Motor unit recruitment may be defined as "the successive activation of
the same and additional motor units with increasing strength of voluntary
muscle contraction." (American Association of Electrodiagnostic Medicine)
The central nervous system can increase the strength of muscle
contraction by the following:
- Increasing the number of active motor units (ie, spatial
recruitment)
- Increasing the firing rate at which individual motor units fire to
optimize the summated tension generated (ie, temporal recruitment)
Both mechanisms occur concurrently. The primary mechanism at lower
levels of muscle contraction strength is the addition of more motor units,
even though this increases the firing rate of the initially recruited
motor units. The recruitment of different units
takes precedence over increase in firing rate until nearly all motor units
are recruited. At this level and beyond, motor
units may be driven to fire in their secondary range to rates greater than
50 Hz.
The terms firing rate and firing frequency are used interchangeably.
Recruitment frequency is the firing frequency of the first motor unit
when the second unit just begins to fire regularly. The term "recruitment
rate" is used interchangeably.
Recruitment interval is the time difference between 2 motor unit
potentials belonging to the first firing motor unit when the second unit
first appears. The recruitment interval is the reciprocal of the
recruitment frequency.
Most extremity muscles have a recruitment interval of about 90-100 ms,
corresponding to a recruitment frequency
of about 10-11
Hz. Facial muscles are an exception to this
rough guide. MUAPs of facial muscles have shorter recruitment intervals
(around 40 ms) and higher recruitment frequencies (about 25 Hz).
in order of their
size. When the muscle is activated initially,
the first motor units to fire are small in size and weak in the degree of
tension they can generate. Starting with the smallest motor units,
progressively larger units are recruited with increasing strength of
muscle contraction. The result is an orderly addition of sequentially
larger and stronger motor units resulting in a smooth increase in muscle
strength.
This orderly recruitment of sequentially larger motor units is referred
to as the "Henneman size principle." Recording from the ventral rootlets
in cats and measuring the amplitudes of motor axon spikes, Henneman et al
concluded that motor axon diameter, conduction velocity and, by further
inference, motor neuron cell size all increase with functional threshold
(i.e. neural activation threshold).
The
3 main types of motor units, which have different physiologic and staining properties,
include the following:
- Type I or type S (slow) - Slow twitch, fatigue-resistant units with
smallest force or twitch tension and slowest contraction; contain
oxidative enzymes
- Type IIa or type FR (fast, resistant) - Fast twitch,
fatigue-resistant units with larger forces and faster contraction times;
contain oxidative and glycolytic enzymes
- Type IIb or type FF (fast, fatigable) - Fast twitch, easily
fatigable units with largest force and fastest contraction; contain
glycolytic enzymes
The recruitment sequence is thought to begin with type I motor units
analogous to type S units, to progress to type II units that first include
type FR (type IIa), and to end with units analogous to type FF (type IIb),
which are active only at relatively high force output.
In an EMG study, the term "size" of a motor unit usually refers
to the amplitude of the motor unit action potential (MUAP). In
rather general terms, the later recruited type II fibers, especially the
FF type, have larger diameter muscle fibers generating higher potentials
than the smaller, slow twitch type I units. Because of the small uptake
area of standard EMG needle electrodes, however, the size of consecutively
recruited MUAPs during an EMG study varies considerably.
Damage may occur to the neural portion of a motor unit, anterior horn
cell, or corresponding axon. Such injury may result in wallerian
degeneration of the motor axon, and all the muscle fibers previously
innervated by this axon will be denervated. As a result of such motor unit
loss, fewer motor units are available for muscle activation.
Normally, when the first recruited motor unit reaches a firing
frequency of 10 Hz, a second unit should begin firing with increasing
muscular effort. In a neurogenic condition, this second unit is missing
and an increase in force can be achieved only by increasing the firing
rate of the first unit (see
Image
1). Successful activation of a second motor unit occurs only at a
higher level of muscular effort than in the normal condition. The
recruitment frequency, defined above as the firing rate of the first motor
unit at the point when the second motor unit is activated, is therefore
increased in a neurogenic lesion. Such an abnormally fast firing motor
unit is called "rapid firing unit" (RFU). Because in such cases fewer
MUAPs are active than expected, given the first motor unit firing rate,
this pattern is called "decreased recruitment" or "reduced recruitment."
This pattern of decreased recruitment may occur whenever a lesion
results in a reduced number of functionally intact motor neurons and
axons, whether it is the result of actual motor unit loss or temporary
conduction block as in neurapraxia. It is an early finding after acute
nerve injury (eg, radiculopathy from disk herniation or nerve trauma) and
may precede other evidence of denervation in the EMG study.
In muscle diseases such as polymyositis or muscular dystrophies, muscle
fibers are damaged. A number of motor units are unaffected but the muscle
fiber content of each motor unit is reduced; therefore, the force output
of each unit is diminished. The number of units required to maintain a
given force increases in proportion to the inefficiency of the individual
motor unit discharge. Compensation occurs by having multiple motor units
begin firing simultaneously (see
Image
2).
In a myopathy, isolating a single firing motor unit often is
impossible. Even with minimal muscular effort, typically 2 or more units
may be activated. This recruitment pattern in myopathic conditions is
called "early recruitment" or "increased recruitment." The recruitment
frequency is decreased.
With increasing effort, the firing frequency of individual motor units
increases and progressively more and larger units are activated. In a
healthy subject providing maximal voluntary effort of the muscle under
investigation, the action potentials of individual motor units no longer
can be separated from each other but are mixed with the signals of other
units. The recruitment pattern with maximal voluntary contraction is
called "interference pattern" because of the increasing degree of
superimposition of action potentials from different units. With increasing
force, the EMG becomes continuously denser and the maximal peaks in the
signal have a higher amplitude.
American Association of Electrodiagnostic Medicine
defines the interference pattern as "electric activity
recorded from the muscle with a needle electrode during maximal voluntary
effort."
During a maximal voluntary muscle contraction of a healthy individual,
a "full" or "complete" interference pattern is present. No individual
MUAPs can be identified clearly (this is normal). The baseline is obscured
completely by motor unit activity.
Incomplete interference pattern may be divided as follows:
A single motor unit
fires at rapid rate during maximum voluntary effort.
An incomplete interference pattern typically signifies a decreased
number of MUAPs being activated with maximal effort. This may be
suggestive of a neurogenic lesion resulting in a decreased number of
functional motor units. It may, however, occur with incomplete effort of
muscle contraction, possibly as a result of poor cooperation or pain. In
myopathic conditions, the interference pattern is typically complete, even
though low-amplitude MUAPs may be noted on the recording of the
interference pattern; however, in very advanced stages of muscle
disorders, the interference pattern may be incomplete because of marked
loss of muscle fibers.