Fundamentals of Biomechanics

outs. We will see in the next section that
muscle mechanical properties also change
in response to activity and inactivity.
THREE MECHANICAL CHARAC-
TERISTICS OF MUSCLE
Previously we discussed the passive tension
in an MTU as it is passively stretched.
Now it is time to examine the tensile forces
the MTU experiences in the wide variety
actions, lengths, and other active conditions
encountered in movement. The force potential
of an MTU varies and can be described
by three mechanical characteristics. These
characteristics deal with the variations in
muscle force because of differences in velocity,
length, and the time relative to activation.
Force–Velocity Relationship
The Force–Velocity Relationship explains
how the force of fully activated muscle
varies with velocity. This may be the most
important mechanical characteristic since
all three muscle actions (eccentric, isometric,
concentric) are reflected in the graph.
CHAPTER 4: MECHANICS OF THE MUSCULOSKELETAL SYSTEM 79
We will see that the force or tension a muscle
can create is quite different across actions
and across the many speeds of movement.
The discovery and formula describing
this fundamental relationship in concentric
conditions is also attributed to A. V.
Hill. Hill made careful measurements of
the velocity of shortening when a preparation
of maximally stimulated frog muscle
was released from isometric conditions.
These studies of isolated preparations of
muscle are performed in what we term in
vitro (Latin for “in glass”) conditions.
Figure 4.7 illustrates the shape of the complete
Force–Velocity Relationship of skeletal
muscle. The Force–Velocity curve essentially
states that the force the muscle can
create decreases with increasing velocity of
shortening (concentric actions), while the
force the muscle can resist increases with
increasing velocity of lengthening (eccentric
actions). The force in isometric conditions
is labeled P 0 in Hill's equation. The
right side of the graph corresponds to how
the tension potential of the muscle rapidly
decreases with increases in speed of concentric
shortening. Also note, however, that
Figure 4.7. The in vitro Force–Velocity Relationship of muscle. Muscle force potential rapidly decreases with increasing
velocity of shortening (concentric action), while the force within the muscle increases with increasing velocity
of lengthening (eccentric action). The rise in force for eccentric actions is much higher than illustrated.