Architectural and functional features of human
triceps surae muscles during contraction
YASUO KAWAKAMI, YOSHIHO ICHINOSE, AND TETSUO FUKUNAGA
Department of Life Sciences (Sports Sciences), The University of Tokyo, Tokyo 153, Japan
pennate muscle; gastrocnemius and soleus muscles; lengthforce relationship; ultrasonography
in bundles (fascicles) that
extend from the proximal to distal tendons, comprising
a whole muscle. In many cases, when investigators
refer to muscle fiber length, they are actually referring
to fascicle length (4, 11, 12, 24, 25, 29), although some
muscle fibers have been shown to terminate midfascicularly (17, 31). Intrafascicle muscle fibers are, however,
serially connected to make one functional unit, which
has the same length as that of a fascicle (31). Changes
in fiber length by contraction are thus expressed as
fascicle length changes.
In pennate muscles, fascicles are arranged obliquely
with respect to the tendon, and this angulation (pennation angle) changes by contraction. The forces exerted
by muscle fibers are therefore modified at the fascicle
level to characterize the force-generating capabilities of
a muscle. Pennate muscles also have long tendons and
aponeuroses with substantial compliance (9), which
modulate the force-generating capabilities of a muscle
by causing changes in fascicle length as force is exerted
at a given joint angle. These factors make it difficult to
estimate muscle actions from sole observation of joint
performance (6).
Attempts have been made to determine the geometric arrangement of muscle fibers or fascicles (muscle
architecture) in humans, and many attemtps have been
based on measurements of cadaver specimens (3, 4, 11,
12, 29, 32). However, it has been shown in animals (9)
MUSCLE FIBERS ARE PACKED
398
as well as in humans (6) that muscle architecture
changes by contraction even in isometric actions. Therefore, available data on human muscle architecture
based on human cadaver specimens might not accurately represent the profile of actively contracting
muscles; consequently, there are particular advantages
in using noninvasive techniques to determine the muscle
architecture in living subjects.
The triceps surae muscles are the main synergists for
plantar flexion (6, 18), but they have different architectural properties, such as muscle length, fascicle length,
and pennation angles (3, 4, 32). In addition, the gastrocnemius muscles are two-joint muscles crossing both the
knee and ankle joints, whereas the soleus is a singlejoint plantar flexor. Consequently, the relationships
among joint angles (knee and ankle), muscle (fascicle)
lengths, and pennation angles are highly specific to
individual muscles. Information on muscle architecture related to joint positions is essential for the study
of muscle functions, but to date very few data are
available in humans in vivo (16). The purpose of the
present study is to quantitatively describe the relationships between joint angles and muscle architecture
(lengths and angles of fascicles) of human triceps surae
muscles in vivo in passive (relaxed) and active (contracting) conditions and to discuss their functional implications.
METHODS
Subjects. Six healthy men [age, 21–53 yr; height, 175 6 5
(SD) cm; and weight, 71 6 7 kg] participated as subjects. The
nature and possible consequences of the study were explained
to each subject before informed consent was obtained.
Joint position settings and torque measurement. Each
subject’s right foot was firmly attached to an electric dynamometer (Myoret, Asics), and the lower leg was fixed to a test
bench. The ankle joint was fixed at 15° dorsiflexion (215°)
and 0, 15, and 30° plantar flexion. The knee joint was
positioned at 0 (full extension), 45, and 90°. Thus the following measurements were performed in 12 conditions. In each
condition, the subject was asked to relax the plantar flexor
muscles (passive condition), and passive plantar flexion torque
was recorded from the output of the dynamometer by a
computer (PC-9801, NEC). The passive plantar flexion torques
were greater when the ankle joint was in a less flexed position
and the knee joint was in a less flexed position [0 (ankle, 30°;
knee; 90°) vs. 12 (ankle, 215°; knee, 0°) N · m; average of 6
subjects]. We assumed that there was no muscle activity in
the passive condition.
After performace in the passive condition, the subject was
encouraged to perform maximal voluntary isometric plantar
flexion (active condition), and torque output was recorded.
The passive plantar flexion torque was subtracted from
maximal voluntary plantar flexion torque to give active
torque produced by the muscles.
8750-7587/98 $5.00 Copyright r 1998 the American Physiological Society
http://www.jap.org
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Kawakami, Yasuo, Yoshiho Ichinose, and Tetsuo
Fukunaga. Architectural and functional features of human
triceps surae muscles during contraction. J. Appl. Physiol.
85(2): 398–404, 1998.—Architectural properties of the triceps
surae muscles were determined in vivo for six men. The ankle
was positioned at 15° dorsiflexion (215°) and 0, 15, and 30°
plantar flexion, with the knee set at 0, 45, and 90°. At each
position, longitudinal ultrasonic images of the medial (MG)
and lateral (LG) gastrocnemius and soleus (Sol) muscles were
obtained while the subject was relaxed (passive) and performed maximal isometric plantar flexion (active), from which
fascicle lengths and angles with respect to the aponeuroses
were determined. In the passive condition, fascicle lengths
changed from 59, 65, and 43 mm (knee, 0°; ankle, 215°) to 32,
41, and 30 mm (knee, 90° ankle, 30°) for MG, LG, and Sol,
respectively. Fascicle shortening by contraction was more
pronounced at longer fascicle lengths. MG had greatest
fascicle angles, ranging from 22 to 67°, and was in a very
disadvantageous condition when the knee was flexed at 90°,
irrespective of ankle positions. Different lengths and angles of
fascicles, and their changes by contraction, might be related
to differences in force-producing capabilities of the muscles
and elastic characteristics of tendons and aponeuroses.
ARCHITECTURE OF HUMAN PLANTAR FLEXORS DURING CONTRACTION
399
Measurement of lengths and angles of fascicles. In each
position, longitudinal ultrasonic images of the triceps surae
[medial (MG) and lateral (LG) gastrocnemius and soleus (Sol)
muscles] were obtained (SSD-2000, Aloka) (Fig. 1) at the
proximal levels 30 (MG and LG) and 50% (Sol) of the distance
between the popliteal crease and the center of the lateral
malleolus. Each level is where the anatomic cross-sectional
area of the respective muscle is maximal (7). At that level,
mediolateral widths of MG and LG were determined over the
skin surface, and the position of one-half of the width was
used as a measurement site for each muscle. For Sol, the
position of the greatest thickness in the lateral half of the
muscle was measured at the level mentioned above. Figure 2
shows the calf with planes of ultrasonograms for the three
muscles. The echoes from interspaces of fascicles and from
the superficial and deep aponeuroses were visualized, and the
ultrasonic images were printed onto calibrated recording
films (SSZ-305, Aloka). By visualizing the fascicles along
their lengths from the superficial to the deep aponeuroses,
one can be convinced that the plane of the ultrasonogram is
parallel to the fascicles (14); otherwise, the fascicle length
Fig. 1. Ultrasonic images of longitudinal sections of medial (MG; top)
and lateral (LG; middle) gastrocnemius and soleus (Sol; bottom)
muscles. In each image, thin subcutaneous adipose tissue layer and
superficial and deep aponeuroses are visualized, between which
parallel echoes from fascicles are arranging diagonally. Fascicle
length was determined as length of echoes from fascicles. Fascicle
angles were defined as angles at which fascicles arose from deep (MG
and LG) and superficial (Sol) aponeuroses.
would be overestimated and the fascicle angle would be
underestimated (25). The echoes from interspaces of the
fascicles were sometimes imaged more clearly along the
length of fascicles when the plane was changed slightly
diagonally to the longitudinal line of each muscle, in which
case the recreated image was used. In the printed images, the
length of the fascicles and fascicle angles [the angle at which
the fascicles arose from the deep (MG and LG) and superficial
(Sol) aponeuroses] were measured, the former by the use of a
curvimeter (Comcurve-8, Koizumi) and the latter by the use
of a protractor. The fascicles were somewhat curvilinear in all
muscles (particularly MG) at shorter lengths. The length of a
fascicle was always measured along its path, with the curvature, if present, taken into consideration. For the fascicle
angle, a line was drawn tangentially to the fascicle at the
contacting point onto the aponeurosis. The angle made by the
line and aponeurosis was measured as the fascicle angle.
Some authors have approximated a fascicle as a straight line
between its origin and insertion to determine fascicle angles
(10), but we considered that the angle defined in the present
study would be more appropriate for studying the impact of
pennation on the force transmission from fascicles to aponeuroses. The reliability of fascicle length and angle measure-
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Fig. 2. Schematic representation of calf with planes of ultrasonograms (heavy black lines; direction of ultrasonic transducer) for MG,
LG, and Sol.
400
ARCHITECTURE OF HUMAN PLANTAR FLEXORS DURING CONTRACTION
ment have been confirmed elsewhere from a comparison with
manual measurements on human cadavers (14, 19) as well as
the reproducibility of measurement (7, 14, 15). In the present
study, ultrasonic measurement was repeated three times for
each subject and averaged values were used. The coefficients
of variation of three measurements were in the range of 0–2%.
Statistical analyses. A three-way ANOVA with repeated
measures was used to analyze lengths and angles of fascicles
as well as plantar flexion torque (2 3 3 3 4, activation 3 knee
positions 3 ankle positions). F-ratios were considered significant at P , 0.05. Significant differences among means at P ,
0.05 were detected by using Tukey’s post hoc tests. The
relationships between muscle length change (Dlmus; described
in RESULTS ) and fascicle lengths and torques were examined
with linear regression analysis with a significance detected
by correlation coefficients at a level of P , 0.05.
RESULTS
Table 1. Fascicle lengths and fascicle angles
of MG, LG, and Sol
Muscle Type/
Ankle Position, °
Passive Condition
Active Condition
Knee position, °
Knee position, °
0
45
90
0
45
90
Fascicle length, mm
MG
215
0
15
30
LG
215
0
15
30
Sol
215
0
15
30
59 6 5
52 6 7
45 6 7
40 6 5
47 6 7
42 6 6
38 6 5
35 6 5
38 6 7
35 6 5
33 6 4
32 6 4
38 6 6
31 6 5
28 6 5
26 6 4
30 6 6
26 6 3
26 6 4
25 6 4
27 6 3
26 6 3
26 6 4
25 6 4
65 6 8
56 6 8
51 6 8
47 6 7
59 6 6
51 6 6
46 6 6
43 6 5
53 6 4
46 6 4
42 6 5
41 6 5
46 6 8
38 6 6
33 6 5
30 6 6
40 6 6
34 6 4
31 6 5
28 6 5
35 6 5
31 6 5
29 6 5
27 6 6
43 6 4
38 6 4
33 6 5
29 6 5
43 6 4
39 6 4
34 6 4
29 6 5
43 6 5
38 6 4
33 6 5
30 6 5
31 6 4
26 6 3
24 6 1
23 6 2
31 6 4
26 6 3
24 6 2
22 6 2
32 6 4
26 6 3
23 6 2
23 6 2
Fascicle angle, °
MG
215
0
15
30
LG
215
0
15
30
Sol
215
0
15
30
22 6 2
24 6 2
27 6 3
31 6 3
26 6 4
29 6 4
34 6 4
38 6 5
34 6 7
39 6 9
42 6 9
45 6 7
33 6 5
40 6 4
46 6 6
48 6 7
44 6 8
51 6 6
55 6 5
58 6 4
54 6 11
62 6 8
65 6 7
67 6 6
12 6 2
13 6 1
15 6 2
16 6 2
13 6 1
14 6 2
15 6 1
16 6 2
12 6 2
14 6 1
16 6 2
17 6 3
19 6 3
24 6 4
28 6 5
31 6 6
21 6 2
25 6 3
29 6 4
34 6 6
25 6 4
29 6 6
31 6 5
35 6 6
19 6 3
21 6 3
25 6 3
29 6 3
19 6 2
22 6 2
25 6 3
29 6 3
19 6 1
21 6 1
24 6 1
28 6 3
33 6 3
40 6 4
45 6 3
49 6 3
33 6 3
39 6 3
45 6 4
49 6 3
33 6 4
40 6 4
45 6 3
49 6 3
Values are means 6 SD; n 5 6 men. MG, medial gastrocnemius;
LG, lateral gastrocnemius; Sol, soleus.
D lmus 5 lFp ? cos ap 2 lfa ? cos aa
where lFp and lFa are fascicle lengths in passive and
active conditions, and ap and aa are fascicle angles in
passive and active conditions, respectively. The Dlmus
ranged from 12 6 4 (SD) to 24 6 4 mm for MG, from
16 6 2 to 21 6 5 mm for LG, and from 11 6 4 to 15 6 5
mm for Sol. In each muscle, Dlmus was greater when the
fascicle lengths were longer (MG: r 5 0.73, LG: r 5 0.47,
and Sol: r 5 0.69 for all measurements in 6 subjects and
MG: r 5 0.92, LG: r 5 0.83, and Sol: r 5 0.78 for the
pooled values of the subjects).
The plantar flexion torque decreased almost linearly
as the ankle was plantar flexed (Fig. 3). Knee and ankle
positions significantly affected the torque, and there
was a significant interaction between knee and ankle
positions. There was no significant difference in torque
between the knee positions at 45 and 90° at all ankle
positions. When the knee was flexed over 45°, torque at
ankle joint angles of 15 and 30° did not differ significantly. The torque was greater when the fascicle lengths
were longer. The Dlmus (averaged for MG, LG, and Sol)
was significantly correlated with the plantar flexion
torque (r 5 0.65 for all measurements in six subjects
and r 5 0.91 for the pooled values of the subjects;
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Table 1 shows average fascicle lengths of MG, LG,
and Sol. Fascicle lengths were longest when the ankle
joint angle was 215° with the knee fully extended, and
shortest when the knee was flexed at 90° and the ankle
joint angle was 30°. When the knee was straight and
the ankle joint angle was 215°, LG had the longest
fascicle lengths, followed by MG and Sol, all significant
in this order.
The degree of fascicle length change was not identical
for the three muscles. The effects of knee and ankle
joint positions on fascicle lengths were significant for
MG and LG, and there was also a significant interaction between knee and ankle positions in these muscles.
In other words, in MG and LG, changes in fascicle
length because of ankle position changes were larger
with the straight- compared with the bent-knee condition. In the active condition, when the knee was flexed
at 90°, fascicle lengths of MG at four ankle positions
were not different, although in the passive condition
the difference was significant. The fascicle length of Sol
was affected by ankle joint angles, but not by knee joint
angles.
The fascicle angles of MG demonstrated the greatest
variation in three muscles, ranging from 22° (passive;
knee, 0°; ankle, 215°) up to 67° (active; knee, 90°;
ankle, 30°) (Table 1). The effects of activation and ankle
positions were significant in all three muscles with a
significant interaction. The differences in MG fascicle
angles because of changes in ankle positions were not
significant among 0, 15, and 30° both in the passive and
active conditions. Fascicle angles of LG differed among
different ankle positions in the active condition but not
in the passive condition, except between 215 and 30°.
For Sol, fascicle angles were affected by activation and
ankle joint angles but not by knee joint angles.
Shorter fascicle lengths and steeper fascicle angles in
the active compared with the passive condition show
internal shortening of fascicles by contraction. From
these parameters, the Dlmus was estimated by the
following formula, i.e.
ARCHITECTURE OF HUMAN PLANTAR FLEXORS DURING CONTRACTION
Fig. 3), which suggests that the muscle force and the
estimated muscle length change in a similar way.
DISCUSSION
The relationship between joint angles and torque is
influenced by such factors as the length-force relationship of muscle fibers, geometric arrangement of muscles
with respect to the joint, and architectural characteristics of the muscle. Muscle architecture, together with
intrinsic properties such as fiber composition, also
affects functional characteristics of muscle (e.g., maximal shortening velocity and maximal force) (1, 21).
Previous studies have not found a clear relationship
between muscle architecture and fiber composition (2,
21), but the variation in force-generating capabilities
among limb muscles is influenced more by differences
in their architecture than those in their fiber types (1,
2, 23). Attempts have been made to determine muscle
architecture in humans; however, few of them have
related it to joint performance. Furthermore, many of
the previous reports have been based on cadaver specimens (3, 4, 11, 12, 29, 32), and little data are available
on muscle architecture in living human muscles, especially during contraction. Recently, the authors have
developed a technique for determining the length and
angles of fascicles in vivo in humans (6, 14, 15). In the
present study, we used this technique to determine
architectural characteristics of the human triceps surae muscles in passive and active conditions and re-
lated architectural changes to joint positions so as to
discuss their functional implications.
The pennation angle has been defined as the angle
made by fascicles and the line of action (pull) of muscle
(12, 21, 28). According to this definition, the present
fascicle angles are not equal to pennation angles because, in the present study, the angles of aponeuroses
with respect to the line of action of muscle were not
considered. Some studies, however, have reported that
the influence of aponeurosis angulation on force transmission from fascicles to tendon is negligible and that
the angle of fascicles arising from the aponeuroses can
be used as the pennation angle (8, 29). In addition, from
our observation, the aponeuroses in intact muscles
were not so slanted as they are in the removed preparations (12) because of the existence of adjacent muscles
and bones. Thus we consider that the present fascicle
angles can be substitutable for pennation angles, at
least for the triceps surae muscles.
The LG had the longest fascicle lengths in the triceps
surae muscles. This means that the number of sarcomeres in series is the largest for this muscle, which
illustrates eminent velocity potential of LG, as suggested previously (11, 12, 32). On the other hand, MG
was characterized by shorter fascicle lengths and larger
fascicle angles. The MG can thus pack more fibers
within a certain volume and hence would have greater
force potential. These results are in accordance with
the previous report that the physiological crosssectional area of MG is 2.5 times greater than that of
LG, whereas the muscle volume difference between
them is only 1.7 times (7). The maximal shortening
velocity of a muscle is also influenced by fiber type
composition (28). However, because fiber type composition of MG and LG is similar (13), maximal shortening
velocity and maximal force would be principally determined by their architectural properties.
As the ankle was plantar flexed, changes in fascicle
lengths became smaller in MG and LG both in passive
and active conditions. This result appears contradictory because the moment arm length of the Achilles
tendon increases as a result of plantar flexion (22), and
excursion of the muscles connected to the Achilles
tendon for a certain displacement of ankle joint angles
should increase in a more plantar flexed position.
Smaller fascicle length changes could then be due to
1) increase in fascicle angles that augments tendon
excursion relative to fascicle length change (8),
2) slackness of fascicles at the plantar flexed position,
and 3) decreased tendon elongation due to decreased
muscle force. In MG, fascicle angle increased greatly,
and at shorter lengths the difference between passive
and active conditions became smaller, resulting in
smaller Dlmus. These results would suggest that all of
the above possibilities are influential. When the knee
joint angle was 90°, the fascicle length of MG in the
active condition did not change irrespective of ankle
positions. In this position, fascicle angles of MG were
,60°. The force of muscle fibers is transmitted to the
tendon by a factor of the cosine of the pennation angle
(7, 8, 14). In this position, the factor for MG is ,0.5; i.e.,
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Fig. 3. Relationship among ankle joint angles, plantar flexion torque,
and estimated muscle length change (difference between passive and
active conditions) at different knee positions [0 (j), 45 (p), and 90°
(r); mean values from 6 subjects]. See text for calculation of muscle
length change.
401
402
ARCHITECTURE OF HUMAN PLANTAR FLEXORS DURING CONTRACTION
only one-half of the force exerted by fibers is effectively
transmitted to the tendon. Therefore, in this position
the contribution of MG to Achilles tendon force would
be considerably smaller. No difference in torque with
the knee positioned at 45 and 90° might reflect an
insignificant contribution of MG.
Figure 4 shows the relationships between fascicle
length in the passive condition and Dlmus for MG, LG,
and Sol. The Dlmus corresponds to the tendinous movement of each muscle, which would result from elongation of the tendinous tissues (tendons and aponeuroses)
during isometric contraction (5). The tendon elongation
is a function of the linear force at the tendon (30).
Considering also that the averaged Dlmus of the three
muscles was correlated with plantar flexion torque, the
larger Dlmus would result from greater muscle force.
Because Dlmus was larger when the fascicle lengths
were longer, it is suggested that the force exerted by the
muscle is greater with longer fascicle lengths. It thus
follows that Fig. 4 might roughly represent the lengthforce relationships of the three muscles. Of course, it is
impossible to quantitatively assess muscle force from
Dlmus because Dlmus would depend on the compliance of
tendons, the total lengths of which are unknown from
the present data. One should also be mindful that the
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Fig. 4. Relationships between fascicle lengths in passive condition
and estimated muscle length change for MG, LG, and Sol [for 3 knee
positions: 0 (j), 45 (q), and 90° (r)].
length-force characteristics of the tendon is nonlinear,
with greater tendon length change at lower force (30).
However, the tendency of Dlmus to increase as the
fascicle length increases would indicate that the muscle
force is greater when the fascicle length is longer.
Further dorsiflexion beyond the present setting (knee
0° with 15° dorsiflexion) caused discomfort or localized
pain in the gastrocnemius muscles in the subjects,
which prevented active force production. The fascicle
lengths at this position therefore appear to be their
maxima in the physiological range. The data therefore
suggest that each muscle uses only a part of the
length-force relationship. This speculation is in line
with a previous report (3) on human cadavers that
showed that the lower limb muscles use approximately
the ascending limb and plateau region of the lengthforce relationship. However, the present data cannot
give precise information on which part of the relationship is used in vivo because muscle forces and sarcomere lengths are unknown. Furthermore, due to the
fascicle angles, the component of the force exerted by
fascicles in the direction of the tendon is smaller when
fascicle lengths are shorter. Thus the relationship
between fascicle lengths and Dlmus might be modified by
this angulation effect, especially at shorter fascicle
lengths.
The length range of the triceps surae muscles is more
limited during daily activities, such as locomotion. In
the stance phase of walking (foot contact-push-off), the
knee joint angle changes between 0 and 30°, and the
ankle joint angle ranges between 15° dorsiflexion and
0° plantar flexion (33). From the present results, in this
range fascicles of MG and LG are ,70–100% of their
maximal lengths and the fascicle length of Sol ranges
from 85 to 100%, where Dlmus of each muscle is largest
and apparently constant (Fig. 4). It appears, therefore,
that the gastrocnemius and Sol muscles operate with
moderate variation in length and force during locomotion (and possibly in other daily activities as well) to
exert force more effectively.
The nonlinear nature of tendon elongation by applied
force (less elongation as force increases) suggests relatively small Dlmus at longer fascicle lengths because the
passive tension of the muscle at those lengths would
already have taken up the relatively large tendon
compliance. However, passive plantar flexion torque
ranged only up to 12 N · m (7% of the maximal torque);
thus the active muscle force would have generated
larger length changes, regardless of the reduced compliance. The fascicle length of MG in the active condition
did not change, irrespective of ankle positions when the
knee was flexed at 90°, which contrasts to the result in
the passive condition. This might imply that when the
knee is flexed to 90°, the MG fibers reach a length close
to their active slack length, and little force is produced.
The other possibility is that the very high MG fascicle
angle and hence a very small muscle force component
in the direction of the tendon prevented fibers from
taking up the in-series compliance. On the other hand,
maximal Dlmus of MG was the largest among the three
muscles, which would come from the prominent force
ARCHITECTURE OF HUMAN PLANTAR FLEXORS DURING CONTRACTION
3) the medial head of the gastrocnemius is in a very
disadvantageous condition when the knee is flexed at
90°, irrespective of ankle positions.
Address for reprint requests: Y. Kawakami, Dept. of Life Sciences
(Sports Sciences), The Univ. of Tokyo, Komaba 3–8–1, Meguro, Tokyo
153-8902, Japan (E-mail: [email protected]).
Received 16 June 1997; accepted in final form 24 March 1998.
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17. Loeb, G. E., C. A. Pratt, C. M. Chanaud, and F. J. R.
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18. Murray, M. P., G. N. Guten, J. M. Baldwin, and G. M.
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potential and/or compliant tendon and aponeurosis of
MG. Although MG and LG comprise one muscle unit,
these two heads are thus contrasted in architectural
and force-generating characteristics. It is expected
from the present results that the mechanical properties
of the series elastic component can be assessed with the
present technique, as suggested previously (5). However, determination of the passive length-force characteristics requires the total length and cross-sectional
area of the tendon as well as the force applied to it,
which deserves further study.
The curvature of fascicles observed during contraction would have occurred by the intramuscular pressure and stretch of the aponeuroses (20). This observation also evidences the elongation of in-series compliance
in the active condition. The results that larger fascicle
curvature was observed in all muscles at shorter lengths
and that MG showed the most pronounced curvature
would imply that the curvature might be related to the
degree of pennation. Larger pennation angles would
result in greater force component perpendicular to the
aponeuroses, which might cause more intramuscular
pressure to develop.
In the present study, the length and angle of fascicles
were determined from the midbelly of the muscles.
Although it has been shown that there is marked
uniformity in fiber length (4, 32) and fascicle length (4,
24) throughout a muscle, some studies have reported
heterogeneity of fascicle angles (24) and even fascicle
lengths (11, 21) along the length of the muscle. Thus the
relationships between joint angles and fascicle arrangement might differ in the different portions of the
muscle. Furthermore, the triceps surae muscles have
some variations in internal fascicle arrangement. The
Sol, for example, consists of two portions: portioposterior and -anterior, the latter of which (although much
smaller in volume) has a bipennate architecture (27).
The LG consists of three heads, and the direction of
fascicles has been shown to be different among heads
(26). In the present study we studied the portioposterior for Sol and the ‘‘A head’’ (see Ref. 26; lateral
portion) for LG, which were greatest in volume. The
kinetics of fascicles, which are presently being studied
in our laboratory, might vary within a muscle.
Finally, it should be noted that the behavior of muscle
fibers might not always be the same as that of fascicles.
Although muscle fibers terminating intrafascicularly
are serially connected within a fascicle to act as a
functional unit, adjacent fibers do not necessarily belong to the same motor unit (31). Future studies may
focus on the existence of inhomogeneous fiber lengths
that might be present within a fascicle, especially
during submaximal contractions.
In conclusion, from the present results it was suggested that 1) the architecture of the triceps surae
muscles is considerably different, possibly reflecting
their functional roles; 2) lengths and angles of fascicles
change by contraction in a dissimilar manner between
muscles, which might be related to the differences in
force-producing capabilities of the muscle and elastic
characteristics of the tendons and aponeuroses; and
403
404
ARCHITECTURE OF HUMAN PLANTAR FLEXORS DURING CONTRACTION
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skeletal muscle. In: Exercise Sport Science Review, edited by
K. B. Pandolf. New York: Macmillan, 1988, p. 89–137.
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23. Sacks, R. D., and R. R. Roy. Architecture of the hind limb
muscles of cats: functional significance. J. Morphol. 173: 185–
195, 1982.
24. Scott, S. H., I. E. Brown, and G. E. Loeb. Mechanics of feline
soleus: I. Effect of fascicle length and velocity on force output. J.
Muscle Res. Cell Motil. 17: 207–219, 1996.
25. Scott, S. H., C. M. Engstrom, and G. E. Loeb. Morphometry of
human thigh muscles. Determination of fascicle architecture by
magnetic resonance imaging. J. Anat. 182: 249–257, 1993.
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A. W. English. Anatomical partitioning of three multiarticular
human muscles. Acta Anat. (Basel) 142: 261–266, 1991.
27. Sekiya, S. Muscle architecture and intramuscular distribution
of nerves in the human soleus muscle. Acta Anat. (Basel) 140:
213–223, 1991.
28. Spector, S. A., P. F. Gardiner, R. F. Zernicke, R. R. Roy, and
V. R. Edgerton. Muscle architecture and force-velocity characteristics of cat soleus and medial gastrocnemius: implications for
motor control. J. Neurophysiol. 44: 951–960, 1980.
29. Spoor, C. W., J. L. van Leewen, W. J. T. M. van der Meulen,
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30. Trestik, C. L., and R. L. Lieber. Relationship between Achilles
tendon mechanical properties and gastrocnemius muscle function. J. Biomech. Eng. 115: 225–230, 1993.
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