Functional muscle synergies constrain force production during postural tasks

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Previous work demonstrated five functional muscle synergies characterize cat hindlimb activity and forces during postural responses.
• This characterization relied on the assumption that synergy force vectors rotate with the limb axis across different postures.

Purpose of the Study:

• To biomechanically validate the assumption of rotating synergy force vectors using a 3D hindlimb model.
• To compare the force-generating capability of a muscle synergy control strategy versus individual muscle control.

Main Methods:

• A detailed, 3D static hindlimb model was used to simulate varying postures.
• Simulated synergy force vectors were analyzed for rotation with the limb axis.
• Feasible force sets (FFSs) were compared between models with and without muscle synergy organization.

Main Results:

• Simulated synergy force vectors rotated monotonically with the limb axis across postures (r2=0.94+/-0.08), confirming biomechanical plausibility.
• Muscle synergy organization significantly reduced FFS volumes compared to individual muscle control (p<<0.01).
• Synergy-limited FFS shapes changed with posture, aligning with experimental active force measurements.

Conclusions:

• A neural strategy utilizing functional muscle synergies is biomechanically plausible for postural control.
• Muscle synergy organization reduces the available force-generating capacity but accurately reflects experimentally observed forces.
• This suggests an internal model coordinates invariant muscle synergies for reduced-dimension postural control.