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.
References
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
Questions
-
Do you agree that a phase difference between CoP and CoM is evidence of
voluntary rather than passive control?
-
If so, how do you account for the zero lag in this experiment?
-
If not, what do you think is the cause of any phase lag between CoM and
CoP?
Email your answers to [n/a] 
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