The global foot orthotic industry was estimated around $3 billion in 2017. Biomechanists and clinical researchers have used foot orthoses (FOs) to study their effects on kinetic, kinematic, and muscle activity, although the mechanisms defining how a FO functions remain unclear. Recently, a neuromotor paradigm has been proposed suggesting that a FO augments the sensory feedback from foot sole skin and subsequently reduces muscle activity while optimizing movement. Although this paradigm currently lacks supporting evidence, neurophysiological research has linked foot sole skin to motorneuron pools of the lower extremity. Furthermore, one potential method of intentionally stimulating sensory feedback from the foot sole is to target the activation of cutaneous mechanoreceptors within the design of FOs. Thus, the overall objective of this dissertation was to design a series of experimental studies which uses texture in FO design to intentionally stimulate mechanoreceptors in foot sole skin, and then measure the modulation of lower leg and plantar foot intrinsic muscle activity during locomotor tasks. By studying the effects of textured foot orthoses (FOTs) on muscle activity, this novel approach to FO design will support or refute the defining principles of the neuromotor paradigm, while also providing mechanistic insight into the provision of FOTs.
The results of study 1 confirmed that texture placed under distinct regions of the foot sole can modify lower leg muscle activity during walking. More specifically, distinct regions of tactile feedback demonstrated stimulation-site and gait-phase specificity in the modulation of tibialis anterior, peroneus longus, medial gastrocnemius, extensor digitorum longus, extensor hallucis longus, tibialis posterior, flexor digitorum longus, and flexor hallucis longus during walking. These results support the topographical organization of cutaneous mechanoreceptors in foot sole skin and is the first study exploring how texture can be used in FO design to target the modulation of lower leg muscle activation during locomotion. Study 2 explored the use of texture along the entire length of the foot sole while measuring muscle activity of 4 plantar intrinsic foot muscles (abductor hallucis, transverse head of adductor hallucis, flexor digitorum brevis, and abductor digiti minimi)
during walking. Similar to study 1, results of this study demonstrated phasic modulation of foot intrinsic musculature that was apparent throughout the stance and swing phases of gait. In studies 3 and 4, previously collected data from studies 1 and 2 subdivided the results by foot posture. Using a commonly adopted clinical tool, the Foot Posture Index, results from these studies provided evidence which supports the variability in lower leg and plantar intrinsic foot muscle activation across the foot posture spectrum. Future research exploring texture in FO design is encouraged to consider foot posture when designing experimental protocols.
Overall, this dissertation provides evidence which supports the use of textured materials in FO design while increasing the data available to the scientific community in developing new FO-related research questions. In the interest of distilling the connection between cutaneous mechanoreceptors of the foot sole and muscles’ of the lower extremity and foot, this dissertation suggests the following revision to the neuromotor paradigm: “FOs can modify sensory output to the central nervous system (CNS), and subsequently facilitate and/or inhibit motorneuron pool activation of lower extremity and foot intrinsic musculature during movement”. Future research is now encouraged to ask new questions and develop new experimental protocols which support or refute this paradigm.
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