Fundamentals of Biomechanics

improve performance (De Koning et al.,
1985; Fitts & Widrick, 1996). Weight training
with high loads and few repetitions primarily
shifts the force–velocity curve up
near isometric conditions (Figure 4.8),
while fast lifting of light loads shifts
the curve up near V max , which is the maximum
velocity of shortening for a muscle.
Another area where the Force–Velocity
Relationship shows dramatic differences in
muscle performance is related to muscle
fiber types. Skeletal muscle fibers fall on a
continuum between slow twitch (Type I)
and fast twitch (Type II). Type I are also
called Slow-Oxidative (SO) because of their
high oxidative glycolysis capacity (considerable
mitochrondion, myoglobin, triglycerides,
and capillary density). Type II fibers
are also called Fast-Glycolytic (FG) because
of their greater anaerobic energy capacity
(considerable intramuscular ATP and glycolytic
enzymes). Muscle fibers with intermediate
levels are usually called FOG (Fast-
CHAPTER 4: MECHANICS OF THE MUSCULOSKELETAL SYSTEM 81
Figure 4.8. Training shifts the Force–Velocity curve upward and is specific to the kind of training. Heavy weight
training primarily shifts the curve upward for isometric and slow concentric actions, while speed training improves
muscle forces at higher concentric speeds.
Oxidative-Glycolytic) fibers. Muscle fibers
type have been classified in many ways
(Scott, Stevens, & Binder-Macleod, 2001),
but biomechanics often focuses on the
twitch response and velocity of shortening
characteristics of fiber types. This is because
the force potential of fast and slow
twitch fibers per given physiological crosssectional
area are about the same. The timing
that the muscle fibers create force and
speed of shortening, however, are dramatically
different. This fact has major implications
for high-speed and high-power movements.
The easiest way to illustrate these differences
is to look at the twitch response of
different fiber types. If an in vitro muscle
fiber is stimulated one time, the fiber will
respond with a twitch. The rate of tension
development and decay of the twitch depends
on the fiber type of the fiber. Figure
4.9 illustrates a schematic of the twitch responses
of several fiber types. A fiber at the