Postural Steadiness and Ankle Force Variability in Peripheral Neuropathy

Ankle torque variance is a better indicator of balance control performance than plantar perceptual sensitivity threshold

We explored whether ankle torque variability or plantar perceptual threshold explains human balance control more effectively. We hypothesized that ankle torque variance is a better indicator of center of pressure (COP) velocity variance than plantar perceptual sensitivity. Two conditions were tested: loaded (23-kg vest added) and unloaded, as loading should diminish plantar sensitivity and increase COP velocity variability. We created a linear feedback model to assess the noise change in the sensorimotor loop induced by loading. Plantar sensitivity was quantified using a psychophysical approach while participants stood barefoot. A linear motor applied a force impulse on the participant's heel. A "yes-no" method of limits was selected to identify plantar sole sensory thresholds in both conditions. We observed reduced plantar sensitivity in loaded compared with unloaded conditions. In the loaded condition, participants exhibited greater COP velocity variance, with significant positive Pearson's correlations confirming a substantial association between ankle torque and COP velocity variances for both loaded [variance accounted for (VAF): r2 = 44.56%, P = 0.018] and unloaded conditions (VAF: r2 = 58.83%, P = 0.004). No significant correlation existed between COP velocity variance and plantar sensitivity threshold for both loaded (VAF: r2 = 0.002%, P = 0.99) and unloaded conditions (VAF: r2 = 21.81%, P = 0.35). The model confirmed an ∼88% rise in sensorimotor loop noise in the loaded condition. Ankle torque variance assesses the precision of nonperceptual and perceptual detection mechanisms in evaluating whole body motions and the accuracy in converting sensory cues into ankle torque.NEW & NOTEWORTHY Plantar cutaneous information contributes to balance control by modulating motor commands, but plantar perceptual sensitivity is a suboptimal indicator of balance performance. Multiple sensory cues encode whole body dynamics, guiding sensorimotor mechanisms to minimize body sway variability. Ankle torque variance is proposed as a superior measure for explaining balance control performance and evaluating the sensorimotor loop's functioning in balance control.

Balancing sensory inputs: somatosensory reweighting from proprioception to tactile sensation in maintaining postural stability among older adults with sensory deficits

Background

Sensory deficits increase the risk of falls among older adults. The purpose of this study was to investigate the correlations of lower extremity muscle strength, proprioception, and tactile sensation to postural stability among older adults with and without sensory deficits, to understand the contribution of each factor to postural stability, and to explore sensory reweighting among the two populations.

Methods

A total of 103 participants were recruited and divided into two older adult groups with (female = 24, male = 26, age = 69.1 ± 3.15 years, height = 162.72 ± 6.94 cm, body mass = 64.05 ± 9.82 kg) and without sensory deficits (female = 26, male = 27, age = 70.02 ± 4.9 years, height = 163.76 ± 7.60 cm, body mass = 65.83 ± 10.31 kg), based on whether a 5.07 Semmes-Weinstein monofilament could be detected at foot soles. Their Berg Balance Scale (BBS), lower extremity muscle strength, proprioception, and tactile sensation were tested and compared between the two groups. Pearson's or Spearman's correlations were used to explore the relationships between the BBS and each variable. Factor analysis and multivariate linear regression were used to verify the degrees of correlation between the generated factors and the postural stability.

Results

Low BBS (p = 0.003, η² = 0.088) scores and higher proprioception thresholds (knee flexion: p = 0.015, η² = 0.059; knee extension: p = 0.011, η² = 0.065; ankle plantarflexion: p = 0.006, η² = 0.075; ankle dorsiflexion: p = 0.001, η² = 0.106) were detected among older adults with sensory deficits compared with those without sensory deficits. Lower extremity muscle strength (ankle plantarflexion: r = 0.342, p = 0.002; hip abduction: r = 0.303, p = 0.041) and proprioception (knee flexion: r = -0.419, p = 0.004; knee extension: r = -0.292, p = 0.049; ankle plantarflexion: r = -0.450, p = 0.002; ankle dorsiflexion: r = -0.441, p = 0.002) were correlated with BBS among older adults without sensory deficits, while lower extremity muscle strength (ankle plantarflexion: r = 0.501, p<0.001; hip abduction: r = 0.302, p = 0.041) and tactile sensation (great toe: r = -0.388, p = 0.008; 5th metatarsal: r = -0.301, p = 0.042) were correlated with BBS among older adults with sensory deficits.

Conclusion

Older adults with sensory deficits have poorer proprioception and postural stability. Somatosensory reweighting occurs from proprioception to tactile sensation among older adults with sensory deficits in maintaining postural stability.

Visuomotor Correction is a Robust Contributor to Force Variability During Index Finger Abduction by Older Adults

We examined aging-related differences in the contribution of visuomotor correction to force fluctuations during index finger abduction via the analysis of two datasets from similar subjects. Study (1) Young (N = 27, 23 ± 8 years) and older adults (N = 14, 72 ± 9 years) underwent assessment of maximum voluntary contraction force (MVC) and force steadiness during constant-force (CF) index finger abduction (2.5, 30, 65% MVC). For each trial, visual feedback of the force (VIS) was provided for 8–10 s and removed for 8–10 s (NOVIS). Visual gain of the force feedback at 2.5% MVC was high; 12- and 26-fold greater than the 30 and 65% MVC targets. Mean force, standard deviation (SD) of force, and coefficient of variation (CV) of force was calculated for detrended (<0.5 Hz drift removed) VIS and NOVIS data segments. Study (2) A similar group of 14 older adults performed discrete, randomly-ordered VIS or NOVIS trials at low target forces (1–3% MVC) and high visual gain. Study (1) For young adults the CV of force was similar between VIS and NOVIS for the 2.5% (4.8 vs. 4.3%), 30% (3.2 vs. 3.2%) and 65% (3.5 vs. 4.2%) target forces. In contrast, for older adults the CV of force was greater for VIS than NOVIS for 2.5% MVC (6.6 vs. 4.2%, p < 0.001), but not for the 30% (2.4 vs. 2.4%) and 65% (3.1 vs. 3.3%) target forces. At 2.5% MVC, the increase in CV of force for VIS compared with NOVIS was significantly greater (age × visual condition p = 0.008) for older than young adults. Study (2) Similarly, for older adults performing discrete, randomly ordered trials the CV of force was greater for VIS than NOVIS (6.04 vs. 3.81%, p = 0.01). When visual force feedback was a dominant source of information at low forces, normalized force variability was ~58% greater for older adults, but only 11% greater for young adults. The significant effect of visual feedback for older adults was not dependent on the order of presentation of visual conditions. The results indicate that impaired processing of visuomotor information underlies the greater motor variability observed in older adults during lab-based isometric contractions of a hand muscle.