Mechanical sensibility of the sole of the foot determined with vibratory stimuli of varying frequency.

The mechanoreceptive properties of the sole of the foot were determined by measuring the detection thresholds to vibratory stimuli of 20, 80, and 240 Hz frequency and 300 ms duration. The thresholds were measured at six different sites on the left sole and at toes 1 and 3 with probes of 2 and 8 mm diameter connected to the moving coil of an electromechanical vibrator. The subject sat in an armchair during the experiments, with the left leg supported horizontally by a vacuum cast positioned on a table. Six subjects participated in the experiments. A simple method of limits was used to make the measurements. Lower average thresholds were obtained with higher vibration frequencies, the average thresholds varying between 40-90 microns at 20 Hz and well below 10 microns at 240 Hz. The major decrease in thresholds occurred between 20 and 80 Hz. Interindividual variability in thresholds was large, but the threshold curves obtained from different subjects and from different stimulation points were of the same general shape. The highest thresholds were measured from the toes, but this regional differences in sensibility was obtained only at the higher vibration frequencies. Comparison of the threshold values at the sole with those found with similar stimuli at the thenar eminence and middle fingertip indicates that the mechanoreceptor mechanisms transmitting information about low-frequency vibration in the sole are similar to those in the palmar skin of the hand.