EXSC 408L
Fall ‘11
Introduction to Biomechanics
Lab 6  Total Body Kinetics
Page 1 of 7
Lab #6  Total Body Kinetics
Purpose:
The purpose of this lab is to fully comprehend, through personal experience, the relationship between the
net force acting on the body and the acceleration of the total body center of mass (TBCM). In this lab, we
will focus on vertical ground reaction forces and the subsequent linear motion of the TBCM. Upon
completion of this lab you will:
•
Understand the relationship between force, mass, and acceleration.
•
Know how to interpret a forcetime curve.
•
Be able to draw a freebody diagram and use it to calculate net force.
•
Understand and be able to solve projectile motion problems.
Introduction:
In many skilled tasks, the performer needs to perform a series of movements while in contact with the
ground and during flight. As we have learned in the previous labs, the mechanical objectives of the phases
during foot contact are generally to control and generate the linear and angular momentum necessary for the
subsequent flight phase. The final condition of the foot contact phase (velocity at takeoff) becomes the
input or initial condition of the flight phase. During the flight phase, the path of the total body center of
mass (TBCM) follows the laws of projectile motion.
I.
NET FORCE = MASS * ACCELERATION
In addition to Newton's third law of motion referred to as the Law of Action/Reaction, you need to
understand Newton's second law of motion referred to as the Law of Acceleration.
Newton's Second Law:
Σ
F = m*a
where:
Σ
F = sum of the external forces
m = mass
a = acceleration
This mathematical formula expresses the relationship between the external forces applied to an object and
the linear acceleration that the forces produce. It must be noted that the “a” in the above equation is the
acceleration of the TBCM. In addition, the acceleration must be in the direction of the net external force, F.
Newton's second law of motion, in conjunction with his third law (which states that for every action there is
an equal and opposite reaction), provides the mathematical basis for what happens when someone performs
a movement on a force platform.
Σ
F = ma
substitute in
Σ
F
y
= R
y
+ (BW)
R
y
+ (BW) = ma
R
y
= BW + ma
where:
BW = body weight of person
R
y
= vertical ground reaction force
For the case in which a person is in a stationary (static) position, the two forces (BW
t
and R
y
) must be equal
in magnitude and opposite in direction (
Σ
F
y
= R
y
BW = m*a = 0).
If the person starts to move up (positive
acceleration of the TBCM), then
Σ
F = ma > 0 and the magnitude of the ground reaction force will be
greater than the magnitude of the body weight. With initiation of downwards movement (negative
acceleration of the TBCM),
Σ
F = ma < 0 and the magnitude of the ground reaction force will be less than
the magnitude of the body weight.
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EXSC 408L
Fall ‘11
Introduction to Biomechanics
Lab 6  Total Body Kinetics
Page 2 of 7
If we take the entire body of a person as the system, our free body diagram (FBD) of a person standing on a
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 '10
 McNittGray
 Force, Ry, Ground reaction force, Total Body Kinetics, TBCM

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