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Chapter 2 Applied Anatomy of the Spine 53
3. Grunhagen T, Shirazi-Adl A, Fairbank JC, Urban JP. Intervertebral
disk nutrition: a review of factors inuencing concentrations of
nutrients and metabolites. Orthop Clin North Am.
2011;42(4):465-477, vii.
4.
Ibrahim DA, Myung KS, Skaggs DL. Ten percent of patients with
adolescent idiopathic scoliosis have variations in the number
of thoracic or lumbar vertebrae. J Bone Joint Surg Am.
2013;95(9):828-833.
5.
Lehman RA Jr, Kang DG, Lenke LG, Gaume RE, Paik H. The
ventral lamina and superior facet rule: a morphometric analysis
for an ideal thoracic pedicle screw starting point. Spine J.
2014;14(1):137-144.
6.
Magro E, Gentric JC, Talagas M, et al. Venous organization in
the transverse foramen: dissection, histology, and magnetic
resonance imaging. J Neurosurg. 2015;123(1):118-125.
7.
Osmotherly PG, Rivett DA, Mercer SR. Revisiting the clinical
anatomy of the alar ligaments. Eur Spine J. 2013;22(1):60-64.
8.
Trimba R, Spivak JM, Bendo JA. Conjoined nerve roots of the
lumbar spine. Spine J. 2012;12(6):515-524.
9.
Tubbs RS, Wellons JC 3rd, Banks J, Blount JP, Oakes WJ.
Quantitative anatomy of the transverse ligament tubercles.
J Neurosurg. 2002;97(3 suppl):343-345.
10.
Zhong W, Driscoll SJ, Wu M, et al. In vivo morphological
features of human lumbar discs. Medicine (Baltimore).
2014;93(28):e333.
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AMATPAMADP PForce
++ →+ +++
Skeletal Muscle: Architectural Design,
SECTION
3
CHAPTER
Spinal muscles generate movement of the spine and provide
the stability needed to protect vital anatomic structures, in
coordination with the rest of the neuromusculoskeletal system
(vertebrae, tendons, ligaments, and the nervous system). For
example, large movements of the head require appropriate
muscle strength, vertebral geometry (e.g., facet joint orientation), ligament compliance, and neural control. Spinal muscles
have been described as one of three subsystems (along with
passive spinal column and neural control) that must work
together to stabilize the spine.
Spinal musculature dysfunction is hypothesized to be the
cause of a variety of pathologic conditions, such as segmental
instability, low back or neck pain, and degenerative disc syndromes. However, the mechanisms that relate muscle function
(or dysfunction) to pathologic processes are unclear. Some
factors that lead to pathologic processes may be elucidated by
biomechanical analyses of spine kinematics along with the
associated tissue strains and loads. Such analyses rely heavily
on accurate knowledge of muscle forces, moment arms, and
activation patterns to calculate loads and displacements,
which is frequently unavailable for spinal muscles. Oen,
spinal muscles are ignored or overly simplied (e.g., modeled
together as one muscle) because the anatomy of these muscles
is considered too complex to represent realistically. However,
the complex anatomy and architecture of spinal muscles
profoundly inuence their function; thus, this information
must be incorporated into analyses in order to accurately
predict the role of these muscles in spinal function and
dysfunction.
In this chapter, an overview of skeletal muscle contractile
properties is presented rst. en, a description of the important, and oen neglected, principles of skeletal muscle architecture along with the ways that architecture determines
muscle function are provided. In Chapter 4, specic information about the anatomy and architecture of the spinal musculature is provided, as well as information on the implications
of spinal muscle anatomy and architecture for motor control,
injury, and pain.
1
Physiology, and Function
Samuel R. Ward
Anita Vasavada
Scott Delp
Richard L. Lieber
Cross-Bridge Cycle
e basic force-generating event in skeletal muscle is the
cyclic formation of cross-bridges between the lamentous
proteins actin and myosin. Most of our understanding of the
mechanism of muscle contraction has come from excellent
biochemical studies performed in the 1950s and the decades
that followed.
isolating specic muscle proteins were developed, together
with methods for measuring their physicochemical and biochemical properties. In simple terms, biochemical experiments
on muscle contractile proteins have shown that, during the
cross-bridge cycle, actin (A) combines with myosin (M) and
adenosine triphosphate (ATP) to produce force, adenosine
diphosphate (ADP), and inorganic phosphate (Pi), which can
be represented as a chemical reaction in this form:
It is obvious that if ATP is either not present or is rapidly
depleted with the muscle cell, the number of A-M crossbridges, and therefore force, will decrease. is is manifested
in muscles as “fatigue.” Importantly, many factors can cause
ATP depletion and fatigue, which may predispose muscle
tissue to injury.
Muscle Fiber Types
Overwhelming evidence indicates that skeletal muscle bers
are heterogeneous. In the early 1800s, it was observed that the
gross appearance of dierent skeletal muscles ranged in color
from pale white to deep red. In fact, one of the earliest classication schemes for muscle was based on color; thus, muscles
were classied as “red” or “white” (Table 3.1). However, as
experimental methods became more sophisticated, it became
clear that numerous other dierences existed between muscles.
For example, certain muscles contract rapidly, whereas others
contract more slowly; certain muscles maintain force for a
2-4
It was during this period that methods for
i
(Eq. 1)
I
57
58 BASIC SCIENCE
TABLE 3.1 Fiber Type Classication Schemes
Basis for Scheme FIBER TYPE SPECTRUM Reference
Metabolic SO FOG FG Peter et al., 1972
Morphology and physiology Slow red Fast white Fast white Ranvier, 1873
Z-line width Red Intermediate White Gauthier, 1969
Histochemistry III II I Romanul, 1964
Histochemistry Type 1 Type 2A Type 2B Brooke and Kaiser, 1972
M-band bridging pattern Five bridges Three inner, two faint outer bridges Three bridges Sjöström et al. 1982
Immunohistochemistry Type 1 Types 2A and 2X Type 2B Schiano et al., 1989
Gene sequencing Type 1 Types 2A and 2X Type 2B Schiano and Reggiani,
FG, fast glycolytic; FOG, fast oxidative glycolytic; SO, slow oxidative.
1993
46
39
40
41
42
43
44
45
long period of time, whereas others fatigue rapidly; and certain
muscles generate large forces, while others generate small
forces. ese functional properties led to descriptions of
muscle as “fast twitch” or “slow twitch,” and “fatigable” or
“nonfatigable.” In addition, with the advent of light microscopy
and histochemistry, it became possible to classify individual
bers based on their appearance following a particular staining protocol. Unfortunately, many of these classication
schemes did not correlate simply with muscle color. In fact,
many of them did not correlate at all with one another. e
main problem with ber-type classication schemes is that the
classied feature may correlate with one physiologic or biochemical property, but may have no relationship to others.
e current view of muscle ber types is that skeletalmuscle bers possess a wide spectrum of morphologic, contractile, and metabolic properties.5 e appropriate view of
any classication scheme, therefore, is that it is an articial
system superimposed on a continuum of properties for our
convenience. Probably the most useful “ber-typing” scheme
is one that describes contractile speed and muscle oxidative
capacity. ere are a variety of ways to do this; several schemes
are listed in Table 3.1. e most denitive method for typing
human skeletal muscle (not necessary the most physiologic,
but very consistent among labs) is to use the type of myosin
heavy chain expressed in the bers. One of the most convenient methods to type a muscle based on myosin heavy chains
is to homogenize large-muscle biopsies and separate the
myosin isoforms on protein gels.6 Using this method, it is
possible to identify the three ber types in human muscle:
types 1, 2A, and 2X (see Table 3.1). Although there are four
human myosin heavy chain gene—type 1, type 2A, type 2X,
and type 2B—only the former three are expressed.7 is
strange phenomenon can make it dicult to compare animal
muscle studies with human muscle studies.
Fiber Type Distribution of Paraspinal Muscles
Since ber types develop in response to functional demands,
the ber type of paraspinal and lumbar extensor muscles
provides insight into their function and use patterns. It has
been clearly shown that the paraspinal and lumbar extensor
muscles are largely composed of type 1 and type 2A bers.
us, these muscles are predicted to be relatively slow contracting and fatigue resistant compared to most limb muscles.
In fact, recent analysis of paraspinal muscles showed that
multidus, longissimus, and iliocostalis muscles comprised
more than 60% type 1 muscle bers8; this proportion is greater
than many limb muscles, such as exor digitorum profundus
(35.5 ± 4.8%), triceps (42.4 ± 3.6%), brachioradialis (45.5 ±
6.4), or vastus lateralis (48.3 ± 9.5%), but approaching the
values of other tonic, postural muscles such as soleus (88%).9
Fiber-type distributions of dierent spinal muscles are
described further in Chapter 4.
Muscle Injury
Injury to muscle bers can occur as a result of trauma, disease,
application of myotoxic agents (e.g., local anesthetics), inammatory processes, or intense exercise. e degree to which
muscle injury relates to low back pain or neck pain is unknown.
However, muscle injury, and the pain that accompanies it,
have been studied extensively. Skeletal-muscle injury and
soreness frequently occur when a muscle is rapidly lengthened
while it is activated. Active lengthening of muscle (also called
eccentric contraction) has been used to study injuries in
animals and humans for more than 30 years. Muscle pain
accompanying eccentric exercise peaks 24 to 48 hours aer
the exercise bout. Several studies have reported that eccentric
exercise results in a signicant increase in serum creatine
kinase levels 24 to 48 hours aer the exercise bout
persist for 3 to 6 days, depending on the precise nature of the
exercise. e appearance of creatine kinase in the serum is
interpreted as a result of increased permeability, or breakdown
of the muscle cell membrane surrounding the muscle cell.
Eccentric training of a specic muscle group prevents,
or at least attenuates, the magnitude of muscle injury that
occurs aer eccentric exercise in that muscle group. However,
general increased tness does not prevent or reduce eccentric
contraction-induced muscle injury.
Experimental studies of skeletal muscles directly subjected to eccentric exercise suggest that early mechanical
events result in muscle injury.
12,13
For example, during cyclic
eccentric exercise of the rabbit tibialis anterior, signicant
10,11
that may
Chapter 3 Skeletal Muscle: Architectural Design, Physiology, and Function 59
muscle
Gluteus
ABC
biomechanical changes were observed in the rst 5 to 7
minutes of exercise.14 Other studies have revealed structural
disruption of the cytoskeleton within the bers during these
early time periods,
15,16
which may provide further insights into
the damage mechanism.
Animal and human studies have provided evidence for
selective damage of fast-ber types aer eccentric exercise.
17-19
In human studies, this damage was generally conned to the
type 2 muscle bers; but in animal studies, damage has been
further localized to the fastest of the fast-ber subtypes.
Because these are also the most highly fatigable muscle bers,20
it has been speculated that the high degree of fatigability of
these bers may predispose them to injury. In fact, several
clinical studies have proposed that the fatigability of spinal
muscles may be a predisposing factor to injury. However, it is
dicult to test this idea directly because of the many other
dierences between these bers and other ber types. Further
studies are required to elucidate the basis for ber type–specic
injury to skeletal muscle and to document the relationship
between spinal muscle injury and back and neck pain.
Muscle Architecture
While muscle ber types are much more widely studied and
reported (probably due to the ease with which ber-type data
can be obtained), muscle functional properties are much more
highly determined by muscle architecture—the number and
arrangement of muscle bers relative to the axis of force
generation.
21–23
Whereas muscle ber size (which is directly
proportional to force generation) is relatively consistent
among muscles of dierent sizes, architectural dierences
between muscles are much more variable and much more
strongly aect function. us, muscle architecture is a primary
determinant of muscle function, and understanding this
structure–function relationship is of great practical importance not only in clarifying the physiologic basis for production of force and movement but also in providing a scientic
rationale for surgery or rehabilitation. Muscle architectural
studies also guide electrode placement for electromyographic
measures of muscle activity, explain the mechanical basis of
muscle injury during movement, and aid in the interpretation
of histologic specimens obtained from muscle biopsies.
Basic Architectural Denitions
e various types of architectural arrangement are as numerous as the muscles themselves. For discussion purposes,
however, we present three general classes of muscle-ber
architecture. Muscles composed of bers that extend parallel
to the muscle’s force-generating axis are described as having
a parallel or longitudinal architecture (Fig. 3.1A). Muscles
with bers that are oriented at a single angle relative to the
force-generating axis are described as having unipennate
architecture (Fig. 3.1B). e angle between the ber and forcegenerating axis has been measured at resting length in dierent
mammalian muscles and varies from about 0 to 30 degrees.
It becomes obvious when performing muscle dissections that
most muscles fall into the third and most general category,
SECTION
I
medius
Vastus
lateralis
ML
Biceps
FL
ML
ML = FL
FIG. 3.1 Three general types of skeletal muscle architecture. (A) Longitudinal architecture in which muscle
bers run parallel to the muscle force-generating axis. A typical example is the biceps brachii. (B) Unipennate
architecture in which muscle bers run at a xed angle relative to the muscle’s force-generating axis. The vastus
lateralis muscle is shown. (C) Multipennate architecture in which muscle bers run at several angles relative to
the muscle’s force-generating axis. The gluteus medius muscle is shown. FL, ber length; ML, muscle length.
FL
60 BASIC SCIENCE
multipennate muscles—muscles constructed of bers that are
oriented at several dierent angles relative to the axis of force
generation (Fig. 3.1C). Obviously, these three designations
are oversimplied, but they provide a vocabulary with which
to describe muscle architecture. Because bers may not be
oriented along any of the classic anatomic axes, determination
of muscle architecture is impossible from a single biopsy or
even magnetic resonance imaging (MRI), computed tomography (CT), or ultrasonography, because these methods cannot
account for the variations in ber length and orientation
that occur along the muscle length. us, other methods,
which are described in some detail later, have been developed to characterize the architectural properties of skeletal
muscle.
Experimental Determination of Skeletal Muscle Architecture
Quantitative studies of muscle architecture were pioneered by
Gans and his colleagues,
ology for dening muscle architecture based on microdissection of whole muscles. e parameters usually included in an
architectural analysis are muscle length, ber or fascicle
length, pennation angle (i.e., the ber angle relative to the axis
of force generation), and physiologic cross-sectional area
(PCSA). Typically, muscles are chemically xed in formalin to
maintain ber integrity during dissection.
Pennation angle (θ) is measured by determining the average
angle of the bers relative to the axis of force generation.
Usually only the pennation angle of bers on the supercial
muscle surface is measured, although pennation angles may
vary from supercial to deep and also from proximal to
distal. In fact, variation in pennation from supercial to deep
bers has been documented in several spinal muscles (see
Chapter 4). Although more sophisticated methods could be
developed for measurement of pennation angle (e.g., diusion
tensor imaging), it is doubtful they would provide a great
deal more insight into muscle function because variations in
pennation angle do not dramatically aect function.
Muscle length is dened as “the distance from the origin of
the most proximal muscle bers to the insertion of the most
distal bers.”25 Fiber length represents the number of sarcomeres in series, and experimental evidence suggests that
muscle ber length is proportional to ber contraction veloc-
24,26
it y.
Muscle length and ber length are not always the same
because there is a variable degree of “stagger” seen in muscle
bers as they arise from and insert onto tendon plates (see Fig.
3.1). Muscle ber length can only be determined by microdis-
section of individual bers from xed tissues or by laborious
identication of bers by glycogen depletion followed by serial
sections along the length of the muscle.27 Unless investigators
are explicit when they refer to muscle ber length, they are
probably referring to muscle ber bundle length (also known
as fascicle length) because it is extremely dicult to isolate
intact bers that run from origin to insertion, especially in
mammalian tissue.
muscle suggest that individual muscle bers do not extend the
23,24
who developed a precise method-
23
27,28
Experimental studies of mammalian
entire muscle length and may not even extend the entire
length of a fascicle.
27,28
Detailed studies of individual muscle
ber lengths have not been conducted in human spinal
muscles, but studies in feline neck muscles illustrate that
muscle bers are oen arranged in series, ending in tendinous
inscriptions within the muscle or terminating intrafascicu-
29,30
larly.
Although the terms ber length and fascicle length
are oen used interchangeably, technically they are identical
only if muscle bers span the entire length of a fascicle. In
muscle architecture studies, bundles consisting of 5 to 50
bers are typically used to estimate ber length, which may be
reported as either ber length or fascicle length.
e nal and crucial experimental step required to perform
architectural analysis of a whole muscle is to measure the
sarcomere length within the isolated bers. is is necessary
to compensate for dierences in muscle length that occur
during xation. In other words, to conclude that a muscle has
“long bers” one must ensure that it was not merely xed in
highly stretched position corresponding to a long sarcomere
length. Similarly, muscles with “short bers” must be further
investigated to ensure that they were not simply xed at a short
sarcomere length. To permit such conclusions, ber length
measurements should be normalized to a constant sarcomere
length. Fiber (or fascicle) lengths are usually normalized to the
sarcomere length at which the sarcomere generates maximum
force. is normalized ber length is referred to as optimal
ber (or fascicle) length and provides a reference value that
can be related back to the physiologic length if the relationship
between muscle length and joint position is noted. en,
based on measured architectural parameters and joint properties, the relationship between sarcomere length and joint angle
can be calculated. As an alternative to measurements in cadavers, sarcomere lengths can be measured in live humans using
intraoperative laser diraction31 or less invasively with microendoscopy.
32,33
Because sarcomere length strongly inuences
muscle force generation, an understanding of the relationship
between sarcomere length change and movement has been
used in many studies to provide added understanding of
muscle design.
31,34–37
e physiologic cross-sectional area (PCSA) is the main
architectural calculation. eoretically, PCSA represents the
sum of the cross-sectional areas of all the muscle bers within
the muscle, and thus it is the only architectural parameter that
is directly proportional to the maximum tetanic tension generated by the muscle. e PCSA is almost never the same as
the cross-sectional area of the muscle measured in any of the
traditional anatomic planes, as would be obtained using a
noninvasive imaging method such as MRI, CT, or ultrasonography. It is calculated as muscle volume divided by ber length
and has units of area. Because bers may be oriented at a
pennation angle relative to the axis of force generation, not all
of the ber tensile force is transmitted to the tendons. Specically, if a muscle ber is pulling with X units of force at a
pennation angle θ relative to the muscle axis of force genera-
θ) will
actually be transmitted along the muscle axis. us, the
volume/length is oen multiplied by cosineθ (pennation
angle) to yield PCSA.
Chapter 3 Skeletal Muscle: Architectural Design, Physiology, and Function 61
B
40
Maximum tension (N)
120
40
Maximum tension (N)
B
540
Maximum tension (N)
120
540
Maximum tension (N)
Mechanical Properties of Muscles
With Dierent Architectures
As stated earlier, muscle force is proportional to PCSA and
muscle velocity is proportional to ber length. By stating that
velocity is proportional to ber length, it is implicit that the
total excursion (active range) of a muscle is also proportional
to ber length. us, it is important to understand how these
two architectural parameters, PCSA and ber length, aect
muscle function.
Comparison of Two Otherwise Identical
Muscles With Dierent PCSAs
Suppose that two muscles have identical ber lengths and
pennation angles but one muscle has twice the mass (equivalent to saying that one muscle has twice the number of bers,
thus twice the PCSA). For the sake of simplicity, suppose that
these two muscles have identical ber-type distributions and
that they generate the same force per unit area. e functional
dierence between these two muscles is shown in Fig. 3.2.
e muscle with twice the PCSA has an isometric length–
tension curve with the same shape, but it is amplied upward
by a factor of 2. us, the maximal tetanic tension (Po) of the
larger muscle will be twice that of the smaller muscle. Similarly, comparison of isotonic force–velocity curves indicates
that the dierences between muscles will simply be an upward
shi in Po for the larger muscle.
Comparison of Two Otherwise Identical
Muscles With Dierent Fiber Lengths
For two muscles with identical PCSAs and pennation angles
but dierent ber lengths, the schematic in Fig. 3.3 demonstrates that the eect of increased ber length is to increase
muscle excursion and velocity. Peak force of the length–tension
curves is identical between muscles, but the range of lengths
over which the muscle generates active force is dierent. For
the same reason that increased ber length increases the active
muscle range of the length–tension relationship, it also results
in an increase in the muscle’s maximum velocity (V
max
).
Interplay of Muscle Architecture and Moment Arm
In addition to its architecture, the moment generated by a
muscle is inuenced by its moment arm. Moment arm, the
SECTION
I
FIG. 3.2 Two muscles with dierent physiologic cross-sectional areas
(PCSAs) but identical mass. (A) Comparison of isometric length–tension
properties. (B) Comparison of isotonic force–velocity properties. The eect
of increased PCSA with identical ber length is to shift the absolute
length–tension and force–velocity curves to higher values, but with
retention of the same range and intrinsic shape.
100
80
60
40
20
0
05 10 15 20 25 30 35
A
120
100
80
60
40
20
0
05 10 15 20 25 30 35
Muscle length (mm)
Large PCSA
Small PCSA
Muscle velocity (mm/s)
Large PCSA
Small PCSA
100
80
60
40
20
0
510152025303
A
120
100
80
60
40
20
0
510152025303
FIG. 3.3 Two muscles with dierent ber lengths but identical physiologic
cross-sectional areas. (A) Comparison of isometric length–tension properties.
(B) Comparison of isotonic force–velocity properties. The eect of increased
ber length is to increase the absolute range of the length–tension curve
and absolute velocity of the force–velocity curve, but with retention of the
same peak force and intrinsic shape. The dotted vertical line in (B)
demonstrates that, for an equivalent absolute velocity, the muscle with
longer bers will generate a greater force.
Muscle length (mm)
Long fibers
Short fibers
Muscle velocity (mm/s)
Long fibers
Short fibers
62 BASIC SCIENCE
Full flexion
Full extension
C
“mechanical advantage” of a muscle, is the distance from a
muscle’s line of action to the joint axis of rotation and is
directly related to a muscle’s change in length with joint rotation.38 In other words, the amount of muscle ber length
change that occurs as a joint rotates—and, consequently, the
range of joint angles over which the muscle develops active
force—both depend on the muscle moment arm. is idea can
be explained by comparing the situation in which two muscles
with identical ber lengths have dierent moment arms at a
joint (Fig. 3.4). In the case in which the moment arm is greater,
muscle bers will change length much more for a given change
in joint angle compared with a muscle with a shorter moment
arm. As a result, the range of joint motion over which the
muscle develops active force will be smaller for the muscle
with the larger moment arm in spite of the fact that the muscular properties of both muscles are identical. e architectural
design of a muscle and its placement in relation to skeletal
geometry are both important determinants of its function.
Although it is true that muscles with longer bers can generate
force over a greater range of lengths than muscles with shorter
bers (e.g., Fig. 3.3A), this does not necessarily indicate that
muscles with longer bers are associated with joints that have
larger ranges of motion. Muscles that appear to be designed
for speed based on their long bers may not actually produce
large joint velocities if they are placed in the skeleton with a
very large moment arm, because joint excursion and joint
angular velocity are inversely related to moment arm. A large
moment arm results in a large joint moment, so that the
muscle would be highly suited for torque production but at
low angular velocities. Similarly, a muscle that appears to be
designed for force production due to a large PCSA, if placed
in a position with a very small moment arm, may actually
produce high joint excursions or angular velocities. Dierences between muscle–joint systems thus require complete
analysis of both joint and muscular properties. ese interrelated concepts of architecture and moment arm (gross
anatomy) must be considered when examining the design and
function of spinal muscles. is concept is fairly complex and
it takes practice to become facile with such functional anatomic descriptions.
Summary
In this chapter, the basic functional properties of muscle sarcomeres that generate force via the cross-bridge cycle were
reviewed. Muscle bers are heterogeneous, and descriptors of
human muscle typically refer to types 1, 2A, and 2X, relating
to the type of myosin heavy chain expressed in the bers.
Finally, the major anatomic property that predicts skeletalmuscle function is muscle architecture. is arrangement of
muscle bers and placement of the muscle in relation to
skeletal geometry determines the moments generated at all
joints—whether relatively simple, such as the elbow, or
complex, such as intervertebral movement. Application of
these concepts to the spine and spinal muscles is presented in
more detail in Chapter 4.
(θ = 40°)
(θ = 80°)
A
Full flexion
(θ = 50°)
Full extension
(θ = 75°)
B
1.0
0.5
Relative muscle force
0.0
30 40 50 60 70 80 90
Joint angle (degrees)
FIG. 3.4 Eect of changing moment arm on active range of motion (ROM).
In this example, a schematic muscle (shown as a sarcomere in series with
some tendon) is attached with two dierent moment arms. (A) 40 degrees
range of motion for “normal” muscle (from 40 degrees to 80 degrees).
(B) Moment arm increase results in a decrease in range of motion to 25
degrees muscle (from 50 degrees to 75 degrees). In (B), the active ROM is
smaller since the moment arm is greater; therefore, more sarcomere length
change occurs for a given angular rotation. (C) Comparison of force versus
joint angle (ROM) for muscles with short or long moment arms.
Long moment arm (B)
Short moment arm (A)
KEY REFERENCES
1. Burke RE, Levine DN, Tsairis P, Zajac FE. Physiological types and
histochemical proles in motor units of the cat gastrocnemius.
J Physiol. 1973;234:723-748.
This publication is the classic reference that describes the
relationship between the anatomic, physiologic, and biochemical
properties of the mammalian motor unit. This work helps to explain
the orderly recruitment of motor neurons during normal movement.
2.
Evans WJ, Meredith CN, Cannon JG, et al. Metabolic changes
following eccentric exercise in trained and untrained men.
J Appl Physiol. 1986;61:1864-1868.