Depleting Energy Recruitment
As a result of depleting energy, another type of motor unit recruitment occurs. For
example, during contractions of a muscle group against light resistance, each repetition steadily
depletes the working motor units of energy, and other motor units are called upon to assist the
fatigued motor units in the same order as before. First the red, fast twitch motor units assist the
already working slow twitch motor units. As the red, slow and red, fast twitch motor units are
continuing to tire, the white, fast twitch motor units are called upon. By the time the white, fast
twitch motor units start working, the entire muscle group is rapidly exhausting. It is also quite
probable that by the time the white, fast twitch motor units are called upon, lactic acid and free
phosphate are accumulating, thus inhibiting further contraction. This type of recruitment minimizes
the involvement and growth stimulation of the white, fast twitch motor units, which are known
to have the greatest potential for growth. To experience this type of motor unit recruitment,
pick a relatively light weight and begin to perform repetitions. As the muscle becomes more and
more fatigued, you will begin to notice a burning sensation, and a slight pump. As the exercise
becomes more difficult, especially when performing a movement involving a very large amount
of muscle tissue, such as the squat, your heart rate will become elevated and your breathing
labored. As the movement becomes even more difficult with each repetition, there are more
white, fast twitch motor units coming into play, since the these motor units are reserved until
last when the work is more strenuous. When you finish the set, after performing as many as 25
repetitions, the white, fast twitch motor units may only have performed optimally for two to three
of those last repetitions. This is hardly enough to stimulate optimum involvement.
Contractile Speed Recruitment
Yet another type of motor unit recruitment is based on the speed of contraction. A
maximum contraction against a sub-maximal resistance will, in theory call on the red and white
“fast twitch” motor units due to their faster contractile speeds. This can be accomplished
through compensatory acceleration training, plyometrics, or by performing Olympic style weight
lifting. Olympic style weight lifting has been applied to athletes for years by strength trainers. It is
used more frequently when training for specific events requiring explosive types of movements,
and without proper execution, can increase the risk of injury. Therefore, the frequency of this
type of training should be controlled. It is the National Federation of Professional Trainer’s
(NFPT) position that the risk of injury far outweighs the benefits to be gained from Olympic lifting,
especially among non-athletes or by athletes without the benefit of professional and experienced
instruction. Non-load barring “plyometric” (multi-directional) training is the suggested option to
enhancing power and speed.
Resistance Exercise and Contractile Failure
Neuromuscular Failure (Power and Speed) (All Players)
This type of failure results more in the enhancement of nerve impulse transmission rather
than muscle strength and/or endurance. Examples of activities that involve neuromuscular failure
include Olympic style lifting such as power cleans, cleans, jerks, clean and jerks, and other load
barring compensatory acceleration activities. These training methods are also commonly referred
to as being “ballistic.” These exercises have been used in the past and are still used, with the