SPARC Workshop - Falls Prevention

Technology Review

Dr Ben Heller

Sheffield Hallam University

Posturography (I)Static and dynamic

• Posture is the orientation of any body segment relative to the gravitational vector (Winter, 1995)

• In static posturography, postural control is assessed while subjects maintain stance in a relatively unperturbed state (usually quiet stance on a fixed support surface).

• In dynamic posturography, the postural response of a standing subject is monitored as they are subjected to externally applied balance perturbations.

Posturography IICOP and COM

• Quantified either by movement of the centre of mass (COM) or the centre of pressure (COP).

• COM is the point in (3D space) about which the mass of the body is evenly distributed. About 55-57% of height from floor (4cm inferior to naval).

• Often approximated by 2D movement of the sacrum.

• The COP is defined as the point of application of the ground reaction forces under the feet

• COP-COM is considered to be an error signal (Winter), and has diagnostic value (Corriveau 2000)

• Winter stresses COM is the true measure of sway.

COP vs. COM

Centre of Pressure measurement techniquesForce Platforms

• General biomechanics platforms (e.g. Kistler) traditional approach to measure postural sway – accurate but expensive

• Alternatives include lower cost platforms, balance specific platforms and pressure-sensing mats

• Nintendo WiiFit. Low-cost, Bluetooth link to PC, free-drivers. Demonstrated by Clark et al. 2009 to produce equivalent results to force platforms.

• BUT – no shear force measurement.

Centre of Mass measurement techniques IPen sensor

• Physical movement detected by a pen on paper

• + low cost

• - not genuine COM, no automated analysisintrusive

Centre of Mass measurement techniques IIComputerised stereophotogrammetry

• gold standard (Vicon, MAC, CODA)

• + genuine COM.

• - expensive, hard to use

Centre of Mass measurement techniques IIIVideo

• Extraction of silhouettes from video (Goffredo et al. 2006)

• + genuine COM, low cost

• - experimental

Centre of Mass measurement techniques IVSway Pen

• Sway Pen (Heller and Bacon).

• + low cost, accurate, easy to use

• - not genuine COM.

Centre of Mass measurement techniques VGravity line projection (GLP) method

• The movement of the COP and COM are related by Newtonian mechanics d2x/dt2 = Fx/M

• Double integration doesn’t allow the boundary position of the COM to be determined, but when shear forces are zero, the COP is directly underneath the COM.

• Lafond et al. (2004) support this approach.

Centre of Mass measurement techniques VIAccelerometry

• Accelerometers measure acceleration AND gravity.

• In recent years MEMS accelerometers have become much pervasive – low cost, good performance

• No fixed reference, so useful for gait

• Moe-Nilssen and Helbostad (2002) describe a technique to remove the effect of gravity. Can discriminate different conditions, but can’t be directly compared to traditional measures of COM movement (no absolute position information)

Accelerometry II

• Accelerometers and rate gyroscopes ideally suited to free-living movement and activity monitoring.

• E.g. Actigraph, Minimod, ActivPal, Xsens

• Improved ecological validity at cost of loss of experimental control.

• See: De Bruin et al.(2008) Wearable systems for monitoring mobility-related activities in older people: a systematic review. Clinical Rehabilitation; 22:878–95

Dynamic posturography

• Movable platforms and controllable visual fields allow proprioceptive and visual sensory inputs to be perturbed and investifgation of sensory organisation.

– Balance Master from Neurocom (~$50k)Equitest sway-referenced surround (~$100k)

– Biodex balance system – tilting surface

– CAREN system from Motek• hydraulic 6 DOF motion base

• 2 m diameter platform with force plates

• 8 camera real-time motion capture system

• flat-screen projection system

• surround sound system

Fall detection

• By using accelerometer(s) and/or rate gyroscope(s), information can be obtained we can detect falls, or the imminent occurrence of a fall.

– Can automatically alert emergency services

– Can alert if not worn

– Can implement protective measures:A Wearable Airbag to Prevent Fall InjuriesTamura 2009

– Must be near 100% sensitivity and specificitythey claim 93% sensitivity, unstated specificitybut falsely triggered by running and jumping.

Computer games

• Rewarding activities can improve a participant’s motivation to practice (Betker et al., 2006).

• A potential approach to making balance-based exercises rewarding is to introduce a gaming element to the activity. – Exploiting real-time biofeedback information from force platforms for example

• Betker et al. (2006)– Position of the COP used to control computer games which require users to explore their boundaries of stability and which challenge equilibrium.

– Improved balance and reduced risk of falls – Increased attention during training and improved motivation

Pilot study with healthy young participants

x

y

Force Transducers

300 mm 225 mm

225 mm

Results with young participants

• Significant improvements (with large effect sizes) seen in balance computer game play

• Could hint at improvements in dynamic balance function

• However, could simply be a learning effect

• COP results much less clear• Significant interaction for APSD during two-feet eyes open

– Trend towards improvement for all variables in two-feet eyes open (effect sizes: 0.07-0.17 - small to moderate effects)

• Improvements in postural sway as a result of computer game and biofeedback based exercise have been reported previously (e.g. Betkar et al., 2006; Sihvonen et al., 2004).

• Now working with older people, more age appropriate activities (virtual dance).

Nintendo Wii Fit

• Recently released by Nintendo

• Four force sensors pick up total force (weight) and position of centre of pressure on balance-board.

• Link to Wii via Bluetooth (CAN LINK TO PC – vast games selection).

• Comes with suite of games / training applications

• Costs about £70 (+ £180 for the Wii)

Balance rehabilitation in virtual environments

•Computer games can provide a controlled, safe and challenging stimulus for rehabilitation.

•Most computer games not suited to an older clientele.

•Virtual worlds can present any real or imagined environment – can be age and culture appropriate for the target population and provide socialisation.

•Second Life (Linden Labs) example of virtual world

•Dance is an age and culture-appropriate, single or multiple participant activity that has significant rehabilitation benefits.

Convergence

Sensor Technology

Virtual worlds

Movement-Mapped Avatars

Dance Therapy

Second Lives

for the

Third Age

Virtual Dance for Older People Pilot study

• Two modified weighing scales per participant were used to collect centre of pressure (COP).

• Subjects’ balance rehabilitation movements were mapped on to the Avatar’s dance movements

• Piloted with 6 women (ages 80-91) undertaking exercise classes for balance rehabilitation

• Cohort had no previous computer experience

Results

• All were able to control basic avatar movements, some able to control complex dance movements

• All enjoyed the experience

• This, together with appropriate sensors, may motivate maintenance and rehabilitation exercise.

• Now funded to look at broadening this approach beyond dance into multi-participant ‘life narratives’, still controlled by targeted physical movement.

Future trendsJumping on the games bandwagon - Nintendo

• Nintendo WiiFit £70 wireless balance board with 4 vertical sensors – some balance games.

• Nintendi Wiimote £30 – wireless triaxial accelerometer and autotracking camera (up to 4 infra-red points, 100 fps, 128x96 raw resolution ‘subpixel analysis’ => 1024x768)

• now has dual axis rate gyro in Motionplus attachment

• Free library – Brian Peekhttp://www.codeplex.com/WiimoteLib

Future trendsJumping on the games bandwagon - Sony

• SonyEye provides crude motion detection, can be used for rehabilitation gaming

• Playstation Move soon to be released (Fall 2010) – will track (6 dof?) up to 4 illuminated spheres

• Augmented reality applications

• Specialist hardware, not soeasy to develop.

Future trendsJumping on the games bandwagon - Xbox

• Project Natal – Microsoft’s 3D motion tracking add-on for Xbox. Scheduled for November 2010 release £50 (?!!!)

• Depth tracking camera, so producesa 3D ‘point cloud’ at 30 fps.

• ‘State of the art’ computer vision human tracking algorithms.

• ‘Learns’ postures froma giant database!

Thank you!