Fundamentals of Biomechanics

elastic mechanisms in the SSC are preventing
energy loss or maintaining muscle efficiency
(Ettema, 2001), rather than an energy-saving
mechanism. Animal studies (e.g.,
wallabies and kangaroo rats) have been
used to look at the extremes of evolutionary
adaptation in muscletendon units related to
SSC movement economy (Biewener, 1998;
Biewener & Roberts, 2000; Griffiths, 1989;
1991). A special issue of the Journal of Applied
Biomechanics was devoted to the role of
stored elastic energy in the human vertical
jump (Gregor, 1997). Recent in vivo studies
of the human gastrocnemius muscle in SSC
movements has shown that the compliant
tendon allows the muscle fibers to act in
near isometric conditions at joint reversal
and while the whole muscle shortens to allow
elastic recoil of the tendinous structures
to do more positive work (Kubo,
Kanehisa, Takeshita, Kawakami, Fukashiro,
& Fukunaga, 2000b; Kurokawa, Fukunaga,
& Fukashiro, 2001). The interaction of tendon
and muscle must be documented to
fully understand the benefits of the SSC action
of muscles (Finni et al., 2000).
Another mechanism for the beneficial
effect of SSC coordination is related to the
timing of force development. Recall that the
rate of force development and the
Force–Time Relationship have dramatic effect
on high-speed and high-power movements.
The idea is that if the concentric
movement can begin with near-maximal
force and the slack taken out of the elastic
elements of the MTU, the initial acceleration
and eventual velocity of the movement
will be maximized. While this is logical, the
interaction of other biomechanical factors
(Force–Length Relationship, architecture,
and leverage) makes it difficult to examine
this hypothesis. Interested students should
see papers on this issue in the vertical jump
(Bobbert, Gerritsen, Litjens, & van Soest,
1996; Bobbert & van Zandwijk, 1999) and
sprint starts (Kraan, van Veen, Snijders, &
Storm, 2001).
CHAPTER 4: MECHANICS OF THE MUSCULOSKELETAL SYSTEM 91
The most influential mechanism for the
beneficial effect of an SSC will likely depend
on the movement. Some events like
the foot strike in sprinting or running jump
(100 to 200 ms) require high rates of force
development that are not possible from rest
due to the Force–Time Relationship. These
high-speed events require a well-trained
SSC technique and likely have a different
mix of the four factors than a standing vertical
jump.
Plyometric (plyo=more metric=length)
training will likely increase the athlete's
ability to tolerate higher eccentric muscle
forces and increase the potentiation of initial
concentric forces (Komi, 1986). Plyometrics
are most beneficial for athletes in
high-speed and power activities. There has
been considerable research on the biomechanics
of lower-body drop jumping plyometrics
(Bobbert, 1990). Early studies
showed that jumpers tend to spontaneously
adopt one of two techniques (Bobbert,
Makay, Schinkelshoek, Huijing, & van
Ingen Schenau, 1986) in drop jumping exercises.
Recent research has focused on technique
adaptations due to the compliance of
the landing surface (Sanders & Allen, 1993),
the effect of landing position (Kovacs et al.,
1999), and what might be the optimal drop
height (Lees & Fahmi, 1994). Less research
has been conducted on the biomechanics of
upper body plyometrics (Newton, Kraemer,
Hakkinen, Humphries, & Murphy,
1996; Knudson, 2001c). Loads for plyometric
exercises are controversial, with loads
ranging between 30 and 70% of isometric
muscular strength because maximum power
output varies with technique and the
movement (Cronin, McNair, & Marshall,
2001a; Izquierdo, Ibanez, Gorostiaga,
Gaurrues, Zuniga, Anton, Larrion, &
Hakkinen, 1999; Kaneko, Fuchimoto, Toji,
& Suei, 1983; Newton, Murphy, Humphries,
Wilson, Kraemer, & Hakkinen, 1997;
Wilson, Newton, Murphy, & Humphries,
1993).