Ting - 2005 - A limited set of muscle synergies for force control during a postural task

Citation

Ting LH, Macpherson JM. A limited set of muscle synergies for force control during a postural task. J Neurophysiol. 2005 January; 93(1): 609–613. DOI

10 Word Summary

Synergies are muscle group primitives for endpoint force in cats.

Abstract

Recently developed computational techniques have been used to reduce muscle activation patterns of high complexity to a simple synergy organization and to bring new insights to the long-standing degrees of freedom problem in motor control. We used a nonnegative factorization approach to identify muscle synergies during postural responses in the cat and to examine the functional significance of such synergies for natural behaviors. We hypothesized that the simplification of neural control afforded by muscle synergies must be matched by a similar reduction in degrees of freedom at the biomechanical level. Electromyographic data were recorded from 8-15 hindlimb muscles of cats exposed to 16 directions of support surface translation. Results showed that as few as four synergies could account for >95% of the automatic postural response across all muscles and all directions. Each synergy was activated for a specific set of perturbation directions, and moreover, each was correlated with a unique vector of endpoint force under the limb. We suggest that, within the context of active balance control, postural synergies reflect a neural command signal that specifies endpoint force of a limb.

Notes

• Active balance control in cats shows synergies that correlate to endpoint forces of the limb.
• What is the biomechanical significance of a synergy?
• Possible typo in the methods section about the EMG filtering?
• APR at 60-135 ms after platform onset. Active force 120-195 ms after platform onset (60 ms later than APR)
• Does gradient search (fmincon) have any problems with NNMF?
• Why four synergies?
  • W1 = downward and outward force = extensors of hip, knee and ankle = max@220°
  • W2 = upward and inward force = flexors of hip, knee and ankle = max@80°
  • W3 = lateral force = biarticular thigh muscles (hamstrings) = max@180°
  • W4 = not correlated with endpoint force = biarticular thigh muscles (quads) = max@290°
• Directional tuning coefficients don't appear to match between Fig 2 and text. Why?