Teach-in '98: The Ground Reaction Vector & Centre of Mass
In normal quiet standing, the centre of pressure (CoP) weaves
around the center of mass (CoM) to maintain balance. In this experiment,
the task is made a little more complicated by asking the subject to balance
on one leg. The difference between the CoP (base of the force vector) and
CoM (the small cross in the pelvis) is proportional to the linear
acceleration of the body, and in this way the CoP can be seen to
act as the controller of the CoM (Winter, 1995).
Looking at the upper end of the ground reaction vector (GRV), we can
see that it too weaves around the CoM, creating an alternating moment
of force about it, which is responsible for an angular acceleration
of the body. The discrepancy between the CoM and GRV at the same horizontal
level is measured in the animated upper graph (directly below the stick
figure).
The same phenomenon occurs during normal gait:
As the lower graph shows, the CoP leads the CoM at the beginning of
stance (causing a braking deceleration of the CoM, i.e. loading
response), but trails it towards toe-off (causing a propulsive acceleration,
i.e.
push-off). Note that at the beginning and end of stance there
are some strange effects, because the other foot is in contact with the
floor and there are two vectors at these times. Once again, the animated
upper graph shows the discrepancy between the CoM and the GRV at the same
level. Clearly, the GRV also leads the CoM at in early stance, and
trails it from mid to late stance.
Questions
-
What will be the effect of this discrepancy between GRV & CoM?
-
Should the discrepancy be equal and opposite at each end of stance?
-
Can you think of any practical significance, or clinical application, for
measurement?
Email your answers to [n/a] 
Notes
The animations on this page were made using BodyBuilder software
on a Vicon 370 motion analysis system (Oxford
Metrics, UK). The model used to calculate body CoM was kindly supplied
by Richard Baker
& Michael Eames (Musgrave Park
Hospital, Belfast, N. Ireland) and was adapted to calculate the discrepancy
between GRV & CoM. It can be downloaded here,
along with the associated model parameters & marker
set files. Please drop them an email if you use their model. The .C3D
motion files used were: Oneleg.c3d & Gait.c3d
from the Hong Kong Polytechnic Univ.
Reference
Winter, DA (1995) A.B.C. (anatomy, biomechanics and control) of balance
during standing and walking, Waterloo, Ont..
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