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44 Biomimetics: Biologically Inspired Technologies

that takes up most of the length changes while storing and returning elastic energy during the
locomotion cycle. Due to the high resilience of these series of elastic structures, this mechanism
allows the muscle to operate more efficiently. Several other studies have revealed that muscles are
also used as brakes (Ahn and Full, 2002), shock absorbers (Wilson et al., 2001), and even (to push
the analogy with motor parts further) as gearboxes (Rome and Lindstedt, 1997; Rome, 1998). In
addition to this, recent modeling studies have pointed out the importance of viscoelastic muscle
properties for the stability of locomotion (van Soest and Bobbert, 1993; Wagner and Blickhan,
1999). The idea postulated in these latter studies is that, due to their inherent stiffness and damping
properties, muscles will act as a first line of defense in response to external perturbations (Loeb
et al., 1999). Understanding muscle function requires a systems approach in which the influences of
the neural control signals, the muscles biochemistry, and its morphology are studied in relation to
the required performance (Dickinson et al., 2000; Full and Meijer, 2001).

2.3 THE FUNCTIONAL UNITS

Muscle function is determined by specific adaptations at all levels of the muscle hierarchy. Muscles
are comprised of distinct functional modules called ‘‘motor units’’ which are controlled individually
by the central nervous system (CNS) via a network of peripheral nerves. A motor unit consists of
motor neuron, which via its axon innervates a distinct set of muscle fibers. From a control
perspective, motor units are the building blocks of muscle function. Force production and modu-
lation occur through discrete and sequential recruitment of individual motor units. An important
property of motor units is that all muscle fibers belonging to a single unit have an identical
biochemical make up. Individual motor units are classified based on their size, speed of contraction,
and fatigue resistance. A typical muscle contains a mix of different motor units, which gives the
CNS the freedom to tailor function to demand. For example, during slow incremental loading tasks,
motor units are recruited according to Henneman’s size principle (Henneman et al., 1965). This
means that the slow, small, fatigue-resistant motor units are recruited first, followed by faster,
larger, and less fatigue-resistant motor units when the load increases. During fast ballistic tasks like
jumping, however, recruitment according to the size principle is not sufficient to accelerate the
limbs fast enough. It has been shown that under these circumstances motor units are recruited
according to a reversed size principle (Wakeling, 2004). Furthermore, motor unit plasticity in
response to use or disuse can alter the motor unit profile of a muscle and thereby its function.
Muscle function is not just influenced by the amplitude of the neural control signal, but also by the
phase of the control signal in relation to the movement kinematics. For example, it has been shown
that neuromuscular system of jumping frogs has evolved phase relationships between the control
signals and the movement kinematics that yield optimal power output (Lutz and Rome, 1994).
Motor unit activity is under control of the CNS, and regulated by reflex activity of several sensory
systems. Therefore, it enables a rich pattern of voluntary and autonomous muscle functions.
Besides neural control, muscle morphology at the macroscopic and microscopic level has a
major impact on muscle function. Muscle fibers are attached to the skeleton via elastic tendons.
Macroscopically, the ratio of muscle fiber length to tendon length is a major determinant of muscle
function (Biewener et al., 1998a). For example, the calf muscles of wallabies have very short
muscle fibers in series with a long tendon. This design appears to be an adaptation to enhance
the storage and return of elastic energy to allow for more efficient locomotion (Biewener et al.,
1998a). At the microscopic level, muscle tissue is highly ordered, typically comprising thousands of
muscle cells embedded in a matrix of basal lamina (Trotter and Purslow, 1992). The muscle cells,
or muscle fibers, are long and slender multinucleated cells in which the contractile proteins are
arranged in highly organized structures called ‘‘sarcomeres.’’ The sarcomeres are the working units
of the muscle fiber. A typical fiber comprises several thousand sarcomeres in series and in parallel.
Microscopically, sarcomere design and the arrangement of sarcomeres within a muscle fiber are