Dr. Knight's Insights: Kinesiology and the 2016 Olympics Part III: Propulsive Impulse

The last post discussed impulse (the product of the average amount of force and the time over which it is applied) when landing from a jump, and how it was important to increase the amount of time over which the force is applied when landing with a "soft" landing to decrease the injury risk. When applying force, especially in a sport like volleyball, the force has to be applied to the ground when jumping or to the ball when attempting a spike over a short time period in order to be successful.

First, let's examine the vertical jump. Volleyball players have to jump repeatedly over the course of a match. If they are jumping to spike a ball or block their opponent's spike, they want to apply a large amount of force to the ground in a very short time period so they can reach the peak of their vertical jump as quickly as possible. If two volleyball players, one on offense, and one on defense, both have the same vertical jump height, but one player can reach their peak height faster (in less amount of time) than the other, then that player will gain an advantage. One way to reach the peak of their jump faster is by applying a large amount of force over a shorter time period than their opponent. The more powerful the person is (power is the product of force and velocity, or how quickly someone can develop force), the more successful they will likely be.

Second, let's examine the volleyball spike. When a player spikes a volleyball, their hand is in contact with the ball for a very short period of time. They have to apply as much force as possible over a very small time period. The same principle is true for other striking activities like hitting a baseball/softball, tennis ball, etc. The more velocity the player can generate (the faster the can swing their arm), the greater the momentum they can transfer to the ball, which will result in a greater amount of impulse, because change in momentum is equal to the change in impulse (a post for another day). The first graph on the bottom figure demonstrates how in the volleyball spike the propulsive impulse is generated by applying a large amount of force over a very short period of time. The second part of the first graph relates to the last blog post regarding landing, which was applying a smaller amount of force over a longer period of time. The second graph would relate more to a person jogging/running at a constant velocity.