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Biomimetics of Muscle Design 49

invertebrate musculoskeletal design, Full (1997) showed that there are specific sarcomere designs
for specific functions or modes of locomotion (Figure 2.4). For example, arthropod limbs have slow
and fast muscles. The slow muscles are mainly used during posture, burrowing, and slow locomo-
tion, while the fast muscles are involved in rapid locomotion and escape. Not surprisingly, the slow
muscles are the ones that have the longest sarcomeres (Full, 1997).
With respect to sarcomere design, vertebrates are pretty conservative. Their sarcomeres typic-
ally have a length between 2 and 3 mm. With myosin filaments having a more or less constant length
of 1.6 mm, much of the variability is due to differences in the length of actin filaments. Their length
ranges from 0.95 mm in chicken to 1.27 mm in humans (Ashmore et al., 1988; Burkholder and
Lieber, 2001; Lieber and Burkholder, 2000; Walker and Schrodt, 1973). Furthermore, in vertebrate
sarcomeres, the ratio of actin to myosin filaments is virtually constant at 2:1. As a consequence,
vertebrates have only a limited capacity to tailor their sarcomeres to meet functional demands and
will have to resort to different mechanisms to achieve this.

2.4.2 Rearranging the Sarcomeres, Muscle Morphology

The function of vertebrate and invertebrate muscle is intimately related to their morphology. To
meet functional demands while at the same time accounting for volume and length constraints set
by (exo)skeletal dimensions, sarcomeres are arranged in specific ways. The basic design options are
the parallel and serial arrangement of the sarcomeres. Figure 2.5 illustrates the functional conse-
quences of these mechanisms. Adding sarcomeres in parallel increases the force of the muscle,
whereas serial addition of sarcomeres increases the operating range of the muscle as well as the
maximal shortening velocity.
Some muscles, like the human hamstrings, are long and slender. They have long parallelly
arranged muscle fibers that contain many sarcomeres in series. They are capable of considerable
shortening while maintaining the ability to generate sufficient force. Interestingly, there appears to
be a limit to the length of individual muscle fibers; one rarely comes across muscle fibers longer
than 10 cm. Muscles whose fleshy belly exceeds this length, like the human and feline sartorius
muscle (Loeb et al., 1987), have tendinous plates that interconnect muscle fibers in series. The exact
reason for this design is thus far unclear. It has been suggested that it has to do with control
problems involved in synchronizing the activation of sarcomeres in very long fibers, but it might
also be a solution to ensure structural integrity of the muscle.
Pennate muscles have relatively short muscle fibers that are orientated at an angle with the line
of work of the muscle. The advantage of this design is that the number of sarcomeres arranged in

dL
parallel 2F
serial F
2dL

2F 2F
P P
F S F
S
dL 2dL v 2v

Figure 2.5 (See color insert following page 302) Functional effects of parallel (P) and serial (S) arrangement of
sarcomeres. F represents force, v represents velocity, and dL represents the length ranges over which the muscle
can generate force.