I agree that these classifications are arbitrary and only useful when
using a very simplified model of human posture and movement.
For instance, muscle synergies originating in the ankle are the primary
stabilizers during quiet stance. The triceps surae fire first with forward
perturbations, the tibialis anterior first with posterior perturbations.
The muscle synergies become even more complex when considering
stabilization during movement. Take for instance the simple movement of
shoulder flexion... prior to the firing of shoulder flexors, the
anticipatory postural muscles of the legs and trunk move the body's center
of mass posteriorly to compensate for the torque at the shoulder joint.
Amazingly, these muscle roles would be reversed if, say, the person were
standing on their hands! Then the wrist and finger muscles would be the
primary balance stabilizers.
We are only just beginning to understand the complex muscle activation
patterns that occur with human movement. It seems we have a lot longer road
ahead than what we've already traveled.
Geoff Mosley, PT
Mt. Vernon, MO
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