Journal of 01
thopaedic Research
10.92C914 Raven Press, Ltd , New York
0 1992 Orthopaedrc Rebearch Society
Physiological Cross-Sectional Area of Human Leg Muscles
Based on Magnetic Resonance Imaging
T. Fukunaga, TR. R. Roy, $§F. G. Shellock, tJ. A. Hodgson, *M. K. Day, ‘IP. L. Lee,
IIH. Kwong-Fu, and *iV. R. Edgerton
Departtnenl of Sport F Sciences, University of Tokyo, Tokyo, Japan; *Department of Plzysiologicul Sciences, fBmin
Research Institute, and A!kpartment oj’ Radiological Sciences, University of Californici, Los Angeles; §Section of
Magnetic Resonance Imaging, Tow3erMusculoskeletul Imaging Center, Cedars-Sinai Medical Center, Los Angeles;
and ‘pet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.
Summary: Magnetic resonance imaging techniques were used to determine the
physiological cross-sectional areas (PCSAs) of the major muscles or muscle
groups of the lower leg. For 12 healthy subjects, the boundaries of each muscle
or muscle group were digitized from images taken at I-cm intervals along the
length of the leg. Muscle volumes were calculated from the summation of each
anatomical CSA (ACSA) and the distance between each section. Muscle length
was determined as the distance between the most proximal and distal images in
which the muscle was visible. The PCSA of each muscle was calculated as
muscle volume times the cosine of the angle of fiber pinnation divided by fiber
length, where published fiber 1ength:muscle length ratios were used to estimate
fiber lengths. The mean volumes of the major plantarflexors were 489,245, and
140 cm3 for the soleus and medial (MG) and lateral (LG) heads of the gastrocnemius. The mean PCSA of the soleus was 230 cm2, about three and eight
times larger than the MG (68 cm2)and LG (28 cm2), respectively. These PCSA
values were eight (soleus), four (MG), and three (LG) times larger than their
respective maximum ACSA. The major dorsiflexor, the tibialis anterior (TA),
had a muscle volume of 143 cm2, a PCSA of 19 cm?, and an ACSA of 9 cm2.
With the exception of the soleus, the mean fiber length of all subjects was
closely related to muscle volume across muscles. The soleus fibers were unusually short relative to the muscle volume, thus potentiating its force potential. Using the relationship between PCSA and fiber length to represent the
maximum force-velocity potential of a muscle and assuming a similar moment
arm, the soleus, MG, and LG would be expected to produce -71, 22, and 7%
of the force and 54, 30, and 16% of the power of the major plantarflexors.
These data illustrate some of the major limitations in the use of ACSA measurements to predict the functional properties of a muscle. Key Words: Magnetic r e s o n a n c e imaging-Muscle
volume-Muscle
architecturePhysiological cross-sectional area-Human.
The force-velocity characteristics of a muscle reflect its architectural design
- as well as the phvsio~logical properties within each of its sarcomeric
the maximum force exstructures. For
erted by a muscle is closely related to the total
Received April 13, 1990; accepted April 29. 1992.
Addre\\ correspondence and reprint requests to Dr. R. R. Roy
at Brain Research Institute, UCLA School of Medicine, Center
for the Health Sciences, 10833 Le Conte Avenue, Los Angeles,
CA 90024-1761, U . S . A .
Y26
92 7
ANATOMY OF H U M A N LEG MUSCLES
cross-sectional area of all fibers, and the maximum
rate of shortening is closely related to the length of
the longest fibers within the muscle (4,5,10,32,35).
These relationships have been verified in correlative studies involving the in situ testing of contractile properties in one hindlimb and the measurement
of the architectural properties in a contralateral
limb in a variety of laboratory animals (1,4,6,23.27,
32).
In human subjects, there has been a problem in
obtaining accurate and sufficient muscle architectural data. The data available are limited because
muscle volumes or masses are subject to morphological changes due to fixation and other treatment
artefacts ( 8 , l l ) . Also, assessments of size based on
a measure from a single cross-section of muscles
are known to be too inaccurate to be physiologically
meaningful. In addition, these materials usually
have been obtained from elderly subjects in which
muscle atrophy and myopathies are likely to have
been present before death (3,5.7,10,34).
Improved imaging techniques, such as magnetic
resonance imaging (18,24), computed tomography
(14,20,22.25,30), and ultrasound (17,37), have been
used to estimate the output potential of muscles or
muscle groups. However, most studies have relied
on a single cross-sectional image of a muscle group
as a measure of its functional potential rather than
multiple cross-sectional scans along the length of
the muscle or muscle group. Even in those studies
in which more than one scan were obtained (19,24),
muscle or muscle group volumes were not determined and the functional data were expressed relative to the area of a single cross-section. This approach has severe limitations (10).
Magnetic resonance imaging is considered to be
the most useful and safest noninvasive imaging device to estimate in vivo human muscle volumes
(31). Not only docs it have excellent resolving
power for differentiating muscle, fat, connective tissue? and bone without pain and other adverse biological effects (31), but multiple scans can be obtained without moving the subject, thus improving
the precision of the three-dimensional reconstruction of the data. In the present study, muscle volumes of human legs were estimated in vivo in
healthy subjects using magnetic resonance imaging.
Based on the assumption that the published data on
relative muscle fiber lengths (i.e.? fiber length to
muscle length ratios) from cadavers are representative of human subjects (5,11,34), the physiological
cross-sectional areas of individual leg muscles were
also calculated and compared with the volume determinations. Some preliminary results have been
published (9,12,28).
METHODS
Twelve healthy adults (11 men and one woman)
volunteered for the present study and followed human consent procedures of Cedars Sinai Medical
Center. The physical characteristics of the subjects
(mean ? SD) were as follows: age (32.6 k 8.2 years,
range 20-459, height (176.4 k 6.2 cm), and weight
(73.5 i 9.4 kg).
Magnetic resonance imaging was performed with
a 1.5-T164 MHz scanner (Signa MR Systems, General Electric Medical Systems, Milwaukee, WI,
U.S.A.) with a transmit and receive quadrature
body coil. T I-weighted spin-echo, axial-plane imaging was performed with the following variables: TR
600 ms; TE 20 ms; number of excitations. two; matrix 256 x 192; field of v ~ e w18 cm; slice thickness
10 mm; and interslice gap 0 mm. These variables
were selected to optimize image quality in order to
clearly delineate the border of each muscle and
bone and to identify fat or connective tissue. T1weighted images are typically used for the determination of anatomy, as well as to provide good soft
tissue contrast between fat/muscle interfaces. The
magnetic resonance scanner was checked for
proper spatial calibration (in Houndsfield units) every 8 h using a standardized imaging sequence and
a saline-filled plexiglass quality assurance phantom.
There were no significant deviations (<10%) from
expected test values.
The subjects were imaged in a prone position
with the ankle and knee held at -120 and 180", respectively, with 180" being full extension at each
joint. N o active muscle contraction was apparent
during measurement. Contiguous, axial 1-cm sections of the right lower leg were measured. The
number of sections obtained for each subject was
41-49 (Fig. 1). The outline of each muscle or muscle
group was digitized (Sun 31280 Taacl) and the anatomical cross-sectional area (ACSA) was determined by integrating pixels (each pixel = 0.41 mm'j
(Fig. 2). The muscles investigated were as follows.
Ankle plantarflexors included the medial gastrocnemius (MG). lateral gastrocnemius (LG), soleus,
flexor hallucis longus (FHL). tibialis posterior (TP),
and flexor digitorum longus (FDL); also included
were the major ankle dorsiflexor, the tibialis anterior (TA), and the popliteus (not present in Fig. 2).
The remaining musculature in the anterior (i.e., the
I Orthup Rrs, Vol. 10, IVO.6, I992
928
T . FUKUNAGA ET A L .
FIG. 1. Position of axial plane slices along the length of the
leg is shown for one subject. The total number of images per
subject ranged from 41 to 49 depending on the length of the
leg.
extensor digitorum longus, extensor hallucis longus, and the peroneus tertius) and lateral (i.e., the
peroneus longus and peroneus brevis) compartments were not easily separable in the images of
every subject and thus were outlined as a single
mass (EDL complex).
To determine the repeatability in measuring
ACSA, the legs of seven male subjects were studied
on two occasions over a period of several months
by the same observer (T.F.). The ACSA data for
the cross-section located 35% distal from the most
proximal end of the tibia for the two sessions arc
shown in Table 1. The correlation coefficient between the two measures for each muscle or muscle
group was significant and ranged from 0.92 for the
EDL complex to 0.99 €or the TA.
Muscle length was measured as thc distance between the most proximal and the most distal images
in which the muscle was visible. Muscle volume
was determined by summing the ACSA of each image times the thickness of each section. Muscle fiber length was calculated as muscle length times the
ratio of fiber length to muscle length reported by
Wickiewicz et al. (34) (‘Table 2). It should be noted
that the data for only two soleus muscles were included in the Wickiewicz et al. (34) article. These
data were supplemented for Ihe present study. In an
additional two cadavers, the mean fiber lengths (adjusted to a 2.2 pm sarcomere length) for the soleus
were 2.0 and 2.3 cm, values consistent with fiber
lengths of 1.9 and 2.0 reported by Wickiewicz et al.
(34). The physiological cross-sectional area (PCSA)
of each muscle was calculated as muscle volume
times the cosine of the angle of pinnation divided by
the mean fiber length. The pinnation angles used
were those reported by Wickiewicz et al. (34) (Table 2). The mean fiber lengths and angles of fiber
pinnation were taken from Wickiewicz et al. (34) to
ensure consistency in the techniques used for the
architectural determinations (.i.c., all data were derived from the same laboratory).
Pearson product correlation coefficients were
used to determine the relationships between pairs of
variables. A paired t test was used to determine
differences between repeat measures of ACSA. For
TABLE 1. Repeatability of measrrring muscle ACSA
rising magnetic resonance imaging techniyr~es
FIG. 2. An example of an image from a single axial plane
(35% distal from the most proximal end of the tibia) used to
identify individual muscles in the leg. The outline of the following muscles or muscle groups were digitized and outlined: MG, LG,soleus, FHL, TP, FDL, TA, and EDL, including
the extensor hallucis longus, peroneus longus, peroneus
brevis, and peroneus tertius. Note that the popliteus is absent
at this level.
J Orthop R e s . Vid. 10, N o . 6, 1992
Mude
Session I (cm2)
Session 2 (cm’)
Y
Soleus
MG
LG
FDL
T1’
TA
EDL complex
29.2 5 4.7
12.3 Ifr 2.2
8.6 2 1.5
1.8 f 0.4
5.5 2 1.1
10.3 2 1.7
9.8 i 2.1
29.6 t 4.5
12.9 i 2.1
9.2 t 1.5
1.9 f 0.4
5.4 i 1.4
10.0 -t 1.7
9.9 t 1.9
0.97
0.93
0.97
0.93
0.96
0.99
0.92
All correlation coefficients 0.) were significant (p < 0.05).
There were no significant differences between the mean values
between sessions for any muscle or muscle group complex (p >
0.05, paired t test).
ANATOMY OF HUMAN LEG MUSCLES
TABLE 2. Comparison of the unntomicul feature5
Muscle
Refeience
Soleus
34
II
26
7
2
34
26
11
MG
Muscle
weight (6)
215
390
264
22 1
LG
16
2
34
TP
FHL
FDL
TA
129
91
53.5 i 12.7
103.9
62
21.5 i 5.7
74.5
16.3 i 4.9
29.4
65.7 z 17.8
11
POP
26
7
2
34
26
of huinari leg muscles in cadavers, .from the literature
Muscle
length (cmj
Fiber
lenglh (cmj
FLIML
( x 10')
31.0 -+ 0.2
33.8 t 4.6
36.6
2.0 2 0.1
3.0 t 0.1
3.8
6 2 0
9 i 1
10
31.3
24.8 i- 1.8
22.3 -t 1.8
46.8
24.4 2 3.0
3.5 ? 0.4
3.9 2 0.4
5.7
5.2 2 0.1
14 2 2
182 I
12
21
21.7 2 1.9
21.8 i 1.8
46.7
21.3 f 1.7
24.5
5.1 rf- 1.0
6.1 i 2.3
6.1
6.3 -t 0.1
23 t 3
28 t 8
13
30
25.4 i 4.6
27.0 t 2.8
49.2
2.4 2 0.7
2.9 i 0.9
3.9
11 i 2
22.2 z 0.9
22.4 t 4.0
53.5
26.0 2 2.7
23.5 t 4.2
51.0
29.8 i 2.1
28.4 2 0.8
39.4
26.1
3.4 t 0.3
4.3 -t 1.0
4.8
2.7 2 0.1
3.8 i 1.3
4.1
7.7 -t 1.4
7.3 t 0.6
8.2
10.8 2 1.0
12.1
2.9 t 1.0
9.6
142
11
26
16
7
2
34
11
26
2
34
I1
26
34
11
26
34
154
110
20.1 i 3.4
29.4
92 Y
9z:!
8
1.5 k 2
192 I
9
11 t 1
16 i 3
8
26 2 3
26 i- 1
21
Angle of
pinnation (")
25
? 7
32
IS
19
20
17 2 8
6
15
18 i- 7
16
8 2 3
18
10
9 t 4
II
8
12 i- 3
19
20
20
10 2 5
19
20
7i-3
9
20
5 k n
12
10
PCSA
(cm')
58
122
187
_+
91
33.8 i 23.7
34.5
31.3 i 3.9
11.5 t 4.0
34.5
14.3 t 0.8
20.7 i 5.2
22.8 i. 5.5
50.9
5.27 i 1.00
13.75 i 6.8
26.5
5.07 i 1.20
6.13 2 0.38
12.9
9.87 2 2.51
12.7 i 5.9
39.5
7
27
&
80
6
8
0
0
7.90 = 1.98
5.51
Values are meclns t SD. The number of cadaver5 for each study wds three (34): two ( I 1); unknown, data derived from modeling using
a variety of sourccs (26); eight (16); three (7); and one (2)
all statistical comparisons, p < 0.05 was chosen as
the level of significance.
RESULTS
The mean ACSA (+SD) at each axial section of
the entire leg musculature and along the length of
each muscle o r muscle group in all subjects is
shown in Fig. 3. Several features were evident in
this illustration. The general shape of each muscle
or muscle group across subjccts was distinct. Differences in the location of the largest ACSA along
the length of the leg for individual muscles were
apparent, as are the differences in muscle volumes.
The fleshy portion of the gastrocnemius (both MG
and LG) muscle originated above the knee and terminated approximately midway between the knee
and ankle. Generally, the MG was slightly longer
than the LG. The most proximal position of the
soleus was about 10% of the length of the leg below
the knee and extended for almost the entire distance
of the leg. The largest ACSA of the soleus was located at the distal cnd of the LG and the most proximal end of the FHL. l n addition, the origin, termination, and peak ACSA of the soleus were coincident with those of its major antagonist, the TA. The
TP and FHL had the highest length to volume ratios
(i.e., long, narrow muscles). The FDL had the
smallest ACSA.
The mean anatomical features of each muscle and
the EDL complcx are listed in Table 3. Mean muscle lengths ranged from -19 (FDL) to 32 cm (soleus), except for the considerably shorter popliteus,
which was only 9 cm long. Mean fiber lengths were
more than twice as long for the major dorsiflexor
(i.e., the TA) than for the major plantarflexors (i.e.,
the MG, LG, and soleus), being -8 vs. 3.5 cm,
rebpectively . The soleus was the longest plantarflexor (32.4 cm) and had the shortest fibers (2 cm),
J Orthop Re5, L h l . 10, N u , 6 , 1992
930
T . FUKUNAGA ET AL.
r
l--.Umh
POP
!B I, >I
lSXl 2 n
21 24 *I
w 1,
.l__-__
PI
2s yl',>'lZ
,.
aY uI
xu40 4 x 4 2
uu
1>18.1
Distance (crn )
FIG. 3. Anatomical cross-sectional areas of all muscles combined (top) and of each muscle at each slice along the length
of the leg. Each bar represents the mean (+SD)for all subjects (n = 12). Slice 1 was identified for each subject by the
proximal edge of the patella.
thus having the smallest fiber length to muscle
length ratio.
The plantarflexors comprised 72% of the total
muscle volume of the leg (Table 3). Among the triceps surae, the soleus had the largest mean muscle
volume (-490 cm3), followed by the MG (-240
cm3) and LG (-140 cm3). Thus the soleus and the
gastrocnemius comprised 46 and 36% of the plantarflexor muscle volume, respectively. The TA volume was -10% of the total leg MV.
The soleus had the largest maximum ACSA (-30
cm2) of any muscle in the leg, and this value was
similar to the maximum ACSA of the entire gastrocnemius muscle, 16 and 11 cm2 for the MG and LG,
respectively. The TA had a maximum ACSA that
was about one third that of the soleus. The largest
total ACSA for the lower leg was 74 5 9 cm2, located -35% distally along the length of the leg
(Figs. 1 and 3) and was considerably smaller than
J Ortlzop R e s , Vol. 10, No. 6, 1992
the sum of the maximum ACSA of each muscle in
the leg, 93.7 cm2 (Table 3).
The largest PCSA was observed in the soleus
(-230 cm2)and this value was about three and eight
times larger than the PCSA of the MG and LG,
respectively. The PCSA of the triceps surae was
-326 5 44 cm2. The PCSA of the TA was 18.5 cm2,
that is, -8% of the soleus PCSA. The soleus, MG,
and LG accounted for -70, 20, and 10% of the total
PCSA of the triceps surae, respectively.
The PCSA ranged from two to eight times larger
than the maximum ACSA across all muscles (Table
3). The largest PCSAimaximum ACSA ratios were
observed in the soleus, TP, and FDL, ratios of -8,
7, and 6, respectively. These muscles had relatively
short mean fiber lengths of 2.0, 2.8, and 2.1 cm,
respectively. The TA had the longest mean fiber
length (-8 cm) and the lowest ratio (-2) of any
muscle studied.
Correlation coefficients for muscle length,
ACSA, and PCSA for the major plantarflexors and
dorsiflexor are listed in Table 4. Gcnerally, the MG
had anatomical measures that were highly correlated with its synergists and antagonists. In contrast. the LG was the muscle showing the lowest
relationships with other muscles. Significant correlations also were observed between the PCSA and
maximum ACSA of the MG (Y = 0.91), LG (Y =
0.67), soleus ( r - 0.91), and TA ( r = 0.97) (Table
3). Note that the LG had the lowest correlation coefficient among these muscles. It should also be reemphasized that the point along the proximodistal
axis at which the maximum ACSA occurred varied
considerably among the muscles (Fig. 3).
For each muscle studied, the relationship between fiber length, which is proportional to the
maximum velocity of shortening of a muscle (4,6,
13,16,32,36),and muscle volume is shown in Fig. 4.
With the exception of the soleus, there was a close
relationship between fiber length and muscle volume across muscles. In the soleus, an additional
strategy appears to be incorporated in its basic architectural design; compared with other leg muscles, the mean fiber length of the soleus was unusually short relative to muscle volume. thus enhancing
its force and power potential. A close relationship
between muscle volume and muscle length was also
evident for each muscle identified (Table 3).
Muscle power is defined as the product of force
and velocity. Because force is dependent on PCSA
and velocity on fiber length, thc power potential of
a muscle is related to the product of these two vari-
ANATOMY OF HUMAN LEG MIJSCLES
931
TABLE 3. Meun unutomical .features of human leg muscles, prewnt Atudy
Muscle
Soleus
MG
LG
TP
FHL
FDL
TA
POP
EDL complex
ML (cm)
FL (cm)
MV (ern')
Maximum
ACSA (cm2)
Mean
ACSA (cm')
PCSA
(ern')
PCSAi
maximum ACSA
32.4
(2.9)
23.9
(2.3)
21.4
(1.7)
30.4
(2.7)
24.9
(3.6)
19.2
(2.1)
29.9
(2.6)
9.3
(0.9)
-
2.0
(0.2)
3.4
(0.3)
5.0
(0.4)
2.8
(0.3)
3.8
(0.5)
2.1
(0.2)
7.7
(0.7)
2.5
(0.3)
-
489. I
(64.5)
243.7
(33.0)
140.8
(27.7)
104.2
(23.7)
74.0
(10.6)
18.7
(5.6)
142.5
(27.7)
21.3
(4.5)
233.4
(35.4)
29.97
(3.70)
16.49
(2.11)
11.24
(1.58)
5.40
(1.41)
4.85
(1.06)
1.59
(0.54)
9.54
( I .59)
4.43
(0.78)
10.4
15.23
(2.43)
10.20
(1.08)
6.53
(0.98)
3.43
(0.72)
3.00
(0.44)
0.93
(0.30)
4.73
(0.94)
2.28
(0.42)
6.W
(1.13)
230.02
( 36.69)
68.34
(7.26)
27.78
(4.16)
36.33
(7.72)
19.32
(2.82)
9.12
(2.77)
18.52
(3.66)
8.61
(1.58)
-
7.70
(0.52)
4.20
(0.23)
2.5,
(0.28)
6.90
(0.55)
4 10
(0.77)
5.80
(0.79)
1.90
10.10)
1.90
(0.20)
-
(1.9)
-
MV %
33.4
(1.7)
16.7
(1.6)
9.6
(1.4)
7.1
(1.1)
4.1
(0.5)
1.3
(0.3)
9.7
(1.1)
1.4
(0.2)
15.9
(1.1)
Values are means (SD). n = 12 subjects.
ML, muscle length; FL. fiber length; MV. muscle volume: maximum ACSA, maximum anatomical cross-sectional area; mean ACSA,
MVIMI,: PCSA, physiological cross-sectional area = MV X cos OIFL, where 0 1 7 the angle of fiber pinnation (29). MV (%), MV of each
muscleitotal muscle volume of the leg.
ables. The interrelationships among the PCSA, velocity [fiber length corrected for angle of pinnation
(adjusted fiber length)], and power (PCSA x adjusted fiber length) for the major plantarflexors and
dorsiflexor are shown in Fig. 5. For a comparison of
the relative velocity and power potential among
these muscles, all data shown in Fig. 5 are exTABLE 4. Correlation coqflicients of muscle length,
muscle volume, maximum ACSA, und PCSA among the
major plantarflexors and dorsiflrxor of the hrrman leg
LG
Muscie length
MG
LG
Soleus
TA
Muscle volume
MG
LG
Soleus
TA
Maximum ACSA
MG
LG
Soleus
TA
PCSA
MG
LG
Soleus
TA
Soleus
TA
0.70"
0.31
0.25
0.57"
0.53
-
0.58"
0.55
-
0.66"
0.36
0.79"
pressed relative to soleus values. The velocity potential of the TA was about four times that of the
soleus. In contrast, the power potential of the soleus was -2.5 times that of the TA. With respect to
both velocity and power, the MG and LG had intermediate values. It is interesting that the relationship between the adjusted fiber length and the
PCSA across muscles resembles the typical forcevelocity relationship observed within a muscle.
These data suggest design constraints among the
I
0.26
-
0.62"
-
" Significant correlation at p < 0.05.
0.63"
0.13
0.65"
0.38
-
0.65"
0.39
0.63"
0.62"
0.13
0.53
X X
10
100
bluscle Volume
1000
(an")
FIG. 4. Relationship between muscle fiber length (FL) and
muscle volume (MV) for all of the muscles studied. Data from
12 subjects are shown. Note that the ordinate and abscissa
are represented as logarithmic scales.
J Orthup Rr6, Vul. 10, Nu. 6, 1992
932
T . FUKUNAGA ET AL.
rFL
-G--
PCSAxrFL
500
400
-s
h
-I
Lt
300
200
100
04
0
20
40
60
80
100
120
PCSA ("10)
FIG. 5. Muscle velocity (adjusted fiber length, rFL) and
power potentials (the product of PCSA and rFL) relative to
the force potential (PCSA) for each muscle are shown. Adjusted fiber length (rFL) = FL x cos 0, where 0 is the pinnation angle (29).
combinations of options (i.e., fiber length and muscle volume) that may be used to define the functional characteristics of the musculature of the
lower leg. These data also illustrate that the power
potential of the musclcs studied is largely a function
of PCSA.
DISCUSSION
Numerous attempts have been made to obtain accurate information on the force and velocity potcntials of human skeletal muscle. To achieve this objective, one of the more essential but difficult parameters to obtain is an accurate measurement of
the mass of the muscle. In addition, the maximum
torque-velocity in the same subjects from which
moment arms are measured and fiber lengths estimated make it possible to estimate the output capability of muscles in subjects differing in size and in
the functional status of the neuromuscular system.
The present data are unique in that muscle volumes, lengths, and shapes were obtained from
healthy subjects. Although each of these parameters have been determined previously from one or
more muscles in a few cadavers, the samples were
usually taken from older individuals with unknown
health histories and after widely varying methods of
fixation (2,5,7,8,11,16,26,34). A variation in I'CSA
has been evident between individual cadaveric material within as well as across samples in these studies. Because muscle lengths and fiber lengths in
these same studies were similar, it would appear
that the differences in PCSA can be attributed to
variations in muscle mass. Based on the results of
J Orthop Res. Vul. 10, No. 6, I Y Y 2
the present study, magnetic resonance imaging appears to be a uscful procedure in determining muscle mass. Although fiber lengths must be determined indirectly when using magnetic resonance
imaging techniques. this seems feasible because the
ratios of fiber length to muscle length are relatively
consistent from individual to individual and across
studies [Table 2; also note that the FLiML ratios
from Pierrynowski (26) are consistently at the extreme of the range of values across studies]. Thus,
assuming an absence of complications related to
fluid shifts, it appears that the crucial morphological
parameters related to muscle function can be obtained using magnetic resonance imaging techniques.
The present data also demonstrate a limitation in
the physiological significance that can be derived
from a single cross-sectional area image from which
ACSA can be measured. For example, in some
muscles the ACSA and PCSA differ severalfold
(Table 3 ) . However, equally cignificant is the observation that the maximum ACSA of each muscle differs with respect to a given proximodistal location
along the leg (Fig. 3). Furthermore, when muscle
adapts in response to functional perturbations, the
shape of the muscle can change (21).
The PCSA, which conceptually is the crosssectional area of all the fibers at right angles to their
long axis, presumably represents the number of
half-sarcomeres in parallel and, consequently,
would be related directly to the amount of tension
that the muscle can produce (13,15,33). However,
because muscle fibers are inclined at an angle to the
tendon of insertion, at least in cadaver specimens,
only a resolved component of the tension may be
developed by fibers in line with the tendon. In such
a case, the PCSA should be multiplied by the cosine
of the angle between the fibers and the tendon. The
PCSA in the present study included this angle adjustment [using the data of Wickiewicz et al. (34),
see Table 21. On the other hand, the maximum
shortening velocity of a muscle fiber is related to
the number of sarcomeres in series and, therefore,
by the fiber length. In much the same way, the fiber
length should be considered as a component of the
velocity vectors relative to the tendon. The relation
between I'CSA and the adjusted fiber length (Fig. 4)
provides an estimate of the maximum forcevelocity potential of each muscle. The larger the
PCSA, the more the muscle is optimized for force
production, whereas the longer the fiber length. the
more the muqcle is designed for displacement or
ANATOMY OF HUMAN LEG MUSCLES
velocity of shortening (16,19,29,34,35). The soleus
seems to be designed for tension production at the
expense of velocity, whereas there seems to be a
relative force priority for MG and velocity priority
for LG. The longer mean fiber length and smaller
PCSA of the TA suggests a priority in design for
displacement or velocity at the expense of force
output.
The muscle power potential, expressed as the
product of PCSA and adjusted fiber length, related
linearly to PCSA (Fig. 5 ) , suggesting that the power
potential is largely a reflection of the force potential. The major plantarflexors had a power potential
that was about threefold higher than the major dorsiflexor. Obviously, these estimates of force, velocity, and powcr potentials of individual muscles and
groups of muscles can be combined with measures
of moment arms to predict joint kinetics under
known mechanical constraints.
The present data indicate that magnetic resonance imaging techniques can be used to determine
muscle volumes. When there techniques are combined with the architectural data on individual muscles and with the appropriate moment arms, it also
appears that under well-controlled experimental
conditions, reasonable estimates of the physiological properties of muscle groups in human subjects
can be made. The prcscnt data also clearly demonstrate the inability t o predict functional properties
of muscle based on ACSA.
Acknowledgment: We t h a n k t h e subjects who volunteered for t h e s e studies, a n d Dr. Martin Pfaff, Sharlene
L a u r e t z , and Heidi Bloom for their technical assistance.
This work was supported by National Aeronautics a n d
Space Administration G r a n t NAG 2-450 and National Institutes of Health G r a n t NS16333.
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