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An introduction to plyometrics

Hadyn Luke posted this on Friday 10th of August 2012 Hadyn Luke 10/08/2012

Tags: Training methods

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Today’s fitness blog will look at the use of Plyometrics.  Plyometrics describes how a muscle is used during the stretch reflex aspect of a muscle contraction. 

The most familiar stretch reflex is the knee jerk test, when a doctor hits the patella tendon of the knee, which responds by pulling the quadriceps very fast, causing it to stretch. The brain then sends a signal back to the muscle telling it to shorten in order to avoid damage, which causes the leg to kick out in the classic knee jerk reaction.

Plyometrics are linked to the same sort of involuntary stretch reflex phenomenon as the knee jerk test. When a physical trainer incorporates plyometrics as part of a training programme, it allows their clients to produce more power, which can be used for both everyday and sporting activities.

Plyometric training versus stretch reflex training

For an exercise to be classed as plyometrics, the stretch reflex must take place in less than 0.3 seconds. Otherwise, it’s simply a stretch reflex contraction. An example of stretch reflex training would be if you jumped in the air, landed, absorbing the energy for a second (or more than 0.3 seconds), and then used your muscle fibres to jump again. With plyometrics, you jump once and then take off again within 0.3 seconds of landing on the ground.

The amortisation phase

The 0.3 seconds is known as the amortisation phase, when your bodyweight is being absorbed down into the muscle and you are able to use that downward force to produce upward force and jump again. If you take longer than 0.3 seconds, the kinetic energy will be absorbed into the muscle and you’ll need to use your muscle to jump again. Generally, the shorter the amortisation phase the quicker the reflex and the higher the power output.

Muscle contractions during plyometric training

As you land in a squat jump position, there will be an eccentric muscle contraction in your lower legs. In particular, the muscles in your quadriceps and gluteus maximus will lengthen. The amortisation phase covers the time it takes to go from that muscle contraction into a static isometric contraction and then into a concentric muscle contraction, which is when the muscle shortens as you jump again.

How this can be used in the gym environment

If a personal trainer has a client who does a lot of strength training in their lower body, it doesn’t automatically mean that they can produce the strength quickly, for example when lifting weights. By showing the client some plyometric training exercises, a fitness instructor can help them convert this strength into power.

Preparing a client for plyometric training

Personal trainers should ensure that clients have sufficient underlying strength before embarking on plyometric training. The preparatory work should include endurance training, a hypertrophy phase and a strength phrase. Depending on the client’s level of fitness and the intensity of plyometrics to be under taken, this could take anything from three to six months.

Training could begin with the concentric part of the exercise: for example, the fitness instructor would ask the client to squat, hold the position, then focus on jumping straight up. This will allow them to develop the initial starting power needed for plyometric training.

Carrying out multiple jumps on the spot will help the client learn the correct way to jump and the best techniques for landing safely. In time this will develop into plyometric training by increasing the speed of the exercise.

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