Sway-dependent changes in standing ankle stiffness caused by muscle thixotropy

KEY POINTS: The passive stiffness of the calf muscles contributes to standing balance, although the properties of muscle tissue are highly labile. We investigated the effect of sway history upon intrinsic ankle stiffness and demonstrated reductions in stiffness of up to 43% during conditions of increased baseline sway. This sway dependence was most apparent when using low amplitude stiffness‐measuring perturbations, and the short‐range stiffness component was smaller during periods of high sway. These characteristics are consistent with the thixotropic properties of the calf muscles causing the observed changes in ankle stiffness. Periods of increased sway impair the passive stabilization of standing, demanding more active neural control of balance. ABSTRACT: Quiet standing is achieved through a combination of active and passive mechanisms, consisting of neural control and intrinsic mechanical stiffness of the ankle joint, respectively. The mechanical stiffness is partly determined by the calf muscles. However, the viscoelastic properties of muscle are highly labile, exhibiting a strong dependence on movement history. By measuring the effect of sway history upon ankle stiffness, the present study determines whether this lability has consequences for the passive stabilization of human standing. Ten subjects stood quietly on a rotating platform whose axis was collinear with the ankle joint. Ankle sway was increased by slowly tilting this platform in a random fashion, or decreased by fixing the body to a board. Ankle stiffness was measured by using the same platform to simultaneously apply small, brief perturbations (<0.6 deg; 140 ms) at the same time as the resulting torque response was recorded. The results show that increasing sway reduces ankle stiffness by up to 43% compared to the body‐fixed condition. Normal quiet stance was associated with intermediate values. The effect was most apparent when using smaller perturbation amplitudes to measure stiffness (0.1 vs. 0.6 deg). Furthermore, torque responses exhibited a biphasic pattern, consisting of an initial steep rise followed by a shallower increase. This transition occurred earlier during increased levels of ankle sway. These results are consistent with a movement‐dependent change in passive ankle stiffness caused by thixotropic properties of the calf muscle. The consequence is to place increased reliance upon active neural control during times when increased sway renders ankle stiffness low

Idiosyncratic Characteristics of Postural Sway in Normal and Perturbed Standing

OBJECTIVE: Are people with a characteristically large physiological sway rendered particularly unstable when standing on a moving surface? Is postural sway in standing individuals idiosyncratic? In this study, we examine postural sway in individuals standing normally, and when subtle continuous sinusoidal disturbances are applied to their support platform. We calculate consistency between conditions to verify if sway can be considered characteristic of each individual. We also correlate two different aspects of participants’ responses to disturbance; their sway velocity and their regulation of body orientation. METHODS: Nineteen healthy adults (age 29.2 ± 3.2 years) stood freely on footplates coaxially aligned with their ankles and attached to a motorized platform. They had their eyes closed, and hips and knees locked with a light wooden board attached to their body. Participants either stood quietly on a fixed platform or on a slowly tilting platform (0.1 Hz sinusoid; 0.2 and 0.4 deg). Postural sway size was separated into two entities: (1) the spontaneous sway velocity component (natural random relatively rapid postural adjustments, RMS body angular velocity) and (2) the evoked tilt gain component (much slower 0.1 Hz synchronous tilt induced by the movement of the platform, measured as peak-to-peak (p-p) gain, ratio of body angle to applied footplate rotation). RESULTS: There was no correlation between the velocity of an individual’s sway and their evoked tilt gain (r = 0.34, p = 0.15 and r = 0.30, p = 0.22). However, when considered separately, each of the two measurements showed fair to good absolute agreement within conditions. Spontaneous sway velocity consistently increased as participants were subjected to increasing disturbance. Participants who swayed more (or less) did so across all conditions [ICC((3,k)) = 0.95]. Evoked tilt gain also showed consistency between conditions [ICC((3,k)) = 0.79], but decreased from least to most disturbed conditions. CONCLUSION: The two measurements remain consistent between conditions. Consistency between conditions of two very distinct unrelated measurements reflects the idiosyncratic nature of postural sway. However, sway velocity and tilt gain are not related, which supports the idea that the short-term regulation of stability and the longer-term regulation of orientation are controlled by different processes

Individual differences in intrinsic ankle stiffness and their relationship to body sway and ankle torque.

When standing, intrinsic ankle stiffness is smaller when measured using large perturbations, when sway size is large, and when background torque is low. However, there is a large variation in individual intrinsic ankle stiffness. Here we determine if individual variation has consequences for postural control. We examined the relationship between ankle stiffness, ankle torque and body sway across different individuals. Ankle stiffness was estimated in 19 standing participants by measuring torque responses to small, brief perturbations. Perturbation sizes of 0.2 & 0.9 degrees (both lasting 140 ms) measured short- and long-range stiffness respectively, while participants either stood quietly on a fixed platform or were imperceptibly tilted to reduce stability (0.1 Hz sinusoid; 0.2 & 0.4 deg). The spontaneous body sway component (natural random relatively rapid postural adjustments) and background ankle torque were averaged from sections immediately before perturbations. The results show that, first, intrinsic ankle stiffness is positively associated with ankle torque, and that this relationship is stronger for long-range stiffness. Second, intrinsic ankle stiffness is negatively associated with body sway, but, in contrast to the relationship with torque, this relationship is stronger for short-range stiffness. We conclude that high short-range intrinsic ankle stiffness is associated with reduced spontaneous sway, although the causal relationship between these two parameters is unknown. These results suggest that, in normal quiet standing where sway is very small, the most important determinant of intrinsic ankle stiffness may be stillness. In less stable conditions, intrinsic ankle stiffness may be more dependent on ankle torque

EMG activity ratio (relative to normal condition).

<p>(*) indicates significance of P<0.05, and (**) indicates P<0.001.</p

Experimental setup.

<p>(A) The position servo motor was installed horizontally and applied perturbations to the crank, thus rotating the platform and footplates. Separate load cells measured torque for each ankle. They transmitted all forces between the platform and footplates, directly above the axis of rotation. A potentiometer attached to the axis of rotation measured anteroposterior rotation of the footplate. An accelerometer attached underneath the left footplate measured its acceleration. Two laser-reflex sensors placed at left mid-tibia and umbilicus level tracked the anteroposterior shin and body tilt. (B) During study 1 (top figures), the standing platform was level and the participant altered body position. During study 2 (bottom figures), the standing platform was rotated upwards by 15 deg during the dorsiflexion condition. Only left lower limb recordings were used for stiffness and sway analysis, and surface EMG was recorded from the medial gastrocnemius and tibialis anterior muscles. (C) Example of averaged ankle angle (continuous line), angular velocity (dashed line) and angular acceleration (dotted line) data used to estimate mechanical intrinsic ankle stiffness. The time-window (70 ms) used for the analysis are indicated by the thin vertical lines. The starting point coincides with the stimulus onset. (D) Ankle torque response (dotted line) and, on top of it, reconstructed torque (continuous line) obtained from the second order model used to estimate stiffness. The bottom horizontal line indicates 14.5 Nm.</p

Representative data.

<p>Effects of active ankle torque and passive tendon stretch on ankle angle (footplate minus shin angle), left ankle torque, rectified left medial gastrocnemius and tibialis anterior EMG. Top panel are data from study 1 and bottom panel are data from study 2, all taken from one participant. The horizontal line beneath the torque traces represents 0 Nm. For study 2, ankle angle equals 90 deg when the footplate is levelled; it decreases (in this case to ~ 73 deg) when the footplate rotates upwards from 0 deg to 15 deg. The difference (from 75 deg) is due to body and leg movement associated with the toes-up stance.</p

Body sway size (deg) and sway velocity (deg s^-1).

<p>Body sway size (deg) and sway velocity (deg s^-1).</p

Mean ankle torque (Nm).

<p>(*) indicates significance of P<0.05, and (**) indicates P<0.001. This and the following box plots show first (bottom), second (band inside the box) and third (top) quartiles; whiskers show 1.5 IQR (Tukey box plot).</p

Litter Reduction during Beach Closure in the Context of the COVID-19 Pandemic: Quantifying the Impact of Users on Beach Litter Generation

This study aimed to quantify marine litter before and during the COVID pandemic found on urban touristic beaches closed to beachgoer access in northeastern Brazil. Litter identification and quantification was conducted during April, June, and August 2019, when 3583 items were sampled, and replicated during the same months in 2020, when access to the beaches studied was prohibited and a significant reduction in the amount of litter was found, 1812 items (49% decrease). Transects were used to monitor and classify litter according to its source, namely: autochthonous (litter that was locally discarded) and allochthonous (litter from other sites and sources). All beaches were classified as “very clean” and presented a smaller amount of litter during the beach closure period. The highest total marine litter reduction between the periods studied was 83%, while autochthonous litter in particular showed the most significant reduction, 88%. The comparison between the quantity and type of litter found in both periods showed greater specific anthropic pressure from beach users