Among young individuals, elderly individuals and transradial amputees, lateral areas on the forearms were relatively more sensitive than medial areas in an analysis of stimulation area and stimulation intensity, according to study results.

The “lack of information on residual sensitivity and the lack of technologies for developing cognitive skills to replace residual sensibility mean that the disabled will continue to have troubles adapting to ordinary life. In order for the elderly and the disabled to promptly respond to changes in their external environment by using their residual sensibility and raise the efficiency of their rehabilitation, it is necessary to understand the characteristics of the skin’s residual sensibility in relation to external stimuli,” the researchers wrote. “Therefore, this study intends to analyze the effects of changes in vibration stimulation intensity in accordance with frequency changes on the subjective response of somatic sensation in the elderly and the transradial amputees who have an acquired loss caused by trauma.”

Sensitivity results

Researchers set up a mechanical vibration system that utilized eight-channel stimulators with a frequency range from 37 Hz to 223 Hz, a reception system for measuring and monitoring, as well as a subjective response evaluation with a one-to-ten scale to measure change of residual somatic sensibility in 20 transradial amputees, 30 young participants without musculoskeletal and neurological disorders and 30 elderly participants with good cognitive function. The effect of random mechanical stimulation on 25% of the proximal forearm’s area was also measured.

Regarding the channels of the vibration device, channels 1 to 4 were related to the lateral area of the forearm, whereas channels 5 to 8 were related to the medial areas. Sensitivity to stimulation was displayed in channels two and three for young participants, while elderly men participants were more sensitive to stimulation in channels four and five. Amputee men experienced the most sensitivity to channel two and amputee women to channels one and two. Consequently, among the amputee and young group, the lateral area of the forearm was more sensitive vs. the medial area. However, the elderly group’s sensitivity was the highest between the lateral and medial areas.

Ten-point subjective response scales (SR-scales) tended to increase with the increase in frequency from 37 Hz to 149 Hz in young men. However, from 176 Hz and upward, their SR-scales did not change in accordance with increases in frequency, according to the researchers. Study results also showed that the SR-scales of healthy women continuously increased along with increases in frequency. In both elderly men and women, saturation occurred rapidly, even at low stimulation frequencies, compared with the young women group. According to study results, saturation occurred around 198 Hz in elderly men, between 120 Hz and 149 Hz in elderly women and around 149 Hz in amputee men, while SR-scales continually increased among amputee women.

Additional research

According to study results, saturation was at a lower frequency in the amputee men compared with young and older men. The researchers believe that additional research is necessary to find the reasons for this result. Additional research is also necessary to verify whether response differences to vibration stimulation among the groups is due to the fact that the external diameter of bones become large and the thickness of the skin becomes thinner with age, creating a closer distance between the bone tissue within the forearm and external vibration stimulation source in the elderly group but not in the young and amputee groups.

Researchers said that future somatic sensation experiments would involve a stimulation delivery module that uses optimal stimulation intensity, intensity area and optimal stimulation time in the consideration of temporal and spatial summation to apply to hand and foot prostheses. Diverse approaches should determine the best methods and placements for the stimulation delivery module, such as applying the module outside a socket using armbands or including it within the socket, according to researchers. — by Casey Murphy

For more information:
Bae TS. Clin Biomech. 2012;doi:10.1016/j.clinbiomech.2012.11.012.

Disclosure: The researchers have no relevant financial disclosures.

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