The Motor Unit
Anatomy and Physiology
ARTHUR WM. ENGLISH
and STEVEN L. WOLF*
The physiological and anatomical properties of mammalian motor units are
discussed, and the results of human and animal studies are compared. A
physiological organization of motor units based on the mechanical properties of
their associated muscle units is examined. It is concluded that such an organizing
principle has broad universal application in both animal and human studies. An
anatomical organization of muscle units by their association with their physiological properties and the histochemical profile of their muscle fibers is also
considered. The anatomical organizing principle also has broad-ranging applicability. The organization of muscle units according to muscle architecture and
innervation patterns is described and its potential applicability to considerations
of muscle structure and function is discussed. It is concluded that a number of
gaps in our knowledge of muscle unit organization have been identified, especially among human studies, but the potential to fill these gaps rapidly is great.
Key Words: Motor activity; Motor units, anatomy and physiology; Muscle units.
The motor unit was termed by Sherrington the
"final common path," because it is through motor
units that all motor commands must be passed to
produce movement.1 A motor unit consists of a motor
neuron, its axon, and what is referred to as the muscle
unit, that is, all of the musclefibersthe motor neuron
innervates.2 The structural and functional organization of the central elements of motor units, the motor
neurons, is discussed by Burke,3 Henneman,4 and
Stein and Bertold.5 This article is a discussion of our
current knowledge of the anatomical and physiological organization of the peripheral elements, muscle
units, and a comparison of animal and human research in this area. It is hoped that this review might
also provide a general basis for careful and detailed
analyses of muscle structure and function that include
consideration of the anatomical and physiological
properties of muscle units.
PHYSIOLOGICAL PROPERTIES OF
MUSCLE UNITS
Animal Studies
One of the most widely accepted views in all of
motor unit physiology has been the classification of
* Direct correspondence to Dr. Wolf.
Volume 62 / Number 12, December 1982
motor units proposed in various studies by Burke and
co-workers.6-8 Their studies, as well as several other
studies, have given rise to the classification scheme
shown in Table 1. Motor units are classified mainly
according to the mechanical properties of their associated muscle units. Two reliable criteria used to type
units are the waveform of contractile force during
unfused tetani ("sag") and the resistance of the muscle
unit to fatigue. Correlated with these two criteria for
motor unit identification are a number of other variables, such as maximum tetanic tension, twitch-tetanus ratio, twitch potentiation, and twitch contraction
time.
Burke et al, in different studies, have thus classified
cat hindlimb motor units as fast- or slow-twitch, and
as fatigable or fatigue-resistant.7, 8 They have proposed that the sag in tension records, noted when a
muscle unit is stimulated at intervals slightly longer
(1.25x) than its twitch contraction time, is the most
reliable way of differentiating fast-twitch from slowtwitch units. Fast-twitch units show a sag (Fig. 1);
slow-twitch units do not. Stephens and Stuart9, 10 and
Goslow et al11 have argued persuasively that twitch
contraction time provides a less ambiguous description of muscle unit types. Fast-twitch units have timeto-peak tension of less than 40 msec; slow-twitch units
contract more slowly.
Widespread agreement exists about identification
of the fatigability of muscle units, and a fatigue index
1763
TABLE 1
Physiological and Histochemical Properties of Muscle Unitsa
Unit type
Fast-twitch fatigable
(FF)
Fast-twitch fatigueresistant (FR)
Fast-twitch intermediate (Fint)
Slow-twitch (S)
a
b
c
Twitch
Maximum
Contraction Tetanic
Tension
Time
(gm-F)
(msec)
Sag in
Unfused
Tetanus
Twitch
Potentiationb
Twitch :Tetanus
Ratio
Fatigue
Index
Fiber
Type
35
75
yes
moderate
0.58
<0.25
llb
41
29
yes
large
0.43
>0.75
llac
35
73
71
8
yes
no
small
0.31
0.25-0.75
>0.75
llac
I
Data for cat medial gastrocnemius muscle as reported by Burke7 and Burke et al 8,31 unless noted.
Discussed in Stephens and Stuart.9
Supposition.
has been calculated.12 The fatigue index is the ratio
of the tetanic tension produced after two minutes of
intermittent stimulation (1 train/sec, 13 pulses/train
at 40 pps) to that produced before such repeated
stimulation. Fatigue indexes greater than 0.75 are
indicative of fatigue-resistant units, and fatigable
units have fatigue indexes of less than 0.25.
Three groups of motor units were originally proposed by Burke et al: fast-twitch fatigable (FF), fasttwitch fatigue-resistant (FR), and slow twitch (S).
Type S units (Fig. 1) generally have a time-to-peak
tension greater than 40 msec, produce small twitch
and tetanic tensions, display no sag in unfused tetani,
and are very resistant to fatigue (fatigue index >
0.75). Type FF units have a time-to-peak tension of
less than 40 msec, produce very large twitch and
tetanic tensions, display a sag during an unfused
tetanus, and are readily fatigued by repeated stimuli
(fatigue index < 0.25). Type FR units are somewhat
intermediate: they are fast contracting but slightly
slower than type FF units; they are resistant to fatigue
(fatigue index > 0.75); and they produce twitch and
Fig. 1. Panels A through D demonstrate criteria for classification of mammalian muscle units. Panels A and B are
isometric force records obtained from stimulation of a single muscle unit at an interstimulus interval that is 1.25 times
its twitch contraction time. The solid lines are the unfused tetanic responses of the unit. The dashed lines are the
superimposed single twitches. Fast-twitch units display a "sag" in the tension record (A), slow-twitch units do not and
often show some potentiation (B). Panels C and D show the results of a fatigue test. The units were stimulated at 40
pps for 330 msec every second for two minutes. The larger force record is from the first stimulus presentation, the
lower record from the 120th. Fatigable units (C) display a drop in force to less than 0.25 of the initial level. Fatigueresistant units (D) show a drop that is not more than 0.75 of the initial level. All data are from English and Weeks.18
(Scale bars: A-C, horizontal = 1 0 0 msec, vertical = 1 0 gm-F; D, horizontal = 100 msec, vertical = 1 gm-F.)
1764
PHYSICAL THERAPY
Muscle
Cat
Medial gastrocnemius
Tibialis posterior
Lateral gastrocnemius
Tibialis anterior
Extensor digitorum
longus
Flexor digitorum
longus
1st lumbrical
Rat
Tibialis anterior
SAG
>0.75
>0.75
>0.75
>0.75
Flb
30
31
35
31
TCTa
+
+
+
+
SAG
0.25-0.75
0.25-0.75
0.25-0.75
0.25-0.75
Flb
37
73
47.5
>40
42
TCTa
>0.75
>0.75
>0.75
>0.75
>0.75
11,16
Table 1
12,15
18
11,16
TABLE 2
Physiological Properties of Typed Mammalian Muscle Units
TCTa
+
+
+
+
>0.75
20
21
S
Flb
41
34
40
34
+
>0.75
>0.75
Fint
SAG
<0.25
<0.25
<0.25
<0.25
30
56.2
>20
FR
TCTa
+
+
+
+
<0.25
0.25-0.75
0.25-0.75
FF
35
30
<40
30
+
+
+
SAG
30
33.6
<20
22
>0.75
>0.75
X).75
+
+
20
35.1
<20
11.2
<0.25
<0.25
11.2
+
+
<0.25
36.6
<20
11.2
Flb
Source
(ref)
Volume 62 / Number 12, December 1982
Twitch contraction time (msec).
Fatigue index.
Human motor units with the following characteristics tend to be classified as tonic motor units: low
threshold for maintained voluntary contraction, long
contraction times, low twitch-tension, high resistance
to fatigue, small amplitude action potentials, and slow
conduction velocities. Conversely, phasic motor units
tend to be recruited at high levels of voluntary contraction, display short contraction times and high
twitch-tensions, are not fatigue-resistant, and show
large amplitude action potentials and fast conduction
velocity. In this sense, human motor units share many
of the characteristics delineated in animal studies. As
shown in Table 3, the number of studies designed to
assess physiological components of human motor
units is somewhat limited. This situation has been
dictated mainly by limitations in technique and technology. In particular, it is difficult to accurately assess
phasic motor units because the high levels of muscle
tension often needed to recruit phasic motor units can
be maintained only for a few seconds before the
recording electrodes may become dislodged or reoriented. Even highly selective single-fiber electrodes
may lose muscle-fiber stability for a contraction exceeding 30 percent of maximal voluntary contraction.23 On the other hand, inaccuracies in recording
peak tensions among slow-twitch units may occur
because those units generate very little force. Despite
these technical limitations, refined microstimulation
applied to motor points has enabled researchers to
describe the same motor unit types for human muscle
as described from animal studies.24-27
The values noted for properties of human motor
units in Table 3 show a broad range. How these
b
Human Studies
a
tetanic tensions that are smaller than those produced
by type FF units and larger than those of type S
units. Several researchers have identified units with
fatigue properties between those of FF and FR/S
units (fatigue index > 0.25 and < 0.75) and have
considered these a fourth type of motor unit which
they termed Fint units.6, 12-14
Although this classification scheme recently has
been applied to various muscles, most of the available
information about the physiological organization of
mammalian motor units has come from studies of the
medial gastrocnemius muscle of the cat. The reason
this classification scheme is so widely accepted is that,
almost without exception, it has proven a useful
means of examining all mammalian motor units (Tab.
2). Studies of motor units in other cat hindlimb
muscles,12, 15-20 cat foot muscles,21 cat forelimb muscles (B. Botterman, 1981, personal communication),
and different rat hindlimb muscles22 have all shown
that this same basic physiological organizing principle
can be applied.
1765
TABLE 3
Physiological Properties of Human Motor Unitsa
Twitch Contraction Average Twitch
Tension (gm)
Time (msec)
Muscle (Motor Unit
Group)
Medial gastrocnemius
FF
FR
S
Tibialis anterior
Extensor digitorum
brevis
1 st dorsal interosseous
FF
FR
S
Abductor digiti minimi
a
69.6
64.5
>99
40-80
46.3
13.8
11.5
35-100
30-100
<70
<70
≥70
40-100
2-14
1-50
3.72
2.41
2.0
Adapted from Grimby and Hannerz.
Fatigue
Index
<0.63
>0.82
>0.84
Source (ref)
24
84
30-55
85,86
68
<1
≥1
≥1
88
87
75
values relate to specific physiological characteristics
of human motor units is uncertain, primarily because
of the limited amount of available data. However, the
recordings made from 18 medial gastrocnemius mus­
cle units by Garnett and colleagues indicate that type
S units have contraction times that are greater than
100 msec and produce peak twitch tensions of less
than 10 g.24 Type FF units contract within 80 msec
and produce a wide range of peak tensions (from 2 to
over 100 g).
Thus, the available evidence indicates that a phys­
iological classification of all mammalian motor units
according to the mechanical properties of their asso­
ciated muscle units is likely to continue to be useful.
"Normal" human motor units appear to be organized
in a manner similar to that of other mammals, but at
the present time, reliable data on the physiological
properties of human motor units in health and disease
are far less abundant than those derived from animal
studies.
ANATOMICAL PROPERTIES OF
MUSCLE UNITS
Animal Studies
A major breakthrough in our understanding of the
anatomical properties of muscle units came with the
development of a means of visualizing the muscle
fibers of a single muscle unit. This method, known as
the glycogen depletion technique, was developed in
several labs28"30 but has been most effectively used by
Burke et al2, 8, 31 and Kugelberg et al.22, 32 It involves
repeated activation of a single motor unit—from an
intracellular penetration of a single motor neuron,8
from a dissected ventral root filament,12 or from
intramuscular microstimulation24—thereby causing
the muscle fibers innervated to use their stored gly­
cogen. If stimulation is continued according to the
appropriate conditions, the innervated muscle fibers
will be depleted of their glycogen and later can be
1766
Axonal Conduction
Velocity (m/sec)
visualized using a histochemical stain for glycogen, a
process much like the periodic acid-Schiff (PAS)
reaction.8, 33 Innervated fibers belonging to the muscle
unit will be seen as lightly stained or unstained fibers
against a background of fibers that are intensely
stained for glycogen and, therefore, belong to other
muscle units. An example of the results of such a
glycogen depletion experiment is shown in Figure 2.
Use of the glycogen depletion method has led to
the identification of two major principles in the ana­
tomical organization of single muscle units. The first
principle is that the muscle fibers composing a single
unit are not grouped, as some previous physiological
studies have suggested. The muscle fibers of a single
muscle unit are considerably scattered (Fig. 2), and
the probability of even two or three fibers lying
exactly adjacent to each other is low.34 That the
distribution of muscle fibers in a single muscle unit is
scattered also means that any given volume of muscle
is occupied by several muscle fibers belonging to
muscle units whose "territories" overlap.
The second major principle elucidated by the gly­
cogen depletion method is that all of the muscle fibers
of a muscle unit have the same histochemical com­
position. By reaction of histological sections adjacent
to those used for the PAS reaction for different en­
zymes in the metabolic pathways used by skeletal
muscle fibers, it was shown that muscle fibers belong­
ing to a single muscle unit contain a uniform histo­
chemical composition.8, 32 It was shown further that
these histochemical profiles correlate well with the
physiological type of the muscle unit. Fast-twitch
units (types FF and FR) innervate muscle fibers that
react strongly to alkaline-stable myosin adenosinetriphosphatase (ATPase), and fatigue-resistant units
(types FR and S) innervate fibers that react strongly
to oxidative enzymes. Type Fint units are presumed
to contain muscle fibers that are histochemically be­
tween those of type FF and type FR units. The
myosin ATPase reaction, if carried out at an optimal
acid pH,35, 36 may also be used to differentiate between
PHYSICAL THERAPY
the three fiber types: type I fiber—type S unit; type
IIb fiber—type FF unit; and type IIa fiber—type FR
(and Fint) unit (Tab. 1). In a recent series of excellent
studies, Nemeth and colleagues confirmed those earlier studies, and by using quantitative biochemical
techniques for assaying a number of different enzymes in type-identified units, they have shown that
the muscle fibers of each muscle unit may contain a
unique composition of metabolic enzymes.37, 38
Like the physiological organization of muscle units
described above, these two principles of the anatomical organization of muscle units have been shown to
be generally accepted. Studies of various muscles in
various animals have all shown that the muscle fibers
in a single muscle unit are spatially separate from one
another, forming a "mosaic" pattern, and that they
are all histochemically distinct.
In addition to these two major anatomical organizing principles, a third principle is emerging of the
structural organization of muscle units. Observations
of the patterns of glycogen depleted fibers in published records first led Letbetter to postulate that the
muscle fibers of individual muscle units are distributed in a "mosaic," but not throughout the entire
muscle.39 Instead, they are organized into different
regions, or compartments, within the muscle. Each
compartment is supplied by a primary branch of the
muscle nerve and contains a unique population of
muscle units. Studies of the histochemical composition of different muscles also indicated that the fibertype composition (and thus motor-unit type) is not
uniform but differs in different parts of the same
muscle, or is "compartmentalized."40"44 Detailed studies of the anatomy, the innervation patterns, and the
histochemical profile of a single lateral gastrocnemius
(LG) muscle of a cat have also described the organi-
zation of muscle units into compartments.8, 33, 36 In
these studies it was also postulated that the organization of muscle units into compartments provides an
explanation for the histochemical "compartmentalization" of muscles observed by others.
All of these studies have emphasized the need for
careful consideration of the architecture of muscles.
In the LG muscle of a cat, for example, as in nearly
all muscles, the muscle fibers course between aponeurotic surfaces of origin and insertion.45 In the case of
the LG muscle, the aponeurosis of insertion forms the
tendocalcaneus (Fig. 3). The muscle fibers belonging
to different muscle units are organized into individual
compartments within this scheme, and each compartment is supplied by a primary branch of the whole
muscle nerve. The compartments lie in parallel to
each other so that the muscle units in different compartments produce the same mechanical effects. A
similar anatomical organization of muscles into compartments has been demonstrated in cat hamstring
muscles46 and in the calf muscles of squirrel monkeys.47 A compartmentalization of the human LG
muscle much like that in other animals is suggested
by its nerve branching pattern (Gotto and Wolf, 1981,
unpublished observations). Thus, the organizational
principle that is emerging is that whole muscles are
subdivided into smaller compartments in parallel arrangement in the muscle. Each compartment is a
collection of single motor units and is supplied by a
primary branch of the muscle nerve.
Human Studies
Use of the glycogen depletion technique has resulted in extending some of the observations on muscle unit anatomy made in animal studies to human
Fig. 2. Results of glycogen depletion of a single muscle unit in cat lateral gastrocnemius muscle.18 A. A photomicrograph of a muscle cross section stained with the PAS reaction, which shows the unstained, depleted muscle fibers of
a single, type FF muscle unit. B. A tracing of an entire cross section of the muscle that maps the distribution of
depleted fibers.
Volume 62 / Number 12, December
1982
1767
Fig. 3. The distribution of fibers in a single, type FF muscle unit from cat lateral gastrocnemius muscle is shown
relative to the distribution of fibers innervated by different primary branches of the muscle nerve. The series of tracings
at the top of the figure shows the distribution of muscle fibers in the unit. The series of tracings at the bottom of the
figure shows the distribution of fibers supplied by muscle nerve branches, as indicated in the nerve branch diagram
to the right. (S—soleus; P—plantaris; MG—medial gastrocnemius.) In the center is a diagram of a dorsal view of the
left triceps surae complex (knee to the right, ankle to the left) that serves as a reference to the locations of the cross
sections. Note that the muscle fibers of this motor unit are distributed throughout and restricted to the territory
supplied by the lateral gastrocnemius branch of the muscle nerve. All data are adapted from English and Weeks18 and
English and Letbetter.33, 36
studies. For example, muscle fibers of individual human muscle units have been shown to be spatially
separated into a mosaic and to be of the same histochemical type.24 Also, motor units identified by physiological criteria have been shown to contain muscle
fibers with a correlated histochemical profile. Thus,
it is thought that human type S motor units contain
type I musclefibers;that type FF motor units contain
type IIb musclefibers;and, although no direct evi1768
dence is available, that type FR (and, presumably,
type Fint) units contain type IIa muscle fibers.
The anatomical disposition of muscle units with
respect to the mechanics of muscles has been studied
more in cats than in humans. This has been the case
mainly because of the inherent limitations in studying
human material with biopsy techniques and, possibly,
because of a diminished interest in the use of the
microstimulation technique initially developed by
PHYSICAL THERAPY
Taylor and Stephens27 for functional isolation of single muscle units. This technique is postulated to
stimulate the axon of a single motor unit and cause
orthodromic invasion of all of its terminal branches.
As a technique it is not without problems, as has been
pointed out in this article and by Garnett and colleagues.24 However, if some of these problems can be
addressed using refined electrode designs in conjunction with signal processing techniques, the study of
the spatial distribution of human muscle units might
be advanced. Of special interest might be whether
both large and small human motor units are organized into compartments in a manner similar to that
described from animal studies.
FUTURE PERSPECTIVES
Histochemical Analyses
Within the past decade considerable efforts have
been directed toward ascertaining the histochemical,
and therefore the muscle unit composition, of many
human muscles. For example, it has been clearly
demonstrated that most human muscles display a
wide variation in type I and II muscle fiber composition.48"50 Histochemical profiles of men and women
are similar, with men showing generally larger fiber
diameters. Female athletes have a higher percentage
than their nonathletic counterparts of slow-twitch
oxidative fibers.51 Furthermore, glycogen content is
similar in slow-twitch and fast-twitch muscle at rest,52
and myosin ATPase activity from human muscles of
inactive and active subjects53 is similar. Age may be
associated with increased type I fiber composition,
but this change is not associated with decreased muscle enzymatic activity.54
Most histochemical analyses, however, have centered around changes in fiber-type composition resulting from various forms of exercise. These studies
generally indicate that intense physical training can
increase the aerobic capacity and composition of
human muscle.55"60 Data such as these have created
great enthusiasm among clinicians and athletes concerned with improved performance but also can be
criticized on several grounds. The duration of such
enzymatic and fiber-type "conversions" following
cessation of consistent, repetitive training is probably
limited, so that the long-range significance of such
aerobic and composition changes has not been clarified. The magnitude of the changes, though often
statistically significant, has not been considered from
the point of view of muscle function and muscle unit
recruitment, and therefore the changes may be of
questionable functional significance. Another criticism relates to the location from which biopsy samples
have been taken. Generally, the samples have been
limited to a specific site within the vastus lateralis or
Volume 62 / Number 12, December 1982
gastrocnemius muscles. Recent evidence indicates
that repeated biopsies from the same area of muscle
will display considerable variability in histochemical
profiles, even under sedentary conditions.61, 62
Whether the compartmentalization of muscle units is
indicative of a central organizing principle influencing motor control is not yet apparent. However, the
implications of the anatomical disposition of muscle
units into compartments may bear strongly on muscle
structure and function, from both clinical and basic
points of view. If histochemical analyses of human
muscle can provide significant and meaningful information, the importance of muscle architecture and
muscle unit anatomy must be recognized.
Clinical interest in histochemical analyses of muscle has been extensive, but elementary questions that
govern basic concepts have not been investigated.
Questions now being posed by basic scientists but
receiving virtually no attention from clinical investigators include: Do the histochemical and physiological properties of muscle units demonstrate differing
organization principles in muscles with vastly different fiber architectures? Is the motor unit composition
at the proximal location of a two-joint muscle different from the composition at the distal location? Does
a unique relationship exist between motor-unit type
and muscle-spindle density within that motor unit
territory? Answers to these basic questions could have
considerable impact on therapeutic interventions. The
manner in which therapeutic exercise of a muscle is
conducted might be more justifiably ascertained if,
for example, it is shown that in humans, as in cats,36
the knee flexor component of the gastrocnemius muscle is heavily invested with fast-twitch motor units
(type II fibers) and muscle units attaching to the
aponeurosis contiguous with the tendocalcaneus (the
distal part of the muscle) show a preponderance of
slow-twitch (type I fibers).
Motor Unit Recruitment
From both basic and clinical perspectives uncertainty exists about how motor units are best used
during volitional tasks. It is generally believed that
motor units are recruited according to the size principle proposed by Henneman and co-workers63; that
is, during a progressive contraction, tonic or type S
motor units are activated before phasic or fast-twitch
motor units. From recordings using percutaneous
electrodes in humans, phasic motor units appear to
have larger amplitudes and greater discharge frequencies than their tonic counterparts. That they are generally recruited after tonic motor units has been
clearly documented,64-69 and this recruitment order
persists during isotonic as well as isometric contractions70 and maintained voluntary contractions.71
A reversal of recruitment order, with phasic motor
units activated prior to tonic motor units, has also
1769
been demonstrated. Smith et al have shown that fasttwitch muscle units are activated preferentially over
slow-twitch units during rapid paw movements in
cats.72 Other study results showing the use of cat LG
muscle units from different compartments during posture and locomotion suggest a certain amount of
independent control of some fast-twitch units.73
Grimby and Hannerz have observed "reversals"
among individuals with spinal lesions.74 Recruitment
order can also be reversed by temporary disruption
of sensory input,75 cutaneous electrical stimulation,76
or using a muscle as a synergist rather than as a prime
mover.77 During prolonged voluntary contraction,
tonic motor units are recruited at a constant or reduced level of tension and phasic motor units are
recruited at increasing levels of tension. Under this
condition, phasic motor units are often observed to
"drop out." If, however, a rapidly reversed voluntary
contraction is made during a maintained contraction,
the recruitment order can be reversed.78 Although
considerable practice is necessary to achieve this reversal under the condition of a rapidly reversed voluntary movement, the occurrence implies that a central programming of motor outflow may also be an
important factor in motor unit recruitment. To date,
systematic study of alternative recruitment patterns
has not been undertaken, and the clinical significance
of this phenomenon has yet to be elucidated.
Most analyses of single motor unit behavior have
been undertaken with "normal" humans. Although
aberrant patterns of motor unit discharge rates have
been noted among patients with spasticity,78 tremor,79
and Huntington's chorea,80 the clinical importance
and the implications of these observations for testing
efficacy of pharmacological or physical therapies are
not clear. In neurogenic or myogenic pathological
conditions, among which a reduction of total motor
units or an increase in muscle fiber density of remaining motor units can be observed, it becomes difficult
to study features of high threshold (phasic) motor
units because normal values for their action potential
characteristics are lacking.
Debate continues about the relationship between
motor unit recruitment and purposeful movement.
Muscle tension can increase through two primary
mechanisms: recruitment of additional motor units
1770
and increase in discharge frequency (rate coding).
During weak to moderate tonic voluntary contraction,
most participating motor units discharge at about the
same rate. Differences in discharge frequency appear
only during a phasic contraction or with a strong and
sustained contraction. Although evidence for recruitment at both moderate69 and high tensions68 has been
presented, it appears that recruitment at high contraction levels occurs primarily when subjects are not
fatigued.
The picture has been further complicated by recent
observations that recruitment or rate coding may be
a function of the muscle under examination.81, 82 In
humans, for example, the small and predominantly
slow-twitch adductor pollicis muscle is incapable of
recruiting additional motor units above 50 percent of
maximal voluntary contraction, so rate coding must
account for force modulation at higher tensions. On
the other hand, the large, histochemically mixed biceps brachii muscle is capable of recruiting additional
motor units up to 88 percent of maximal voluntary
contraction and thus generates considerable force
through recruitment rather than rate coding mechanisms. Recent animal studies have shown that considerable rate coding occurs during locomotion in both
histochemically mixed muscles, such as vastus lateralis and rectus femoris, and in muscles containing
predominantly slow-twitch muscle units, such as vastus intermedins.83 Data such as these indicate that our
knowledge of motor unit contributions to muscle
tension is still in its infancy, and the relevance of rate
coding and recruitment to motor control has yet to be
understood.
SUMMARY
The basic principles in the anatomical and physiological organization of mammalian muscle units that
have been developed from animal studies have broadranging applicability both to further animal studies
and to studies of motor units in humans. A number
of gaps in our knowledge of muscle unit organization
have been identified, especially among human studies, but the potential to fill many of these gaps rapidly
is great.
PHYSICAL THERAPY
REFERENCES
1. Sherrington CG: Flexion-reflex of the limb, crossed extension
reflex and reflex stepping and standing. J Physiol (Lond)
40:28-121, 1910
2. Burke RE, Tsairis P: Anatomy and innervation ratios in motor
units of cat gastrocnemius. J Physiol (Lond) 234:749-765,
1973
3. Burke RE: Motor unit recruitment: What are the critical factors? Progress in Clinical Neurophysiology 9:61-84, 1981
4. Henneman E: Recruitment of motoneurons: The size principle. Progress in Clinical Neurophysiology 9:26-60, 1981
5. Stein RB, Bertold I: The size principle: A synthesis of neurophysiologies data. Progress in Clinical Neurophysiology
9:85-96, 1981
6. Burke RE, Edgerton VR: Motor unit properties and selective
involvement in movement. Exerc Sport Sci Rev 3:31-81,
1975
7. Burke RE: Motor unit types of cat triceps surae muscle. J
Physiol (Lond) 193:141-160, 1967
8. Burke RE, Levine DN, Tsairis P, et al: Physiological types
and histochemical profiles in motor units of the cat gastrocnemius. J Physiol (Lond) 234:723-748, 1973
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