Tania E. Sakanaka

Bio: Tania E. Sakanaka is an academic researcher from University of Birmingham. The author has contributed to research in topics: Ankle & Balance (ability). The author has an hindex of 2, co-authored 4 publications receiving 40 citations.

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

Abstract: 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.

Intrinsic ankle stiffness during standing increases with ankle torque and passive stretch of the Achilles tendon

Abstract: Individuals may stand with a range of ankle angles. Furthermore, shoes or floor surfaces may elevate or depress their heels. Here we ask how these situations impact ankle stiffness and balance. We performed two studies (each with 10 participants) in which the triceps surae, Achilles tendon and aponeurosis were stretched either passively, by rotating the support surface, or actively by leaning forward. Participants stood freely on footplates which could rotate around the ankle joint axis. Brief, small stiffness-measuring perturbations (<0.7 deg; 140 ms) were applied at intervals of 4-5 s. In study 1, participants stood at selected angles of forward lean. In study 2, normal standing was compared with passive dorsiflexion induced by 15 deg toes-up tilt of the support surface. Smaller perturbations produced higher stiffness estimates, but for all perturbation sizes stiffness increased with active torque or passive stretch. Sway was minimally affected by stretch or lean, suggesting that this did not underlie the alterations in stiffness. In quiet stance, maximum ankle stiffness is limited by the tendon. As tendon strain increases, it becomes stiffer, causing an increase in overall ankle stiffness, which would explain the effects of leaning. However, stiffness also increased considerably with passive stretch, despite a modest torque increase. We discuss possible explanations for this increase.

Idiosyncratic Characteristics of Postural Sway in Normal and Perturbed Standing.

Abstract: 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.

Causes of variation in intrinsic ankle stiffness and the consequences for standing

Abstract: Previous studies have shown that the intrinsic mechanical stiffness of the ankles is less than necessary to fully stabilize the body in the upright position (Loram and Lakie, 2002a; Morasso and Schieppati, 1999; Morasso and Sanguineti, 2002; Casadio et al., 2005). Following these studies, research about the controlling mechanisms of standing (the maintenance of an upright posture by a combination of intrinsic and active mechanisms) has developed considerably (Lakie et al., 2003; Loram et al., 2005a,b, 2011; Masani et al., 2006; Maurer and Peterka, 2005; Peterka, 2002). However, very little attention was given to the intrinsic mechanisms themselves. Here I tackled this issue by manipulating the ankle (and its surrounding tissues) in various ways. The objective was to investigate ankle stiffness dependency on mechanical properties that are particular to muscles and tendons. Within-individual differences were confirmed in various conditions. I have shown that in standing, intrinsic ankle stiffness is affected by movement amplitude and history of movement, as well as active ankle torque and passive tendon stretch. I have found no dependency of ankle stiffness on localized cooling. With regards to the effect that differences in intrinsic ankle stiffness may cause to standing sway, a between-individual analysis showed an inverse correlation between ankle stiffness and sway magnitude.

Human Postural Control.

Abstract: From ancient Greece to nowadays, research on posture control was guided and shaped by many concepts. Equilibrium control is often considered part of postural control. However, two different levels have become increasingly apparent in the postural control system, one level sets a distribution of tonic muscle activity ("posture") and the other is assigned to compensate for internal or external perturbations ("equilibrium"). While the two levels are inherently interrelated, both neurophysiological and functional considerations point toward distinct neuromuscular underpinnings. Disturbances of muscle tone may in turn affect movement performance. The unique structure, specialization and properties of skeletal muscles should also be taken into account for understanding important peripheral contributors to postural regulation. Here, we will consider the neuromechanical basis of habitual posture and various concepts that were rather influential in many experimental studies and mathematical models of human posture control.

Sensorimotor control of standing balance.

Abstract: For most individuals, balancing upright is a simple task that requires little effort. The inherent difficulties associated with standing balance are not revealed until a pathology or injury impairs its control. Fundamentally, standing upright requires us to balance our unstable whole-body load within a small base of support. Small movements of the upright body are detected by various sensory receptors, all encoding these movements through their own coordinate system with specific dynamics. The balance controller filters, processes, and integrates sensory cues of body motion to produce an error signal between predicted and actual sensory consequences of balance-related movements. Compensatory motor commands are generated in response to this error to maintain upright standing. In the present review, we first briefly describe the biomechanics and sensor dynamics of standing balance. We further review sensorimotor and perceptual approaches revealing operational principles of the balance system, along with computational approaches that explore control processes underlying upright stance. Finally, we present robotic tools that virtualize the sensory consequences, biomechanics, and/or environmental factors inherent to the standing balance task. Throughout, we emphasize works that combine sensorimotor, computational, and/or robotics approaches to highlight the task dependency, multisensory cue combinations, cortical-subcortical contributions, and internal representations underpinning balance control.

Shear wave elastography reveals different degrees of passive and active stiffness of the neck extensor muscles.

Abstract: Purpose: The neck extensor muscles contribute to spinal support and posture while performing head and neck motion. Muscle stiffness relates to passive elasticity (support) and active tensioning (posture and movement) of muscle. It was hypothesized that support and motion requirements are reflected in the distribution of stiffness between superficial and deep neck extensor muscles. Methods: In ten healthy participants, shear modulus (stiffness) of five neck extensor muscles was determined in prone at rest and during isometric head lift at three intensities using shear wave elastography. Results: Shear modulus differed between muscles (P < 0.001), and was larger for the deeper muscles: (median (interquartile range)) trapezius 7.7 kPa (4.4), splenius capitis 6.5 kPa (2.5), semispinalis capitis 8.9 kPa (2.8), semispinalis cervicis 9.5 kPa (2.5), multifidus 14.9 kPa (1.4). Shear modulus differed between the resting condition and head lift (P < 0.001) but not between levels of head lift intensity. Conclusion: Shear wave elastography revealed highest passive and active stiffness of the deep neck extensor muscles most close to the spine. The highest active increase of stiffness during the head lift was found in the semispinalis cervicis muscle. The non-invasive, clinically applicable estimates of muscle stiffness have potential for the assessment of muscular changes associated with neck pain/injury.

A review of center of pressure (COP) variables to quantify standing balance in elderly people: Algorithms and open-access code.

Abstract: Postural control is often quantified by recording the trajectory of the center of pressure (COP)-also called stabilogram-during human quiet standing. This quantification has many important applications, such as the early detection of balance degradation to prevent falls, a crucial task whose relevance increases with the aging of the population. Due to the complexity of the quantification process, the analyses of sway patterns have been performed empirically using a number of variables, such as ellipse confidence area or mean velocity. This study reviews and compares a wide range of state-of-the-art variables that are used to assess the risk of fall in elderly from a stabilogram. When appropriate, we discuss the hypothesis and mathematical assumptions that underlie these variables, and we propose a reproducible method to compute each of them. Additionally, we provide a statistical description of their behavior on two datasets recorded in two elderly populations and with different protocols, to hint at typical values of these variables. First, the balance of 133 elderly individuals, including 32 fallers, was measured on a relatively inexpensive, portable force platform (Wii Balance Board, Nintendo) with a 25-s open-eyes protocol. Second, the recordings of 76 elderly individuals, from an open access database commonly used to test static balance analyses, were used to compute the values of the variables on 60-s eyes-open recordings with a research laboratory standard force platform.

Implementation of a Central Sensorimotor Integration Test for Characterization of Human Balance Control During Stance

Abstract: Balance during stance is regulated by active control mechanisms that continuously estimate body motion, via a "sensory integration" mechanism, and generate corrective actions, via a "sensory-to-motor transformation" mechanism. The balance control system can be modeled as a closed-loop feedback control system for which appropriate system identification methods are available to separately quantify the sensory integration and sensory-to-motor components of the system. A detailed, functionally meaningful characterization of balance control mechanisms has potential to improve clinical assessment and to provide useful tools for answering clinical research questions. However, many researchers and clinicians do not have the background to develop systems and methods appropriate for performing identification of balance control mechanisms. The purpose of this report is to provide detailed information on how to perform what we refer to as "central sensorimotor integration" (CSMI) tests on a commercially available balance test device (SMART EquiTest CRS, Natus Medical Inc, Seattle WA) and then to appropriately analyze and interpret results obtained from these tests. We describe methods to (1) generate pseudorandom stimuli that apply cyclically-repeated rotations of the stance surface and/or visual surround (2) measure and calibrate center-of-mass (CoM) body sway, (3) calculate frequency response functions (FRFs) that quantify the dynamic characteristics of stimulus-evoked CoM sway, (4) estimate balance control parameters that quantify sensory integration by measuring the relative contribution of different sensory systems to balance control (i.e., sensory weights), and (5) estimate balance control parameters that quantify sensory-to-motor transformation properties (i.e., feedback time delay and neural controller stiffness and damping parameters). Additionally, we present CSMI test results from 40 subjects (age range 21-59 years) with normal sensory function, 2 subjects with results illustrating deviations from normal balance function, and we summarize results from previous studies in subjects with vestibular deficits. A bootstrap analysis was used to characterize confidence limits on parameters from CSMI tests and to determine how test duration affected the confidence with which parameters can be measured. Finally, example results are presented that illustrate how various sensory and central balance deficits are revealed by CSMI testing.