Tuesday, 18 November 2008

Vertical Jump tests: how to perform correctly the Bosco tests

I decided to write this post after having seen numerous tests reports in which the results of squat jumps appear equal and sometimes higher than the counter movement jumps. This seems to be a common mistake, so I will provide in this post some details on how to execute correctly the tests and how to make sure the results are correct.

First of all, I would like to make clear that it is IMPOSSIBLE that an athlete jumps higher with a squat jump then with a counter movement jump. The simple reason for it is explained in more than 200 research papers which have shown the effect of pre-stretch on muscle performance. It all started from the work of Giovanni Cavagna who described the effects of pre-stretching a muscle on its ability to perform more work during the concentric phase. The stretch-shortening cycle (SSC) can be defined as an active stretch (eccentric contraction) of a muscle followed by an immediate shortening (concentric contraction) of that same muscle.

The increased performance benefit associated with muscle contractions that take place during SSCs has been the focus of much research in order to determine the true nature of this enhancement. Many studies followed the early work of Cavagna, all of which showed that performing a jump with a countermovement (CMJ) produces an higher jump then performing the same from a static position (SJ). The physiological mechanisms responsible for such result have been debated during the years, in particular a special issue of the Journal of Applied Biomechanics in 1997 when a target article received the replies of many experts in the field.

I was very fortunate to work alongside the individual who invented the testing protocol and developed all the formulas and information to use such tests to assess vertical jumping ability: the late Prof. Carmelo Bosco. Prof. Bosco, who was my PhD supervisor and mentor, indicated that he had already suggested in the 80s a combination of mechanical and neural aspects connected to the improvements in jumping ability observed when performing a countermovement. I am not going to discuss the physiological mechanisms in this article, what I really want to do is to provide the following information to everyone using and/or planning to use vertical jump tests:

1) make sure you are aware of the correct testing protocols and make sure you understand the results!

2) if your results tell you that an athlete has a SJ value higher than a CMJ you and/or the athlete or the equipment did something wrong!

Let’s look at the testing protocols now.

Squat Jump

The Squat Jump (SJ) is a jump performed from a starting position of 90 degrees knee angle without allowing any counter movement. The hands are held on the hips during the jump, thus avoiding any arm swing.

How to perform it:

Enter the mat or the force platform. Place hands on the hips, bend knees to 90 degrees and stand still for about 1 second, the jump as high as possible without performing a countermovement.

Squat Jump Bosco

From Bosco (1992)

In the above image, it is clear that the athlete needs to reach the jumping position, and from then just move upwards, rapidly extending the legs and the hips. It is important that no countermovement (pre-stretch) is performed around the knee and hip joints. In fact, if this happens, the jump cannot be considered a “true” and valid squat jump.

An animated clip of a correct execution of the Squat Jump, created by the University of Bourgogne is available here:

http://www.u-bourgogne.fr/EXPERTISE-PERFORMANCE/SJ.htm

Landing Instructions

Independently of the equipment used and the mathematical approach used to calculate the height of rise of the centre of gravity, the subjects should always take off and land in the same position. So the requirement is to land with straight legs and perform a couple of rebounds to avoid injuries. Bending the knees while in the air to land can in fact alter the score by increasing the flight time (more on this later on).

Typical Mistakes

This is a very difficult jumping test to perform. In fact, you will find out that many athletes will perform all sorts of movements before jumping. In particular with the knees, hips and with the trunk. A well executed SJ requires only the full extension of the legs and trunk to take off without previous counter-movements.

Counter Movement Jump

The Counter Movement Jump (CMJ) is performed standing with straight legs and performing a jump beginning with a counter movement down to a knee angle of 90 degrees. The hands are held on the hips during the jump to avoid any effect of arm-swing. Counter Movement Jump differs from Squat Jump by the fact that the starting position is standing still and a quick countermovement is performed (stretch-shortening cycle) before take off. The optimal range of movement for testing should be up to 90 degrees knee flexion. This is an important aspect as normally athlete will perform a very short-range countermovement not resulting in a maximal height of jump.

CMJ Bosco

From Bosco (1992).

As for the SJ, the University of Bourgogne has developed an animation available here:

http://www.u-bourgogne.fr/EXPERTISE-PERFORMANCE/CMJ.htm

Landing Instructions

Independently of the equipment used and the mathematical approach used to calculate the height of rise of the centre of gravity, the subjects should always take off and land in the same position. So the requirement is to land with straight legs and perform a couple of rebounds to avoid injuries. Bending the knees while in the air to land can in fact alter the score by increasing the flight time (more on this later on).

Force Platform vs.. Contact/flight mats

The tests can be performed using the following equipment:

1) Force Platform

2) Contact or infrared mat

3) Accelerometers

I will briefly discuss the differences between the first 2 as they are nowadays the most commonly used methods.

Generally speaking, simpler force plates measure the vertical component of the force in the geometric centre of the platform. Advanced (and more expensive) force plates measure the force at the centre of pressure (COP) and the location of the COP. Finally, the most advanced ones measure all three spatial components of the force vector and torques for the three spatial axes.

image

The above is a typical example of Squat Jump performed on a Force Platform that measures only the vertical component of Force. As you can see, it is possible to calculate no only the height of the jump but also instantaneous velocity and power using the Force/Time trace.

Contact mats and/or infrared mats are simple timing devices that measure fly time and contact time and allow the calculation of the height of rise of the centre of gravity using simple Newtonian laws.

Ergojump

There is an advantage of using a force platform over contact mat to calculate jump height.

1) When using the force platform the height is calculated through integration of the ground reaction force. Therefore it does not matter how the subject lands after a jump on the platform.

2) It is possible to identify Peak Force, Time to Peak force, Rise of Force Development and also it is possible to calculate power and velocity and any point in time

With a contact mat, the height of the jump is calculated from the flight time. If the jumps are performed correctly on the contact mat, the result will however be exactly the same with both methods.

Be aware that the height of centre of gravity measured with the force plate will be slightly different from the jump height given by the contact mat method. The reason is that, in the last few milliseconds just before take off, the toes are still touching the ground. The contact mat will not start to count flight time before the toes actually leave the ground. However, at this stage the centre of gravity has already risen 5-10 cm from the starting position because the subject is standing on her/his toes at the moment of take off.

Calculations

Using the fly time method (valid for both the Force Platform and the Contact/infrared mats) it is possible to calculate the height of rise of the centre of gravity using the following formula:

hcg= Fly Time^2 x 1.226 (from Bosco, 2002)

If using a force platform, the fly time can be identified as the difference between the time of landing-the time of take off as indicated in the following image:

FTIme curve

This is an example of a CMJ. The following phases are identified: a:start of movement; b:take off;, c:landing. The difference between c and b provides the fly time value needed to calculate the hcg.

When the height of CMJ and SJ are determined, it is possible to calculate the % gain from the stretch shortening cycle with the following formula:

% Gain = [(CMJ - SJ) * 100 / (CMJ)]

There should always be a difference between the CMJ and SJ, with the CMJ always being higher. Bosco’s work identified during the years how jumping ability and the effect of the stretch-shortening cycle changes with age in the general population:

CMJ SJ graph

Of course training and performance level can provide higher values than the ones presented above. The main thing to keep in mind is that a CMJ should always be higher than a SJ. If your numbers’ don’t show that, there is something wrong with the test and/or the athlete did not perform a maximal jump.

If any reader is interested in knowing more about calculations when using force plates, a useful article written by Dr. Linthorne (Currently at Brunel University in London) is available for download at this link.

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