Abstract
In a previous experiment, we showed that among young and healthy subjects, thin plantar inserts improve postural control and modify vergence amplitudes. In this experiment, however, significant inter-individual variability was observed. We hypothesize that its origin could be attributed to a different reliance upon feet cutaneous afferents. In order to test this hypothesis, we re-analyzed the data relative to 31 young (age 25.7 ± 3.8) and healthy subjects who participated in the first experiment after having classified them into two groups depending on their Plantar Quotient (PQ = Surface area of CoPfoam/Surface area of CoPfirm ground × 100). Foam decreases the information arising from the feet, normally resulting in a PQ > 100. Hence, the PQ provides information on the weight of plantar cutaneous afferents used in postural control. Twelve people were Plantar-Independent Subjects, as indicated by a PQ < 100. These individuals did not behave like the Normal Plantar Quotient Subjects: they were almost insensitive to the plantar stimulations in terms of postural control and totally insensitive in terms of oculomotor control. We conclude that the inter-individual variability observed in our first experiment is explained by the subjects' degree of plantar reliance. We propose that plantar independence is a dysfunctional situation revealing inefficiency in plantar cutaneous afferents. The latter could be due to a latent somatosensory dysfunction generating a noise which prevents the CNS from correctly processing and using feet somatosensory afferents both for balance and vergence control: Plantar Irritating Stimulus. Considering the non-noxious nature and prevalence of this phenomenon, these results can be of great interest to researchers and clinicians who attempt to trigger postural or oculomotor responses through mechanical stimulation of the foot sole.
Highlights
The control of posture involves a multisensory system in which somatosensory information arising from the feet plays an important role (Nashner et al, 1982; Kavounoudias et al, 2001)
The only persisting postural effect of the inserts among the Plantar-Independent Subjects” (PIS) is a backward shift of the mean (Y) position of the Center Of Pressure (CoP), and only with Medial Arch Support (MAS) compared to Control
In absolute terms, the Surface area changed to a greater degree in the PIS group than among the Normal Plantar Quotient Subjects” (NPQS), most especially with MAS compared to Control
Summary
The control of posture involves a multisensory system in which somatosensory information arising from the feet plays an important role (Nashner et al, 1982; Kavounoudias et al, 2001) In their experiment, Kavounoudias et al (2001) stimulated foot sole mechanoreceptors and ankle tendons. Plantar Exteroceptive Efficiency Explains Idiosyncrasy muscle spindles with low amplitude mechanical vibrations They concluded that the regulation of small-amplitude body sways is mainly assigned to feet tactile afferents, whereas ankle muscle proprioception is more involved in the regulation of larger body sways. Foam decreases the information arising from the feet: Yi and Park (2009) have shown that when healthy subjects are standing on foam, it induces a decrease in plantar cutaneous sensitivity to microfilament touch, similar to the one shown by patients suffering from peripheral sensory neuropathy
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