Neuromuscular and biomechanical factors codetermine the solution to motor redundancy in rhythmic multijoint arm movement

Aymar de Rugy, Stephan Riek, Yalchin Oytam, Timothy J. Carroll, Rahman Davoodi, Richard G. Carson
Exp Brain Res. 2008-06-11; 189(4): 421-434
DOI: 10.1007/s00221-008-1437-2

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1. Exp Brain Res. 2008 Aug;189(4):421-34. doi: 10.1007/s00221-008-1437-2. Epub 2008
Jun 11.

Neuromuscular and biomechanical factors codetermine the solution to motor
redundancy in rhythmic multijoint arm movement.

de Rugy A(1), Riek S, Oytam Y, Carroll TJ, Davoodi R, Carson RG.

Author information:
(1)Perception and Motor Systems Laboratory, School of Human Movement Studies, The
University of Queensland, Room 424, Building 26, St Lucia, Brisbane, QLD, 4072,

How the CNS deals with the issue of motor redundancy remains a central question
for motor control research. Here we investigate the means by which neuromuscular
and biomechanical factors interact to resolve motor redundancy in rhythmic
multijoint arm movements. We used a two-df motorized robot arm to manipulate the
dynamics of rhythmic flexion-extension (FE) and supination-pronation (SP)
movements at the elbow-joint complex. Participants were required to produce
rhythmic FE and SP movements, either in isolation, or in combination (at the
phase relationship of their choice), while we recorded the activity of key
bi-functional muscles. When performed in combination, most participants
spontaneously produced an in-phase pattern of coordination in which flexion is
synchronised with supination. The activity of the Biceps Brachii (BB), the
strongest arm muscle which also has the largest moment arms in both flexion and
supination was significantly higher for FE and SP performed in combination than
in isolation, suggesting optimal exploitation of the mechanical advantage of this
muscle. In a separate condition, participants were required to produce a rhythmic
SP movement while a rhythmic FE movement was imposed by the motorized robot.
Simulations based upon a musculoskeletal model of the arm demonstrated that in
this context, the most efficient use of the force-velocity relationship of BB
requires that an anti-phase pattern of coordination (flexion synchronized with
pronation) be produced. In practice, the participants maintained the in-phase
behavior, and BB activity was higher than for SP performed in isolation. This
finding suggests that the neural organisation underlying the exploitation of
bifunctional muscle properties, in the natural context, constrains the system to
maintain the « natural » coordination pattern in an altered dynamic environment,
even at the cost of reduced biomechanical efficiency. We suggest an important
role for afference from the imposed movement in promoting the « natural » pattern.
Practical implications for the emerging field of robot-assisted therapy and
rehabilitation are briefly mentioned.

DOI: 10.1007/s00221-008-1437-2
PMID: 18545990 [Indexed for MEDLINE]

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