Demands of Sport: Part I - Force your way to the top

NEWTON’S LAWS

Newton’s third law states that for every action (force), there is an equal and opposite reaction. When athletes apply pressure to the surface of a force plate, the electrical sensors measure the change in signal. Force plates measure the ground reaction force (GRF) that occurs in the opposite direction to the force applied by the athlete. When an athlete is standing still on a force plate, the GRF equals their body weight, i.e., F=ma:  mass multiplied by acceleration caused by gravity (~ 9.8 m/s²).

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NEWTON’S LAWS

While the athlete is aerial, gravity acts to accelerate the COM downwards. Therefore, the upwards motion of the COM will start to slow down as soon as they leave the ground until reaching zero velocity at the top of the flight and speed up again on the descent. The factor that determines jump height is take-off velocity. To produce the maximum possible take-off velocity, the athlete relies on the application of the impulse-momentum theorem.

IMPULSE MOMENTUM THEOREM

Momentum is the quantity of motion a mass has and is the product of mass and velocity. In the CMJ, the athlete has zero momentum in the starting position (zero velocity) and aims to achieve the maximum possible upwards velocity at take-off. The area under the force-time curve illustrates the change in momentum dependent on the impulse of the force produced.

The period between initiation and take-off can be subdivided into three main phases (see figure 2):

1. The unweighting phase (blue) is a period of negative impulse, where GRF is < body weight.
2. The braking phase (yellow) is a period of positive impulse that is equal in magnitude to the unweighting phase. At the end of the braking phase, the negative and positive impulses cancel each other out, and the athlete’s momentum is again zero as they reach a momentary stop at the bottom of the squat.
3. The propulsive phase (green) occurs as the athlete travels upwards. The subtraction of the negative impulse from the positive propulsive impulse is a net positive result that determines how high the athlete’s COM will travel in the air.

Figure 2: The velocity-time curve (red line) and force-time curve (black line) during the unweighting (blue), braking (yellow), and propulsion (green) phases of the countermovement jump

 CLINICAL IMPLICATIONS

Ballistic and plyometric training is a vital component of RTS rehabilitation as they invoke the stretch-shortening cycle mechanism inherent to most sporting activities. Assessing the CMJ provides clinicians with insight into the tissue capacity during rehabilitation and may guide decision-making. Progressive programming of plyometric exercise is critical to ensuring safe and effective training.

Return to sport testing requires robust, objective measures to support the goal of tissue capacity restoration. Practitioners use metrics such as peak or mean force, peak velocity, and impulse within each phase of the CMJ as considerations in return to sport decision making. If a dual force plate system is available, the athlete performs the CMJ while standing with each foot on separate plates. In that case, practitioners can analyze interlimb differences in force and impulse. Therefore, the CMJ provides clinicians with an easy-to-use, practical, and sport-relevant tool to assess tissue capacity and performance throughout rehabilitation.

REFERENCES

1. Sports Act. Living. 2020;2(12).
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6. Strength and Conditioning Journal. 2018;4(40):96-106.