MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 152

Figure 6.9 Profiles of activity of two antagonist muscles, with cross-hatched area
representing the cocontraction. See the text for detailed discussion and analysis.

where M antag and M agon are the moments of force of antagonists and ago-
nists, respectively.
In the example reported, the antagonist activity results in an equal increase
in agonist activity; thus, the unnecessary activity must be twice that of the
antagonist alone. If the antagonists created a 20 N · m extensor moment

and the agonists generated a 50 N · m flexor moment, %COCON would

be 40 / 70 57%. However, most movements involve continuously

changing muscle forces; thus, an agonist muscle at the beginning of the
movement will likely reverse its role and become an antagonist later on in
the movement. Joint moments of force, as seen in many graphs in Chapter 4,
reverse their polarities many times; thus, a modification of Equation (5.12)
is needed to cope with these time-varying changes. Figure 6.9 demonstrates
the profile of activity of two antagonistic muscles during a given movement.
The cross-hatching of muscles A and B shows a common area of activity that
indicates the cocontraction area. Thus, the percent cocontraction is defined as:

× 100% =

common area A & B
area A

%COCON = 2 × × 100% (6.13)

+ area B

If EMG is the primary measure of relative tension in the muscle, we can
suitably process the raw EMG to yield a tension-related activation profile
(Milner-Brown et al., 1973; Winter, 1976). The activity profiles of many
common muscles look very much like those portrayed in Figure 6.9. In this
case, muscle A is the tibialis anterior and muscle B is the soleus as seen
over one walking stride. Using Equation (6.13) for the profiles in Figure 5.9,
%COCON was calculated to be 24%.

6.1.1.2 Isometric Contractions against Gravity. In normal dynamic
movement, there is minimal muscle activity that can be attributed to holding
limb segments against the forces of gravity. This is because the momentum