Teach-in '99 : Control of Standing
by Chris Kirtley, The Hong Kong Polytechnic University

It is well-known that the centre of pressure (CoP) weaves about the centre of mass (CoM) during standing. The two signals are also in-phase during quiet standing, which has led one group of researchers to claim that this is evidence of a "passive stiffness" mechanism (Winter et al, 1997, 1998). They assert that the CoM is an output variable controlled by the CoP, which is a measure of the response of the neuromuscular system. Due to the conduction delays from CNS to the (mainly ankle) leg musculature, a phase delay between CoM and CoP would, they claim, be detectable if a central control process was active.

In this experiment, a subject stood on a pair of force platforms and voluntarily rocked back and forward slowly in time with a metronome. CoM was measured using a 6-camera Vicon system and full-body BodyBuilder model (Johan, Eames et al, 1999). By the logic of the above, we would expect to see a phase (time) delay (probably around 150-250 ms) between CoM and CoP since the motion is consciously generated by the subject.

In fact, as you can see above, there is no time delay (phase lag) between the two signals. Plotting the cross-correlation in MATLAB confirms this (r = 0.99, lag = 0). Click the graph to zoom to a higher scale.


Winter DA, Patla AE, Prince F, Ishac M, Gielo-Perczak K (1998) Stiffness Control of Quiet Standing. North American Congress on Biomechanics.

Winter, D.A., Prince, F., Patla, A. E: Stiffness Control of Balance During Quiet Standing (1998) Journal
Neurophysiology 80: 1211-1221.

Eames MHA, Cosgrove A & Baker R. (1999) Comparing methods of estimating the total body centre of mass in three-dimensions in normal and pathological gaits . Human Movement Science 18 (5): 637-646

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