Our thanks to Jim Dowling at McMaster University and Garry Allison from
Curtin University in Western Australia for responding to our request for
advice on an operational definition for muscle cocontraction. Both suggested
normalizing the amplitude of the paired muscles prior to computation of a
cocontraction ratio. We have decided on the following formula

Mean Amplitude Muscle A
Mean Amplitude MVIC Muscle A
% Cocontraction = ----------------------------------- X 100%
Mean Amplitude Muscle B
Mean Amplitude MVIC Muscle B

This formula will be used following full-wave rectification and demeaning of
raw EMG from paired muscles in a scenario in which one of the two muscles is
always appreciably less active than the second (Muscle B). The normalization
will be done using maximum voluntary isometric muscle contractions (MVIC)
with the muscle positioned in a length that approximates the median length of
the muscle during the test activity. We recognize the problem created by
using a static muscle activation to normalize a dynamic effort, but are
willing to proceed with this limitation as an estimate of cocontraction that
is normalized to some maximum activation of muscle.

We would appreciate any additional suggestions regarding operational
definitions for muscle cocontraction.

Best regards,
Michael Gross
CB #7135
Division of Physical Therapy
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599
EMAIL: Mike=Gross%PT%MAH@css.unc.edu