Partial Unweighting in Obese Persons Enhances Tactile Transmission From the Periphery to Cortical Areas: Impact on Postural Adjustments

Abstract

Tactile plantar information is known to play an important role in balance maintenance and to contribute to the setting of anticipatory postural adjustments (APAs) prior to stepping. Previous studies have suggested that somatosensory processes do not function optimally for obese individuals due to the increased pressure of the plantar sole resulting in balance issues. Here, we investigated whether decreasing the compression of the mechanoreceptors by unweighting the plantar sole would enhance tactile sensory processes leading to an increased stability and an accurate setting of the APAs in obese individuals. More specifically, we tested the hypothesis that the somatosensory cortex response to electric stimulation (SEP) of the plantar sole in standing obese persons will be greater with reduced body weight than with their effective weight. The level of unweighting was calculated for each participant to correspond to a healthy body mass index. We showed an increase SEP amplitude in the unweighted condition compared to the effective body weight for all participants. This increase can be explained by the reduction of weight itself but also by the modified distribution of the pressure exerted onto the foot sole. Indeed, in the unweighted condition, the vertical ground reaction forces are evenly distributed over the surface of the foot. This suggests that decreasing and equalizing the pressure applied on the plantar mechanoreceptors results in an increase in somatosensory transmission and sensory processes for obese persons when unweighted. These sensory processes are crucial prior to step initiation and for setting the anticipatory postural adjustments (i.e., thrust). These cortical changes could have contributed to the observed changes in the spatiotemporal characteristics of the thrust prior to step initiation.

Keywords: EEG; balance; obesity; plantar sole afferents; somatosensory evoked potentials (SEP); unweighting.

Figure 1

(A) (left panel) Position of stimulation electrodes underneath left foot. (right panel) Average of the somatosensory evoked potential for all participants over Cz electrode in 100% BW and unweighted conditions synchronized to the electric stimulation (dashed line). Note that for the figure, only we have superimposed the two conditions relative to the P50 amplitude. (B) Mean amplitude of the averaged P50N90 SEP evoked by the electric stimulation during 100% BW and unweighted condition for all participants (error bars are standard error across participants). (C) Amplitude of the averaged P50N90 SEP for each participant.

Figure 2

(A) Whisker plot of the mean vertical ground reaction force during 100% BW and unweighted conditions in the 2 directions (mediolateral and anteroposterior) for all participants (error bars are standard error across participants) during a 3s period encompassing the whole stimulation in the standing task. (B) Whisker plot of the mean head acceleration during 100% BW and unweighted conditions in the 4 directions (forward, backward, right, and left) for all participants (error bars are standard error across participants) during a 3-s period encompassing the whole stimulation in the standing task. (C) Average of the rear forces and of the somatosensory evoked potential (SEP) in the 100% BW and unweighted conditions for a representative participant. The SEP was synchronized to the electric stimulation (dashed line). The deflection observed in the SEP curve at the moment of the stimulation corresponds to the electrical stimulation artifact.

Figure 3

(A) Mean vertical ground reaction force during 100% BW and unweighted conditions under the right and the left feet for one participant in the stepping task. The participant is stepping with the right foot moving forwards. This figure shows the loading/unloading mechanism. The increase signifies a loading of the weight on the supporting foot, and the decrease represents a simultaneous unloading of the foot to be moved. Note in black the pre-push mode of body weight transfer. Speakers represent the onset of the 3-sound signals with 1-s interval. (B) Whisker plot of the mean amplitude (in absolute value) of the left and right vertical forces during 100% BW and unweighted conditions for all participants (error bars are standard deviation across participants). (C) Percentage of the occurrence of the pre-push mode for each participant in 100% BW and unweighted conditions.