Proprioceptive Threshold Research Articles

Alteration of ankle proprioceptive threshold during gait in the presence of acute experimental pain.

ObjectiveHuman gait requires complex somatosensory processing of various inputs such as proprioception. Proprioception can be altered in the presence of pain. This has been shown mostly during controlled tasks, thereby limiting the influence of external perturbations. While controlling the environment is sometimes warranted, it limits the ecological validity of the data. Using robotic orthoses to apply perturbations during movements seems a promising tool to functionally assess proprioception, where the complex somatosensory processing required in real-life situations is at play. The main objective of this study was to compare the proprioceptive threshold of healthy participants during gait in the presence and absence of an acute experimental pain.Methods36 healthy participants walked on a treadmill while wearing a robotized ankle–foot orthosis (rAFO) around their right ankle. The rAFO applied torque perturbations of graded magnitudes during the swing phase of gait. Participants had to report the presence/absence of such perturbations, as a measure of proprioceptive threshold. Following initial assessment, they were randomly assigned to one of three experimental groups: Control (no stimulation), Painless (non-nociceptive stimulation) and Painful (nociceptive stimulation). Electrodes placed on the right lateral malleolus delivered an electrical stimulation during the second assessment for Painless and Painful groups. A Kruskal-Wallis was used to compare the percentage of change of the three groups between the two assessments.ResultsA 31.80±32.94% increase in proprioceptive threshold, representing an increase of 1.3±1.2 Nm in the detection threshold, was observed for the Painful group only (p<0.005), with an effect size of 1.6.ConclusionFindings show that the presence of pain at the ankle can alter participants’ proprioceptive threshold during gait. Clinical assessment of proprioception should therefore carefully consider the presence of pain when evaluating a patient’s performance using clinical proprioceptive test and consider the negative effect of pain on proprioceptive threshold for test interpretation.

Phase response analyses support a relaxation oscillator model of locomotor rhythm generation in Caenorhabditis elegans.

Neural circuits coordinate with muscles and sensory feedback to generate motor behaviors appropriate to an animal's environment. In C. elegans, the mechanisms by which the motor circuit generates undulations and modulates them based on the environment are largely unclear. We quantitatively analyzed C. elegans locomotion during free movement and during transient optogenetic muscle inhibition. Undulatory movements were highly asymmetrical with respect to the duration of bending and unbending during each cycle. Phase response curves induced by brief optogenetic inhibition of head muscles showed gradual increases and rapid decreases as a function of phase at which the perturbation was applied. A relaxation oscillator model based on proprioceptive thresholds that switch the active muscle moment was developed and is shown to quantitatively agree with data from free movement, phase responses, and previous results for gait adaptation to mechanical loadings. Our results suggest a neuromuscular mechanism underlying C. elegans motor pattern generation within a compact circuit.

Ankle proprioception during gait in individuals with incomplete spinal cord injury.

IntroductionProprioception is known to be affected after a spinal cord injury (SCI). However, it is currently assessed during simple tasks that do not reflect activities of daily living. To better understand how proprioception affects movement, assessing it during a functional sensorimotor task such as walking is therefore of primary importance.Therefore, the objectives of this study were as follows: (a) measure the protocol reliability of a new robotic test in nondisabled controls; (b) evaluate the effect nonlesion‐related factors such as sex, age, pain, and gait speed on ankle proprioception; and (c) assess ankle proprioception during walking in individuals with SCI.MethodsIn the current study, ankle proprioception was assessed during gait in individuals with an incomplete spinal cord injury (iSCI; n = 15) using an electrohydraulic robotized ankle–foot orthosis (rAFO). Ankle proprioceptive threshold was quantified as the participants’ ability to detect torque perturbations of varied amplitude applied during swing by the rAFO. In addition, test–retest reliability and the potential effect of nonlesion‐related factors (sex, age, pain, and gait speed) were evaluated in nondisabled (ND; n = 65) participants.ResultsDuring gait, individuals with iSCI had a 53% poorer proprioceptive threshold than ND controls (p < .05). Test–retest reliability was good (ICC = 0.78), and only gait speed affected proprioceptive threshold (p = .018).ConclusionThis study is the first to show that ankle proprioception assessed during gait is impaired in individuals with an iSCI. The developed test can now be used to better characterize proprioception in population with other neurological conditions and has potential to maximize functional recovery during gait training in those populations.

Failure on the Foam Eyes Closed Test of Standing Balance Associated With Reduced Semicircular Canal Function in Healthy Older Adults.

Standing on foam with eyes closed (FOEC) has been characterized as a measure of vestibular function; however, the relative contribution of vestibular function and proprioceptive function to the FOEC test has not been well described. In this study, the authors investigate the relationship between peripheral sensory systems (vestibular and proprioception) and performance on the FOEC test in a cohort of healthy adults. A total of 563 community-dwelling healthy adults (mean age, 72.7 [SD, 12.6] years; range, 27 to 93 years) participating in the Baltimore Longitudinal Study of Aging were tested. Proprioceptive threshold (PROP) was evaluated with passive motion detection at the right ankle. Vestibulo-ocular reflex (VOR) gain was measured using video head impulses. Otolith function was measured with cervical and ocular vestibular-evoked myogenic potentials. Participants stood on FOEC for 40 sec while wearing BalanSens (BioSensics, LLC, Watertown, MA) to quantify center of mass sway area. A mixed-model multiple logistic regression was used to examine the odds of passing the FOEC test based on PROP, VOR, cervical vestibular-evoked myogenic potential, and ocular vestibular-evoked myogenic potential function in a multisensory model while controlling for age and gender. The odds of passing the FOEC test decreased by 15% (p < 0.001) for each year of increasing age and by 8% with every 0.1 reduction in VOR gain (p = 0.025). Neither PROP nor otolith function was significantly associated with passing the FOEC test. Failure to maintain balance during FOEC may serve as a proxy for rotational vestibular contributions to postural control. Semicircular canals are more sensitive to low-frequency motion than otoliths that may explain these relationships because standing sway is dominated by lower frequencies. Lower VOR gain and increased age independently decreased the odds of passing the test.

Enhancing simulations with intra-subject variability for improved psychophysical assessments.

Psychometric properties of perceptual assessments, like reliability, depend on stochastic properties of psychophysical sampling procedures resulting in method variability, as well as inter- and intra-subject variability. Method variability is commonly minimized by optimizing sampling procedures through computer simulations. Inter-subject variability is inherent to the population of interest and cannot be influenced. Intra-subject variability introduced by confounds (e.g., inattention or lack of motivation) cannot be simply quantified from experimental data, as these data also include method variability. Therefore, this aspect is generally neglected when developing assessments. Yet, comparing method variability and intra-subject variability could give insights on whether effort should be invested in optimizing the sampling procedure, or in addressing potential confounds instead. We propose a new approach to estimate intra-subject variability of psychometric functions by combining computer simulations and behavioral data, and to account for it when simulating experiments. The approach was illustrated in a real-world scenario of proprioceptive difference threshold assessments. The behavioral study revealed a test-retest reliability of r = 0.212. Computer simulations without considering intra-subject variability predicted a reliability of r = 0.768, whereas the new approach including an intra-subject variability model lead to a realistic estimate of reliability (r = 0.207). Such a model also allows computing the theoretically maximally attainable reliability (r = 0.552) assuming an ideal sampling procedure. Comparing the reliability estimates when exclusively accounting for method variability versus intra-subject variability reveals that intra-subject variability should be reduced by addressing confounds and that only optimizing the sampling procedure may be insufficient to achieve a high reliability. This new approach allows computing the intra-subject variability with only two measurements per subject, and predicting the reliability for a larger number of subjects and retests based on simulations, without requiring additional experiments. Such a tool of predictive value is especially valuable for target populations where time is scarce, e.g., for assessments in clinical settings.

Carbon Ankle Seven spring orthoses in children affected by diplegic cerebral palsy: Effect on gait efficiency and symmetry

Distal symmetric polyneuropathy increases fall risk due to inability to cope with perturbations. We aimed to 1) identify the frontal plane lower limb sensorimotor functions which are necessary for robustness to a discrete, underfoot perturbation during gait; and 2) determine whether changes in the post-perturbed step parameters could distinguish between fallers and non fallers.Forty-two subjects (16 healthy old and 26 with diabetic PN) participated. Frontal plane lower limb sensorimotor functions were determined using established laboratory-based techniques. The subjects’ most extreme alterations in step width or step length in response to a perturbation were measured. In addition, falls and fall-related injuries were prospectively recorded.Ankle proprioceptive threshold (APrT; p = .025) and hip abduction rate of torque generation (RTG; p = .041) independently predicted extreme step length after medial perturbation, with precise APrT and greater hip RTG allowing maintenance of step length. Injured subjects demonstrated greater extreme step length changes after medial perturbation than non-injured subjects (percent change = 18.5 ± 9.2 vs. 11.3 ± 4.57; p = .01).The ability to rapidly generate frontal plane hip strength and/or precisely perceive motion at the ankle is needed to maintain a normal step length after perturbation, a parameter which distinguishes between subjects sustaining a fall-related injury and those who did not.

Development and reliability of a measure evaluating dynamic proprioception during walking with a robotized ankle-foot orthosis, and its relation to dynamic postural control

BackgroundProprioception is important for proper motor control. As the central nervous system modulates how sensory information is processed during movement (sensory gating), proprioceptive tests performed at rest do not correlate well with performance during dynamic tasks such as walking. Proprioception therefore needs to be assessed during movement execution. Objectives1) To develop a test evaluating the ability to detect movement errors during walking, and its test-retest reliability; 2) to quantify the relationship between proprioceptive threshold (obtained with this new test) and performance in a standardized dynamic balance task. MethodThirty healthy subjects walked on a treadmill while wearing a robotized ankle-foot orthosis (rAFO) for 2 bouts of 6min on 2 evaluation sessions (test-retest reliability). Force perturbations resisting ankle dorsiflexion during swing were applied to the ankle via the rAFO (150ms duration, variable amplitude). Participants pushed a button when they detected the perturbations. The Star Excursion Balance Test (SEBT) was used to evaluate dynamic balance. AnalysisAngular differences between perturbed and non-perturbed gait cycles were used to quantify movement error. Detection threshold was defined as the minimal movement error at which 50% of the perturbations were perceived. Intraclass correlation coefficients (ICCs) estimated test-retest reliability, and Pearson coefficients were used to determine the correlation between detection threshold and SEBT. ResultsDetection threshold was 5.31±2.12°. Good reliability (ICC=0.70) and a moderate to strong correlation to SEBT (r=−0.57 to −0.76) were found. ConclusionForce perturbations produced by the robotized AFO provides a reliable way of evaluating proprioception during walking.

Does Sensory Function Decline Independently or Concomitantly with Age? Data from the Baltimore Longitudinal Study of Aging.

Objectives. To investigate whether sensory function declines independently or in parallel with age within a single individual. Methods. Cross-sectional analysis of Baltimore Longitudinal Study of Aging (BLSA) participants who underwent vision (visual acuity threshold), proprioception (ankle joint proprioceptive threshold), vestibular function (cervical vestibular-evoked myogenic potential), hearing (pure-tone average audiometric threshold), and Health ABC physical performance battery testing. Results. A total of 276 participants (mean age 70 years, range 26–93) underwent all four sensory tests. The function of all four systems declined with age. After age adjustment, there were no significant associations between sensory systems. Among 70–79-year-olds, dual or triple sensory impairment was associated with poorer physical performance. Discussion. Our findings suggest that beyond the common mechanism of aging, other distinct (nonshared) etiologic mechanisms may contribute to decline in each sensory system. Multiple sensory impairments influence physical performance among individuals in middle old-age (age 70–79).

Accuracy of Clinical Techniques for Evaluating Lower Limb Sensorimotor Functions Associated With Increased Fall Risk

BackgroundIn prior work, laboratory-based measures of hip motor function and ankle proprioceptive precision were critical to maintaining unipedal stance and fall/fall-related injury risk. However, the optimal clinical evaluation techniques for predicting these measures are unknown. ObjectiveTo evaluate the diagnostic accuracy of common clinical maneuvers in predicting laboratory-based measures of frontal plane hip rate of torque development (HipRTD) and ankle proprioceptive thresholds (AnkPRO) associated with increased fall risk. DesignProspective, observational study. SettingBiomechanical research laboratory. ParticipantsA total of 41 older subjects (aged 69.1 ± 8.3 years), 25 with varying degrees of diabetic distal symmetric polyneuropathy and 16 without. AssessmentsClinical hip strength was evaluated by manual muscle testing (MMT) and lateral plank time, defined as the number of seconds that the laterally lying subject could lift the hips from the support surface. Foot/ankle evaluation included Achilles reflex and vibratory, proprioceptive, monofilament, and pinprick sensations at the great toe. Main Outcome MeasuresHipRTD, abduction and adduction, using a custom whole-body dynamometer. AnkPRO determined with subjects standing using a foot cradle system and a staircase series of 100 frontal plane rotational stimuli. ResultsPearson correlation coefficients (r) and receiver operator characteristic (ROC) curves revealed that LPT correlated more strongly with HipRTD (r/P = 0.61/<.001 and 0.67/<.001, for abductor/adductor, respectively) than did hip abductor MMT (r/P = 0.31/.044). Subjects with greater vibratory and proprioceptive sensation, and intact Achilles reflexes, monofilament, and pin sensation had more precise AnkPRO. LPT of <12 seconds yielded a sensitivity/specificity of 91%/80% for identifying HipRTD < 0.25 (body size in Newton-meters), and vibratory perception of <8 seconds yielded a sensitivity/specificity of 94%/80% for the identification of AnkPRO >1.0°. ConclusionsLPT is a more effective measure of HipRTD than MMT. Similarly, clinical vibratory sense and monofilament testing are effective measures of AnkPRO, whereas clinical proprioceptive sense is not.

Measuring ankle proprioception DURING walking using a robotized ankle-foot orthosis: Towards a quantitative method to assess clinical populations

Evaluation of lower limb proprioception is usually done under static conditions. Sensory inputs are partially gated during movements, however. This gating might explain the lack of correlation between static proprioceptive testing and motor performance in clinical populations. The goal of the present study was therefore to determine if proprioception could reliably be estimated during actual walking. Twenty healthy subjects walked on a treadmill while wearing a robotized ankle-foot orthosis (see Noel et al., 2008; 2 visits within 2–7 days). Short-duration force perturbations (150 ms; n = 96) of graded amplitude (1–7 Nm; producing ankle deviations of 1–12°) were applied during the swing phase of gait, once every 5–10 strides. Participants pressed a hand-held button each time they felt a perturbation. To determine proprioceptive threshold, % detection was then plotted as a function of ankle deviations. A sigmoidal curve was fitted to individual subject data. Proprioceptive threshold was defined as the ankle deviation at which subjects detected 50% of the perturbations. Intraclass correlation coefficient (ICC) were then calculated to determine the test-retest reliability. Median proprioceptive threshold was 5.25° (range 1.04°–9.14°; n = 20). The ICC was 0.792, indicating a good reliability. This pilot study suggests that a threshold for movement error detection can be reliably obtained during walking in healthy subjects. This proprioceptive threshold is higher than what is reported during static tasks, likely reflecting the sensory gating occurring during movement, and emphasizing the need to perform this assessment during walking. Further studies are now underway to determine if this method can be used as a tool for proprioception assessment in clinical populations.

Hip strength: Ankle proprioceptive threshold ratio predicts falls and injury in diabetic neuropathy

We determined lower limb neuromuscular capacities associated with falls and fall-related injuries in older people with declining peripheral nerve function. Thirty-two subjects (67.4 ± 13.4 years; 19 with type 2 diabetes), representing a spectrum of peripheral neurologic function, were evaluated with frontal plane proprioceptive thresholds at the ankle, frontal plane motor function at the ankle and hip, and prospective follow-up for 1 year. Falls and fall-related injuries were reported by 20 (62.5%) and 14 (43.8%) subjects, respectively. The ratio of hip adductor rate of torque development to ankle proprioceptive threshold (Hip(STR) /AnkPRO ) predicted falls (pseudo-R(2) = .726) and injury (pseudo-R(2) = .382). No other variable maintained significance in the presence of Hip(STR) /AnkPRO . Fall and injury risk in the population studied is related inversely to Hip(STR) /AnkPRO . Increasing rapidly available hip strength in patients with neuropathic ankle sensory impairment may decrease risk of falls and related injuries.

Step length after discrete perturbation predicts accidental falls and fall-related injury in elderly people with a range of peripheral neuropathy

AimsDistal symmetric polyneuropathy increases fall risk due to inability to cope with perturbations. We aimed to 1) identify the frontal plane lower limb sensorimotor functions which are necessary for robustness to a discrete, underfoot perturbation during gait; and 2) determine whether changes in the post-perturbed step parameters could distinguish between fallers and non fallers. MethodsForty-two subjects (16 healthy old and 26 with diabetic PN) participated. Frontal plane lower limb sensorimotor functions were determined using established laboratory-based techniques. The subjects’ most extreme alterations in step width or step length in response to a perturbation were measured. In addition, falls and fall-related injuries were prospectively recorded. ResultsAnkle proprioceptive threshold (APrT; p=.025) and hip abduction rate of torque generation (RTG; p=.041) independently predicted extreme step length after medial perturbation, with precise APrT and greater hip RTG allowing maintenance of step length. Injured subjects demonstrated greater extreme step length changes after medial perturbation than non-injured subjects (percent change = 18.5 ± 9.2 vs. 11.3 ± 4.57; p = .01). ConclusionsThe ability to rapidly generate frontal plane hip strength and/or precisely perceive motion at the ankle is needed to maintain a normal step length after perturbation, a parameter which distinguishes between subjects sustaining a fall-related injury and those who did not.

Enhanced Proprioceptive Acuity at the Knee in the Competitive Athlete

Controlled laboratory study: cross-sectional. To determine if proprioception, measured by the threshold to detection of passive motion (TDPM), differed in individuals who regularly participate in moderate-intensity exercise for fitness as compared to individuals involved in high-intensity skilled exercise. Previous research has been equivocal as to whether exercise training is associated with superior proprioceptive acuity, in particular, exercise that includes dynamic postural challenges such as cutting and pivoting. Two groups of 25 healthy individuals (18-32 years old) were recruited. One group consisted of individuals who performed moderate-activity level exercises for 5 to 10 hours per week. Participants in the other group performed high-activity level exercises, including high-speed cutting and pivoting activities, at least 10 hours per week. Proprioception was determined using TDPM, in which the knee was slowly extended or flexed at an angular velocity of 0.5°/s or less from a starting position of 40° of knee flexion. Individuals participating in competitive, high-intensity skilled exercise demonstrated better acuity (average of both limbs) of TDPM (mean ± SD, 0.81° ± 0.38°; P<.001) than those participating in moderate-intensity exercise for fitness (1.53° ± 0.58°). A low but statistically significant association (r = -0.38, P = .006) was found between weekly duration of exercise and proprioceptive threshold as measured by TDPM. These results suggest that perceptual thresholds of passive movement may be enhanced, depending on activity level and associated postural challenge, and that higher level and increased amount of exercise may promote enhanced neurosensory processing in these individuals. Consequently, high-intensity skilled training may deserve further emphasis in orthopaedic rehabilitation.

Fibular motor nerve conduction studies and ankle sensorimotor capacities

Nerve conduction studies provide information regarding the status of peripheral nerves, but relationships with sensorimotor capacities that influence mobility have not been defined. A secondary analysis was conducted on data from 41 older subjects (20 women and 21 men, age 69.1 ± 8.3 years), including 25 with diabetic neuropathy of varying severity and 16 without diabetes or neuropathy. Measurements included routine fibular motor nerve conduction studies and laboratory-based determination of ankle inversion/eversion proprioceptive thresholds and ankle inversion/eversion motor function. Independent of age, fibular amplitude correlated robustly with ankle inversion/eversion proprioceptive thresholds (R(2) = 0.591, P < 0.001), moderately with ankle inversion and eversion rates of torque generation (R(2) = 0.216; P = 0.004 and R(2) = 0.200; P = 0.006, respectively), and more strongly when fibular motor amplitude was normalized for body mass index (R(2) = 0.350; P < 0.001 and R(2) = 0.275; P = 0.001). Fibular motor amplitude was strongly associated with ankle sensorimotor capacities that influence lateral balance and recovery from perturbations during gait. The results suggest that nerve conduction study measures have potential for an expanded clinical role in evaluating mobility function in the population studied.

Which Lower Limb Frontal Plane Sensory and Motor Functions Predict Gait Speed and Efficiency on Uneven Surfaces in Older Persons With Diabetic Neuropathy?

To identify which frontal plane lower limb sensorimotor functions predict gait speed and efficiency (step-width-to-step-length ratio) on an uneven surface. Cross-sectional observational study. A biomechanics research laboratory. Thirty-three subjects (14 women [42.4%]; 21 with diabetic distal symmetric peripheral neuropathy [63.6%]), with a spectrum of lower limb sensorimotor functions that ranged from normal to marked diabetic neuropathy. Independent variables included ankle inversion-eversion proprioceptive thresholds, and normalized measures of maximum voluntary strength and maximum rate of torque development (RTD) of hip abduction-adduction and ankle inversion-eversion. Kinematic data were obtained by using an optoelectronic system as subjects walked over an uneven 10-m surface. Dependent variables included gait speed and efficiency (determined by step-width-to-step-length ratio) on an uneven surface. Hip adduction RTD and ankle inversion RTD predicted 54% of gait speed, with the former predicting the majority (44%). Ankle inversion RTD was the only significant predictor of gait efficiency, which accounted for 46% of its variability. Age did not predict gait speed or efficiency. The rapid generation of strength in the frontal plane at the hip and ankle is responsible for the successful negotiation of irregular surfaces in older persons. Age demonstrated no independent influence. Training regimens in older persons should include maneuvers that rapidly develop strength in hip adductors and ankle invertors if navigation of uneven surfaces is a functional goal.

Frontal plane hip and ankle sensorimotor function, not age, predicts unipedal stance time

Changes occur in muscles and nerves with aging. In this study we explore the relationship between unipedal stance time (UST) and frontal plane hip and ankle sensorimotor function in subjects with diabetic neuropathy. UST, quantitative measures of frontal plane ankle proprioceptive thresholds, and ankle and hip motor function were tested in 41 subjects with a spectrum of lower limb sensorimotor function ranging from healthy to moderately severe diabetic neuropathy. Frontal plane hip and ankle sensorimotor function demonstrated significant relationships with UST. Multivariate analysis identified only composite hip strength, ankle proprioceptive threshold, and age to be significant predictors of UST (R(2) = 0.73), explaining 46%, 24%, and 3% of the variance, respectively. Frontal plane hip strength was the single best predictor of UST and appeared to compensate for less precise ankle proprioceptive thresholds. This finding is clinically relevant given the possibility of strengthening the hip, even in patients with significant peripheral neuropathy.

Do Ankle Orthoses Improve Ankle Proprioceptive Thresholds or Unipedal Balance in Older Persons with Peripheral Neuropathy?

To determine whether ankle orthoses that provide medial and lateral support, and have been found to decrease gait variability in older persons with peripheral neuropathy, decrease (improve) frontal plane ankle proprioceptive thresholds or increase unipedal stance time in that same population. Observational study in which unipedal stance time was determined with a stopwatch, and frontal plane ankle (inversion and eversion) proprioceptive thresholds were quantified during bipedal stance using a foot cradle system and a series of 100 rotational stimuli, in 11 older neuropathic subjects (8 men; age 72 +/- 7.1 yr) with and without ankle orthoses. The subjects demonstrated no change in combined frontal plane (inversion + eversion) proprioceptive thresholds or unipedal stance time with vs. without the orthoses (1.06 +/- 0.56 vs. 1.13 +/- 0.39 degrees, respectively; P = 0.955 and 6.1 +/- 6.5 vs. 6.2 +/- 5.4 secs, respectively; P = 0.922). Ankle orthoses that provide medial-lateral support do not seem to change ankle inversion/eversion proprioceptive thresholds or unipedal stance time in older persons with diabetic peripheral neuropathy. Previously identified improvements in gait variability using orthoses in this population are therefore likely related to an orthotically induced stiffening of the ankle rather than a change in ankle afferent function.

Frontal plane ankle proprioceptive thresholds and unipedal balance

Reliable unipedal balance is fundamental to safe ambulation. Accordingly, older persons with peripheral neuropathy (PN), who are at increased risk for falls, demonstrate impaired unipedal balance. To explore the relationship between afferent function and unipedal balance, frontal plane proprioceptive thresholds at the ankle were quantified in 22 subjects (72.5 +/- 6.3 years; 11 with PN and 11 matched controls) while they were standing using a foot cradle system and a staircase series of 100 rotational stimuli. PN subjects, as compared to controls, demonstrated shorter median unipedal balance times (3.4 +/- 2.7 vs. 14.3 +/- 8.9 s; P = 0.0017) and greater (less precise) combined ankle inversion/eversion proprioceptive thresholds (1.17 +/- 0.36 vs. 0.65 +/- 0.37 degrees ; P = 0.0055). Combined ankle inversion/eversion proprioceptive thresholds explained approximately half the variance in unipedal balance time (R2 = 0.5138; P = 0.0004). Given prior work demonstrating a similarly strong relationship between ankle torque generation and unipedal balance, neuropathy-associated impairments in ankle frontal plane afferent and efferent function appear to be equally responsible for the inability of older persons with PN to reliably balance on one foot. They therefore provide distinct targets for intervention.

62. Unipedal balance, neuropathy, and frontal plane ankle proprioceptive thresholds

62. Unipedal balance, neuropathy, and frontal plane ankle proprioceptive thresholds

Pronounced overestimation of support surface tilt during stance

A veridical internal notion of the kinematic state of the foot support is essential for postural control. The means by which this is obtained is still a matter of debate. We therefore measured the conscious perception of support tilt during transient anterior-posterior rotations of a motion platform in six healthy subjects, using a psychophysical matching procedure. Furthermore, we evaluated subjects' postural responses (in terms of displacement of subjects' center of mass, COM, and their ankle torque, as represented by the center of foot pressure, COP). The platform tilts were applied in absence of visual and auditory orientation cues. The platform rotations consisted of smoothed position ramps with different dominant frequencies (0.025, 0.05, 0.1, 0.2, 0.4, and 0.8 Hz) and different amplitudes (0.125 degrees, 0.25 degrees, 0.5 degrees, 1 degree, 2 degrees, 4 degrees, and 8 degrees) for the forward and backward directions, which yielded a 6x14 stimulus matrix. The stimuli were repeated five times in a random order. For the matching procedure, subjects tried to maintain an upright body orientation, while trying to orient a light-weight rod, which was attached to a belt around their waists, parallel to the perceived platform surface. We measured the stimulus-evoked angular excursions of the rod and of the subjects' COM as well as the COP shift. We found that the subjects' rod indications overestimated the platform tilts, particularly with small stimulus amplitudes. To characterize the overestimation, we compared the rod indications obtained while subjects stood on the tilting platform, to rod indications in a situation in which they stood next to the platform and tried to match the rod angle to the now visually perceived platform angle. From this comparison, we inferred that the subjects' kinesthetically derived notion of platform tilt overestimates the actual tilt by a factor of approximately 4. The estimates were linearly related to the angle between body (COM) and platform, i.e., to approximately the angle of the ankle joint, a finding which suggests a proprioceptive source of the overestimation. Further analyses supported this view; they showed that the onset latencies of the rod indications could be approximated by a theoretical indication mechanism with a reaction time of about 0.31 s, a velocity threshold of 0.099 degrees/s, and a displacement threshold of 0.12 degrees. These threshold values are well in line with previous work on the leg proprioceptive detection threshold of conscious perception of body sway. We therefore assume that the phenomenon of support tilt overestimation reflects a still unknown mechanism of leg proprioception in postural control.