Introduction to Sports Biomechanics: Analysing Human Movement ...

The stretch–shortening cycle
Many muscle contractions in dynamic movements in sport undergo a stretch–shortening
cycle, in which the eccentric phase is considered to enhance performance in the
concentric phase (Figure 6.15). The mechanisms thought to be involved are pre-load,
elastic energy storage and release (mostly in tendon), and reflex potentiation. The
stretch–shortening effect has not been accurately measured or fully explained. It is
important not only in research but also in strength and power training for athletic
activities. Some evidence shows that muscle fibres may shorten while the whole
muscle–tendon unit lengthens. Furthermore, the velocity of recoil of the tendon during
the shortening phase may be such that the velocity of the muscle fibres is less than that
of the muscle–tendon unit. The result would be a shift to the right of the force–velocity
curve (Figure 6.14) of the contractile component. These interactions between tendinous
structures and muscle fibres may substantially affect elastic and reflex potentiation
in the stretch–shortening cycle, whether or not they bring the muscle fibres closer to
their optimal length and velocity. There have been alternative explanations for the
phenomenon of the stretch–shortening cycle. Differences of opinion also exist on the
amount of elastic energy that can be stored and its value in achieving maximal performance.
The creation of larger muscle forces in, for example, a counter-movement jump
compared with a squat jump is probably important both in terms of the pre-load effect
and in increasing the elastic energy stored in tendon.
Muscle force components and the angle of pull
THE ANATOMY OF HUMAN MOVEMENT
In general, the overall force exerted by a muscle on a bone can be resolved into three
force components, as shown in Figure 6.16. These are:
Figure 6.16 Three-dimensional muscle force components: (a) side view; (b) force components; (c) front view.
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