Co-contraction

Co-contraction may be defined as a simultaneous contraction of antagonists and agonists (prime movers) during the performance of a movement. It is often assumed that antagonists are inhibited during a movement, but antagonist co-contraction is quite common during strong and/or rapid movements, and when precise movements are required. Antagonist co-contraction would appear to be counterproductive, especially for strength tasks. However, the ability of antagonist co-contraction to limit, by reciprocal inhibition, the action of agonists may be a protective mechanism in activities involving strong or rapid contractions. Antagonist co-contraction may also contribute to joint stability during forceful movements.

One way to get the index of co-contraction is by calculating the value of the integral of the enveloped EMG signal (trapezoidal method) of each of the muscles and after applying the equation proposed by Winter (1990), as follows:

where:
%COCON = percentage of co-contraction between the two antagonistic muscles.
area A = area under the enveloped EMG muscle A curve.

area B= area under the enveloped EMG muscle B curve.

commonarea A & B= common area of activity between two antagonistic muscles.

Here's a link to download Matlab Code with a sub-program implemented in environment Matlab©.
To use the function provided just save the file "cocon_gbiomech.m" in the same folder as the data to be analyzed. Then just call in your Matlab routine sub-program as follows in the example below:

Cocon_Muscle1_Muscle2=cocon_gbiomech(Muscle1,Muscle2);

where:
Cocon_Muscle1_Muscle2 = output variable with the percentage of co-contraction.
Muscle1 = input variable respective to their Agonist Muscle.
Muscle2 = input variable respective to their Antagonist Muscle.

Note: you can specify the window which will be examined by inserting an index on the input variables.

References:
Winter D. Biomechanics and motor control of human movement. Wiley-Interscience: Toranto-Ontario, 1990.

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