THE ABSENCE OF VISION, BUT NOT A DISTURBANCE OF CUTANEOUS FEEDBACK, IMPAIRED THE STEADINESS OF SUBMAXIMAL, ISOMETRIC CONTRACTIONS

Presented at NACOB 98:
North American Congress on Biomechanics
Canadian Society for Biomechanics - American Society of Biomechanics

University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998

THE ABSENCE OF VISION, BUT NOT A DISTURBANCE OF CUTANEOUS FEEDBACK,
IMPAIRED THE STEADINESS OF SUBMAXIMAL, ISOMETRIC CONTRACTIONS

S.J. De Serres, L. F. Shapiro, M.A. Pflum, R.M. Enoka
Department of Kinesiology, University of Colorado
Boulder, CO 80309-0354

INTRODUCTION

The influence of afferent feedback on the motor output of the first dorsal interosseus muscle was examined in human subjects. Young and older adults performed a simple motor task under three experimental conditions in the presence and absence of vision: control condition, with electrical stimulation that activated the cutaneous afferents, and following anesthesia of the cutaneous afferents. The performance was compared across these conditions to determine the age-related role of cutaneous feedback in the steadiness of submaximal, isometric contractions.

REVIEW AND THEORY

It has been suggested that some of the decline in motor performance with age can be attributed to the decrease in steadiness that is evident during submaximal, isometric contractions. A reduction in steadiness, which was expressed as an increase in force fluctuations, was found in a human hand muscle of elderly subjects for isometric contractions at 5 and 20% of the maximal voluntary contraction force (Galganski et al. 1993). The mechanisms commonly thought to influence steadiness include unfused contractions of motor units, reflex instability, synchronization of motor units, recurrent inhibition, plastic changes in spinal-cord connectivity, and alternating activation of the agonist and antagonist muscles (Vallbo & Wessberg, 1993; Windhorst, 1988).

However, the aging somatosensory system can impair dexterity, as tactile sensibility is diminished in older persons and this can influence motor performance (Cole, 1991). Two methods can be used to study how afferent feedback is involved in a specific task: transient removal of the feedback, or selective stimulation of the afferents. For example, Garnett et al. (1981) have shown previously that a continuous low-intensity electrical stimulation of the skin surface alters the recruitment threshold of motor units.

The aim of this study was to determine the influence of cutaneous afferent feedback on the steadiness of submaximal, isometric contractions in young and older adults.

PROCEDURES

Young (n = 3, 23 ± 3 yrs) and older (n = 4, 71 ± 2 yrs) adults each participated in two experimental sessions. In each session, the subjects were seated with the left arm abducted so that the hand and forearm were pronated and resting on a table. Subjects were asked to perform submaximal, isometric contractions of the first dorsal interosseus muscle at forces that were 5, 10, 20 and 35% of the maximal voluntary contraction (MVC) force. The abduction force exerted by the index finger was displayed on an oscilloscope along with a target force that was to be achieved in the trial. Subjects were instructed to maintain the contraction as steady as possible for 15 seconds, after which they closed their eyes and attempted to keep a similar level of effort for another 15 seconds.

In one experimental session, a continuous electrical stimulation was applied on the index finger while the subject performed the task. Stimuli (50 Hz, 0.1 ms pulse duration) were delivered through two spring-type ring electrodes that were placed around the index finger, with one electrode on each side of the proximal interphalangeal joint. Stimulus strength was set at 4x the perception threshold. Trials with electrical stimulation were randomly interspersed between control trials, when no electrical stimulation was applied. In the other experimental session, a digital nerve-block procedure was performed to anesthetize the cutaneous afferents. Lidocaine (2%) was injected at two locations at the base of the index finger.

Mean and standard deviation of the force were determined for each 15-s segment (vision and no vision) of each trial. The variability in force was expressed as the coefficient of variation (CV, ratio of the standard deviation to the mean force). An increase in CV indicated a decrease in steadiness during the constant force contraction.

RESULTS AND DISCUSSION

Preliminary results showed a large increase in force fluctuations for both groups when the visual feedback was removed (see fig.). However, no difference was observed across the three experimental conditions, suggesting that feedback from cutaneous afferents does not represent an important input in the execution of this task, even in the absence of visual feedback. This conclusion is supported by the result that even the older subjects did not show a change in steadiness when the cutaneous afferent pathway was perturbed or blocked.

Figure 1. Mean ((SE) CV for young (top) and older (bottom) adults relative to the level of submaximal force. ES: condition with electrical stimulation; AA: condition with anesthesia of afferents.

REFERENCES

Cole, J. Mot. Behav., 23, 251-258, 1991.

Galganski et al. J. Neurophysiol., 69, 2108-2115, 1993.

Garnett and Stephens, J. Physiol., 311, 463-473, 1981.

Vallbo and Wessberg, J. Physiol., 469, 673-691, 1993.

Windhorst, How brain-like is the spinal cord? Berlin: Springer-Verlag, 1988.

ACKNOWLEDGMENTS

This study was supported by NIA Grant AG-09000 to R.M.E. and a fellowship from the FRSQ to S.J.D.