How the body applies force looks at the forces the body applies to the ground in order to cause movement. How the body applies force particularly relates to Newtown’s third law: for every action (force) there is an equal and opposite reaction. In order for our bodies to move, either upwards or horizontally (downwards is caused by gravity most of the time), we need to apply forces to the ground and then move as a result of the equal and opposite reaction force.
The body creates force by manipulating the gravitational force on the body and by generating force using its muscles. Muscular contraction provides the force used to create movement around joints. Most of the forces of the body act using a lever system which converts a smaller input force (from your muscles) into a larger output force.
In order to complete the vertical jump test the athlete will first load the legs by squatting down allowing gravity to push the athlete towards the ground. The athlete will then contract the muscles in their legs and butt in order to push down into the ground and also contract their arm muscles to swing their arms vertically into the air.
Gravity and the lower body muscle contractions create a force in the downward direction into the ground. This force is then met by an equal and opposite reaction force that propels the athlete into the air. The greater the downward force the greater the reaction force, and the higher the athlete jumps. Further upward force is generated by the vertical acceleration of the arms to create a thrust in the vertical direction.
If the force applied to the ground is not directly vertical, then the athlete will move in the opposite direction of the force. So if they create a force that is both downwards and backwards, they will jump forward.
For sprinting events it is important for the athlete to get out of the blocks quickly. For this to happen, the athlete needs to apply a force in a downward and rear direction in order to have a reaction force that propels them forward. In order to increase this force, the athlete gets low to the ground and angles their body forward so that they can generate a larger force in the backwards direction and accelerate their body in a forwards direction (as seen below).