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Summary Chapter 17
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Summary Plyometric and SpeedTraining
– minor PT
Book: Chapter 17
Because so many injuries occur as the result of an inability to control declarative
forces, The use of both plyometric and speed training, with their emphasis on the
efficient production and use of ground reaction forces, should be considered an
integral part of any program whose goal is injury prevention. Additional benefits of
incorporating plyometric and speed training components include overall enhanced
coordination, increased agility, and improved anaerobic and general conditioning.
A plyometric movement is a quick, powerful movement consisting of an eccentric
muscle action, also known as a countermovement or pre-stretch, followed by an
immediate powerful concentric muscle action. Speed is simply the ability to achieve
high velocity. Both plyometric and speed rely heavily on the stretch-shortening cycle
(SSC) to elicit the desired outcome.
Speed training exercises are designed to use these same mechanical and
neurophysiologic components, in concert with technique and muscular strength, to
produce larger ground forces, thereby allowing clients to run faster.
Plyometric Mechanics and Physiology
The term used to define this force–speed relationship is power (see biomechanics).
When used correctly, plyometric training has consistently demonstrated the ability to
improve the production of muscle force and power. This increased production of
muscular power is best explained by two proposed models as discussed in this
section— mechanical and neurophysiological.
Different models:
Mechanical Model of Plyometric exercise
Neurophysiological Model of Plyometric exercise
Stretch-Shortening Cycle
Mechanical Model of Plyometric exercise
In the mechanical model, elastic energy is stored following a rapid stretch and then
released during a subsequent concentric muscle action, thereby increasing the total
force production. There are three mechanical components:
Series elastic components (SEC) – a primary contributor to force production
during plyometric exercises.
Parallel elastic components (PEC)
Contractile components (CC)
,While the series elastic component includes some muscular components (actin and
myosin), it is composed mainly the tendon. When the musculotendinous unit is
stretched, as during an eccentric muscle action, the SEC acts as a spring and is
lengthened, storing elastic energy. If the muscle then immediately begins a concentric
muscle action, the stored energy is released, contributing to the total force production
by naturally returning the muscles and tendons to their resting configuration. If a
concentric muscle action does not occur immediately following the eccentric action, or
if the eccentric phase is too long or requires too great a motion about the given joint,
the stored energy dissipates and is lost as heat. Consequently, no plyometric effect will
occur.
Neurophysiological Model of Plyometric Exercise
The neurophysiological element involves a change in the force velocity characteristics
of the muscle’s contractile components caused by stretch; concentric muscle force is
increased with the use of the stretch reflex. The stretch reflex is the body’s involuntary
response to an externa stimulus that causes a rapid stretching of the muscle. In
response to this rapid stretch, a signal is sent to the spinal cord, which in turn sends a
message back, resulting in a concentric contraction of the same overstretched muscle.
Example: A quick stretch of the patellar tendon (knee) occurs when the reflex hammer
comes in contact with the tendon. The quadriceps muscle then senses this stretch and
responds with an involuntary concentric contraction, resulting in the knee jerk as seen
by the observer.
The faster the muscle is stretched, the greater the concentric force following the
stretch, resulting in increased power output.
Stretch Shortening Cycle
The SSC is a model explaining the energy storing capabilities of the SEC and
stimulation of the stretch reflex that facilitate a maximal increase in muscle recruitment
over a minimal amount of time. The SSC involves three distinct phases.
1. The eccentric phase – the deceleration phase – involves preloading the agonist
muscle groups. During this phase, the SEC stores elastic energy and the muscle
spindles are stimulated.
2. The amortization – or transition – phase – is the time between the eccentric and
concentric phases, the time from the end of the eccentric phase to the initiation
of the concentric muscle action. This is the turn-around time from landing to
take off and is the most important part of the plyometric exercise, as it is critical
for power development. There is a delay between the eccentric and concentric
muscle actions during which the spinal cord begins to transmit signals to the
agonist—stretched—muscle group.
3. The concentric phase is the body response to the events occurring during the
eccentric and amortization phases. During the final phase of the SSC, the
energy stored in the SEC during the eccentric phase is either used to increase
the force of the subsequent movement or is dissipated as heat.
The stretch shortening cycle describes the stretch reflex and stored elastic energy
induced increases in concentric force production that follow a rapid eccentric muscle.
, Illustration of the stretch–shortening cycle (SSC) with the events of the mechanical
model (row 2) and neurophysiological model (row 3) that occur during each of its three
phases (row 4). For example, during the eccentric phase of the SSC (column 2)—that
is, the client’s countermovement—the series elastic component (SEC) undergoes a
rapid stretch that the muscle spindles detect, which then send a signal to the spinal
cord.
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