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Stretch-Shortening Cycle
The last time we met, we went over some of the basics
and history of plyometrics. We talked a little about
eccentric, isometric, and concentric muscle contractions.
This time, we will discuss an inter-related concept – the
stretch-shortening cycle. Basically, this is a moderately
complex term for something as easy to understand
as shooting an elastic band at your buddy in class.
Teacher looks the other way, you hook the elastic
on your thumb, pull back, and wham! Your buddy doesn’t
know what hit him. The concept here is: the farther
you pull the elastic back, the faster the elastic
returns to its original length, and the further it
flies. Obviously, the human musculature is much more
complex than an elastic band, but the basic concept
holds true.
When we discuss the stretch-shortening cycle, there
are two important factors that must be understood.
The first of these is that muscle is elastic in nature
and more elastic in series than in individual fibres
(muscle elasticity includes that of tendons as well).
Secondly, there are sensors in the muscle called muscle
spindles (or proprioceptors) that preset muscle tension
and send messages to the spinal cord regarding rapid
stretching that activates the “stretch reflex”.
In order to discuss muscle elasticity, we’ll
have to think back to grade 11 Physics class (may not
be much of a stretch for some of you, but it’s
a long way back for old coaches like me!) and the lessons
on potential and kinetic energy. If kinetic energy
is the energy of movement, then potential energy must
be stored energy waiting to be transferred to some
type of movement. Now, back to our previous discussion,
with our high jumper as the example, and you’ll
remember that muscles can store energy for a short
period of time (this is our jumper in the 0.12 seconds
prior to takeoff). Well, guess what folks? That stored
energy is actually potential elastic energy. The downside
is that if we linger too long in the amortization phase,
the potential energy we have just created is lost to
the production of heat. Again, the best illustration
of this concept is the simple elastic band...stretch
one out and there exists the possibility of a rapid,
and occasionally painful, return to its original length.
The second of the important factors in the stretch-shortening
cycle is the stretch, or myotatic, reflex. This is
the system responsible for the knee jerk response.
The doctor taps your knee with a rubber mallet. This
stretches the quadriceps tendon, which is sensed by
the muscle spindle in the quadriceps group of muscles,
which, in turn, contract causing the knee to jerk.
The stretch reflex is among the fastest in the human
body due to the direct connection between the muscles
and the spinal cord via the muscle spindles. The muscle
spindles get the message back to the cells responsible
for contraction with minimal delay. The stretch reflex,
then, is key in allowing the muscles to contract faster
during a stretch-shortening cycle than with any other
method of contraction. A voluntary muscle contraction
in response to a muscle stretch would occur too late
to be of any use to an athlete and far too late to
thwart a potential injury.
All this theory begs the question, “How will
training the stretch-shortening cycle make me a better
athlete?” While its true that the response time
of the stretch reflex does not change substantially,
even with training, the factor that does exhibit the
most significant change is the strength of the response.
While we, as coaches, can’t increase the speed
at which the muscle spindles transmit their messages
(that is, to a certain extent, genetically predetermined),
what we can do is get your muscles to respond more
forcefully to the messages they receive from the muscle
spindles.
So, what is the most important thing to remember about
the stretch-shortening cycle? The faster a muscle is
stretched eccentrically, the greater its concentric
force. Remember this, it is vital information to all
athletes! I’ll say it again, the faster the stretch,
the more forceful the contraction. This little bit
of information will play a huge factor in our discussion
of the vertical jump. How about a real life example
of the stretch-shortening cycle at work. An athlete
can jump higher using a depth jump technique than a
standing start. A depth jump involves stepping off
a medium height plyo box to the ground and jumping
as high as possible. Typically, athletes will jump
higher using this technique than in their vertical
jump. Why? A faster stretch cycle results in a more
forceful shortening cycle!
Next time we get together we’ll go over some
basic plyometric drills, until then...get after it!
Coach Jeff Jensen
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