Endpoint Force Fluctuations Reveal Flexible Rather Than Synergistic Patterns of Muscle Cooperation

• Published in: Journal of neurophysiology Vol. 100; no. 5; pp. 2455 - 2471
• Main Authors: Kutch, Jason J, Kuo, Arthur D, Bloch, Anthony M, Rymer, William Z
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01.11.2008; American Physiological Society
• Subjects: Biomechanical Phenomena; Electromyography; Feedback; Female; Fingers - innervation; Humans; Isometric Contraction - physiology; Male; Metacarpophalangeal Joint - innervation; Models, Biological; Movement - physiology; Muscle, Skeletal - physiology; Posture - physiology; Psychophysics; Stress, Mechanical
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.90274.2008

1 Applied and Interdisciplinary Mathematics and 2 Mechanical Engineering and Biomedical Engineering, University of Michigan, Ann Arbor, Michigan; and 3 Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois Submitted 15 February 2008; accepted in final form 10 September 2008 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. Address for reprint requests and other correspondence: J. Kutch, Ronald Tutor Hall, RTH-402, 3710 S. McClintock Ave., Los Angeles, CA 90089-2905 (E-mail: kutch{at}usc.edu )