Adelle C. F. Coster

Accelerometry: providing an integrated, practical method for long-term, ambulatory monitoring of human movement

Accelerometry offers a practical and low cost method of objectively monitoring human movements, and has particular applicability to the monitoring of free-living subjects. Accelerometers have been used to monitor a range of different movements, including gait, sit-to-stand transfers, postural sway and falls. They have also been used to measure physical activity levels and to identify and classify movements performed by subjects. This paper reviews the use of accelerometer-based systems in each of these areas. The scope and applicability of such systems in unsupervised monitoring of human movement are considered. The different systems and monitoring techniques can be integrated to provide a more comprehensive system that is suitable for measuring a range of different parameters in an unsupervised monitoring context with free-living subjects. An integrated approach is described in which a single, waist-mounted accelerometry system is used to monitor a range of different parameters of human movement in an unsupervised setting.

Classification of basic daily movements using a triaxial accelerometer.

A generic framework for the automated classification of human movements using an accelerometry monitoring system is introduced. The framework was structured around a binary decision tree in which movements were divided into classes and subclasses at different hierarchical levels. General distinctions between movements were applied in the top levels, and successively more detailed subclassifications were made in the lower levels of the tree. The structure was modular and flexible: parts of the tree could be reordered, pruned or extended, without the remainder of the tree being affected. This framework was used to develop a classifier to identify basic movements from the signals obtained from a single, waist-mounted triaxial accelerometer. The movements were first divided into activity and rest. The activities were classified as falls, walking, transition between postural orientations, or other movement. The postural orientations during rest were classified as sitting, standing or lying. In controlled laboratory studies in which 26 normal, healthy subjects carried out a set of basic movements, the sensitivity of every classification exceeded 87%, and the specificity exceeded 94%; the overall accuracy of the system, measured as the number of correct classifications across all levels of the hierarchy, was a sensitivity of 97.7% and a specificity of 98.7% over a data set of 1309 movements.

Insulin Increases Cell Surface GLUT4 Levels by Dose Dependently Discharging GLUT4 into a Cell Surface Recycling Pathway

ABSTRACT The insulin-responsive glucose transporter GLUT4 plays an essential role in glucose homeostasis. A novel assay was used to study GLUT4 trafficking in 3T3-L1 fibroblasts/preadipocytes and adipocytes. Whereas insulin stimulated GLUT4 translocation to the plasma membrane in both cell types, in nonstimulated fibroblasts GLUT4 readily cycled between endosomes and the plasma membrane, while this was not the case in adipocytes. This efficient retention in basal adipocytes was mediated in part by a C-terminal targeting motif in GLUT4. Insulin caused a sevenfold increase in the amount of GLUT4 molecules present in a trafficking cycle that included the plasma membrane. Strikingly, the magnitude of this increase correlated with the insulin dose, indicating that the insulin-induced appearance of GLUT4 at the plasma membrane cannot be explained solely by a kinetic change in the recycling of a fixed intracellular GLUT4 pool. These data are consistent with a model in which GLUT4 is present in a storage compartment, from where it is released in a graded or quantal manner upon insulin stimulation and in which released GLUT4 continuously cycles between intracellular compartments and the cell surface independently of the nonreleased pool.

Detection of daily physical activities using a triaxial accelerometer

Triaxial accelerometers have been employed to monitor human movements in a variety of circumstances. The study considered the use of data from a single waist-mounted triaxial accelerometer to distinguish between activity states and rest. A method using acceleration magnitude was applied to data collected from 26 normal subjects performing sit-to-stand and stand-to-sittransitions and walking. The effects of three parameters were investigated: the length n of a smoothing median filter, the width w of the averaging window used to process the signal and the value of the acceleration magnitude threshold th. These were found to be inter-related, and sets of parameters that resulted in accurate discrimination were determined by the relationship between th and the product of w and n, and by the relationship between n and w. The subjects were randomly divided into control (N=13) and test (N=13) groups. Optimum parameter sets were determined using the control group. Eleven sets of parameters yielded the same optimum results of a sensitivity of 1.0 and a specificity of 0.96 in the control group. Upon application to the test group, using these parameters, the system successfully distinguished between activity and rest, giving sensitivities greater than 0.98 and specificities between 0.88 and 0.94.

A pilot study of long-term monitoring of human movements in the home using accelerometry

We assessed the feasibility of using a waist-mounted, wireless triaxial accelerometer (TA) to monitor human movements in an unsupervised home setting to detect changes in functional status. A pilot study was carried out with six healthy subjects aged 80–86 years. The subjects wore a TA unit every day for two to three months. Each morning they carried out a short routine of directed movements that included standing, sitting, lying and walking. Important movement variables were measured. During the rest of the day, subjects were monitored for falls, and variables such as metabolic energy expenditure were measured. All subjects remained healthy; there was no overall change in functional status and there were only slight fluctuations in health status. No longitudinal changes were detected in any of the variables measured during the directed routine. There was a moderate correlation between weekly self-reported health status and energy expenditure: subjects reported a lower health status for weeks in which they expended less energy. The TA system was found to be practical for long-term, unsupervised home monitoring. All subjects found the system simple to use and the TA unit unobtrusive and comfortable to wear. High compliance rates were achieved: the TA units were worn on 88% of the days in the study, for an average of 11.2 hours per day.

Impedance spectroscopy of interfaces, membranes and ultrastructures

For the past century, impedance spectroscopy has provided a non-invasive means of characterizing the electrical properties of many systems. Even today, it often provides the only non-invasive method for detailed structural-functional studies of these systems. This is especially so of systems in which important processes occur at the molecular level, such as those processes associated with biological and synthetic membranes and interfaces that form between solutions and various solids (e.g. metals and colloid particles). The fundamental concepts of impedance spectroscopy are re-examined and a review is given of the role that impedance spectroscopy has played in the development of our understanding of cellular and synthetic membranes, cell biophysics and ionic systems in general. Special emphasis is given to the problems associated with solution-electrode interfaces, as well as unstirred layers, which can plague measurements on biological systems and have led to much confusion in the past. A description is given of a new computer-controlled, four-terminal digital impedance spectrometer, which provides resolutions in impedance magnitude and phase of 0.002% and 0.001 ° respectively over a frequency range of 10−2 to 105Hz and for impedances ranging from 10 to 109 Ω. We also describe impedance dispersions in terms of transfer functions which, when plotted along the negative frequency axis, yield “spectra” with distinct sharp peaks that identify fundamental frequency constants of the system. This “control engineering” form of presentation of impedance spectra demystifies the impedance analyses of these systems. The spectra and changes in these which occur as a result of perturbations to the system can be readily assessed and interpreted.

Determining activity using a triaxial accelerometer

Triaxial accelerometers have been employed to monitor human movements in a variety of circumstances. It is necessary, however, to be able to extract useful information from the acceleration measurements. We show that we can, in a controlled environment, use an energy expenditure method to identify and separate dynamic activities, including changes in posture and ambulation.

Insulin Stimulates the Entry of GLUT4 into the Endosomal Recycling Pathway by a Quantal Mechanism

The insulin‐sensitive glucose transporter GLUT4 mediates the uptake of glucose into adipocytes and muscle cells. In this study we have used a novel 96‐well plate fluorescence assay to study the kinetics of GLUT4 trafficking in 3T3‐L1 adipocytes. We have found evidence for a graded release mechanism whereby GLUT4 is released into the plasma membrane recycling system in a nonkinetic manner as follows: the kinetics of appearance of GLUT4 at the plasma membrane is independent of the insulin concentration; a large proportion of GLUT4 molecules do not participate in plasma membrane recycling in the absence of insulin; and with increasing insulin there is an incremental increase in the total number of GLUT4 molecules participating in the recycling pathway rather than simply an increased rate of recycling. We propose a model whereby GLUT4 is stored in a compartment that is disengaged from the plasma membrane recycling system in the basal state. In response to insulin, GLUT4 is quantally released from this compartment in a pulsatile manner, leaving some sequestered from the recycling pathway even in conditions of excess insulin. Once disengaged from this location we suggest that in the continuous presence of insulin this quanta of GLUT4 continuously recycles to the plasma membrane, possibly via non‐endosomal carriers that are formed at the perinuclear region.

Kinetic Evidence for Unique Regulation of GLUT4 Trafficking by Insulin and AMP-activated Protein Kinase Activators in L6 Myotubes

In L6 myotubes, redistribution of a hemagglutinin (HA) epitope-tagged GLUT4 (HA-GLUT4) to the cell surface occurs rapidly in response to insulin stimulation and AMP-activated protein kinase (AMPK) activation. We have examined whether these separate signaling pathways have a convergent mechanism that leads to GLUT4 mobilization and to changes in GLUT4 recycling. HA antibody uptake on GLUT4 in the basal steady state reached a final equilibrium level that was only 81% of the insulin-stimulated level. AMPK activators (5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR) and A-769662) led to a similar level of antibody uptake to that found in insulin-stimulated cells. However, the combined responses to insulin stimulation and AMPK activation led to an antibody uptake level of ∼20% above the insulin level. Increases in antibody uptake due to insulin, but not AICAR or A-769662, treatment were reduced by both wortmannin and Akt inhibitor. The GLUT4 internalization rate constant in the basal steady state was very rapid (0.43 min−1) and was decreased during the steady-state responses to insulin (0.18 min−1), AICAR (0.16 min−1), and A-769662 (0.24 min−1). This study has revealed a nonconvergent mobilization of GLUT4 in response to activation of Akt and AMPK signaling. Furthermore, GLUT4 trafficking in L6 muscle cells is very reliant on regulated endocytosis for control of cell surface GLUT4 levels.

Skeletal muscle and plasma lipidomic signatures of insulin resistance and overweight/obesity in humans.

Alterations in lipids in muscle and plasma have been documented in insulin‐resistant people with obesity. Whether these lipid alterations are a reflection of insulin resistance or obesity remains unclear.