WHAT IS A COUNTERMOVEMENT JUMP (CMJ)?
A CMJ is a plyometric movement that enables a muscle to reach its maximum potential, in the shortest time possible and is often utilised due to its similarity to what is seen in most sports that incorporate a triple extension pattern, when the hip knee and ankle joints extend. This sporting movement can also be used as an objective measure when determining explosive power when looking for tangible markers within some testing protocols.
WHAT IS INVOLVED DURING A CMJ AND HOW DO PLYOMETRICS WORK?
An important part of plyometrics is the stretch shortening cycle (SSC), as this is paramount when conducting any plyometric movement. The SSC is when the muscle being used lengthens under tension, stores and releases energy quickly, in order to conduct the explosive nature of fast muscle action; it does this by utilising the contractile force from the series elastic component (SEC) and stretch reflex when the muscles are stimulated (stretched), (see video).
CMJ VIDEO – MOVEMENT EXPLAINED
As seen in the video the athlete is required to accelerate towards the floor in a short sharp movement, produce a short ground contact and a high external force this is then combated by the athlete remaining stiff in the tendons to elicit the best gains, in order to explode into the air. During this phase the muscle spindles are also innervated and react to the stretch of the muscle by causing the reflexive muscle action.
CMJ VIDEO - SSC EXPLAINED:
As the athlete in the video counters the jump by lowering/dipping into a ¼ squat (eccentric phase) the muscle spindles in the lower limb detect a stretch and send a signal to the spinal cord via the afferent nerve. Upon reaching the spinal cord, the signal then synapses (passes a message) (amortization phase) and relays this information to the efferent nerve, and in turn back to the muscle, causing muscle activity in the form of a contraction (concentric phase) resulting in force production. If trained consistently and over a progressive time period, this synapse will speed up, ensuring a faster more powerful athlete. However, in order for an athlete to be competitive, this message is to be passed <250ms for the athlete to be considered as reacting quickly. By enhancing this muscle rate of force development (RFD) within the CMJ through training, the athlete can speed up their reaction and improve force production capabilities from the contractile tissue (muscle and tendons), if this length-tension relationship (the pre-stretch of the muscle) is also short in duration this will help to increase the force production from the athlete.
STRETCH SHORTENING CYCLE: (FIGURE 1)
The SSC can be broken down into three distinct phases, where the muscle spindles and the stiffness of the tendons work together to elicit gains in jump height.
A. Eccentric phase - A rapid eccentric contraction and lengthening of the muscle whilst it is under tension, causing a rapid stretch reflex.
B. Amortization phase - Transition between the eccentric phase and the last phase (concentric) where the muscle has to switch from lengthening to shortening in a rapid fashion.
C. Concentric phase - The muscle shortens, contracts and the athlete explodes into the air.
HOW TO BECOME MORE EXPLOSIVE?
The athlete with the quickest response will usually have stiffer tendons and therefore a faster ground contact, as the athlete who spends more time preloading the muscle will generally lose energy as it will be dissipated as heat. (However athlete genetics, training status and body type will also play a part in the explosive nature of any athlete). In order to gain the advantage of powerful legs, stiffer tendons with a quicker more dominant response, it is suggested that a considerable amount of gym strength along with plyometric training in a programme is paramount to athlete success. However, prior to taking part in plyometrics; the coach should first evaluate and try to ascertain whether the athlete is competent with regards to control and the movement patterns required with plyometrics. If athlete control is good, then plyometrics are to be utilised in a programme starting with low level exercises, however the coach may also look to consider a perodised programme starting with strength, as this will help to ensure that the athlete is strong enough prior to placing the high impact exercises into the programme, as some athletes are generally not suitable for medium to high level plyometrics.
Coaches are to ensure that the correct parameters for training sessions are set by the following guidelines with regards to their athlete:
BIOLOGICAL AGE, TRAINING EXPERIENCE, STRENGTH, TECHNICAL ABILITY, BODY MASS AND GENDER.
The coach should consider the above recommendations when determining the athletes’ strengths and weaknesses prior to conducting a plyometric session. Once the coach is content, a typical training programme may consist of the following exercises: Jumps in place, standing jumps, multiple hops and jumps, bounding, box drills and depth jumps (see video)
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