Biological Mechanisms as Models for Mimicking: Sarcomere Design, Arrangement and Muscle Function - Biomimetics: Biologically Inspired Technologies - page 43

This chapter focuses on the principles that underlie muscle function and plasticity while
considering their potential for new design in actuators. The emphasis will be on the organization
of the contractile proteins and how this is related to functional demands. To this aim, a description
of the principal contractile unit, the sarcomere, will be given. The various sarcomere designs
present in the animal kingdom will be discussed in relation to their functional consequences.
Subsequently, the principles of muscle remodeling and repair in response to use and disuse will
be discussed. The chapter will end with a discussion of the principles that could prove to be relevant
for the design of ''muscle-like actuators.''

2.2 MUSCLE FUNCTION

Muscle force production is characterized by three contraction modes: concentric, isometric, and
eccentric. During concentric contractions muscles generate force while shortening. Force produc-
tion during concentric contractions is described by the force-velocity relationship in which force
production declines with increasing speed. In isometric contractions the muscle generates force
without changing its length, for example, when the task requires holding a certain position. In
eccentric contractions the muscle generates force while being lengthened, for example, when an
animal needs to decelerate a limb.
One of the primary functions of skeletal muscles is to generate force while shortening in order to
power the movement of the attached appendages. Comparative studies have revealed the broad
range in force generating and shortening abilities of skeletal muscle (Full, 1997; Josephson, 1993;
Medler, 2002). Maximal strain ranges from 2 to 200% (Full, 1997). The maximal isometric stress of
muscles (Po) varies by three orders of magnitude from 8 to 2200 kN/m 2 . The maximal rate of
shortening ( V max ) varies by two orders of magnitude from 0.35 to 38 muscle lengths per second
(Josephson, 1993; Medler, 2002). Body size has an important influence on muscle function, with
muscles from smaller animals having larger contractile speed (Medler, 2002). It has been suggested
that this is a consequence of the higher movement frequencies utilized by small animals (Medler,
2002). Operating frequency varies by three orders of magnitude and ranges from less than one to
over a 1000 Hz (Full, 1997).
Recent sophisticated experiments have revealed that during animal locomotion muscles do more
than just generating power. In fact, the multi-functionality of muscle is the key explanation for the
success of animal locomotion (Dickinson et al., 2000; Full and Meijer, 2001). Driven by techno-
logical advances, researchers are now capable of determining muscle function during animal
locomotion. One of the approaches involves direct measurement of muscle function using small
force and length sensors implanted in the muscle of choice (Biewener et al., 1998a,b; Griffiths,
1991; Roberts et al., 1997). Others have determined in vivo 3-D kinematics of animal locomotion
and muscle activity patterns, and used this data to replicate the in vivo muscle length changes and
stimulation patterns in workloop experiments (Ahn and Full, 2002; Josephson, 1985). The emer-
ging picture from these experiments is that muscles are well equipped to meet the basic require-
ments for successful locomotion, that is power generation, stability, maneuverability, and energy
conservation. For example, insect flight muscles operate as tunable springs that keep the thorax at
which the wings attach in resonance. The muscles themselves undergo very small strains and the
design is very effective for operation at high frequencies (100 Hz and above) that are needed to keep
insects airborne. To sustain the high frequencies, these muscles make use of specialized contractile
mechanisms (Josephson et al., 2000). In these muscles there is no direct correspondence between
muscle contraction and muscle action potential; hence they are called asynchronous muscles
(Machin and Pringle, 1959). Some muscles do not even shorten during their daily tasks. For
example, during level running, the calf muscle fibers of turkeys generate force without shortening
(Roberts et al., 1997). Functionally, they work like struts, transmitting energy between body
segments. They use their force to load the elastic structures within the muscle, like the aponeurosis,