MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 234

Figure 9.10 Introduction of the series elastic (S.E.) element. During isometric con-
tractions, the tendon tension reflects a lengthening of the series element and an internal
shortening of the contractile element. During most human movement, the presence of
the series elastic element is not too significant, but during high-performance move-
ments, such as jumping, it is responsible for storage of energy as a muscle lengthens
immediately prior to rapid shortening.
will lengthen slightly as tension increases. However, during dynamic situa-
tions the series elastic element, in conjunction with viscous components, does
influence the time course of the muscle tension.
During isometric contractions, the series elastic component is under tension
and, therefore, is stretched a finite amount. Because the overall length of the
muscle is kept constant, the stretching of the series elastic element can only
occur if there is an equal shortening of the contractile element itself. This is
described as internal shortening. Figure 9.10 illustrates this point at several
contraction levels. Although the external muscle length L is kept constant,
the increased tension from the contractile element causes the series elastic
element to lengthen by the same amount as the contractile element shortens
internally. The amount of internal shortening from rest to maximum tension is
only a few percent of the resting length in most muscles (van Ingen Schenau,
1984) but has been shown to be as high as 7% in others (Bahler, 1967). It
is widely inferred that these series elastic elements store large amounts of
energy as muscles are stretched prior to an explosive shortening in athletic
movements. However, van Ingen Schenau (1984) has shown that the elastic
capacity of these series elements is far too small to explain improved perfor-
mances resulting from prestretch. He argues that some other mechanism at
the cross-bridges must be responsible.
Experiments to determine the force-length characteristics of the series ele-
ment can only be done on isolated muscle and will require dynamic changes
of force or length. A typical experimental setup is shown in Figure 9.11.
The muscle is stimulated to a certain level of tension while held at a certain
isometric length. The load (tension) is suddenly dropped to zero by releasing
one end of the muscle. With the force suddenly removed, the series elas-
tic element, which is considered to have no mass, suddenly shortens to its
relaxed length. This sudden shortening can be recorded, and the experiment