Calf Cutaneous Stimulation Generates Reflexes in the Thigh that Can be Modified by Subthreshold Noisy Vibration.

Imperceptible tactile noise applied to the skin of the feet enhances posture-correcting cutaneous reflexes. This sensory augmentation technique, stochastic resonance (SR), has not been tested in the less-sensitive hairy skin of the leg for its reflex-enhancement ability. The objectives of this study were to determine whether calf skin stimulation produces cutaneous reflexes and if noise can modify the reflex. In 20 participants, electrotactile pulse trains were applied at the calf while participants performed submaximal isometric knee extension. To test SR, five different levels of vibrotactile noise were applied simultaneous to the test input. Muscle activity from the vastus lateralis (VL) was analyzed 60-110 ms after stimulation. Reflex ratios were calculated by dividing the reflex peak activity by the pre-stimulation background muscle activity. A significant reflex response was evoked in 16/20 participants (5.41 2.6% of background muscle activity); these responses varied between individuals with 8 being facilitatory and 8 being inhibitory. In half of the participants, a new reflex appeared at some level of added noise (N=10). The average reflex ratio of the study population was significantly higher at the "optimal" noise level (8.61 ± 4.5) compared to "baseline" (4.70 ± 5.6) (p = 0.002); the optimal level varied across participants. These results suggest that cutaneous reflexes exist at the VL in response to calf skin stimulation and that SR can change cutaneous reflexes at the leg. This study provides an important first step towards SR application in clinical populations with sensory loss such as individuals with lower extremity amputation.