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hof60
06-19-2005, 07:48 PM
Dear Chris, dear BIOMCH-L members,

The issue of electromechanical delay is rather clouded, because it
is
used with at least 3 meanings

1) The time difference between the start of the (raw) EMG and the
first detectable mechanical event, the ON-delay.

As far as I know, this depends of which is the 'event' you detect,
but the common opinion is, that it is very small, between 10 and 20
ms. There was a nice study, which showed that the detection
method was all important, but I forgot the name. Was it Gabriel, Basford, An
(JEK 11:123-129)?

2) Not very often mentioned is the OFF delay: the interval between
the end of the EMG and the time when the force has returned to zero.
It depends of course strongly of your definition of "zero" force, but it
is easy to see that is is very considerable, some 100 - 200 ms!

Inman, Human Walking, 1st ed, gives very simple experiments,
which show both ON- and OFF-delay very nicely (Figures 5.20 and
5-21)

3) The isometric force can quite neatly be reproduced by the
rectified EMG after it has been filtered by a low-pass filter with a
cut-off frequency of some 3 Hz (depends on muscle). This is the
version popularized by Winter. It can also be seen in Fig.5.22 in the
same book of Inman. This figure is reproduced from a paper of
Inman et al. from 1952 (!). A 3 Hz filter corresponds to a time constant of
(3*2*pi)^-1 = 50 ms.

When you apply such a filter (2nd order, critically damped) the
filtered signal shows quite realistic ON and OFF delays, with
respect to the original EMG.

Last remark. The Winter method does not apply no non-isometric
contractions. It has been applied to walking and cycling (Van Ingen
Schenau) but you get different cut-off freqs or time delays every
time. A striking case, about running, I have shown in J. Biomech
36:1031-1038 (2003) in Figures 6 and 7.

Yours, At Hof


Hartelijke Groeten,
At Hof

e-mail: a.l.hof@med.rug.nl
www.ihms.nl

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