Extending the center of pressure to incorporate handhold forces: Derivation and sample application.

Measures describing movement of the center of pressure (CoP) are frequently used to characterize postural control. Estimates of CoP often focus on forces that individuals exert in a single plane through the feet (standing on force plates). However, balance may also be supported by forces other than those developed at the feet, especially when walkers, handrails, and other aids are used. In these cases, it is common to neglect the contributions of handheld supports. Here, we derive and apply equations for an extended CoP that incorporates handhold forces. We then examine the influence of CoP definition (i.e., including or ignoring handhold forces) on common metrics (path length; RMS and maximum excursion; average and maximum velocity) for 12 younger adults with a handrail located lateral to the participants' dominant hand. Participants attempted balance recovery in response to a range of small, medium and large forward and backward platform translations. Significant interactions between perturbation magnitude and CoP definition were found for most metrics. Notably, the interaction of CoP definition and perturbation magnitude significantly affected path length (p-values < 0.001). Post-hoc analyses revealed larger CoP path length when handrail forces were incorporated in CoP estimates compared to ignoring handrail forces at medium (backward: 59.9 vs. 19.0% height; forward: 70.5 vs 22.4% height) and large perturbation magnitudes (backward: 69.9 vs 22.4% height; forward: 103.5 vs 24.6% height). Incorporation of hand forces in CoP calculations can present a different view of postural balance control than relying on a feet-only CoP. This measure could be useful in assessing balance control tasks that involve the use of handrails or hand-held mobility devices.

Development of a stick-on hip protector: A multiple methods study to improve hip protector design for older adults in the acute care environment.

INTRODUCTION: Over 90% of hip fractures in older adults result from falls, and hospital patients are at especially high risk. Specific types of wearable hip protectors have been shown to reduce hip fracture risk during a fall by up to 80%, but user compliance has averaged less than 50%. We describe the development and evaluation of a "stick-on" hip protector (secured over the hip with a skin-friendly adhesive) for older patients in acute care. METHODS: An initial version of the product was evaluated with six female patients (aged 76-91) in a hospital ward, who were asked to wear it for one week. We subsequently refined the product through biomechanical testing and solicited feedback from 43 health professionals on a second prototype. RESULTS: The first prototype was worn by five of six patients for the full week or duration of their hospital stay. The second prototype (20 mm thick, surface area 19 × 15.5 cm) provided 36% force attenuation, more than common garment-based models (20-21%). Feedback from patients and health professionals highlighted usability, comfort, cost, and appearance. CONCLUSIONS: Our results from biomechanical and user testing support the need for further work to determine the value of stick-on hip protectors in acute care.

A novel method for synchronizing motion capture with other data sources for millisecond-level precision.

Synchronization of multiple data collection systems is necessary for accurate temporal alignment of data, and is particularly important when considering rapid movements which occur in less than one second. This paper describes a novel method for synchronizing multiple data collection instruments including load cells and a motion capture system, using a common analog signal. An application of the synchronization method is demonstrated using biomechanical data collected during a rapid reach-to-grasp reaction, where data from motion capture and load cells are collected. Results are provided to validate and demonstrate the accuracy of the synchronization of motion capture with other data collection systems. During the reach-to-grasp trials, delays between the data collection systems ranged from 4ms to 235ms. The large range and variability in delay times between trials highlights the need for synchronization on a continual basis, rather than application of an average or constant value to correct for time delays between systems.

Age-related differences in dynamic balance control during stair descent and effect of varying step geometry.

The incidence of stairway falls and related injuries remains persistently high; however, the risk of stair injuries could be reduced through improved stairway design. The current study investigated dynamic balance control during stair descent and the effects of varying the step geometry. Data were collected from 20 healthy young and 20 older adults as they descended three staircases (riser heights of 7, 7.5 and 8 inches (178, 190 and 203 mm, respectively)). At each riser height, the tread run length was varied between 8 and 14 inches (203 mm and 356 mm) in one-inch (25 mm) increments. Kinematic data provided measures of segmental and whole-body dynamic control. Results demonstrated that older adults had greater lateral tilt of the upper body than young adults, but actually had larger margins of stability than the young in the antero-posterior direction as a result of their slower cadence. Nonetheless, for both age groups, the longer run lengths were found to provide the largest margins of stability. In addition, increase in run length and decrease in riser height tended to reduce forward upper body tilt. These results help to explain the underlying biomechanical factors associated with increased risk of falls and the relationship with step geometry. Considering the importance of stair ambulation in maintaining independence and activity in the community, this study highlights the definite need for safer stair design standards to minimize the risk of falls and increase stair safety across the lifespan.

Interpersonal strategies for disturbance attenuation during a rhythmic joint motor action.

Helping someone carry a table is fairly easy; however, our understanding of such joint motor actions is still poorly understood. We studied how pairs of human subjects (referred to as dyads) collaborate physically to attenuate external mechanical perturbations during a target tracking task. Subjects tracked a target moving in a slow and predictable way using wrist flexion/extension movements, with and without destabilizing torque perturbations. Dyad strategies were classified using interaction torques and muscular activity. During unperturbed interactions (baseline), the dyads tended to stabilize on a particular strategy. The baseline strategy was not the same in all dyads, suggesting that the solution to the task was not global but specific to each particular dyad. After several trials of unperturbed interactions, we introduced mechanical vibrations and analyzed the adaptation process. Dyads showed a tendency to counteract the external disturbances by first increasing co-contraction within each subject (independent co-contraction), and then raising the amount of opposing interaction torques (dyadic co-contraction) with increased perturbation amplitude. The introduction of perturbations impelled dyads to abandon their unperturbed baseline strategy and adopt a more common strategy across dyads, suggesting attractor solutions. Our results establish a framework for future human-human interaction studies, and have implications in human motor control as well as human-robot and robot-robot interactions.

Classification of strategies for disturbance attenuation in human-human collaborative tasks.

Rigorous analyses of the mechanisms human-human physical interaction are only possible if corresponding means of systematically classifying dyad strategies are in place. Previous suggestions for classification of strategies neglect the high level of redundancy that is present when attenuation of external disturbances is required. To address this, we propose a quantitative classification system based on combined interaction force and EMG recordings of the flexion and extension activities of each partner in a given dyad.