Cutaneous afferent feedback from the posterior ankle contributes to proprioception

Estimating Center of Pressure of a Bipedal Mechanism Using a Proprioceptive Artificial Skin around its Ankles

Estimating the Center of Pressure (CoP) under legged robots is useful to control their posture and gait. This is traditionally done using contact sensors at the base of the foot or with sensors on distal joints, which are subject to wear and damage due to impulse forces. In vertebrates, skin and ligament deformation at the ankle is a particularly rich source of sensory information for locomotion. For our bipedal mechanism, afferent signals from sensors on synthetic skin wrapped around the ankles sufficed to estimate the location of the CoP with a mean accuracy >81.5%. For this we used K-Nearest Neighbors (KNN) algorithm trained on the same force magnitude applied at four and nine ground-truth CoP locations. For a single mechanical foot (i.e., single stance), signals from skin or ligaments (i.e., elastic rubber sheets and cables, respectively) also sufficed to calculate the CoP (Mean prediction accuracy >91.3%). Moreover, the visco-elasticity of these elements serves to passively stabilize the ankle. Importantly, training the single leg case with forces of different magnitudes also resulted in similarly accurate mean CoP prediction accuracy >84.5%. We show that using bio-inspired proprioceptive skins and/or ligament arrangements can provide reliable COP predictions, while permitting arbitrary postures of the ankle and no sensors on the sole of the foot prone to wear and damage. This novel approach to estimation of the CoP can be used to improve locomotion control in a new class of bio-inspired rigid, soft and hybrid (soft-rigid) legged robots.

Effects of a 16-week Tai Chi intervention on cutaneous sensitivity and proprioception among older adults with and without sensory loss

This study investigated the effects of a 16-week Tai Chi (TC) intervention on cutaneous sensitivity and proprioception among older adults with and without sensory loss. Thirty-six older adults were divided into sensory loss and control groups, and they underwent a 16-week TC intervention. Significant interactions were detected in heel cutaneous sensitivity (p = 0.046, F = 4.419) and knee flexion (p = 0.043, F = 4.580), extension (p = 0.027, F = 5.529) and ankle plantar-flexion proprioception (p = 0.037, F = 4.860). The post hoc test indicated that in the sensory loss group, heel cutaneous sensitivity threshold (p = 0.034) and knee flexion (p = 0.004), extension (p = 0.002) and ankle plantar-flexion (p = 0.023) proprioception threshold decreased at week 17, whereas in the control group, knee flexion (p = 0.029) proprioception threshold decreased at week 17. TC intervention improved cutaneous sensitivity at more sites and proprioception in more joints among the older adults with sensory loss. TC intervention is a good option for older adults to exercise, and it is more effective among older adults with sensory loss.

Relative Contribution of Proprioceptive and Vestibular Sensory Systems to Locomotion: Opportunities for Discovery in the Age of Molecular Science

Locomotion is a fundamental animal behavior required for survival and has been the subject of neuroscience research for centuries. In terrestrial mammals, the rhythmic and coordinated leg movements during locomotion are controlled by a combination of interconnected neurons in the spinal cord, referred as to the central pattern generator, and sensory feedback from the segmental somatosensory system and supraspinal centers such as the vestibular system. How segmental somatosensory and the vestibular systems work in parallel to enable terrestrial mammals to locomote in a natural environment is still relatively obscure. In this review, we first briefly describe what is known about how the two sensory systems control locomotion and use this information to formulate a hypothesis that the weight of the role of segmental feedback is less important at slower speeds but increases at higher speeds, whereas the weight of the role of vestibular system has the opposite relation. The new avenues presented by the latest developments in molecular sciences using the mouse as the model system allow the direct testing of the hypothesis.

Cutaneous and muscular afferents from the foot and sensory fusion processing: Physiology and pathology in neuropathies

The foot-sole cutaneous receptors (section 2), their function in stance control (sway minimisation, exploratory role) (2.1), and the modulation of their effects by gait pattern and intended behaviour (2.2) are reviewed. Experimental manipulations (anaesthesia, temperature) (2.3 and 2.4) have shown that information from foot sole has widespread influence on balance. Foot-sole stimulation (2.5) appears to be a promising approach for rehabilitation. Proprioceptive information (3) has a pre-eminent role in balance and gait. Reflex responses to balance perturbations are produced by both leg and foot muscle stretch (3.1) and show complex interactions with skin input at both spinal and supra-spinal levels (3.2), where sensory feedback is modulated by posture, locomotion and vision. Other muscles, notably of neck and trunk, contribute to kinaesthesia and sense of orientation in space (3.3). The effects of age-related decline of afferent input are variable under different foot-contact and visual conditions (3.4). Muscle force diminishes with age and sarcopenia, affecting intrinsic foot muscles relaying relevant feedback (3.5). In neuropathy (4), reduction in cutaneous sensation accompanies the diminished density of viable receptors (4.1). Loss of foot-sole input goes along with large-fibre dysfunction in intrinsic foot muscles. Diabetic patients have an elevated risk of falling, and vision and vestibular compensation strategies may be inadequate (4.2). From Charcot-Marie-Tooth 1A disease (4.3) we have become aware of the role of spindle group II fibres and of the anatomical feet conditions in balance control. Lastly (5) we touch on the effects of nerve stimulation onto cortical and spinal excitability, which may participate in plasticity processes, and on exercise interventions to reduce the impact of neuropathy.

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.

We Are Upright-Walking Cats: Human Limbs as Sensory Antennae During Locomotion

Humans and cats share many characteristics pertaining to the neural control of locomotion, which has enabled the comprehensive study of cutaneous feedback during locomotion. Feedback from discrete skin regions on both surfaces of the human foot has revealed that neuromechanical responses are highly topographically organized and contribute to "sensory guidance" of our limbs during locomotion.

Judging heel height: A new test for proprioception while standing reveals that young hypermobile children perform better than controls

BACKGROUND

Children with Generalized Joint Hypermobility (GJH) have been reported to have poorer proprioception than children with normal mobility. However, they were usually tested under unloaded conditions and in an age-group in which pain starts to play a role.

RESEARCH QUESTION

In contrast, some young children with GJH perform well in motor tasks, suggesting they may have good proprioceptive abilities if assessed more ecologically.

METHODS

Children with GJH (Beighton score of  ≥ 5; mean age 8.34 years) were compared to children with a Beighton score of 4 or less. A proprioception test was performed using wedges of different heights to evaluate the ability to judge heel height. A pair of wedges of various heights, was placed under the children's feet at random and they were required to report the higher leg while standing RESULTS: Independent t-test showed that children with GJH performed better (p < 0.01) than controls, suggesting better proprioceptive abilities when assessed under loaded conditions SIGNIFICANCE: Children with GJH do not have inferior proprioception when tested under loaded conditions. The least one can say is that one should be careful in postulating that measuring passive position sense in one particular joint is necessarily the best estimation of proprioception. Body position during standing can be estimated on the basis of knowledge of joint positions (of the ankle in particular in the present test) but also of other information (loading of foot mechanoreceptors for example). In conclusion, the new test may be more suited to evaluate proprioception than the conventional tests, which rely on passive joint position estimation during sitting.

Age-related changes in proprioception of the ankle complex across the lifespan

BACKGROUND

Ankle complex proprioceptive ability, needed in active human movement, may change from childhood to elderly adulthood; however, its development across all life stages has remained unexamined. The aim of the present study was to investigate the across-the-lifespan trend for proprioceptive ability of the ankle complex during active ankle inversion movement.

METHODS

The right ankles of 118 healthy right-handed participants in 6 groups were assessed: children (6-8 years old), adolescents (13-15 years old), young adults (18-25 years old), middle-aged adults (35-50 years old), old adults (60-74 years old), and very old adults (75-90 years old). While the participants were standing, their ankle complex proprioception was measured using the Active Movement Extent Discrimination Apparatus.

RESULTS

There was no significant interaction between the effects of age group and gender on ankle proprioceptive acuity (F (5, 106) = 0.593, p = 0.705, η² _p = 0.027). Simple main effects analysis showed that there was a significant main effect for age group (F (5, 106) = 22.521, p < 0.001, η ² _p = 0.515) but no significant main effect for gender (F (1,106) =  2.283, p = 0.134, η ² _p = 0.021) between the female (0.723 ± 0.092, mean ± SD) and the male (0.712 ± 0.083) participants. The age-group factor was associated with a significant linear downward trend in scores (F (1, 106) = 10.584, p = 0.002, η ² _p = 0.091) and a strong quadratic trend component (F (1,106) = 100.701, p < 0.001, η ² _p = 0.480), producing an asymmetric inverted-U function.

CONCLUSION

The test method of the Active Movement Extent Discrimination Apparatus is sensitive to age differences in ankle complex proprioception. For proprioception of the ankle complex, young adults had significantly better scores than children, adolescents, old adults, and very old adults. The middle-aged group had levels of ankle proprioceptive acuity similar to those of the young adults. The scores for males and females were not significantly different. Examination of the range of the scores in each age group highlights the possible level that ankle complex movement proprioceptive rehabilitation can reach, especially for those 75-90 years of age.

Soleus single motor units show stronger coherence with Achilles tendon vibration across a broad bandwidth relative to medial gastrocnemius units while standing

To probe the frequency characteristics of somatosensory responses in the triceps surae muscles, we previously applied suprathreshold noisy vibration to the Achilles tendon and correlated it with ongoing triceps surae muscle activity (recorded via surface EMG) during standing. Stronger responses to tendon stimuli were observed in soleus (Sol) relative to medial gastrocnemius (MGas) surface EMG; however, it is unknown whether differences in motor unit activity or limitations of surface EMG could have influenced this finding. Here, we inserted indwelling EMG into Sol and MGas to record the activity of single motor units while we applied noisy vibration (10-115 Hz) to the right Achilles tendon of standing participants. We analyzed the relationship between vibration acceleration and the spike activity of active single motor units through estimates of coherence, gain, phase, and cross-covariance. We also applied sinusoidal vibration at frequencies from 10 to 100 Hz (in 5-Hz increments) to examine whether motor units demonstrate nonlinear synchronization or phase locking at higher frequencies. Relative to MGas single motor units, Sol units demonstrated stronger coherence and higher gain with noisy vibration across a bandwidth of 7-68 Hz, and larger peak-to-peak cross-covariance at all four stimulus amplitudes examined. Sol and MGas motor unit activity was modulated over the time course of the sinusoidal stimuli across all frequencies, but their phase-locking behavior was minimal. These findings suggest Sol plays a prominent role in responding to disturbances transmitted through the Achilles tendon across a broad frequency band during standing.NEW & NOTEWORTHY We examined the relationship between Achilles tendon stimuli and spike times of single soleus (Sol) and medial gastrocnemius (MGas) motor units during standing. Relative to MGas, Sol units demonstrated stronger coherence and higher gain with noisy stimuli across a bandwidth of 7-68 Hz. Sol and MGas units demonstrated minimal nonlinear phase locking with sinusoidal stimuli. These findings indicate Sol plays a prominent role in responding to tendon stimuli across a broad frequency band.

Effects of foot position on skin structural deformation

As the largest and most superficial organ, the skin is well positioned for receiving sensory information from the environment. It is conceivable that changes in posture could result in deformations of the skin and subsequent changes in skin material properties. Specifically, the ankle and metatarsophalangeal joints have the capability to undergo large postural alterations with the potential to induce large structural deformations in the skin of the foot. The purpose of this study was to determine the extent to which alterations in foot posture may influence measures of foot sole and dorsum skin stretch, hardness, and thickness in vivo. Ten young and healthy individuals were tested while three static foot postures (plantar flexion, neutral and dorsiflexion) were maintained passively. Skin stretch deformation was quantified across each posture using an 11 × 4 point matrix of 3D kinematic markers affixed to the skin of the foot sole and dorsum. Skin hardness was assessed across each posture at specific locations of the foot sole (1st metatarsal, 5th metatarsal, medial arch, lateral arch and heel) and foot dorsum (proximal, middle and distal) using a handheld Shore durometer. Skin (epidermal + dermal) thickness was measured in each posture from the same test locations using ultrasound images obtained for the foot sole and dorsum. In the plantar flexion ankle posture, the foot sole skin was observed to relax/retract on average (± standard errorr of the mean (SEM) by 9 ± 2% to become both 20 ± 6% softer and 10 ± 6% thicker. In this posture, the foot dorsum skin stretched on average by 7 ± 2% resulting in 84 ± 8% harder and 5 ± 4% thinner skin. In the dorsiflexion ankle posture, the skin of the foot sole was observed to stretch on average by 5 ± 1% to become both 20 ± 8% harder and 4 ± 7% thinner. In this posture, the skin of the foot dorsum relaxed/retracted on average by 9 ± 1% resulting in the skin becoming 27 ± 12% softer and 7 ± 5% thicker. Notably, all of the sites responded with movement in a similar direction, but each site responded to a variable extent. Importantly, it was clear that the majority of skin structural deformation of the foot sole occurred within the 1st metatarsal, 5th metatarsal, and medial arch regions, while deformation was more evenly distributed across regions of the foot dorsum. The results suggest there is location specificity in the retraction and stretch characteristics of the foot skin. While not tested directly, this may suggest that local stretch distributions could be in part due to the underlying dermal and hypodermal structures in these foot regions. With these observed changes in the mechanical structure of the foot sole and dorsum skin tissue matrix, it is possible that corresponding posture-dependent changes in cutaneous mechanoreceptor activation may be present.

Cutaneous afferent innervation of the human foot sole: what can we learn from single-unit recordings?

Cutaneous afferents convey exteroceptive information about the interaction of the body with the environment and proprioceptive information about body position and orientation. Four classes of low-threshold mechanoreceptor afferents innervate the foot sole and transmit feedback that facilitates the conscious and reflexive control of standing balance. Experimental manipulation of cutaneous feedback has been shown to alter the control of gait and standing balance. This has led to a growing interest in the design of intervention strategies that enhance cutaneous feedback and improve postural control. The advent of single-unit microneurography has allowed the firing and receptive field characteristics of foot sole cutaneous afferents to be investigated. In this review, we consolidate the available cutaneous afferent microneurographic recordings from the foot sole and provide an analysis of the firing threshold, and receptive field distribution and density of these cutaneous afferents. This work enhances the understanding of the foot sole as a sensory structure and provides a foundation for the continued development of sensory augmentation insoles and other tactile enhancement interventions.

The importance of cutaneous feedback on neural activation during maximal voluntary contraction

PURPOSE

The purpose of this study was to investigate the importance of cutaneous feedback on neural activation during maximal voluntary contraction (MVC) of the ankle plantar flexors.

METHODS

The effects of cutaneous plantar anaesthesia were assessed in 15 subjects and compared to 15 controls, using a one-day pre/post-repeated measures design. Cutaneous plantar anaesthesia was induced by lidocaine injection at the centre of forefoot, lateral midfoot, and heel. Each subject performed isometric MVCs of the ankle plantar flexors. During each isometric ramp contraction, the following variables were assessed: maximal isometric torque; surface electromyography (EMG) activity of the medial gastrocnemius (MG) and tibialis anterior (TA) muscles; and co-contraction index (CCI) between the MG and TA.

RESULTS

For ankle torque, two-way ANOVA showed no significant interaction between the pre/post-measurements × group (p = 0.166). However, MG activity presented significant interactions between the pre/post-measurements × group (p = 0.014). Post hoc comparisons indicated a decrease of MG activity in the experimental group, from 85.9 ± 11.9 to 62.7 ± 30.8% (p = 0.016). Additionally, the post-anaesthesia MG activity of the experimental group differed statistically with pre- and post-MG activity of the control group (p = 0.027 and p = 0.008, respectively). For TA activity and CCI, two-way ANOVA detected no significant interactions between the pre/post-measurements × group (p = 0.605 and p = 0.332, respectively).

CONCLUSION

Our results indicate that during MVC, cutaneous feedback modulates neural activity to MG muscle, without changing the extent of MG-TA co-contraction.