Macpherson - 1988 - Strategies that simplify the control of quadrupedal stance. II. Electromyographic activity

Citation

Macpherson JM. Strategies that simplify the control of quadrupedal stance. II. Electromyographic activity. J Neurophysiol. 1988 Jul;60(1):218-31. Pubmed

10 Word Summary

Synergies must account for variation in muscle activity patterns.

Abstract

1. This study tested the hypothesis that muscle synergies underlie the invariance in the direction of corrective forces observed following stance perturbations in the horizontal plane. Electromyographic activity was recorded from selected forelimb and hindlimb muscles of cats subjected to horizontal translations of the supporting surface in 16 different directions. The responses of muscles were quantified for each perturbation, and tuning curves were constructed that related the amplitude of muscle response to the direction of platform movement. 2. Muscle tuning curves tended to group into one of two regions, corresponding to the two directions of force vectors. A few muscles showed clearly different recruitment patterns. The same direction of correction force vector was produced by different patterns of muscle activity, and the particular EMG pattern depended on the direction of platform movement. Therefore a simple muscle synergy organization could not account for the invariance in force vector generation. 3. It is concluded that there is a hierarchy of control in the maintenance of stance in which the vector of force exerted against the ground is a high level, task-dependent controlled variable and the selection of muscles to activate in order to produce the vector is controlled at a lower level. It is proposed that muscles are controlled using a modified synergy strategy. In this scheme, a synergy is not simply a fixed group of muscles, constrained to act as a unit. Rather, muscles are organized as a task-dependent synergy that is tuned or modified as needed by the addition or subtraction of other muscles.

Notes

• Invariance is observed in reactionary end-point forces elicited from horizontal plane perturbations.
• "Simple synergies" do not account for variety in muscle activity to different perturbation directions.
• Hypothesis that end-point force is a high-level task-dependent control variable.
• Synergy defined as a set of muscles constrained to act as a unit.
• 0° = backward translation, 90° = leftward translation
• Epoch 1 = 0-30 ms after initial burst, Epoch 2 = time period including all increased activity
• EMGs recorded from left limbs.
• Maximal EMG corresponds fairly well to maximal endpoint force direction (Fig 1)
• GLUT, BIFM, ADFM, LGAS, VMED show surprising amount of continuity in magnitude between 113° and 270°. They all appear to reach a maximum at the 203° translation (upper-right).
• More variability was seen in the flexors than in the extensors (uncontrolled manifold idea?)
• Magnitude of endpoint force matches what is expected from loading direction.
  • Interesting is the additional X pattern overlain on-top of the CoM physics. Is this because of mechanical constraints or neural control?
• Synergies divided into four groups:
  • Backward perturbation - LH quad&ta and RH quad&ta
  • Leftward perturbation - LH quad&ta and RH ham&gas
  • Upward perturbation - LH ham&gas and RH ham&gas
  • Rightward perturbation - LH ham&gas and RH quad&ta