The objective of this study was to develop a quantitative method to assess muscle spindle function. Three groups of subjects were studied: ten young and healthy subjects, 15 older subjects with diabetic neuropathy, and 15 age-matched controls. All subjects performed an ankle-movement matching task with and without muscle vibration. Input from the plantar cutaneous mechanoreceptors was minimized by using a foot-clamping device. The younger subjects tracked the movement very well, but vibration had a significant effect on their performance (P < 0.001). Similar results were seen in the older control subjects, but they were less successful in tracking movement and slightly less affected by vibration. The neuropathic subjects had the most difficulty tracking, and vibration had only a small but still significant effect on their performance. The interaction between the group and the vibration effect was highly significant (P < 0.001), indicating that the performance of the control subjects changed to a greater degree in the presence of vibration than the performance of the subjects with diabetic neuropathy. Muscle spindles are the primary receptors that are involved in the change in tracking performance when vibration is added during an ankle-movement matching task, and we therefore conclude that the procedure described provides a quantitative evaluation of muscle spindle function. The results demonstrate that diabetic neuropathy degrades muscle sensory function, which may contribute to the impaired balance and unsteadiness of gait that has been observed in diabetic neuropathy.