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Macpherson - 1988 - Strategies that simplify the control of quadrupedal stance. I. Forces at the ground


Macpherson J. Strategies that simplify the control of quadrupedal stance. I. Forces at the ground. J Neurophysiol. 1988 Jul;60(1):204-17. PUBMED

10 Word Summary

Muscle coordination results in simplified endpoint force suggesting high-level goal.


1. Postural reactions were studied in six cats subjected to small, linear translations of the supporting surface in each of 16 different directions in the horizontal plane. Directions were specified in a polar coordinate system, with posterior translations being 0 degrees and leftward translations, 90 degrees. The data consisted of the forces exerted by each paw of the cat against the ground, measured in three orthogonal directions, vertical (z-axis), longitudinal (y-axis), and lateral (x-axis). 2. The force traces were analyzed by measuring the area under the curve during the postural reaction and dividing by the time of integration to give an average change in force. These values were normalized and plotted against direction of translation in polar coordinates, to give force tuning curves. The longitudinal and lateral force components were combined to generate force vectors in the horizontal plane. 3. Every cat responded to the platform translations with the same, simple strategy in which each hindlimb actively produced a correction force vector in one of only two possible directions. Participation of the forelimbs in the horizontal plane correction was not obligatory. While the direction of each hindlimb force vector was invariant, the amplitude was modulated according to the direction of platform movement. The resultant force vector, that acts through the center of mass of the animal, was in a direction opposite to the platform movement and directly opposed the perturbation. By this strategy, the cat was able to correct for destabilizing movements of the supporting surface in any direction in the horizontal plane. 4. It is concluded that the generation of forces between the paws and the ground is a high-level parameter that is controlled by the nervous system in a task-dependent manner. By using the strategy of restricting these forces to a set of two direction-invariant vectors, the problem of maintaining stance in the face of horizontal plane disturbances is greatly simplified.


  • Horizontal perturbations to standing cats in 16 directions of the compass.
  • Average forces during the "postural response" were used to create force tuning curves
    • Average force magnitude for a direction
    • Shading showing lines in the direction of the horizontal reaction
  • Reaction forces exhibited by the cat only occurred in two different directions.
  • Force output is thought to be controlled by a regulating a "high-level" parameter in the CNS.
    • This is thought to reduce the complexity of standing balance.
  • Aims of the study:
    • Examine the biomechanical strategies of cats in maintaining stance when perturbed.
    • Test whether muscle activity is grouped into synergies during a postural task.
      • Expect to see similar patterns of muscle groupings across the different perturbations.
  • Platform movement:
    • Velocity = 12.5 cm/s
    • Displacement = 2->3 cm (ML->AP)
  • Responses:
    • Passive = 0-50 ms
    • Active = 50-250 ms
  • Cats in quiet standing put slightly more weight on their fore-limbs.
    • Forelimbs push forward and out
    • Hindlimbs push backward and out
  • Passive forces are in the direction opposite the perturbation
    • Constant response in force to all perturbation directions
    • Response direction 180° opposite perturbation direction
  • Active response has a distinct shift in the direction of the reaction forces
    • The response appears along a diagonal, still point away from direction of perturbation.
    • Hindlimbs produced larger horizontal reaction forces than forelimbs.
    • Forelimbs produced larger vertical reaction forces than hindlimbs.
    • Magnitude of forces changed with perturbation direction
    • Response direction occurred in one of roughly two possible directions for all perturbation directions.  Directions make roughly an X shape for the four paws.
      • Composite "resultant" reaction force was always opposite direction of perturbation
        • CoM being controlled?
  • Cats don't act like a table, even during quiet standing.  Thus the mechanisms for response are guided by neural control and not by mechanics alone.