Forcetime Principle

The applied manifestation of Newton's Second Law of Motion as the Impulse-Momentum Relationship is the Force-Time Principle. If a person can apply force over a longer period of time (large impulse), they will be able to achieve a greater speed (change in momentum) than if they used similar forces in a shorter time interval. Unfortunately, in many human movements there is not an unlimited amount of time to apply forces, and there are several muscle mechanical characteristics that complicate application of this principle. Recall from chapter 4 that maximizing the time force application is not always the best strategy for applying the Force-Time Principle. The movement of interest, muscle characteristics, and the mechanical strengths of tissues all affect optimal application of forces to create motion.

There are a few movements that do allow movers to maximize the time of force application to safely slow down an object. In landing from a jump, the legs are extended at contact with the ground, so there is near maximal joint range of motion to flex the joints and absorb impact forces. A softball infielder is taught to lean forward and extend her glove hand to field a ground ball so that she can absorb the force of the ball over a longer time interval. Figure 6.15 illustrates two people catching balls: which athlete is using a technique that is correctly applying the Force-Time Principle? Young children often catch by trapping the object against the body and even turn their heads in fear. Even professional football players (6.15, below) occasionally rely on their

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Figure 6.15. Catching the ball close to the body (the American football example) is a poor application of the Force-Time Principle because there is minimal time or range of motion to slow down the ball. The softball catcher has increased the time and range of motion that can be used to slow down the ball.

talent or sense of self-preservation more than coaching and use a similar catching technique. For how much time can forces be applied to slow the balls in these cases? The momentum of the ball in these situations is often so great that the force between the person's body and the ball builds up so fast that the ball bounces out of their grasp. If these people extended their arms and hands to the ball, the time the force is applied to slow down the ball could be more than ten times longer. Not only does this increase the chance of catching the ball, but it decreases the peak force and potential discomfort involved in catching.

Figure 6.15. Catching the ball close to the body (the American football example) is a poor application of the Force-Time Principle because there is minimal time or range of motion to slow down the ball. The softball catcher has increased the time and range of motion that can be used to slow down the ball.

Athletes taught to reach for the ground and "give" with ankle, hip, and knee flexion dramatically increase the time of force application in landing and decrease the peak ground reaction forces. Exactly how the muscles are positioned and pre-tensed prior to landing affects which muscle groups are used to cushion landing (DeVita & Skelly, 1992; Kovacs et al, 1999; Zhang, Bates, & Dufek, 2000). How to teach this important skill has not been as well researched. The sound of an impact often tells an athlete about the severity of a collision, so this has been used as a teaching point in catching and landing. It has also been shown that focusing attention on decreasing the sound of landing is an effective strategy to decrease peak forces during landing (McNair, Prapavessis, & Callender, 2000). Increasing the "give" of the cushioning limbs increases the time of force appli-

Activity: Impulse-Momentum Relationship

Fill a few small balloons with water to roughly softball size.Throw the water balloon vertically and catch it.Throw the balloon several times trying to maximize the vertical height thrown. Imagine that the water balloon represents your body falling and the catching motions represent your leg actions in landing.What catching technique points modify the force and time of force application to the balloon to create a vertical impulse to reduce the momentum of the balloon to zero?

cation and decreases the tone and intensity of the sound created by the collision.

In some movements there are other bio-mechanical factors involved in the activity that limit the amount of time that force can be applied. In these activities, increasing time of force application would decrease performance, so the only way to increase the impulse is to rapidly create force during the limited time available. A good example of this is long jumping. Recall that in the kinematics chapter we learned that long jumpers have low takeoff angles (approximately 20°). The takeoff foot is usually on the board for only 100 ms, so there is little time to create vertical velocity. Skilled long jumpers train their neuromuscular system to strongly activate the leg muscles prior to foot strike. This allows the jumper to rapidly increase ground reaction forces so they can generate vertical velocity without losing too much horizontal velocity. Similar temporal limitations are at work in running or throwing. In many sports where players must throw the ball quickly to score or prevent an opponent scoring, the player may make a quicker throw than they would during maximal effort without time restrictions. A quick delivery may not use maximal throwing speed or the extra time it takes to create that speed, but it meets the objective of that situation. Kinesiology professionals need to instruct movers as to when using more time of force application will result in safer and more effective movement, and when the use of longer force application is not the best movement strategy.

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