Effects of hypothermically reduced plantar skin inputs on anticipatory and compensatory balance responses

Abstract

Background: Anticipatory and compensatory balance responses are used by the central nervous system (CNS) to preserve balance, hence they significantly contribute to the understanding of physiological mechanisms of postural control. It is well established that various sensory systems contribute to the regulation of balance. However, it is still unclear which role each individual sensory system (e.g. plantar mechanoreceptors) plays in balance regulation. This becomes also evident in various patient populations, for instance in diabetics with reduced plantar sensitivity. To investigate these sensory mechanisms, approaches like hypothermia to deliberately reduce plantar afferent input have been applied. But there are some limitations regarding hypothermic procedures in previous studies: Not only plantar aspects of the feet might be affected and maintaining the hypothermic effect during data collection. Therefore, the aim of the present study was to induce a permanent and controlled plantar hypothermia and to examine its effects on anticipatory and compensatory balance responses. We hypothesized deteriorations in anticipatory and compensatory balance responses as increased center of pressure excursions (COP) and electromyographic activity (EMG) in response to the hypothermic plantar procedure. 52 healthy and young subjects (23.6 ± 3.0 years) performed balance tests (unexpected perturbations). Subjects' foot soles were exposed to three temperatures while standing upright: 25, 12 and 0 °C. COP and EMG were analyzed during two intervals of anticipatory and one interval of compensatory balance responses (intervals 0, 1 and 2, respectively).

Results: Similar plantar temperatures confirmed the successful implementation of the thermal platform. No significant COP and EMG differences were found for the anticipatory responses (intervals 0 and 1) under the hyperthermia procedure. Parameters in interval 2 showed generally decreased values in response to cooling.

Conclusion: No changes in anticipatory responses were found possibly due to sensory compensation processes of other intact afferents. Decreased compensatory responses may be interpreted as the additional balance threat, creating a more cautious behavior causing the CNS to generate a kind of over-compensatory behavior. Contrary to the expectations, there were different anticipatory and compensatory responses after reduced plantar inputs, thereby, revealing alterations in the organization of CNS inputs and outputs according to different task difficulties.

Keywords: CNS; Compensatory responses; Dynamic balance; Plantar hypothermia; Sensitivity, anticipatory responses; Spinal cord.

Fig. 1

Picture of the apparatus used. Subjects stood with their dominant leg on top of the thermal plate which was mounted on top of the force platform. The force platform was then placed on top of the bottom platform of the Posturomed

Fig. 2

Illustration of all four balance conditions: double leg in medio-lateral (DM), double leg in anterior–posterior (DA), single leg in anterior–posterior (SA) and single leg in medio-lateral (ML) perturbation directions (black arrows). The electro-magnet is depicted as solid black rectangles

Fig. 3

Boxplots of plantar foot temperatures of each anatomical location (Met 1, Met 5, Heel) and for each temperature stage (stage I, II, III) before and after the 12 trials

Fig. 4

Boxplots of COP Total excursions for each condition (DA, DM, SA, SM) and each temperature stage for interval 2. Significant differences are marked with asterisks (*p < 0.017)

Fig. 5

Boxplots of EMG values (RMS for TA, GM and FIB) for each condition (DA, DM, SA, SM) and each temperature stage for interval 2. Significant differences are marked with asterisks (*p < 0.017)