David J. McKeown

Displacement of centre of mass during quiet standing assessed using accelerometry in older fallers and non-fallers

Postural sway during quiet standing is associated with falls risk in older adults. The aim of this study was to investigate the utility of a range of accelerometer-derived parameters of centre of mass (COM) displacement in identifying older adults at risk of falling. A series of instrumented standing balance trials were performed to investigate postural control in a group of older adults, categorised as fallers or non-fallers. During each trial, participants were asked to stand as still as possible under two conditions: comfortable stance (six repetitions) and semi-tandem stance (three repetitions). A tri-axial accelerometer was secured to the lower back during the trials. Accelerometer data were twice integrated to estimate COM displacement during the trials, with numerical techniques used to reduce integration error. Anterior-posterior (AP) and medial-lateral (ML) sway range, sway length and sway velocity were examined, along with root mean squared (RMS) acceleration. All derived parameters significantly discriminated fallers from non-fallers during both comfortable and semi-tandem stance. Results indicate that these accelerometer-based estimates of COM displacement may improve the discriminative power of quiet standing falls risk assessments, with potential for use in unsupervised balance assessment.

Intra- and inter-session reliability of vertical jump performance in healthy middle-aged and older men and women

Abstract Despite its widespread use in performance assessment, the reliability of vertical jump in an ageing population has not been addressed properly. The aim of the present study was to assess intra- and inter-day reliability of countermovement jump in healthy middle-aged (55–65 years) and older (66–75 years) men and women. Eighty-two participants were recruited and asked to perform countermovement jumps on two different occasions interspersed by 4 weeks. The middle-aged groups exhibited excellent absolute reliability for flight height, jump height, peak force, peak power, peak force/body mass, and peak power/body mass, with coefficients of variation ranging from 2.9% to 7.2% in men and from 3.6% to 6.9% in women and moderate-to-high intraclass correlations (0.75 to 0.97 in men; 0.77 to 0.95 in women). The older groups displayed good coefficients of variation (4.2% to 10.8% in men and 3.4% to 9.5% in women), but the intraclass correlations were low-to-high (0.43 to 0.84 in men; 0.42 to 0.93 in women). Overall, intra-session reliability was higher than inter-session reliability. Peak power was by far the most consistent variable, whereas flight and jump height had the most marked variability. The minimum detectable change varied from 10.5% to 33%, depending on the variable examined, suggesting important implications for intervention studies.

Quantitative falls risk estimation through multi-sensor assessment of standing balance

Falls are the most common cause of injury and hospitalization and one of the principal causes of death and disability in older adults worldwide. Measures of postural stability have been associated with the incidence of falls in older adults. The aim of this study was to develop a model that accurately classifies fallers and non-fallers using novel multi-sensor quantitative balance metrics that can be easily deployed into a home or clinic setting. We compared the classification accuracy of our model with an established method for falls risk assessment, the Berg balance scale. Data were acquired using two sensor modalities--a pressure sensitive platform sensor and a body-worn inertial sensor, mounted on the lower back--from 120 community dwelling older adults (65 with a history of falls, 55 without, mean age 73.7 ± 5.8 years, 63 female) while performing a number of standing balance tasks in a geriatric research clinic. Results obtained using a support vector machine yielded a mean classification accuracy of 71.52% (95% CI: 68.82-74.28) in classifying falls history, obtained using one model classifying all data points. Considering male and female participant data separately yielded classification accuracies of 72.80% (95% CI: 68.85-77.17) and 73.33% (95% CI: 69.88-76.81) respectively, leading to a mean classification accuracy of 73.07% in identifying participants with a history of falls. Results compare favourably to those obtained using the Berg balance scale (mean classification accuracy: 59.42% (95% CI: 56.96-61.88)). Results from the present study could lead to a robust method for assessing falls risk in both supervised and unsupervised environments.

Reliability of quantitative TUG measures of mobility for use in falls risk assessment

Recent advances in body-worn sensor technology have increased the scope for harnessing quantitative information from the timed-up-and-go test (TUG), well beyond simply the time taken to perform the test. Previous research has shown that the quantitative TUG method can differentiate fallers from non-fallers with greater success than the manually timed TUG or the Berg Balance Test. In order to advance this paradigm of falls risk estimation it is necessary to investigate the robustness of the quantitative TUG variables. This study investigated the inter-session and intra-session reliability of 44 quantitative TUG variables measured from the shanks and lower back of 33 study participants aged between 55–65 yrs. For intra-session reliability, 25 variables demonstrated excellent reliability (ICC>0.75), and 12 demonstrated “fair to good reliability” with ICCs between 0.4 and 0.75. Analysis of test-retest reliability resulted in ICC > 0.75 for 18 out of 44 variables, with 20 variables showing fair to good reliability. Turn time parameters demonstrated poor reliability. We conclude that this is a reliable instrument that may be used as part of a long-term falls risk assessment, with further work required to improve certain turn parameters.

Taking balance measurement out of the laboratory and into the home: Discriminatory capability of novel centre of pressure measurement in fallers and non-fallers

We investigated three methods for estimating centre of pressure excursions, as measured using a portable pressure sensor matrix, in order to deploy similar technology into the homes of older adults for longitudinal monitoring of postural control and falls risk. We explored the utility of these three methods as markers of falls risk in a cohort of 120 community dwelling older adults with and without a history of falls (65 fallers, 55 non-fallers). A number of standard quantitative balance parameters were derived using each centre of pressure estimation method. Rank sum tests were used to test for significant differences between fallers and non-fallers while intra-class correlation coefficients were also calculated to determine the reliability of each method. A method based on estimating the changes in the magnitude of pressure exerted on the pressure sensor matrix was found to be the most reliable and discriminative. Our future work will implement this method for home-based balance measurement.

Benefits of a worksite or home‐based bench stepping intervention for sedentary middle‐aged adults – a pilot study

The aim of this study was to investigate the benefits of a low‐volume, vigorous intensity bench stepping programme in sedentary middle‐aged adults. Thirty‐one healthy but sedentary adults (12 men; 55–64 years) took part in the study. Participants accumulated up to 9 min per day of stepping exercise on three days per week over the 4‐week experimental period. Parameters of cardiorespiratory fitness, body composition and lower limb muscle strength were measured on three occasions: TS1 (baseline test), TS2 (following a 4‐week control period) and TS3 (following the 4‐week intervention). Data were analysed using a repeated‐measures ANOVA. Adherence to the programme was excellent (96%). Relative to the insignificant changes following the control period, parameters of cardiorespiratory fitness were significantly improved following training. No alterations in body composition or lower limb muscle strength were detected. These results show that less than 30 min per week of bench step exercise, accumulated in short bouts throughout the day, can improve parameters of cardiorespiratory fitness after only 4 weeks in previously sedentary middle‐aged adults. Due to its low‐cost, time‐efficient and discrete aspects, stepping exercise may have important implications for public health initiatives that promote physical activity in a population who commonly report ‘lack of time’ as a barrier to physical activity.

Time-Optimal Control of Flexible Robots Made Robust Through Wave-Based Feedback

This paper presents a new, robust, time-optimal control strategy for flexible manipulators controlled by acceleration-limited actuators. The strategy is designed by combining the well-known, open-loop, time-optimal solution with wave-based feedback control. The time-optimal solution is used to design a new launch wave input to the wave-based controller, allowing it to recreate the time-optimal solution when the system model is exactly known. If modeling errors are present or a real actuator is used, the residual vibrations, which would otherwise arise when using the time-optimal solution alone, are quickly suppressed due to the additional robustness provided by the wave-based controller. A proximal time-optimal response is still achieved. A robustness analysis shows that significant improvements can be achieved using wave-based control in conjunction with the time-optimal solution. The implications and limits are also discussed.

Wave-Based Control - Implementation and Comparisons

Success in controlling flexible system is constrained by unavoidable practical limitations, such as actuator dynamics, exactness of system parameter modeling, and the accuracy of the system feedback available. Ultimately the practicality of implementing a control scheme will be determined by the robustness of the scheme to such non-ideal behavior, and by its ease of implementation. Wave-based control is presented as performing well on all these issues. Comparisons with other schemes for controlling flexible systems are made which show that it is not only relatively easy to implement but is also capable of achieving very fast responses.

Wave-Based Control of a Mass-Restricted Robotic Arm for a Planetary Rover

Here wave-based control is applied to suppress vibrations during repositioning of a flexible robotic arm on a planetary rover in a Martian environment. Typically, such a robotic arm has very low power and mass budgets, implying significant flexibility, with static and dynamic consequences. Meeting precision performance specifications then becomes challenging and active vibration control is required. The example of the DExtrous LIghtweight Arms for exploratioN (DELIAN) arm is the focus of this paper. DELIAN is a general purpose arm, currently in development by SELEX ES for the European Space Agency (ESA). For active vibration control, a wave-based controller is used: a strategy formulated specifically for flexible mechanical systems. It considers actuator motion as launching and absorbing mechanical waves into and from the system. Absorbing the returning wave provides effective, active vibration damping while simultaneously moving the system to the target displacement. It is robust to both actuator performance and modelling errors, very stable, and easy to implement. The controller was found to perform well in limiting the effect of the flexibility during manoeuvres and also when rejecting vibrations due to impacts.