Journal publishes limb study

Feb 21, 2002, vol. 23, no. 4
By Marianne Meadahl

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Results of a study conducted by a team of SFU kinesiologists and researchers in Japan on how limb stiffness is controlled by the body have been published in a recent edition of the journal Nature.

Kinesiologist Ted Milner and former post-doctoral fellow Etienne Burdet worked on the study in conjunction with Mitsuo Kawato from the Advanced Telecomm-unications Reserarch (ATR) Institute International in Japan. Their paper, The Central Nervous System Stabilizes Unstable Dynamics by Learning Optimal Impedance, appeared in November.

Their study grew out of earlier work carried out at SFU by Milner and student David Franklin on predicting features of arm movement. Burdet later went to ATR to develop a practical technique for measuring the stiffness of the arm during movement. ATR has a sophisticated robotic manipulator designed for studying human arm movements. It can be programmed to apply precise displacements of the arm. By measuring the resistance of the arm to these displacements the researchers obtained estimates of its stiffness.

The researchers found that subjects can selectively increase the stiffness of the arm in specific directions. “What this means is that humans have the ability to selectively activate specific combinations of muscles, for the explicit purpose of increasing the postural stability of the arm in a chosen direction,” says Milner. Burdet is now with the department of mechanical engineering at the National University of Singapore.

Milner says the study demonstrates that the central nervous system has tremendous versatile control over the stiffness of a limb. “Once the characteristics of environmental instability have been identified, the central nervous system can adjust muscle activation patterns to counteract the instability,” explains Milner. “Where this is particularly important is when constraints limit access to other means of changing limb stiffness.”

For example, a downhill skier learns to increase the stability of the body by adopting certain postures that provide stability.

A certain amount of knee and hip flexion increases stability in the forward and backward directions, because they increase the effective stiffness at the point where the foot applies force to the ski, compared to an upright posture.

Increasing the width of the skier's stance, at the base of support, increases stability to the left and right sides.

“However, to increase speed, a downhill skier must adopt the most aerodynamic posture,” Milner notes. “This imposes constraints on the use of posture to maximize stability. There may be conflicts between the most stable posture, and the posture which minimizes aerodynamic drag. In this case, the central nervous system may be able to compensate for loss of stability due to posture by activating appropriate sets of muscles to increase stiffness at the point where the foot applies force to the ski.”

Milner says there are many such similar situations that arise in the tasks we perform each day.

“This versatile ability to control the stiffness of limbs increases the repertoire of actions that we can successfully perform,” notes Milner, who will teach a course on the topic at SFU next semester.

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