motor units
will finally assist in the work. This time, pick up a very heavy object, and not only will your bicep
feel extremely hard, but you will also feel the strain against the resistance. In this case, most
if not all, the motor units in the working muscle are involved. The immediate involvement of
varying numbers of motor units based upon the amount of weight moved is one form of motor
unit “recruitment.” Immediately providing extremely heavy resistance will insure an earlier
“recruitment” of the white, fast twitch motor units, which we know to have the greatest potential
for growth. This also tells us that performing light resistance exercise will be of little value in
optimizing growth. There is, however, still some growth stimulation of the red, fast and red, slow
twitch motor units in lighter high repetition training. However, they can experience only limited
growth by comparison, due to their tissues’ differing composition, which will be discussed in
greater detail later.