18th annual conference on manual control.pdf - Acgsc.org

CL{ANGES IN HUMAN JOINT COMPLIANCE DURING PERIPHERAL ISCHEMIA
by
Robert J. Jaeger, Gvan C. Agarwal, and Gerald 5. Gottlieb
Department of Physiology, Rush Medical College
Chicago, IL 6@612
and
Bioengineering Program, University of Illinois
Chicago, IL 6_68@
ABSTRACT
Joint compliance was measured at both the wrist and ankle by
applying various torque waveforms (step, sinusoidal, and
random) about the joint while measuring the resulting joint
angle. Stretch-evoked electromyograDhic activity (EMG) was
recorded from agonist-antagonist muscle pairs at the joint
being tested. Measurements were made on normal human subjects
before and after the development of peripheral ischemia in the
limb. Ischemia is thought to preferentially block conduction
in large diameter nerve fibers. Before ischemia developed,
feedback from primary muscle spindles was intact. As ischemia
developed, this feedback was reduced or eliminated.
Under normal conditions, the compliance measured by sinusoidal
techniques is dependent on input amplitude and the resting
level of contraction in the muscle. However, for any given mean
level of contraction and RMS level of perturbation, the
compliance is often well fitted by a second order linear
system. When ischemia has developed, changes in compliance were
seen at io_ frequencies. In terms of a second order mechanical
system model, these changes were observed in the stiffness and
viscosity parameters, and not in the moment of inertia. The
observations are strongly dependent on the level of muscle
fatigue during tlhe the development of ischemia. There was also
a concomitant loss of stretch-evoked EMG during the ischemia.
The functional role of stretch-evoked EMG has always been
controversial. These data suggest a relationship between
mechanical characteristics of the joint and the stretch-evoked
EMG, and quantify the mechanical contributions stretch-evoked
(i.e. reflex) responses make to joint compliance.
INTRODUCTION
In studying man-machine interactions one is concerned with a
number of issues. One'of these is how quickly a human
operator can voluntarily respond to disturbances in the system.
Another concerns determining a mathematical transfer function
of the human operator which also incorporates the
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