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Kutch - 2008 - Endpoint force fluctuations reveal flexible rather than synergistic patterns ...


Kutch JJ, Kuo AD, Bloch AM, Rymer WZ. Endpoint force fluctuations reveal flexible rather than synergistic patterns of muscle cooperation. 1: J Neurophysiol. 2008 Nov;100(5):2455-71. PUBMED

10 Word Summary

Finger muscles activate individually for specific tasks, instead of together.


We developed a new approach to investigate how the nervous system activates multiple redundant muscles by studying the endpoint force fluctuations during isometric force generation at a multi-degree-of-freedom joint. We hypothesized that, due to signal-dependent muscle force noise, endpoint force fluctuations would depend on the target direction of index finger force and that this dependence could be used to distinguish flexible from synergistic activation of the musculature. We made high-gain measurements of isometric forces generated to different target magnitudes and directions, in the plane of index finger metacarpophalangeal joint abduction-adduction/flexion-extension. Force fluctuations from each target were used to calculate a covariance ellipse, the shape of which varied as a function of target direction. Directions with narrow ellipses were approximately aligned with the estimated mechanical actions of key muscles. For example, targets directed along the mechanical action of the first dorsal interosseous (FDI) yielded narrow ellipses, with 88% of the variance directed along those target directions. It follows the FDI is likely a prime mover in this target direction and that, at most, 12% of the force variance could be explained by synergistic coupling with other muscles. In contrast, other target directions exhibited broader covariance ellipses with as little as 30% of force variance directed along those target directions. This is the result of cooperation among multiple muscles, based on independent electromyographic recordings. However, the pattern of cooperation across target directions indicates that muscles are recruited flexibly in accordance with their mechanical action, rather than in fixed groupings.


  • First dorsal interosseousThe dorsal interosseous muscles are bipennate, with each muscle arising by two heads from the adjacent sides of the metacarpal bones, but more extensively from the metacarpal bone of the finger into which the muscle is inserted. They are inserted into the bases of the proximal phalanges and into the extensor expansion of the corresponding extensor digitorum tendon. - wikipedia
  • Muscle synergy - fixed patterns of activation among multiple muscles acting about the relevant DoF.
  • Difficulties with measuring synergies:
    • Not able to record all active muscles with EMG
    • Task is overly restrictive and does not allow for full range of muscle behaviors
  • Signal dependent noise: SDN - used to characterize endpoint variability by suggesting that each muscle will produce different amounts of signal dependent noise.
  • Model requires these assumptions:
    • Linear superposition
    • Signal dependent noise
    • Low correlation of noise-like forces between muscles
  • In Figure 1(F) why with synergies will only non-target directed variance be observed?
  • Looked at "static" behavior of constant force for >10s
    • Different than the transients that we have been looking at in the lab.
  • In Figure 3(F) I see another aligned direction at 90°, do they mention this?
  • Questions left unanswered:
    • Are target-directed force covariance ellipses associated with EMGs of particular muscles?
    • Do target-directed force covariance ellipses arise from combining muscles with different action directions?
    • Can the force covariance map be predicted from EMGs?