ANTHROPOMETRY - Biomechanics and Motor Control of Human Movement - page 102

4.3.2 Change in Muscle Length during Movement
A few studies have investigated the changes in the length of muscles as a func-
tion of the angles of the joints they cross. Grieve and colleagues (1978), in a
study on eight cadavers, reported percentage length changes of the gastrocne-
mius muscle as a function of the knee and ankle angle. The resting length of
the gastrocs was assumed to be when the knee was flexed 90 ◦ and the ankle
was in an intermediate position, neither plantarflexed nor dorsiflexed. With
40 ◦ plantarflexion, the muscle shortened 8.5% and linearly changed its length
to a 4% increase at 20 ◦ dorsiflexion. An almost linear curve described the
changes at the knee: 6.5% at full extension to a 3% decrease at 150 ◦ flexion.
4.3.3 Force per Unit Cross-Sectional Area (Stress)
A wide range of stress values for skeletal muscles has been reported (Hax-
ton, 1944; Alexander and Vernon, 1975; Maughan et al., 1983). Most of these
stress values were measured during isometric conditions and range from 20
to 100 N / cm 2 . These higher values were recorded in pennate muscles, which
are those whose fibers lie at an angle from the main axis of the muscle. Such
an orientation effectively increases the cross-sectional area above that mea-
sured and used in the stress calculation. Haxton (1944) related force to stress
in two pennate muscles (gastrocs and soleus) and found stresses as high as
38 N / cm 2 . Dynamic stresses have been calculated in the quadriceps during
running and jumping to be about 70 N / cm 2 (based on a peak knee exten-
sor moment of 210 N · m in adult males) and about 100 N / cm 2 in isometric
maximum voluntary contractions (MVCs) (Maughan et al., 1983).
4.3.4 Mechanical Advantage of Muscle
The origin and insertion of each muscle defines the angle of pull of the
tendon on the bone and, therefore, the mechanical leverage it has at the joint
center. Each muscle has its unique moment arm length, which is the length
of a line normal to the muscle passing through the joint center. This moment
arm length changes with the joint angle. One of the few studies done in
this area (Smidt, 1973) reports the average moment arm length (26 subjects)
for the knee extensors and for the hamstrings acting at the knee. Both these
muscle groups showed an increase in the moment length as the knee was
flexed, reaching a peak at 45 ◦ , then decreasing again as flexion increased to
90 ◦ . Wilkie (1950) has also documented the moments and lengths for elbow
flexors.
4.3.5 Multijoint Muscles
A large number of the muscles in the human body pass over more than one
joint. In the lower limbs, the hamstrings are extensors of the hip and flexors