Mechanoreceptor sensory feedback is impaired by pressure induced cutaneous ischemia on the human foot sole and can predict cutaneous microvascular reactivity.

Introduction: The foot sole endures high magnitudes of pressure for sustained periods which results in transient but habitual cutaneous ischemia. Upon unloading, microvascular reactivity in cutaneous capillaries generates an influx of blood flow (PORH: post-occlusive reactive hyperemia). Whether pressure induced cutaneous ischemia from loading the foot sole impacts mechanoreceptor sensitivity remains unknown. Methods: Pressure induced ischemia was attained using a custom-built-loading device that applied load to the whole right foot sole at 2 magnitudes (15 or 50% body weight), for 2 durations (2 or 10 minutes) in thirteen seated participants. Mechanoreceptor sensitivity was assessed using Semmes-Weinstein monofilaments over the third metatarsal (3MT), medial arch (MA), and heel. Perceptual thresholds (PT) were determined for each site prior to loading and then applied repeatedly to a metronome to establish the time course to return to PT upon unload, defined as PT recovery time. Microvascular flux was recorded from an in-line laser speckle contrast imager (FLPI-2, Moor Instruments Inc.) to establish PORH peak and recovery rates at each site. Results: PT recovery and PORH recovery rate were most influenced at the heel and by load duration rather than load magnitude. PT recovery time at the heel was significantly longer with 10 minutes of loading, regardless of magnitude. Heel PORH recovery rate was significantly slower with 10minutes of loading. The 3MT PT recovery time was only longer after 10 minutes of loading at 50% body weight. Microvascular reactivity or sensitivity was not influenced with loading at the MA. A simple linear regression found that PORH recovery rate could predict PT recovery time at the heel (R2=0.184, p<0.001). Conclusion: In populations with degraded sensory feedback, such as diabetic neuropathy, the risk for ulcer development is heightened. Our work demonstrated that prolonged loading in healthy individuals can impair skin sensitivity, which highlights the risks of prolonged loading and is likely exacerbated in diabetes. Understanding the direct association between sensory function and microvascular reactivity in age and diabetes related nerve damage, could help detect early progressions of neuropathy and mitigate ulcer development.

Vestibular input modulates stepping balance reactions early in the pre-step phase through to post-recovery.

Compensatory stepping reactions to recover balance are frequently performed, however, the role of sensory feedback in regulating these responses is not fully understood. Specifically, it is unknown whether vestibular input influences compensatory stepping. Here, we aimed to assess whether step responses utilize vestibular input by combining medio-lateral galvanic vestibular stimulation (GVS) with step-inducing balance perturbations via unpredictable anterior-posterior platform translations. Step responses were assessed for any lateral differences due to the illusory sense of left (LGVS) or rightward (RGVS) postural motion in terms of pre-step weight-shifts, center of mass (COM) motion and step-placement as well as lateral stability when recovering balance. GVS evoked clear differences from the pre-step phase onwards, in an asymmetrical pattern depending on the GVS direction relative to the right step-leg side. RGVS induced a leftwards postural shift to create a larger stability margin to the right (p < 0.0007), opposing the illusory motion and reducing the fall towards the unsupported side during the step; however, RGVS caused no change in step-width. Conversely, LGVS evoked a leftward step placement (p < 0.0001) in the direction of the mis-sensed motion, but without any rightward shift in postural motion. This asymmetry is consistent with vestibular input predictively modulating pre-step lateral weight-shifts and foot-placement in accordance with step mechanics, specifically in controlling frontal plane stability when lifting the foot to step.

Neuromechanical characterization of the abductor hallucis and its potential role in upright postural control.

There is growing evidence to support a role for the abductor hallucis (AH) in standing balance control; however, functional properties of the muscle that may provide more insight into AH's specific contribution to upright posture have yet to be characterized. This study was conducted to quantify functional neuromechanical properties of the AH and correlate the measures with standing balance variables. We quantified strength and voluntary activation during maximal voluntary isometric contractions of the great toe abductor in nine (3 females and 6 males) healthy, young participants. During electrically evoked twitch and tetanic contractions, we measured great toe abduction peak force and constructed a force-frequency curve. We also evaluated peak abduction force, contraction time (CT), half-relaxation time (HRT), rate of force development (RFD), and relaxation rate (RR) from twitch contractions evoked using doublet stimuli. Strength, VA, CT, HRT, RFD, and RR were correlated to centre of pressure standard deviation (COP SD) and velocity (COP VEL) variables of the traditional COP trace and its rambling and trembling components during single-legged stance. AH twitch properties (e.g., CT: 169.8 ± 32.3 ms; HRT: 124.1 ± 29.2 ms) and force-frequency curve were similar to other slow contractile muscles. Contractile speed related negatively with COP VEL, suggesting AH may be appropriate for slow, prolonged tasks such as ongoing postural balance control. Correlation coefficient outcomes for all variables were similar between rambling and trembling components. Our results provide further evidence for the importance of AH neuromechanical function for standing balance control, at least during a challenging single-legged posture.

Heating the skin on the foot sole enhances cutaneous reflexes in the lower limb.

Cutaneous input is important in postural control and balance. Aging and diabetes impair skin sensitivity and motor control. Heat application can improve skin sensation, but its influence on motor control remains unknown. This study investigated the effects of heating the skin of the foot sole on lower limb cutaneous reflexes. Reflexes were evoked in the tibialis anterior muscle of 20 young, healthy adults before and after heating the foot sole to a maximum of 42°C. While holding a 15% maximum root mean square EMG generated during maximum isometric dorsiflexion, a filtered white noise (0-50 Hz) vibration at 10 times the perceptual threshold was applied to the heel to stimulate cutaneous mechanoreceptors. Reflexes were analyzed in both the time (cumulant density) and frequency (coherence, gain) domains. Heat increased foot skin temperature ∼15.4°C (P < 0.001). Cumulant density peak to peak amplitude significantly increased by 44% after heating (P = 0.01) while latencies did not vary (P = 0.46). Coherence and gain were significantly greater in the 30- to 40-Hz range following heating (P = 0.048; P = 0.02). Heating significantly enhances lower limb cutaneous reflexes. This may be due to the increased ability of cutaneous mechanoreceptors to encode in the 30- to 40-Hz range.NEW & NOTEWORTHY Cutaneous input is a known modulator of muscle activity. Targeting skin to intentionally enhance motor output has received little attention. We explored local skin heating to enhance skin sensitivity and found a significant increase in the amplitude, coherence, and gain of cutaneous reflexes in the tibialis anterior. Our current findings provide the first support for the use of heat as a viable and easily integrated modality in rehabilitation technology to improve balance and postural control.

Calf cutaneous stimulation generates reflexes in the thigh that can be modified by subthreshold noisy vibration.

Imperceptible tactile noise applied to the skin of the feet enhances posture-correcting cutaneous reflexes. This sensory augmentation technique, stochastic resonance (SR), has not been tested in the less-sensitive hairy skin of the leg for its reflex-enhancement ability. The objectives of this study were to determine whether calf skin stimulation produces cutaneous reflexes and whether noise can modify the reflex. In 20 participants, electrotactile pulse trains were applied at the calf while participants performed submaximal isometric knee extension. To test SR, five different levels of vibrotactile noise were applied simultaneously to the test input. Muscle activity from the vastus lateralis (VL) was analyzed 60-110 ms after stimulation. Reflex ratios were calculated by dividing the reflex peak activity by the prestimulation background muscle activity. A significant reflex response was evoked in 16/20 participants (5.41 ± 2.6% of background muscle activity); these responses varied between individuals with eight being facilitatory and eight being inhibitory. In half of the participants, a new reflex appeared at some level of added noise (n = 10). The average reflex ratio of the study population was significantly higher at the "optimal" noise level (8.61 ± 4.5) than at "baseline" (4.70 ± 5.6) (P = 0.002); the optimal level varied across participants. These results suggest that cutaneous reflexes exist at the VL in response to calf skin stimulation and that SR can change cutaneous reflexes at the leg. This study provides an important first step toward SR application in clinical populations with sensory loss such as individuals with lower extremity amputation.NEW & NOTEWORTHY Our work showed that cutaneous reflexes, known to be present in response to foot sole stimulation, can also be evoked by stimulation of hairy leg skin. In addition, we demonstrated that adding tactile noise can enhance this reflex response. These findings demonstrate proof-of-concept for potential future applications where tactile stimulation, applied to the leg of an individual with amputation, can enhance postural-relevant reflexes. Improving postural control may reduce the risk of falls in this high-risk population.

Modeling foot sole cutaneous afferents: FootSim.

While walking and maintaining balance, humans rely on cutaneous feedback from the foot sole. Electrophysiological recordings reveal how this tactile feedback is represented in neural afferent populations, but obtaining them is difficult and limited to stationary conditions. We developed the FootSim model, a realistic replication of mechanoreceptor activation in the lower limb. The model simulates neural spiking responses to arbitrary mechanical stimuli from the combined population of all four types of mechanoreceptors innervating the foot sole. It considers specific mechanics of the foot sole skin tissue, and model internal parameters are fitted using human microneurography recording dataset. FootSim can be exploited for neuroscientific insights, to understand the overall afferent activation in dynamic conditions, and for overcoming the limitation of currently available recording techniques. Furthermore, neuroengineers can use the model as a robust in silico tool for neuroprosthetic applications and for designing biomimetic stimulation patterns starting from the simulated afferent neural responses.

Heating the Skin Over the Knee Improves Kinesthesia During Knee Extension.

To determine how heating affects dynamic joint position sense at the knee, participants (n = 11; F = 6) were seated in a HUMAC NORM dynamometer. The leg was passively moved through extension and flexion, and participants indicated when the 90° reference position was perceived, both at baseline (28.74 ± 2.43 °C) and heated (38.05 ± 0.16 °C) skin temperatures. Day 2 of testing reduced knee skin feedback with lidocaine. Directional error (actual leg angle-target angle) and absolute error (AE) were calculated. Heating reduced extension AE (baseline AE = 5.46 ± 2.39°, heat AE = 4.10 ± 1.97°), but not flexion. Lidocaine did not significantly affect flexion AE or extension AE. Overall, increased anterior knee-skin temperature improves dynamic joint position sense during passive knee extension, where baseline matching is poorer. Limited application of lidocaine to the anterior thigh, reducing some skin input, did not influence dynamic joint position sense, suggesting cutaneous receptors may play only a secondary role to spindle information during kinesthetic tasks. Importantly, cutaneous input from adjacent thigh regions cannot be ruled out as a contributor.

Location-specific cutaneous electrical stimulation of the footsole modulates corticospinal excitability to the plantarflexors and dorsiflexors during standing.

Non-noxious electrical stimulation to distinct locations of the foot sole evokes location-specific cutaneous reflex responses in lower limb muscles. These reflexes occur at latencies that may enable them to be mediated via a transcortical pathway. Corticospinal excitability to the plantarflexors and dorsiflexors was measured in 16 participants using motor evoked potentials (MEPs). Spinal excitability was measured in eight of the original participants using cervicomedullary motor evoked potentials (CMEPs). Measurements were collected with and without preceding cutaneous stimulus to either the heel (HEEL) or metatarsal (MET) locations of the foot sole, and evoked potentials were elicited to coincide with the arrival of the cutaneous volley at either the motor cortex or spinal cord. Plantarflexor MEPs and CMEPs were facilitated with cutaneous stimulation to the HEEL for MEPs (soleus p = 0.04, medial gastrocnemius (MG) p = 0.017) and CMEPs (soleus p = 0.047 and MG p = 0.015), but they were unchanged following MET stimulation for MEPs or CMEPs. Dorsiflexor MEPs were unchanged with cutaneous stimulation at either location, but dorsiflexor CMEPs increased with cutaneous stimulation (p = 0.05). In general, the increase in CMEP amplitudes was larger than the increase in MEP amplitudes, indicating that an increase in spinal excitability likely explains most of the increase in corticospinal excitability. The larger change observed in the CMEP also indicates that excitability from supraspinal sources likely decreased, which could be due to a net change in the excitability of intracortical circuits. This study provides evidence that cutaneous reflexes from foot sole skin are likely influenced by a transcortical pathway.

The immediate effects of foot orthosis geometry on lower limb muscle activity and foot biomechanics.

Foot orthoses (FOs) are used to treat clinical conditions by altering the external forces applied to the foot and thereafter the forces of muscles and tendons. However, whether specific geometric design features of FOs affect muscle activation is unknown. The aim of this study was to investigate if medial heel wedging and increased medial arch height have different effects on the electromyography (EMG) amplitude of tibialis posterior, other muscles of the lower limb and the kinematics and kinetics at the rearfoot and ankle. Healthy participants (n = 19) walked in standardised shoes with i) a flat inlay; ii) a standard shape FOs, iii) standard FOs adjusted to incorporate a 6 mm increase in arch height, iv) and standard FOs adjusted to incorporate an 8° medial heel wedging and v) both the 6 mm increase in arch height and 8° increase in medial wedging. EMG was recorded from medial gastrocnemius, peroneus longus, tibialis anterior and in-dwelling tibialis posterior muscles. Motion and ground reaction force data were collected concurrently. Tibialis posterior EMG amplitude reduced in early stance with all FOs (ηp2 = 0.23-1.16). Tibialis posterior EMG amplitude and external ankle eversion moment significantly reduced with FOs incorporating medial wedging. The concurrent reduction in external eversion moment and peak TP EMG amplitude in early stance with medial heel wedging demonstrates the potential for this specific FOs geometric feature to alter TP activation. Medial wedged FOs could facilitate tendon healing in tibialis posterior tendon dysfunction by reducing force going through the TP muscle tendon unit.

Exploring the effect of capsaicin-induced central sensitization on the upper limb nociceptive withdrawal reflex threshold.

The nociceptive withdrawal reflex (NWR) threshold is commonly employed in the lower limb to assess clinical and experimentally induced pain. However, no studies to date have investigated changes in spinal nociception in the upper limb, via the NWR threshold, following experimentally induced central sensitization (CS). We tested the hypothesis that experimentally induced CS of the C5-C6 spinal segment significantly reduces NWR thresholds in muscles of the upper limb. Upper limb NWR thresholds from 20 young, healthy adults were assessed by applying noxious electrical stimuli to the right index finger and recording muscle activity from the biceps brachii (BI), triceps brachii (TRI), flexor carpi ulnaris (WF), and extensor carpi radialis longus (WE) muscles via surface electromyography. Topical cream (either 0.075% capsaicin, or control) was applied to the C5-C6 dermatome of the lateral forearm (50 cm2). NWR thresholds were compared at baseline, and four 10-min intervals after topical application. WF muscle NWR thresholds were significantly reduced in the capsaicin session compared to control, while TRI muscle NWR thresholds were significantly reduced 40 min after capsaicin application only (p < 0.05). There were no significant differences for BI or WE muscle NWR thresholds. We observed poor to moderate test-retest reliability for all upper limb NWR thresholds, a key contributor to the selective reduction in NWR thresholds among muscles. Accordingly, while our findings demonstrate some comparability to previously reported lower limb NWR studies, we concurrently report limitations of the upper limb NWR technique. Further exploration of optimal parameters for upper limb NWR acquisition is needed.

The Anatomical Distribution of Mechanoreceptors in Mouse Hind Paw Skin and the Influence of Integrin α1ß1 on Meissner-Like Corpuscle Density in the Footpads.

Afferent neurons and their mechanoreceptors provide critical sensory feedback for gait. The anatomical distribution and density of afferents and mechanoreceptors influence sensory feedback, as does mechanoreceptor function. Electrophysiological studies of hind paw skin reveal the different types of afferent responses and their receptive fields, however, the anatomical distribution of mechanoreceptor endings is unknown. Also, the role of integrin α1ß1 in mechanoreceptor function is unclear, though it is expressed by keratinocytes in the stratum basale where it is likely involved in a variety of mechanotransduction pathways and ion channel functionalities. For example, it has been shown that integrin α1ß1 is necessary for the function of TRPV4 that is highly expressed by afferent units. The purpose of this study, therefore, was to determine and compare the distribution of mechanoreceptors across the hind paw skin and the footfall patterns of itga1-null and wild type mice. The itga1-null mouse is lacking the integrin α1 subunit, which binds exclusively to the ß1 subunit, thus rendering integrin α1ß1 nonfunctional while leaving the numerous other pairings of the ß1 subunit undisturbed. Intact hind paws were processed, serially sectioned, and stained to visualize mechanoreceptors. Footfall patterns were analyzed as a first step in correlating mechanoreceptor distribution and functionality. Merkel cells and Meissner-like corpuscles were present, however, Ruffini endings and Pacinian corpuscles were not observed. Meissner-like corpuscles were located exclusively in the glabrous skin of the footpads and digit tips, however, Merkel cells were found throughout hairy and glabrous skin. The increased density of Merkel cells and Meissner-like corpuscles in footpads 1 and 3 and Meissner-like corpuscles in footpad 4 suggests their role in anteroposterior balance, while Meissner-like corpuscle concentrations in digits 2 and 5 support their role in mediolateral balance. Finally, a larger density of Meissner-like corpuscles in footpads 3 and 4 in male itga1-null mice compared to wild type controls paves the way for future site-specific single fiber in vivo recordings to provide insight into the role of integrin α1ß1 in tactile mechanotransduction.

Cutaneous tactile sensitivity before and after tail loss and regeneration in the leopard gecko (Eublepharis macularius).

Amongst tetrapods, mechanoreceptors on the feet establish a sense of body placement and help to facilitate posture and biomechanics. Mechanoreceptors are necessary for stabilizing the body while navigating through changing terrains or responding to a sudden change in body mass and orientation. Lizards such as the leopard gecko (Eublepharis macularius) employ autotomy - a voluntary detachment of a portion of the tail - to escape predation. Tail autotomy represents a natural form of significant (and localized) mass loss. Semmes-Weinstein monofilaments were used to investigate the effect of tail autotomy (and subsequent tail regeneration) on tactile sensitivity of each appendage of the leopard gecko. Prior to autotomy, we identified site-specific differences in tactile sensitivity across the ventral surfaces of the hindlimbs, forelimbs and tail. Repeated monofilament testing of both control (tail-intact) and tail-loss geckos had a significant sensitization effect (i.e. decrease in tactile threshold, maintained over time) in all regions of interest except the palmar surfaces of the forelimbs in post-autotomy geckos, compared with baseline testing. Although the regenerated tail is not an exact replica of the original, tactile sensitivity is shown to be effectively restored at this site. Re-establishment of tactile sensitivity on the ventral surface of the regenerate tail points towards a (continued) role in predator detection.

Cutaneous Sensitivity Across Regions of the Foot Sole and Dorsum are Influenced by Foot Posture.

Understanding the processing of tactile information is crucial for the development of biofeedback interventions that target cutaneous mechanoreceptors. Mechanics of the skin have been shown to influence cutaneous tactile sensitivity. It has been established that foot skin mechanics are altered due to foot posture, but whether these changes affect cutaneous sensitivity are unknown. The purpose of this study was to investigate the potential effect of posture-mediated skin deformation about the ankle joint on perceptual measures of foot skin sensitivity. Participants (N = 20) underwent perceptual skin sensitivity testing on either the foot sole (N = 10) or dorsum (N = 10) with the foot positioned in maximal dorsiflexion/toe extension, maximal plantarflexion/toe flexion, and a neutral foot posture. Perceptual tests included touch sensitivity, stretch sensitivity, and spatial acuity. Regional differences in touch sensitivity were found across the foot sole (p < 0.001) and dorsum (p < 0.001). Touch sensitivity also significantly increased in postures where the skin was compressed (p = 0.001). Regional differences in spatial acuity were found on the foot sole (p = 0.002) but not dorsum (p = 0.666). Spatial acuity was not significantly altered by posture across the foot sole and dorsum, other than an increase in sensitivity at the medial arch in the dorsiflexion posture (p = 0.006). Posture*site interactions were found for stretch sensitivity on the foot sole and dorsum in both the transverse and longitudinal directions (p < 0.005). Stretch sensitivity increased in postures where the skin was pre-stretched on both the foot sole and dorsum. Changes in sensitivity across locations and postures were believed to occur due to concurrent changes in skin mechanics, such as skin hardness and thickness, which follows our previous findings. Future cutaneous biofeedback interventions should be applied with an awareness of these changes in skin sensitivity, to maximize their effectiveness for foot sole and dorsum input.

Remote Subthreshold Stimulation Enhances Skin Sensitivity in the Lower Extremity.

Foot sole skin interfaces with the ground and contributes to successful balance. In situations with reduced sensitivity in the glabrous foot skin, stochastic resonance (SR) improves skin sensitivity by adding tactile noise. Some situations, however, involve an interface comprised of hairy skin, which has higher thresholds for sensitivity. For example, in lower extremity amputation the residual limb is comprised of hairy leg skin. The main objective of this study was to determine if SR improves skin sensitivity in hairy skin, and whether a specific intensity of noise is most effective. Secondary objectives were to compare the effect between locations, ages and modalities. In 60 healthy participants a vibrotactile (test) input was delivered at the lower extremity concurrently with a second, noisy stimulus applied more proximally. The presence of a remote SR effect was tested in 15 young participants using electrotactile noise at the calf. Secondary objectives were tested in separate groups of 15 subjects and differed by substituting for one of the three variables: vibrotactile noise, heel site, and with older participants. A forced-choice protocol was used to determine detection ability of the subthreshold vibration test input with varying noise levels applied simultaneously (0, 20, 40, 60, 80, and 100% of perceptual threshold). An SR effect was identified when increased detection of the input was obtained at any level of noise versus no noise. It was found that all four test groups demonstrated evidence of SR: 33-47% of individuals showed better detection of the input with added noise. The SR effect did not appear consistently at any specific noise level for any of the groups, and none of the variables showed a superior ability to evoke SR. Interestingly, in approximately 33% of cases, threshold values fluctuated throughout testing. While this work has provided evidence that SR can enhance the perception of a vibrotactile input in hairy skin, these data suggest that the ability to repeatably show an SR effect relies on maintaining a consistent threshold.

Subthreshold Electrical Noise Applied to the Plantar Foot Enhances Lower-Limb Cutaneous Reflex Generation.

Reflex responses generated by cutaneous mechanoreceptors of the plantar foot are important for the maintenance of balance during postural tasks and gait. With aging, reflex generation, particularly from fast adapting type I receptors, is reduced, which likely contributes to impaired postural stability in this population. Therefore, improving reflex generation from these receptors may serve as a tool to improve balance performance. A mechanism to enhance reflexes may lie in the phenomenon of stochastic resonance, whereby the addition of certain intensities and frequencies of noise stimuli improves the performance of a system. This study was conducted to determine whether tactile noise stimuli could improve cutaneous reflex generation. In 12 healthy young adults, we evoked cutaneous reflex responses using a 0-50 Hz Gaussian noise vibration applied to the plantar heel. Concurrently, we applied one of six subthreshold intensities of electrical tactile noise to the plantar heel [0%, 20%, 40%, 60%, 80% or 100% (threshold)] and were able to analyze data from 0%, 20% and 40% trials. Across participants, it was found that the addition of a 20% perceptual threshold (PT) noise resulted in enhanced reflex responses when analyzed in both the time and frequency domains. These data provide evidence that cutaneous reflex generation can be enhanced via a stochastic resonance effect and that 20% PT is the optimal intensity of noise to do so. Therefore, the addition of noise stimuli may be a valuable clinical intervention to improve reflex responses associated with postural balance in populations with impairments.

Foot sole cutaneous stimulation mitigates neuromuscular fatigue during a sustained plantar flexor isometric task.

Neuromuscular fatigue impairs motor coordination, movement stability, and proprioception, which further decreases performance. A neuromechanical coupling exists between foot sole cutaneous mechanoreceptors and motoneurons of the lower limb, however, the contribution of skin sensory input on muscle fatigue remains unclear. The purpose of this study was to determine if the presence of cutaneous stimulation could mitigate the effect of fatigue of the plantar flexor muscles during a sustained isometric task at 30% maximal voluntary contraction (MVC). Participants (N = 16, age 24.1 ± 2.6 yr) underwent a 30% isometric plantar flexor fatiguing task in a seated position with hip, knee, and ankle angle at 80°, 100°, and 90°, respectively, with intermittent MVCs until task failure. Failure was defined as when the participant could no longer maintain 30% MVC for a minimum of two seconds. Throughout the protocol, electrical stimulation was applied to either the right heel, right metatarsals, or no stimulation. A subset of participants (N = 6) underwent an additional condition with electrical stimulation applied to the left arm. MVCs were also conducted intermittently throughout recovery for 30 min. Foot sole cutaneous stimulation mitigated fatigue, as demonstrated by an ~15% increased time to task failure (TTF) compared with the control condition. When normalized to TTF, MVC torque amplitude was not different at each time epoch, which indicated that each %MVC was maintained longer into the fatigue task during the heel and metatarsal stimulation conditions However, there was no significant effect of cutaneous stimulation on recovery. The results indicate that cutaneous stimulation may serve as a feasible means to mitigate fatigue.NEW & NOTEWORTHY Cutaneous coupling with lower limb motor neurons has long been known. We set out to establish whether this pathway could serve a purpose other than muscular modulation during standing and walking. We found that during a submaximal contraction of the plantar flexor muscles, the addition of intermittent cutaneous stimulation to the skin of the foot sole resulted in an increase in time to task failure by 15%, which was over a minute longer in duration. We conclude that skin stimulation may serve as a mechanism to mitigate fatigue.

A systematic review of the effect of footwear, foot orthoses and taping on lower limb muscle activity during walking and running.

BACKGROUND: External devices are used to manage musculoskeletal pathologies by altering loading of the foot, which could result in altered muscle activity that could have therapeutic benefits. OBJECTIVES: To establish if evidence exists that footwear, foot orthoses and taping alter lower limb muscle activity during walking and running. STUDY DESIGN: Systematic literature review. METHODS: CINAHL, MEDLINE, ScienceDirect, SPORTDiscus and Web of Science databases were searched. Quality assessment was performed using guidelines for assessing healthcare interventions and electromyography methodology. RESULTS: Thirty-one studies were included: 22 related to footwear, eight foot orthoses and one taping. In walking, (1) rocker footwear apparently decreases tibialis anterior activity and increases triceps surae activity, (2) orthoses could decrease activity of tibialis posterior and increase activity of peroneus longus and (3) other footwear and taping effects are unclear. CONCLUSION: Modifications in shoe or orthosis design in the sagittal or frontal plane can alter activation in walking of muscles acting primarily in these planes. Adequately powered research with kinematic and kinetic data is needed to explain the presence/absence of changes in muscle activation with external devices. CLINICAL RELEVANCE: This review provides some evidence that foot orthoses can reduce tibialis posterior activity, potentially benefitting specific musculoskeletal pathologies.

Can texture change joint position sense at the knee joint in those with poor joint position accuracy?

Purpose: Skin contributes to joint position sense (JPS) at multiple joints. Altered cutaneous input at the foot can modulate gait and balance and kinesiology tape can enhance proprioception at the knee, but its effect may be dependent on existing capacity. The effect of texture at the knee, particularly in those with poor proprioception, is unknown. The aim of this study was to determine the effect of textured panels on JPS about the knee. Materials and methods: Eighteen healthy females were seated in an adjustable chair. Their left leg (target limb) moved passively from 65° to a target of flexion (115° or 90°) or extension (40°). Their right leg (matching limb) was passively moved towards this target angle and participants indicated when their limbs felt aligned. We tested three textured panels over the knee of the matching limb and two control conditions. The target limb maintained a control panel. Directional error, absolute error and variable error in matching between limbs were calculated. Results: On average textured panels over the knee increased JPS error compared to control pants for participants with poor JPS. These participants undershot the target at 90° of flexion significantly more with textured panels (-11° ± 3°) versus control (-7° ± 3°, p = 0.04). Conclusions: For participants with poor JPS accuracy, increased JPS error at 90° with a textured panel suggests these individuals utilised altered cutaneous information to adjust joint position. We propose increased error results from enhanced skin input at the knee leading to the perception of increased flexion.

Visual feedback is not necessary for recalibrating the vestibular contribution to the dynamic phase of a perturbation recovery response.

Our recent work demonstrated that vision can recalibrate the vestibular signal used to re-establish equilibrium following a platform perturbation. Here, we investigate whether vision provided during a platform perturbation can recalibrate the use of vestibular reafference during the dynamic phase of the perturbation response. Dynamic postural responses were examined during a series of five forward perturbations to the body, while galvanic vestibular stimulation (GVS) selectively altered vestibular feedback and LCD occlusion spectacles controlled visual availability. Responses with and without vision were compared. The presence of GVS caused 1.78 ± 0.19 cm of medio-lateral (ML) body motion toward the anode during the initial 3 s of the dynamic postural response across perturbations. This dynamic ML response was attenuated across perturbations 1-3 independent of visual availability, resulting in a significant reduction of ML center of mass and pressure deviations (p < 0.01, ƞ2 = 0.27). That is, the vestibular influence on the ML perturbation response could be altered but vision was not necessary for this adaptation. After removing GVS, the ML response component reversed in direction toward the cathode with a magnitude that was not significantly different to the amount of response attenuation seen when GVS was present (- 0.95 ± 0.19 cm; p = 0.99, ƞ2 = 0.00). This suggested that the use of a GVS-altered vestibular signal during dynamic perturbation responses could be recalibrated, but that visual feedback was likely not responsible. Alternative mechanisms to explain the recalibration process are discussed.

Physiological and cognitive measures during prolonged sitting: Comparisons between a standard and multi-axial office chair.

Prolonged sitting, common in many workplaces, reduces blood flow to the lower limb and has negative health outcomes. CoreChair is an active-sitting chair that encourages increased movement to help mitigate these outcomes. Physiological and cognitive measures were recorded in ten subjects over 4 h of sitting in both the CoreChair and a traditional office chair. Sitting in both chairs led to increases in calf circumference (p < 0.0001), reduced tactile sensitivity (p = 0.02), and a cognitive decline in attention (p = 0.035) over time. However, the increase in calf circumference was smaller in the CoreChair at the second (p = 0.017) and third hour (p = 0.012) compared to the traditional chair. Additionally, for the attention task, the traditional chair generated more attention-task errors (p = 0.005), while no changes were observed with the CoreChair (p = 0.13). These findings suggest that during prolonged sitting CoreChair may have modest physiological and cognitive benefits compared to a traditional chair.