Sensory Factors in the Voluntary Application of Pressure

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Abstract
The experiments described attempt to assess the extent of sensory control of voluntary movements of the forearm and finger when the contractions of the muscles concerned are approximately isometric. In the first two experiments, records were obtained of the accuracy with which subjects were able to reproduce a particular pressure with the ulnar border of the hand at different pressure values and rates of application. Results show a statistically significant increase in error of reproduction of pressure as the pressure applied decreases and the rate of application is increased. In the third experiment, using an applied force of 25 gms. weight, a significant difference in accuracy of reproduction of finger pressure between the maximum and a slower rate of application is also shown, but not when the digital nerves are blocked by a local anaesthetic. Furthermore, there is no significant difference in accuracy of reproduction of finger pressure between the anaesthetized and unanaesthetized condition at the fastest rate of application. On the other hand, at the slow rate of application, there is an increasingly significant difference between the anæsthetized and unanaethetized condition as the applied force is reduced below 400 gms. weight, in favour of the control condition. In the fourth experiment, subjects were trained to apply a force of 25 gms. weight using a small area of skin on the palmar surface of the index finger, then tested for their ability to reproduce the same force before and after changing the size of the area of application. Results show a marked change in applied force corresponding to the change in size of skin area used. It is concluded that below an applied force of about 400 gms. weight, the maximum accuracy in reproduction of finger pressure is dependent upon cutaneous receptors. The results of these experiments are discussed in relation to evidence from other experimental, clinical and animal studies and it is suggested that the major function of afferent impulses from a limb in the initiation of voluntary movements is probably facilitatory rather than sensory.

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