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.

The mechanisms thought
to be involved are elastic energy storage and release (mostly in tendon), and
reflex potentiation (e.g. Komi, 1992). 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 whilst 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 of the contractile element (Gregor, 1989), similar
to 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 (Huijing, 1992). There have been alternative
explanations for the phenomenon of the stretch-shortening cycle (e.g. van
Ingen Schenau, 1984). Differences of opinion also exist on the amount of
elastic energy that can be stored (compare van Ingen Schenau, 1984 with
Alexander, 1992) and its value in achieving maximal performance (e.g. Zajac,
1993). 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 (e.g. van Ingen Schenau, 1984) and increasing the
elastic energy stored in tendon (Huijing, 1992). Force enhancement occurs in
dynamic concentric contractions after stretch, such that the force-velocity
relationship shifts towards increasing forces at any given velocity (Chapman,

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