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Reactive Strength Development – The Biomechanics and Neurophysiology of the Jump

by Matt Jordan on May 17, 2012 No comments

Reactive strength development (RSD) is a critical speed strength quality for the elite athlete.  It can also be tied into fitness goals and can even benefit the aging human.  In this article, I am going to review the different phases of a jump, and some of the biomechanical and neurophysiological considerations for the jump.

As I wrote in my first article in this series entitled Plyometric Training – The Most Incorrectly Used Phrase in Training, reactive strength development (RSD) is often incorrectly referred to as plyometrics.

As I explained in this original article, plyometric (pliometric) refers to muscle lengthening actions, miometric refers to muscle shortening actions, and isometric refers to muscle actions with no change in muscle length.  To be scientifically accurate, I won’t use the word Plyometric as it is conventionally used but feel free to bastardize our profession and continue using this word as you wish.  I won’t hold it against you.

In its pure form jumping, which is a staple movement for RSD, involves movements with a rapid pliometric action (eccentric muscle action) followed by a rapid miometric muscle action (concentric muscle action).  This cyclical movement is often referred to as a stretch-shorten-cycle (SSC), and SSC’s are a big part of sport and life.

For example, take the sports of running, hockey, and slalom skiing.  All sports are very distinct in terms of the direction of the ground reaction forces, the corresponding joint torques, and the speed of the movement but one common thread is that they also include movements that require a rapid switching between muscle shortening and lengthening actions.  It is for this reason that jumping movements are often used as a training method in the physical preparation process for a whole range of sports.

Further analysis of a SSC reveals the following distinct phases.  The first phase is often referred to as the Initial Momentum Phase.  This is the phase in which the athlete’s centre of mass is moving with the force of gravity as their centre of mass descends towards the ground.

The second phase is the Ground Contact Phase.  The ground contact phase represents the initial touch down on the ground, which is immediately followed by the Amortization Phase in which the athlete produces a pliometric (eccentric) muscle action to effectively break the continuation of the initial momentum phase.

Following the amortization phase, the athlete performs a rapid miometric (concentric) muscle action.  This explosive and rapid muscle shortening leads to the Final Momentum Phase where the athlete overcomes gravitational forces and leaves the ground.

I’ve provided a visual of these phases.  Figures 1-3 are pictures of an athlete going through each phase.  Figure 1 is the initial momentum phase, Figure 2 is ground contact and the amortization phase, and Figure 3 is midway through the final momentum phase.

For those of you who are a bit more scientific, I’ve also included the ground reaction force of a subject performing a countermovement jump on a force plate (Figure 4).  The ground reaction force (the curve that you see) can be looked at as the resulting forces measured where the foot contacts the ground.  This is the sum of all the muscle forces from the different joints involved in the jump (hip, knee and ankle).

Some of the important observations include the phase in which muscle forces are produced to absorb energy (amortization phase), the phase in which muscle force is produced to overcome gravitational forces (miometric phase and final momentum phase), and the flight phase where the athlete is airborne. You will also notice a large spike in force when the athlete returns to the ground, which represents another initiation of the amortization phase.

Further analysis and consideration of this graph reveals the following important technical considerations for a jump:

The Amortization Phase

The amortization phase is critical to the proper execution of a jump.  This phase is often referred to as the reactive ability or the ability to rapidly switch from a muscle lengthening action to a muscle shortening action.

The important physiological contributors are: the storage and return of elastic energy in connective tissue, spinal reflex mediation (e.g. stretch reflex), and muscular strength.

In most circumstances the total time of the amortization phase is to be kept as short as possible.  This allows for the the greatest contribution from the release of elastic energy and spinal reflexes, which essentially add to the muscle forces to further increase the jump height.

The amortization phase needs to be addressed with proper cueing and technical development.

Total Impulse or Area Under the Curve

Another important observation is the area under the curve.  This represents the impulse.  Through the impulse momentum relationship, the net impulse gives the take off velocity of the subject.  The net impulse is a very important performance variable.

The impulse at various segments of the jump also provide an excellent metric for monitoring neuromuscular fatigue.  I may address this in another blog but the truth of the matter is this is very complicated and advanced.  If you are interested in this for your athletes, contact me to take an advanced course in monitoring and assessing neuromuscular fatigue through my internship program!

Total Ground Contact Time

The ground contact time (GCT), which is the sum of the amortization phase and the miometric phase, is a very important component of a jump.  In fact elastic strength development and jumps are often classified according to the ground contact times.  Long contact jumps are ones with a contact time of 300-500 msec, medium contact jumps are ones with a contact time of 150-300 msec, and short contact jumps are ones with a ground contact time of 150 msec or less.

Often the periodization of elastic strength development is done such that the volume of jumps (often referred to as “The Number of Contacts”) is organized with respect to the ground contact time.

The largest volume of long contact jumps occurs early in the preparatory period, and the largest volume of short contact jumps occurs in later in phases of the preparatory period (i.e. specific preparatory phase).  However, within a microcycle (5-10 day period) short contact jumps are generally performed first as this type of jumping requires that the athlete be in a more rested.

The final consideration is the large ground reaction force that occurs upon impact after the jump.  This peak ground reaction force upon landing provides evidence in support of proper jumping technique and physical preparation to prevent jumping related injuries.

To put this in layman’s terms: muscle lengthening actions (pliometric/eccentric) are associated with large muscular forces. Without proper mechanics and physical preparation injuries often ensue.  These injuries include stress fractures, tendonitis, lower back pain, patellofemoral pain syndrome, and lower leg compartment syndromes.

In summary, reactive strength development and its most commonly used exercise (jumping) is more complicated that it may seem.  There are many considerations that go into the jump from a technical and cueing perspective, to a program planning and periodization perspective, and lastly to a biomechancial and neurophysiological perspective.

What I attempted to show in this blog are the distinct phases of the jump: the initial momentum phase, the amortization phase, the mimometric and final momentum phase, and the flight phase.  All aspects of the jump have their nuances, their important technical considerations, and can be trained individually with various exercises.

Matt JordanReactive Strength Development – The Biomechanics and Neurophysiology of the Jump

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