GRF LAB KIN 3400 GROUND REACTION FORCE Most of our movements ultimately rely upon our interaction with the ground. We are constantly pushing against the ground both vertically and horizontally as we initiate and modify movements of the total body and the body segments. Consider just a few examples of movements, both simple and complex, that depend upon our ability to push against the solid base of the earth: walking, running, reaching up in a cupboard for a glass or dish, a push-up exercise, raising your hand to ask a question, and jumping. Because of the importance of our interactions with the ground in the generation and modulation of our movements, the ground reaction force could arguably be considered the most important external force acting on the body. What is important to keep in mind is that the ground | reaction force is largely under our control via coordinated muscle actions. By producing a certain combination of muscle actions, we ultimately push against the ground which pushes back against the body with an equal and opposite force (remember Newton's 3rd law of motion). some simple vertical movements of the body's center of gravity. The exercise will help you gain a better understanding of how we ultimately interact with the ground to produce movement of the body. While we will focus on vertical motion in this exercise, the same principles apply to horizontal movement. The basic mechanical principle to be studied in this exercise is Newton's 2nd law of motion: 2 F = mh '33,; where 2 F represents the summation of all forces acting on the body (i.e., the net force), mb is the body's mass, and aCg is the acceleration of the body's center of gravity (CG). A simple model of the body illustrates the application of Newton's 2nd law to the vertical motion of an individual during a squatting motion, as if one were about to sit in a chair: 2 F = mh 'acg . (GRFv-BW) = mb {-an where: Weight GRFv is the vertical ground reaction force / BW is the person's weight mb is the person's body mass acg is the vertical acceleration of the Vertical Ground CG Reaction Force (GRFv-BW) = mb meg From Newton's 2nd law: GRF LAB KIN 3400 we can see that if the upward push of the ground is taua] t2 the downward pull of gravity (i.e., the weight of the body), the net force on the body equals zero and resulting acceleration is zero. If the ground reaction force is greater m the weight of the body, there is a net upwards force acting on the body and the acceleration is positive. Finally, if the ground reaction force is lei M the weight of the body, the net force on the body acts downwards and the acceleration is negative. Summarizing: ifGRFH = BW,then 2 F = 0 (no net force) and acg = 0 if GRFH > BW, then 2 F > 0 (net force upwards) and acg > 0 or positive if GRFH