University of Wisconsin, Milwaukee KIN 320 04_Linear_kinetics_lab_FA19

Name: _______Juan Delapaz__________________________ Lab Section: ______801__
University of Wisconsin - Milwaukee
KIN 320 – Biomechanics
Laboratory #4: Ground Reaction ForcesLinear Kinetics
Readings (available on D2L):
1. Luhtanen P, Komi RV. Segmental contribution to forces in vertical jump. Eur J Appl Physiol
Occup Physiol. Apr 15 1978;38(3):181-188.
Introduction
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 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, a push up exercise, raising your hand to ask a question, and jumping (the focus of today's
exercise). Because of the importance of our interactions with the ground in the generation and modulation
of our movements, the ground reaction force (GRF) 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. This is explained by Newton's 3rd law of motion which states that for every action there is
an equal and opposite reaction.
Purposes: 1) to compare the pattern and magnitude of vertical GRFs for a series of vertical movements of
the body or body segments, 2) to consider the relative contribution of individual segment motions to
vertical jump performance, and 3) to consider how a countermovement enhances vertical jump
performance.
Vertical Jump Kinetics: The basic mechanical principle to be studied in this exercise is Newton's 2nd law
of motion: ΣF = ma
where F represents the summation of all external forces acting on a body, m is the body's mass, and a 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 vertical jump:
W
G R F v
F = ma
(GRFv - W) = ma OR GRFv+(W) = ma