Elite Physique The New Science of Building a Better Body

Muscle Rules
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Muscle fibers are arranged in bundles, ranging from around 10 fibers in muscles
around your eyes to thousands of fibers in large muscle groups (e.g., hamstrings).
Each bundle of fibers is connected to one motor neuron, which sends a signal to
contract all those fibers. The combination of the single motor neuron and all the
muscle fibers it activates is a motor unit. Since there are three primary types of
muscle fibers there are three types of motor units (see figure 3.2):
• Slow (S) motor units that contain type I fibers
• Fast fatigue-resistant (FFR) motor units that contain type IIa fibers
• Fast fatigable (FF) motor units that contain type IIx fibers
Large
motor neuron
Medium
motor neuron
Small
motor neuron
Type I fibers
Slow (S)
motor unit
FIGURE 3.2 The three types of motor units.
Type IIa fibers
Fast fatigue-resistant (FFR)
motor unit
Type IIx fibers
Fast fatigable (FF)
motor unit
E8315/Waterbury/F03.02/670185/mh-R2
Your nervous system recruits motor units in an orderly fashion, from smallest
(S motor units) to largest (FF motor units). When a muscle contracts, the S motor
units fire first, producing small increments of force. As more force is needed, larger
motor units are recruited, each contributing progressively more force, with that
force increasing in progressively larger increments:
• Low force activity = S motor units
• Medium force activity = S + FFR motor units
• High force activity = S + FFR + FF motor units
This orderly recruitment of motor units is known as the size principle (see
figure 3.3) and was first proposed by neurophysiologist Elwood Henneman (1957).
When the nervous system determines that a muscle requires relatively little
force, it activates relatively few motor units, and the ones it activates deploy the
muscle’s smallest fibers. When higher levels of force are required, the nervous
system brings in larger motor neurons, which activate more and bigger fibers.
Once all motor units are recruited, the brain sends a stronger signal to the motor