Biomedical subjects

Mark S Redfern

Publications and source records attributed to Mark S Redfern.

At least 19 recordsLinked to original sources

PubMed2007

Effects of slip testing parameters on measured coefficient of friction.

Slips and falls are a major cause of injuries in the workplace. Devices that measure coefficient of friction (COF) of the shoe-floor-contaminant interface are used to evaluate slip resistance in various environments. Testing conditions (e.g. loading rate, timing, normal force, speed, shoe angle) are believed to affect COF measurements; however, the nature of that relationship is not well understood. This study examines the effects of normal force (NF), speed, and shoe angle on COF within physiologically relevant ranges. A polyvinyl chloride shoe was tested using a modified industrial robot that could attain high vertical loads and relatively high speeds. Ground reaction forces were measured with a loadcell to compute COF. Experiment #1 measured COF over a range of NF ( approximately 100-500 N) for two shoe angles (10 degrees and 20 degrees ), four speeds (0.05, 0.20, 0.35, and 0.50 m/s), and two contaminants (diluted detergent and diluted glycerol). Experiment #2 further explored speed effect by testing seven speeds (0.01, 0.05, 0.20, 0.35, 0.50, 0.75, and 1.00 m/s) at a given NF (350 N) and shoe angle (20 degrees ) using the same two contaminants. Experiment #1 showed that faster speeds significantly decreased COF, and that a complex interaction existed between NF and shoe angle. Experiment #2 showed that increasing speed decreased COF asymptotically. The results imply that COF is dependent on film thickness separating the shoe and the heel, which is dependent on speed, shoe angle, and NF, consistent with tribological theory.

Accidental Falls↗

PubMed2006

Kinematics of the fingers and hands during computer keyboard use.

BACKGROUND: Although there has been extensive research about the kinematics of the neck, arm, and wrist during computer keyboarding, there is almost no information concerning the kinematics of the fingers, thumbs, and hands. The purpose of this descriptive study was to establish normative values of the kinematics of the fingers and hands during computer keyboard use. METHODS: This study describes the angles, angular velocities, and angular accelerations of the metacarpophalangeal joints and proximal interphalangeal joints for the right and left hands of 20 computer keyboard users during a word-processing task. A new kinematic variable for computer keyboard use, hand/wrist displacement, is also defined and examined. Hand/wrist displacement refers to the translational movements of the hands in which the entire hand is repositioned to strike the keys. Kinematics of both hands of the keyboard users were captured using a three-dimensional motion capture system. FINDINGS: Metacarpophalangeal joint kinematics in flexion/extension and abduction/adduction are reported during typing. Proximal interphalangeal joint kinematics in flexion/extension are also reported. The means and standard deviations for finger postures, velocities and acceleration were generally not significantly different between the right and left hands, with the exception of the 1st digit (thumb). Hand/wrist displacement was significantly different between the right and left hands for side to side movements. Differences in kinematics among the fingers are discussed in view of their potential to be a risk factor for musculoskeletal disorders. INTERPRETATION: This study establishes baseline understanding of the kinematics of computer keyboard use. This information will be useful in future studies of potential risk factors associated with keyboard use.

Adult↗

PubMed2006

Visually induced postural sway in anxiety disorders.

Postural sensitivity to moving visual environments in patients with anxiety disorders was studied. We hypothesized that patients with anxiety disorders would have greater sway in response to a moving visual environment compared to healthy adults, especially if they have space and motion discomfort (SMD). Twenty-one patients with generalized anxiety without panic (NPA) and 38 patients with panic and agoraphobia (PAG) were compared to 22 healthy controls. SMD was evaluated in all subjects via questionnaire. Subjects stood on a force platform that was either fixed or rotating with the subject (i.e., sway referenced) during exposure to a sinusoidally moving visual surround. Center of pressure (COP) data were computed from force transducers in the platform as a measure of sway. Results showed that patients swayed significantly more in response to the moving visual scene compared to control subjects, with no differences between the NPA and PAG groups. SMD was a predictor of sway response in the patients: patients with high SMD swayed significantly more than both Controls and anxiety patients with low SMD. These results indicate that patients with anxiety disorders, particularly those with SMD, are more visually dependent for balance. This subgroup of patients may be amenable to treatment used for patients with balance disorders (i.e., vestibular rehabilitation) that focuses on sensory re-integration processes that address visual sensitivity.

Adolescent↗

PubMed2006

Spectrally similar periodic and non-periodic optic flows evoke different postural sway responses.

The present study investigated the effect of optic flow periodicity on postural sway. Head and center-of-pressure (COP) displacements in response to an oscillating full-field bullseye-and-checkerboard pattern were recorded in six healthy adults. Scene movement was driven by one of five signals: (1) 0.1 Hz sinusoid, (2) 0.3 Hz sinusoid, (3) 0.5 Hz sinusoid, (4) the periodic sum of these three sinusoids (PSUM), or (5) a non-periodic counterpart (NPSUM = 0.1+ pi/10 + 0.5 Hz). Sway response power at the various stimulus frequencies were compared: (1) among the three pure sinusoidal groups; and (2) between the two sum-of-sinusoid groups. Head and COP responses displayed similar spectral content, though sway magnitude was larger for the head. Sway responses to the moving scenes were significantly larger than those observed during quiet stance. Each sinusoidal moving scene evoked a strong response at the stimulus frequency, as well as increased sway at non-stimulus frequencies, primarily below 0.2 Hz. For the sum-of-sinusoids stimuli, both PSUM and NPSUM signals elicited sway responses at each of their component frequencies. The amplitudes of these responses were similar to one another at 0.1 and 0.3 Hz, but significantly different at 0.5 Hz, with PSUM responses on average four times larger than those for NPSUM. These findings indicate that spectrally similar periodic and non-periodic stimuli elicit quantitatively different sway responses. The observed behaviors may be due to postural sensitivity to the predictability of visual motion, or due to other nonlinear and/or time-varying mechanisms in the postural control system.

Adult↗

PubMed2006

Responses to a virtual reality grocery store in persons with and without vestibular dysfunction.

People with vestibular dysfunction often complain of having difficulty walking in visually complex environments. Virtual reality (VR) may serve as a useful therapeutic tool for providing physical therapy to these people. The purpose of this pilot project was to explore the ability of people with and without vestibular dysfunction to use and tolerate virtual environments that can be used in physical therapy. We have chosen grocery store environments, which often elicit complaints from patients. Two patients and three control subjects were asked to stand and navigate in VR grocery stores while finding products. Perceived discomfort, simulator sickness symptoms, distance traveled, and speed of head movement were recorded. Symptoms and discomfort increased in one subject with vestibular dysfunction. The older subjects traveled a shorter distance and had greater speed of head movements compared with young subjects. Environments with a greater number of products resulted in more head movements and a shorter distance traveled.

Activities of Daily Living↗

PubMed2005

Sensory re-weighting in human postural control during moving-scene perturbations.

The aim of the current study was to further investigate a recently proposed "sensory re-weighting" hypothesis, by evoking anterior-posterior (AP) body sway using visual stimuli during sway-referencing of the support surface. Twelve healthy adults participated in this study. Subjects stood on the platform while looking at a visual scene that encompassed the full horizontal field of view. A sequence of scene movements was presented to the subjects consisting of multiple visual push/pull perturbations; in between the first two push/pull sequences, the scene either moved randomly or was stationary. The peak-squared velocity of AP center-of-pressure (COP) was computed within a 6 s window following each push and pull. The peak-squared velocity was lowest for the push/pull sequence immediately following the random moving scene. These results are consistent with the sensory re-weighting hypothesis, wherein the sensory integration process reduced the contribution of visual sensory input during the random moving scene interval. We also found evidence of habituation to moving scene perturbations with repeated exposure.

Adult↗

PubMed2005

Reciprocal influence of concurrent walking and cognitive testing on performance in older adults.

This study investigated how cognitive and walking performance are influenced when performed concurrently in older adults and what factors are associated with poorer dual-task responses. Using a dual-task study design, 217 subjects aged 74-85 years completed three information processing test conditions--(1) no test; (2) push-button simple reaction-time; and (3) visual-spatial decision reaction-time; and three walking test conditions--(1) seated; (2) walking over a 20m straight course; and (3) walking over 20m with a turn. Push-button and visual-spatial reaction-times increased approximately 21% and 6%, respectively, while walking compared to sitting (p<0.008). Reaction-time tests had a relatively weak impact on walking-time. Poorer dual-task walking-time responses correlated with slower walking velocity and weaker muscle strength (p< or =.062). Findings suggest that walking requires significant cognitive resources among older adults and slower-walking adults and weaker adults may require higher cognitive loading while walking.

Age Factors↗

PubMed2005

The influence of dynamic visual cues for postural control in children aged 7-12 years.

Young children rely heavily on vision for postural control during the transition to walking. Although by 10 years of age, children have automatic postural responses similar to adults, it is not clear when the integration of sensory inputs becomes fully developed. The purpose of this study was to examine this transition in the sensory integration process in children aged 7-12 years. Healthy children and adults stood on a fixed or sway-referenced support surface while viewing full-field optic flow scenes that moved sinusoidally (0.1 and 0.25 Hz) in an anterior-posterior direction. Center of pressure was recorded, and measures of sway amplitude and phase were calculated at each stimulus frequency. Children and adults had significant postural responses during approximately two-thirds of the trials. In adults, there was a 90% decrease in sway on the fixed surface compared with the sway-referenced surface, but only a 50% decrease in children. The phase between the optic flow stimulus and postural response in children led that of adults by 52 degrees at 0.1 Hz and by 15 degrees at 0.25 Hz. Adults and children aged 7-12 years have similar ability to use dynamic visual cues for postural control. However, 7-12-year-old children do not utilize somatosensory cues to stabilize posture to the same extent as adults when visual and somatosensory cues are conflicting.

Adult↗

PubMed2005

Developing an observational instrument to evaluate personal computer keyboarding style.

While computer use is considered to be a risk factor for musculoskeletal disorders of the upper extremity (MSD-UE), there is currently little information on how the fingers, hand and thumb are used during keyboarding, and no means to document their postures and motions. A computer keyboarding observational instrument, termed the Personal Computer Keyboarding Style (PeCKS), is being developed to meet this need. The goal of this paper is to describe the process by which the PeCKS was developed. Literature on MSD-UE was reviewed and discussions were held with content experts to develop criterion-based items considered to be potential risk factors for MSD-UE during keyboard use. These items were systematically reviewed and rated by seven experts using the Delphi technique. These ratings were used to determine which items to retain and which to discard, resulting in 19 items to define keyboarding style. The PeCKS, when further psychometric testing is completed, will be useful to document and assess keyboarding to identify those individuals who may be at risk for MSD-UE.

Computer Peripherals↗

PubMed2005

Ethnic differences in the frequency and circumstances of falling in older community-dwelling women.

OBJECTIVES: To examine ethnic differences in fall rates and fall circumstances in older community-dwelling Caucasian and African-American women. DESIGN: Prospective analysis of incident falls and a nested retrospective analysis of fall circumstances over 5.7 years. SETTING: Monongahela Valley, Pennsylvania. PARTICIPANTS: A total of 1,821 Caucasian and African-American women (mean age+/-standard deviation 76+/-5) enrolled in the Study of Osteoporotic Fractures and participating in 1993/94. Circumstances of 338 falls were collected on a subsample of 197 women who fell. MEASUREMENTS: Fall rates and fall circumstances. RESULTS: Women reported 4,547 falls in 9,508 person-years, averaging 0.48 falls per woman annually (95% confidence interval (CI)=0.43-0.53). Age-adjusted fall rates were nonsignificantly higher in Caucasians than African Americans (relative risk (RR)=1.30, 95% CI=0.93-1.83%). In women younger than 75, fall rates were similar in Caucasians and African Americans (RR=1.17, P=.46). In women aged 75 and older, fall rates were 50% higher in Caucasians than in African Americans (RR=1.50, 95% CI=0.90-2.49), although this difference was not significant (P=.12). Fall circumstances differed by ethnicity. Caucasian women were significantly more likely than African Americans to fall outdoors versus indoors (odds ratio (OR)=1.6, 95% CI =1.0-2.7) and laterally versus forward (OR=2.0, 95% CI =1.1-3.4) but less likely to fall on the hand/wrist (OR=0.6, 95% CI =0.3-1.0). Ninety-eight percent of individuals falling on their hand/wrist reported that they extended their hand to attempt to break their fall. CONCLUSION: Although the circumstances of falling differed for older Caucasian and African-American women, there were no differences in the frequencies of falling. These findings suggest that ethnic differences in fracture risk in older women may be due in part to the different ways in which older Caucasian and African-American women fall, rather than how often they fall. More information will be needed on fall circumstances to determine whether interventions need to be tailored by ethnic group.

Accidental Falls↗

PubMed2005

Clinical measurement of sit-to-stand performance in people with balance disorders: validity of data for the Five-Times-Sit-to-Stand Test.

BACKGROUND AND PURPOSE: People with balance disorders are characterized as having difficulty with transitional movements, such as the sit-to-stand movement. A valid and feasible tool is needed to help clinicians quantify the ability of people with balance disorders to perform transitional movements. The purpose of this study was to describe the concurrent and discriminative validity of data obtained with the Five-Times-Sit-to-Stand Test (FTSST). The FTSST was compared with the Activities-specific Balance Confidence Scale (ABC) and the Dynamic Gait Index (DGI). SUBJECTS AND METHODS: Eighty-one subjects without balance disorders and 93 subjects with balance disorders were recruited for the study. Each subject was asked to stand from a 43-cm-high chair 5 times as quickly as possible. The ABC and DGI scores were recorded. RESULTS: Subjects with balance disorders performed the FTSST more slowly than subjects without balance disorders. Discriminant analysis demonstrated that the FTSST correctly identified 65% of subjects with balance dysfunction, the ABC identified 80%, and the DGI identified 78%. The ability of the FTSST to identify subjects with balance dysfunction was better for subjects younger than 60 years of age (81%). DISCUSSION AND CONCLUSION: The FTSST displays discriminative and concurrent validity properties that make this test potentially useful in clinical decision making, although overall the ABC and the DGI are better than the FTSST at discriminating between subjects with and subjects without balance disorders.

Adolescent↗

PubMed2004

Simulator sickness when performing gaze shifts within a wide field of view optic flow environment: preliminary evidence for using virtual reality in vestibular rehabilitation.

BACKGROUND: Wide field of view virtual environments offer some unique features that may be beneficial for use in vestibular rehabilitation. For one, optic flow information extracted from the periphery may be critical for recalibrating the sensory processes used by people with vestibular disorders. However, wide FOV devices also have been found to result in greater simulator sickness. Before a wide FOV device can be used in a clinical setting, its safety must be demonstrated. METHODS: Symptoms of simulator sickness were recorded by 9 healthy adult subjects after they performed gaze shifting tasks to locate targets superimposed on an optic flow background. Subjects performed 8 trials of gaze shifting on each of the six separate visits. RESULTS: The incidence of symptoms of simulator sickness while subjects performed gaze shifts in an optic flow environment was lower than the average reported incidence for flight simulators. The incidence was greater during the first visit compared with subsequent visits. Furthermore, the incidence showed an increasing trend over the 8 trials. CONCLUSION: The performance of head unrestrained gaze shifts in a wide FOV optic flow environment is tolerated well by healthy subjects. This finding provides rationale for testing these environments in people with vestibular disorders, and supports the concept of using wide FOV virtual reality for vestibular rehabilitation.

Journal Article↗

PubMed2004

Correlation between EMG and COP onset latency in response to a horizontal platform translation.

This study examined the relationship between onset latencies estimates from EMG and center of pressure (COP) in young (five female, five male; mean=24.2+/-2.3 years) and older (six female, four male; 78.4+/-2.3 years) subjects during anterior or posterior platform translations. The latencies to onset of activity were estimated for the tibialis anterior (TA; mean=119.8 ms across both age groups) and COP (mean=139.7 ms across both groups) for anterior translations, and the soleus (SOL; mean=122.4 ms across both groups), gastrocnemius (GAS; mean=126.0 ms for young, and 115.9 ms for old subjects) and COP (mean=160.0 ms across both groups) for posterior translations. Average within-subject correlations (r') among these measures showed a high correlation between TA and COP onset latency (r'=0.667, young; r'=0.482, old), and relatively low correlations between the plantar flexors (SOL and GAS) and COP onset latencies (SOL: r'=0.292 for young, r'=0.249 for old; GAS: r'=0.126 for young, r'=0.143 for old). The SOL and GAS onset latencies correlated well with each other, especially in the older subjects (r'=0.762), suggesting that the contribution of two muscles creates some variability in the relationship with COP onset latency. The strong correlation between TA and COP for anterior perturbations, coupled with the weaker correlations for the plantar flexors suggest that the COP method may be preferable for studies interested in determining timing of postural responses to multidirectional perturbations.

Adult↗

PubMed2004

Cognitive influences in postural control of patients with unilateral vestibular loss.

The aim of this study was to investigate the interference between postural control and cognitive processing in patients with surgically confirmed unilateral vestibular lesions. These patients were well-compensated for vestibular lesions with no symptoms of dizziness or definable postural deficit. We hypothesized that attentional processes would play a greater role in postural control of these patients compared to healthy age-matched controls suggesting that successful compensation for a vestibular impairment involves ongoing increased attentional resources, and is not an automatic process. To explore this hypothesis, we used a dual-task paradigm that combined postural challenges with concurrently performed cognitive tasks. The postural conditions were seated, standing on a fixed floor, standing on a sway-referenced floor, and standing on a translating floor. Cognitive tasks were simple, inhibitory, and forced choice reaction time (RT) tasks. Patients had slower RTs compared to the controls under all conditions, including the seated condition. This effect was particularly large for the choice and inhibitory tasks. Both groups had increased RTs as the postural task became more challenging. Postural sway increased similarly in the patients and controls when performing the RT tasks for all postural conditions. These results suggest that patients with vestibular lesions that are well-compensated require increased attention compared to healthy controls; however, this increased demand on attention extends beyond postural control. The site of action may be at the sensory integration level resolving multiple sensory signals for spatial orientation.

Adult↗

PubMed2004

Effect of preparation on dual-task performance in postural control.

The authors applied an overlapping-task design to study the interaction between postural control and cognitive task processes in young (n = 10) and older (n = 10) adults. A rapid destabilizing floor translation was followed at specific time intervals by a simple auditory reaction time (RT) task. The translations were preceded by either an informational cue or no cue. Interference between postural task demands and the RT task was found only in the first 50 ms. Cueing also had an effect on both the onset of the postural recovery response and RT performance. The results suggest (a) only a brief interference between postural and cognitive processing demands in relatively easy tasks, (b) competition for a common central mechanism, possibly a response-selection mechanism, and (c) no differential impact of aging on that interaction.

Adult↗

PubMed2003

Visual-vestibular stimulation interferes with information processing in young and older humans.

Attention has been implicated in postural control and other tasks requiring sensory integration. The purpose of this study was to investigate the role of attention in sensory-motor processing of vestibular and combined visual-vestibular information during seated rotations using a dual-task interference approach. We hypothesized that auditory information processing would be influenced by concurrent visual-ocular, vestibulo-ocular, or combined visual-vestibulo-ocular processing. We further hypothesized that the effect would be greater in older subjects. Twenty older subjects (10 women, 10 men, 69.3+/-3.2 years) and 20 young subjects (10 women, 10 men, 23.5+/-2.9 years) were asked to perform information-processing tasks while they underwent several types of vestibular, visual-vestibular, and ocular motor paradigms. The information-processing tasks were: (1) an auditory simple reaction-time task (SRT), (2) an auditory go-no-go (disjunctive) reaction-time task (DRT), and (3) an auditory forced-choice task (CRT). The visual-vestibular-ocular motor conditions included: (1) no movement/darkness (NO), (2) no movement/fixation (FIX), (3) no movement/pursuit (P), (4) earth-vertical axis rotation (EVAR) in darkness, (5) EVAR with fixation (E-FIX), (6) off-vertical axis rotation (OVAR) in darkness, and (7) OVAR with fixation (O-FIX). Results showed that older subjects had longer reaction times for all combinations of stimulus condition and reaction-time task compared with young subjects. Compared with the NO baseline, reaction times during EVAR were longer for young and older subjects and during OVAR were longer for the young subjects. For FIX and P, the reaction times during P exceeded those during FIX and during NO for both groups. For E-FIX and O-FIX, reaction times did not differ from those during EVAR and OVAR. The interference with information processing by concurrent vestibular stimulation in the dark may be based upon cortical inhibition of auditory processes by vestibular stimulation. Eye movements induced by EVAR showed an increased phase lead during reaction-time tasks, suggesting altered vestibulo-ocular reflex (VOR) dynamics, possibly based on cerebellar-mediated changes in velocity storage. Since fixation of a head-fixed visual target did not add to the effect of rotation in the dark, a further implication of our results is that VOR-fixation while performing a concurrent information-processing task may be accomplished primarily by VOR suppression rather than by VOR cancellation.

Acoustic Stimulation↗

PubMed2003

Otolith-ocular responses in patients with surgically confirmed unilateral peripheral vestibular loss.

The chronic effects of unilateral peripheral vestibular loss (UPVL) are influenced by vestibular compensation. This study documents the balance-related symptoms and quantitative vestibular laboratory testing of 20 patients with surgically confirmed UPVL. Included are measures of the semicircular canal-ocular reflex, the otolith-ocular reflex, and both static and dynamic semicircular canal-otolith-interaction. This study differs from previous studies of patients with UPVL in that a large number of patients with surgically confirmed lesions were tested with three types of off-vertical axis rotation, several of the patients had anatomically preserved superior vestibular nerves, and self-perceived level of disability related to dizziness and imbalance were available. Results confirmed previously reported changes in the vestibulo-ocular reflex of patients with UPVL. Also, there was no apparent effect of anatomically preserving the superior vestibular nerve during surgical resection of vestibular schwannomas based on either subjective or objective measures of vestibular dysfunction. Further, there were no apparent correlations between subjective measures of dizziness and imbalance and objective measures of vestibulo-ocular function. These results have clinical implications for the management of patients with unilateral vestibular loss and provide insights into the process of vestibular compensation especially with respect to the otolith-ocular reflex.

Adult↗

PubMed2002

Changes in gait when anticipating slippery floors.

Falls precipitated by slipping are listed among the leading causes of injuries. The biomechanical analysis of such events is a necessary component of the slips/falls prevention research. One of the challenges of biomechanical studies is reproducing the unexpected nature of real-life slipping accidents. Thus, the goal of this study was to quantify changes in gait biomechanics when subjects anticipate slippery environments. Foot ground reaction forces and body dynamics of 16 subjects were recorded during level walking and descending ramps of varying frictional properties and inclination. Gait biomechanics were compared among three types of dry trials: (1) baseline (subjects knew the floor was dry); (2) anticipation (subjects were uncertain of the contaminant condition, dry, water, soap or oil); and (3) recovery trials recorded after a contaminated trial (subjects again knew the floor was dry). Subjects were asked to walk as naturally as possible throughout testing. Anticipation trials produced peak required coefficient of friction (RCOF(peak)) values that were on average 16-33% significantly lower than those collected during baseline trials, thus reducing slip potential. During recovery trials, RCOF(peak) values did not return to baseline characteristics (5-12% lower). Postural and temporal gait adaptations, which affected ground reaction forces, were used to achieve RCOF(peak) reductions. Statistically significant gait adaptations included reductions in stance duration (SD) and loading speed on the supporting foot, shorter normalized stride length (NSL), reduced foot-ramp angle and slower angular foot velocity at heel contact. As a result of these adaptations, anticipation of slippery surfaces led to significant changes in lower extremity joint moments, a reflection of overall muscle reactions. Thus, this study suggests that significant gait changes are made when there is a potential risk of slipping even though subjects were asked to walk as naturally as possible. Insights are also gained into the adaptations that are used to reduce the potential of slips/falls.

Accidental Falls↗