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BIOMECHANICS OF HUMAN MOVEMENT 145

Z

Object-space Force platform
y
z
Y (O x , O , O )
(0, 0, 0) X
X
Y
Z
FIGURE 6.14 Relationship between the reference axes of the force
platform and the reference axes of the object-space. In general, the refer-
ence axes will not be aligned. The force platform is located at a distance
O , O , O from the origin of the object-space.
x y z

force platform are used in an inverse dynamics analysis, the data must be synchronized with the kine-
matic observations. Generally, analog data sampled from a force platform are collected at an integer
multiple of the video collection rate.

6.4.6 Example: Results from an Inverse Dynamics Analysis
We close this section by presenting several examples of joint kinematic and kinetic data calculated
using the methods outlined above. The ground reaction forces for a subject walking at a natural
cadence are illustrated in Fig. 6.15. The vertical component of the ground reaction force (GRF) is by
far the largest, with the peak AP component of the GRF next largest in magnitude. Notice how the
AP force component has both a negative phase and a positive phase corresponding to braking and
propulsive phases during stance. The first “hump” of the vertical component of the GRF corresponds
to a deceleration of the whole body COM during weight acceptance (note how this corresponds with
the AP braking force). The second “hump” in the vertical component of the GRF and the positive
phase of the AP force component accelerate the body COM upward and forward as the subject
prepares for push-off at the end of stance.

Ground Reaction Forces
1200
1000
800
Newtons 600 ML
AP
400
Vertical
200
0
–200
0 20 40 60 80 100
% Stance
FIGURE 6.15 Ground reaction forces during the stance phase of natural cadence
walking. The stance phase begins at foot strike and ends when the foot leaves the ground.
ML = medio-lateral, AP = antero-posterior.