MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 226

Figure 9.2 EMG from an indwelling electrode in a muscle as it begins to develop
tension. The smallest motor unit is first recruited, and as its rate increases, a second,
then a third motor unit are recruited. Each motor action potential has a characteristic
shape at a given electrode, which depends on the size of the motor unit and the distance
from the electrode to the fibers of the motor unit (see Section 10.1.3).
The electrical indication is a motor unit action potential; the mechanical result
is a twitch of tension. An increase in tension can, therefore, be accomplished
in two ways: by an increase in the stimulation rate for that motor unit or by
the excitation (recruitment) of an additional motor unit. Figure 9.2 shows the
electromyogram (EMG) of a needle electrode in a muscle as the tension was
gradually increased. The upper tracing shows one motor unit firing, the mid-
dle trace, two motor units, and the lower trace three units. Initially, muscle
tension increases as the firing rate of the first motor unit increases. Each unit
has a maximum firing rate, and this rate is reached well after the next unit is
recruited (Erim et al., 1996). When the tension is reduced, the reverse process
occurs. The firing rates of all recruited units decrease until the last-recruited
unit drops out at a rate usually less than when it was originally recruited.
As the tension further decreases, the remainder of the units drop out in the
reverse order from that in which they were recruited. The firing rate of each
unit increases monotonically with force; however the firing rates increase non-
linearly as force increases from 0 to 100% maximum voluntary contraction
(MVC) (Erim et al., 1996). Mathematical models describing recruitment have
been reported by Wani and Guha (1975), Milner-Brown and Stein (1975),
Fuglevand et al. (1993), and Erim et al. (1996).
9.0.3 Size Principle
Considerable research and controversy has taken place over the past three
decades over how the motor units are recruited. Which ones are recruited first?