CHAPTER 21: IMPACT KINESIOLOGY Scientific Basis of Human Motion, 12th edition Hamilton, Weimar & Luttgens Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University Revised by Hamilton & Weimar Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved. McGraw-Hill/Irwin

Objectives Name common problems associated with the diverse forms of receiving impact. Explain how the work-energy, impulse-momentum, and pressure-area relationships apply to receiving impact either of one’s own body or of external objects. State the principles related to avoiding injury while receiving impact and furnish an application for each.

Objectives State the principles related to maintaining and regaining equilibrium while receiving impact and furnish an application for each. State the principles related to accuracy & control while receiving impact & furnish an application for each.

MEANING OF IMPACT AND ITS’ RECEPTION Derived from the Latin word impingere, “to press together”. Further defined as force of contact, violent collision, striking together. Receiving impact is opposing in some manner the force with which a moving body tends to maintain its speed and direction. Some impacts have a positive effect, such as stimulating bone development.

Problems Problems in receiving impact: Avoiding injury. Maintaining equilibrium. Receiving the object with accuracy and control.

Concepts Kinetic energy-work relationship: When a body or object is “received” work is done equal in amount to the change in kinetic energy of the moving body. (KE = Fd)

Concepts Momentum-impulse relationship: Any change in momentum requires a force applied over a period of time (impulse) and is equal to the product of the force and the time. (Ft = Δmv)

Concepts Pressure-area relationship: The pressure that any part of the body must absorb is inversely proportional to the area over which the force is applied. (P = F/a)

Falls and Landings Important contributors to the gradual decrease in momentum without injury: Landing on “giving” surfaces; mats. Controlled flexion at the joints of the landing extremities through eccentric contraction of the antagonist muscles, activated prior to landing. Relationship of the force of impact to the size of the area that bears the brunt of the impact.

Regaining Equilibrium Control the placement of the limbs in preparation for landing. Requires control to place the feet, or hands and feet, in a position that will provide an adequate base of support. Adequate base of support needs to be established. Depends on the integrity of the bones and joints that receive the force of impact.

Protective Equipment The goal of most protective equipment is to take a blow from a relatively small point of contact and dissipate or distribute that force over a greater surface area and use a compressible material to dissipate/absorb the shock of the impact. By increasing the time over which the force is applied, a smaller force is required to bring about the same change in momentum.

Protective Equipment: Helmets Helmets are designed to protect the head, particularly the brain Classified as: Single impact Multi-impact More foam is not better more foam moves the hard shell further away from the skull and leaves the head and neck open to greater rotational forces. The foam should be soft enough to be comfortable but dense enough not to completely flatten during impact.

Protective Equipment: Body Protection Uses the dispersal of forces over larger areas; less force per unit area. Pads that cover joints dissipate forces over increased surface area. use deformation of foam to absorb energy of impact.

Catching: Avoiding Injury Achieved by increasing the distance over which the object’s kinetic energy is lost. By moving the hands toward the body through a distance of 10” to 20” as a ball is received, the catcher is making it possible for the ball’s kinetic energy to be lost gradually. (F = KE/d) Hand position: Fingers should be pointed either down or up, according to whether the ball is below or above the waist, respectively.

Catching: Maintaining Equilibrium Widen the base of support in the direction of the ball’s flight. Allows weight shift from forward to rear foot at impact. Increases chances of maintaining equilibrium, and contributes to gradual reduction of ball’s motion.

Catching: Accuracy and Control Accurate vision, judgement, and positioning of the body are of vital importance. “Keeping the eye on the ball”. Accurate judgement depends on accurate vision, and accurate adjustment of body position, as well as on agility and smoothness of neuromuscular response. “Hand-eye” & “foot-eye coordination”.

PRINCIPLES OF RECEIVING IMPACT Related to Avoiding Injury The more gradual the loss of momentum (or kinetic energy) of the moving body or object, the less the force exerted on the body. (F = Δmv/t) Fig 21.1

PRINCIPLES OF RECEIVING IMPACT Related to Avoiding Injury Fig 21.3 Fig 21.2

PRINCIPLES OF RECEIVING IMPACT Related to Avoiding Injury The larger the area receiving the impact, the less will be the force per unit of surface area. Repeated impacts may lead to overuse injures. Attention should be given to the nature of the landing surface.

PRINCIPLES OF RECEIVING IMPACT Related to Maintaining and Regaining Equilibrium The larger the base of support in the direction of the impact, the greater will be the body’s equilibrium. Fig 21.4

PRINCIPLES OF RECEIVING IMPACT Related to Maintaining and Regaining Equilibrium At the moment of impact, the line of gravity should be lowered & centered above the base. Fig 21.5

PRINCIPLES OF RECEIVING IMPACT Related to Accuracy and Control The more gradually the loss of velocity of an external object, the less likely the object is to rebound when impact is received. “Keep the eye on the ball” enables one to judge speed and direction and to respond accordingly.

PRINCIPLES OF RECEIVING IMPACT Related to Accuracy and Control Catching an external object with accuracy and control depends largely on the position of the catcher relative to the direction of the approaching object. Fig 21.6