Impulsion from a biomechanical and sports point of view

The definition of momentum/ impulsion is different depending on its field of application : mechanics, psychology, electricity etc. It was Marey who introduced the term impulsion from Newton’s law of motion, by analysing a jump (figure 1). He defined it as a variation of the ground force during the jump.

From then on, momentum/impulsion became a term used in sports and a parameter determining motor performance (momentum of the sprinter in the starting block, momentum of the long-jumper etc).

Figure 1 : Simultaneous impression of pressure from both feet on the ground and the changes in body height during a jump (E.J. Marey, Movement, 1894). Marey introduced impulsion as being the « variation of ground reaction force recorded over the time of a jump ». Since then, the term has become widespread in all sports disciplines.

Impulsion depends on the forces acting on the body. It is characterised by the way these forces evolve in relation to time. Graphically, it is represented by the area beneath the force curve, drawn in relation to time (see figure 2).

Figure 2 : Forces of a horse’s limb at a trot. Maximum value can vary from 60 % to 140 % of the horse’s weight borne by one limb… The first phase of absorbtion is when the horse puts a foot down on the ground, until the canon goes past a vertical line, then comes the propulsion/ thrust phase, which creates acceleration. Impulsion corresponds to the grey area.

To better describe impulsion, we often differentiate between two phases : the first phase is  absorbtion, the second propulsion/ thrust (creating acceleration) (figure 3).

Interest

Studying the variation of these forces over time enables to analyse performance : what is the impact of the forces applied to a limb during a race ? Does it increase speed ? Does it increase spring ? Is it effective or risky for the integrity of the tendons ?...

Measuring equipment

Different devices enable us to assess impulsion : either directly with a force platform or horseshoes equipped with sensors, an estimation with an accelerometer near the center of gravity (Equimetrix® system) or indirectly using kinetics….

Factors of variation of impulsion

Impulsion = force x time...

Impulsion is characterised by its duration, its maximum value or the moment when maximum value is reached.
These characteristics differ depending on the nature of support, on the movement to be carried out, on the environment etc.

For example :

• At a trot, the presence of a rider, delays maximum value of impulsion in the forelimbs.
• Maximum value of impulsion for the forelimbs and hind limbs is higher when at a trot than in passage.

Impulsion also varies depending on the nature of the ground. The value of maximum impulsion will be lower in deep going. The impulsion of the same horse on a treadmill and on a track is different : on the treadmill, impulsion lengthens the flight phase, whereas on a track support time is longer. To every situation there is an effective impulsion. This parameter is determining for motor performance in any sports activity.