Anti-fatigue mats can reduce low back discomfort in transient pain developers.

Complaints of musculoskeletal pain are common among employees who stand for prolonged periods. This study sought to determine if an anti-fatigue mat (AFM) could uniquely affect low back pain (LBP), low back posture, and foot-floor interface responses in individuals prone to developing LBP (termed pain developers (PDs)) during prolonged standing experiments compared to those who do not develop LBP under the same exposures (termed non pain developers (NPDs)). Sixteen volunteers (8 PDs and 8 NPDs) were recruited based on their pain-development tendencies, which were established in previous standing experiments. They visited the laboratory on two separate days for 60 min of light manual work while standing on either a rigid floor or AFM. All participants were asymptomatic at the beginning of each experimental session. The amount of LBP experienced during the standing exposure, measured via a visual analogue scale, was reduced (p = 0.03) in the PD group when on the AFM (3.6 ± 6 mm) compared to the rigid floor (6.8 ± 7 mm). LBP levels remained low and unchanged (p = 0.5) between the AFM (2.4 ± 5 mm) and rigid floor (1.6 ± 2 mm) conditions for the NPD group. Neither postural nor foot-floor interface measures correlated with this unique reduction of LBP for the PD group when standing on the AFM. The AFM did, however, increase centre of pressure excursion (NPD 55% increase; PD 35% increase) and tended to increase the number of body weight shifts (NPD 116% increase; PD 54% increase) in both the PD and NPD groups. These findings suggest that AFMs may selectively benefit individuals prone to developing standing-induced back pain by facilitating subtle movements at the foot-floor interface.

Application of Principal Component Analysis to Forward Reactive Stepping: Whole-body Movement Strategy Differs as a Function of Age and Sex.

BACKGROUND: Differences in reactive stepping strategy to recover balance have been investigated as a function of age and sex, but to date have been measured using discrete step or joint specific measures. It is unknown how whole-body strategy or underlying motor control objectives differ between age and sex groups in forward reactive stepping. RESEARCH QUESTION: Does whole-body movement and/or motor control strategy differ as a function of age or sex in a forward reactive step to maintain balance? METHODS: Forty young and older adults (45 females, 35 males) participated in this study. All participants performed five reactive stepping trials in response to a forward balance perturbation while whole-body kinematics and ground reaction forces were collected. Features of whole-body movement strategy were determined using a principal component analysis model. Average principal component (PC) scores were compared between groups as a measure of whole-body movement strategy and within participant relative standard deviation of PC scores were compared to determine if motor control objectives differed across groups. RESULTS: Significant differences in reactive stepping strategy were observed both as a function of age and sex. Older adults had a greater step length and width, greater anterior trunk and pelvis translation, greater knee flexion angles and anterior translation of the hip joint on the stepping leg compared to young participants. Males had lesser step length and width, as well as greater trunk flexion compared to females. No differences in relative standard deviation of PC scores were observed between age or sex-based groups suggesting that motor control objectives were similar between groups. SIGNIFICANCE: This study demonstrates how whole-body movement strategy differs as a function of age and sex, which explains why previously reported discrete outcomes occur. Additionally, it does not seem that motor control strategy objectives differ between age or sex groups in forward reactive stepping.

The influence of increased passive stiffness of the trunk and hips on balance control during reactive stepping.

BACKGROUND: Age-related changes, which include increased trunk and hip stiffness, negatively influence postural balance. While previous studies suggest no net-effect of trunk and hip stiffness on initial trip-recovery responses, no study to date has examined potential effects during the dynamic restabilisation phase following foot contact. RESEARCH QUESTION: Does increased trunk and hip stiffness, in isolation from other ageing effects, negatively influence balance during the restabilisation phase of reactive stepping. METHODS: Balance perturbations were applied using a tether-release paradigm, which required participants to react with a single-forward step. Sixteen young adults completed two blocks of testing: a baseline and an increased stiffness (corset) condition. Whole-body kinematics were utilized to estimate spatial step parameters, center of mass (COM), COM incongruity (peak - final position) and time to restabilisation, in anteroposterior (AP) and mediolateral (ML) directions. RESULTS: In the corset condition, peak COM displacement was increased in both directions (p < 0.024), which drove reductions in minimum margins of stability (p < 0.032) as step width and length were unchanged (p > 0.233). Increased passive stiffness also increased the magnitude and variability of peak shear ground reaction force, COM incongruity, and time to restabilisation in the ML (but not AP) direction (p < 0.027). SIGNIFICANCE: In contrast to previous literature, increased stiffness resulted in greater peak COM displacement in both directions. Our results suggest increased trunk and hip stiffness have detrimental effects on dynamic stability following a reactive step, particularly in the ML direction. Observed increases in magnitude and variability of COM incongruity suggest the likelihood of a sufficiently large loss of ML stability - requiring additional steps - was increased by stiffening of the hips and trunk. The current findings suggest interventions aiming to mobilize the trunk and hips, in conjunction with strengthening, could improve balance and reduce the risk of falls.

Body configuration as a predictor of centre of mass displacement in a forward reactive step.

In balance perturbations that elicit backwards reactive steps, body configuration at stepping contact is related to likelihood of balance recovery. However, less is known about the relationship between body configuration (at stepping contact) and underlying centre of mass (COM) dynamics during dynamic perturbations requiring a forward reactive step. Accordingly, the primary objective of this study was to characterize the potential relationships between body configuration and COM displacement during simulated trips. Towards determining the robustness of these relationships, trips were simulated in both baseline and increased passive joint stiffness conditions. Sixteen healthy adults participated in this study. Trips were simulated using a tether release paradigm where participants were suddenly released, necessitating a forward step (onto a force plate) to recover their balance. Trials were performed in a baseline unconstrained condition, and in a 'corset' condition to increase passive stiffness of the trunk and hips. In all trials, whole body kinematics and kinetics were collected. Multiple linear regression models were run to assess the relationship of body angles to COM displacement in both the anteroposterior (AP) and mediolateral (ML) planes. Regression models showed a significant association of sagittal plane body configuration to both COM displacement at stepping contact and maximum COM displacement in the AP plane. Across models, the strongest predictor was the trail leg angle. Associations were stronger in the increased passive stiffness condition (average R2 = 0.366) compared to the baseline condition (average R2 = 0.266). Poor association of body configuration to COM displacement was found in the ML plane. The significant associations observed between body configuration and COM dynamics in simulated trips supports the potential downstream application of these models in identifying individuals with impaired balance control and increased fall risk.

Manual patient transfers: factors that influence decisions and kinematic strategies employed by nursing aides.

While extensive literature has characterised factors that influence the acceptable mass of 'boxes' during MMH tasks, less is known about these factors when moving 'people' in healthcare settings. This study examined factors that influence decisions/approaches employed during manual patient transfers. Sixteen nursing aides manually-transferred a standardised 'patient'; patient mass was adjusted (using a weight vest) to determine a maximum acceptable patient mass for this task (massmax). Grip strength was the only worker characteristic significantly associated with massmax (r = 0.48). Older worker age was associated with smaller peak trunk flexion (r = -0.58) and shoulder abduction (r = -0.59), and greater trunk axial twist (r = 0.52). Workers emphasised that patient characteristics (e.g. physical/cognitive status) influenced their decisions when performing transfers. These findings extend previous literature by suggesting that grip strength is a useful predictor of perceived work capacity, older workers adapt protective postural strategies during patient transfers and worker-patient dynamics are crucial during this high-risk occupational task. Practitioner Summary: This study examined manual patient transfers performed by nursing aides. Worker grip strength (but not age or size) was associated with perceptions of maximum acceptable patient mass. Kinematic changes suggested more conservative strategies used by older workers. Workers emphasised that patient characteristics substantially influenced their decisions when performing transfer tasks.

Standing Versus Stepping-Exploring the Relationships Between Postural Steadiness and Dynamic Reactive Balance Control.

While the literature has characterized balance control during quasi-static and/or dynamic tasks, comparatively few studies have examined relationships across paradigms. This study investigated whether quiet-stance postural steadiness metrics were associated with reactive control parameters (during both stepping and restabilization phases) following a lean-and-release perturbation. A total of 40 older adults participated. Postural steadiness (center of the pressure range, root mean square, velocity, and frequency) was evaluated in "feet together" and "tandem stance" positions. During the reactive control trials, the step length, step width, movement time, and reaction time were measured, in addition to the postural steadiness variables measured during the restabilization phase following the stepping response. Out of 64 comparisons, only 10 moderate correlations were observed between postural steadiness and reactive spatio-temporal stepping parameters (P ≤ .05, r = -.312 to -.534). However, postural steadiness metrics were associated with the center of pressure velocity and frequency during the restabilization phase of the reactive control trials (P ≤ .02, r = .383 to .775 for velocity and P ≤ .01, r = .386 to .550 for frequency). Although some elements of quasi-static center of pressure control demonstrated moderate associations with dynamic stepping responses, relationships were stronger for restabilization phase dynamics after foot-contact. Future work should examine the potential association between restabilization phase control and older adult fall-risk.

Stooping, crouching, and standing - Characterizing balance control strategies across postures.

BACKGROUND: While stooping and crouching postures are critical for many activities of daily living, little is known about the balance control mechanisms employed during these postures. Accordingly, the purpose of this study was to characterize the mechanisms driving net center of pressure (COPNet) movement across three postures (standing, stooping, and crouching) and to investigate if control in each posture was influenced by time. METHODS: Ten young adults performed the three postures for 60s each. Kinetic signals were collected via a force platform under each foot. To quantify mechanisms of control, correlations (CorrelLR) were calculated between the left and right COP trajectories in the anterior-posterior (AP) and medio-lateral (ML) directions. To examine the potential effects of time on balance control strategies, outcomes during the first 30s were compared to the last 30s. RESULTS: CorrelLR values did not differ across postures (AP: p = 0.395; ML: p = 0.647). Further, there were no main effects of time on CorrelLR (AP: p = 0.976; ML: p = 0.105). A significant posture-time interaction was observed in the ML direction (p = 0.045) characterized by 35% decreases in CorrelLR over time for stooping (p = 0.022). CONCLUSION: The dominant controllers of sway (i.e., AP: ankle plantar/dorsi flexors; ML: hip load/unload mechanism) are similar across quiet stance stooping, and crouching. Changes in ML control strategies over time suggests that fatigue could affect prolonged stooping more so than crouching or standing.

Use of the Nintendo Wii Balance Board for Studying Standing Static Balance Control: Technical Considerations, Force-Plate Congruency, and the Effect of Battery Life.

The Nintendo Wii Balance Board (WBB) has become popular as a low-cost alternative to research-grade force plates. The purposes of this study were to characterize a series of technical specifications for the WBB, to compare balance control metrics derived from time-varying center of pressure (COP) signals collected simultaneously from a WBB and a research-grade force plate, and to investigate the effects of battery life. Drift, linearity, hysteresis, mass accuracy, uniformity of response, and COP accuracy were assessed from a WBB. In addition, 6 participants completed an eyes-closed quiet standing task on the WBB (at 3 battery life levels) mounted on a force plate while sway was simultaneously measured by both systems. Characterization results were all associated with less than 1% error. R2 values reflecting WBB sensor linearity were > .99. Known and measured COP differences were lowest at the center of the WBB and greatest at the corners. Between-device differences in quiet stance COP summary metrics were of limited clinical significance. Lastly, battery life did not affect WBB COP accuracy, but did influence 2 of 8 quiet stance WBB parameters. This study provides general support for the WBB as a low-cost alternative to research-grade force plates for quantifying COP movement during standing.

Falls and Parkinson's Disease: Evidence from Video Recordings of Actual Fall Events.

OBJECTIVES: To compare the fall characteristics of individuals with and without Parkinson's disease (PD) through the analysis of real-life falls captured on video. DESIGN: Observational cohort study. SETTING: Two long-term care facilities in British Columbia, Canada. PARTICIPANTS: Individuals living in long-term care (N=306; 16 with PD). MEASUREMENTS: Falls captured on video and analyzed (N=906; 71 in participants with PD). Generalized estimating equation models were used to examine differences in fall characteristics between participants with and without PD. RESULTS: Individuals with PD were 1.3 times as likely as those without PD to fall because of incorrect weight shifting (95% confidence interval (CI)=1.03-1.65). Secondary steps during the fall event were 1.5 times as likely to be short in individuals with PD as in those without (95% CI=1.23-1.78). Individuals with PD were also 1.6 times as likely to attempt to recover balance by reaching to grasp an external object (95% CI=1.13-2.15) and 5.0 times as likely to secure grasp of the object (95% CI=1.23-20.0). CONCLUSION: Along with greater likelihood for individuals with PD to fall because of incorrect weight shifting, differences between groups were found in reactive stepping responses. This first detailed evidence of the characteristics of falls in PD should help to inform fall and injury prevention approaches for clinicians and rehabilitation professionals working with individuals with PD.

Age-related differences in movement strategies and postural control during stooping and crouching tasks.

While epidemiologic data suggests that one in four older adults have difficulty performing stooping and crouching (SC) tasks, little is known about how aging affects SC performance. This study investigated differences between young and older adults in lower limb kinematics and underfoot center of pressure (COP) measures when performing a series of SC tasks. Twelve healthy younger and twelve healthy older participants performed object-retrieval tasks varying in: (1) initial lift height, (2) precision demand, and (3) duration. Whole-body center of mass (COM), underfoot COP, and hip and knee angular kinematics (maximum angles and velocities) were analyzed. Compared to younger, older participants moved slower when transitioning into and out of pick-up postures that were characterized by less hip and knee flexion. Older participants also showed a diminished ability to adapt to the changing postural demands of each set of tasks. This was especially evident during longer tasks, whereby older individuals avoided high knee flexion crouching postures that were commonly used by younger participants. Older adults also tended to exhibit faster and more frequent COP trajectory adjustments in the anterior-posterior direction. It is likely that limitations in physical characteristics such as lower limb strength and range of motion contributed to these differences.

Moving beyond quiet stance: applicability of the inverted pendulum model to stooping and crouching postures.

BACKGROUND: Currently, it is unknown whether the inverted pendulum model is applicable to stooping or crouching postures. Therefore, the aim of this study was to determine the degree of applicability of the inverted pendulum model to these postures, via examination of the relationship between the centre of mass (COM) acceleration and centre of pressure (COP)-COM difference. METHODS: Ten young adults held static standing, stooping and crouching postures, each for 20s. For both the anterior-posterior (AP) and medio-lateral (ML) directions, the time-varying COM acceleration and the COP-COM were computed, and the relationship between these two variables was determined using Pearson's correlation coefficients. Additionally, in both directions, the average absolute COM acceleration, average absolute COP-COM signal, and the inertial component (i.e., -I/Wh) were compared across postures. RESULTS: Pearson correlation coefficients revealed a significant negative relationship between the COM acceleration and COP-COM signal for all comparisons, regardless of the direction (p<0.001). While no effect of posture was observed in the AP direction (p=0.463), in the ML direction, the correlation coefficients for stooping were different (i.e., stronger) than standing (p=0.008). Regardless of direction, the average absolute COM acceleration for both the stooping and crouching postures was greater than standing (p<0.002). CONCLUSION: The high correlations indicate that the inverted pendulum model is applicable to stooping and crouching postures. Due to their importance in completing activities of daily living, there is merit in determining what type of motor strategies are used to control such postures and whether these strategies change with age.

The influence of instruction on arm reactions in individuals with Parkinson's disease.

The purpose of this study was to examine whether explicit instruction would facilitate arm reactions in individuals with Parkinson's disease (PD). Individuals with (n = 10) and without (n = 15) PD responded to unexpected support-surface translations. To recover their balance, participants were required to either respond naturally (react natural) or to reach toward a nearby handrail (explicit instruction). Arm reactions were quantified from electromyographic (EMG) and arm kinematic recordings. Results showed that while explicit instruction led to earlier and larger arm reactions, the benefits were not different between individuals with and without PD. Specifically, when explicitly instructed to reach toward a handrail, shoulder EMG responses were 4% earlier (p = .005) and 32% larger (p < .001) compared to when instructed to react naturally. A 44% greater peak wrist medio-lateral velocity (p < .001) and a 29% greater peak shoulder abduction angular velocity (p < .001) were also observed when participants were instructed to direct their arms toward a handrail after an unexpected support-surface translation. Explicit instruction also led to a higher frequency of handrail contact and a 49 ms earlier time to handrail contact compared to the react natural condition (p = .015). These results suggest that providing instruction to promote arm movement may help reduce falls in older adults with and without PD.

The influence of ankle muscle activation on postural sway during quiet stance.

Although balance during quiet standing is postulated to be influenced by multiple factors, including ankle stiffness, it is unclear how different mechanisms underlying increases in stiffness affect balance control. Accordingly, this study examined the influence of muscle activation and passive ankle stiffness increases on the magnitude and frequency of postural sway. Sixteen young adults participated in six quiet stance conditions including: relaxed standing, four muscle active conditions (10%, 20%, 30% and 40% maximum voluntary contraction (MVC)), and one passive condition wearing an ankle foot orthotic (AFO). Kinetics were collected from a force plate, while whole-body kinematics were collected with a 12-sensor motion capture system. Bilateral electromyographic signals were recorded from the tibialis anterior and medial gastrocnemius muscles. Quiet stance sway amplitude (range and root mean square) and frequency (mean frequency and velocity) in the sagittal plane were calculated from time-varying centre of gravity (COG) and centre of pressure (COP) data. Compared to the relaxed standing condition, metrics of sway amplitude were significantly increased (between 37.5 and 63.2%) at muscle activation levels of 30% and 40% MVC. Similarly, frequency measures increased between 30.5 and 154.2% in the 20-40% MVC conditions. In contrast, passive ankle stiffness, induced through the AFO, significantly decreased sway amplitude (by 23-26%), decreased COG velocity by 13.8%, and increased mean COP frequency by 24.9%. These results demonstrate that active co-contraction of ankle musculature (common in Parkinson's Disease patients) may have differential effects on quiet stance balance control compared to the use of an ankle foot orthotic (common for those recovering from stroke).

The influence of handrail predictability on compensatory arm reactions in response to a loss of balance.

The current study examined whether compensatory arm reactions are influenced by the participant's knowledge of the handrail location prior to losing their balance. Thirteen young adults stood on a motor driven platform that could translate in the forward or backward directions. A handrail was positioned in a location that was either predictable (i.e., always on the participant's right) or unpredictable (i.e., on either the participant's right or left) to the participant. Unpredictability of the handrail location was ensured by using liquid crystal goggles to occlude the participant's vision until the onset of each translation. In response to each surface translation, participants were instructed to reach for and grasp the handrail as fast as possible. EMG activity from the posterior and anterior deltoids of the left and right arms as well as kinematic data of the wrist were recorded to quantify the resulting arm responses. It was found that in response to a loss of balance, participants activated the reaching arm 7 ms earlier (p = 0.020) and with a 21-30% greater amplitude (p = 0.010-0.029) during the predictable compared to unpredictable handrail condition. The earlier and larger EMG activity resulted in a 19% earlier initiation of arm movement (p = 0.016) and a 24% earlier handrail contact (p = 0.002) when the handrail was in a predictable compared to unpredictable location. These findings indicate that when a handrail is predictably located, individuals will pre-select their upcoming compensatory arm reactions prior to losing their balance and may be more effective in re-gaining stability.

Changes in spinal excitability during dual task performance.

The authors investigated how the nervous system responds to dual task performance. Because dual tasking is associated with greater postural challenges, it was hypothesized that spinal excitability would be reduced when simultaneously performing 2 tasks. For this experiment, participants maintained a lying or standing posture with or without performing a concurrent cognitive task (i.e., reacting to an auditory tone). Spinal excitability was assessed by eliciting the soleus Hoffmann reflex (H-reflex). Results indicated that the H-reflex was 6.4 ± 2.3% smaller (p = .011) when dual compared to single tasking. The reduced H-reflex amplitude, indicating a depressed spinal excitability, when dual tasking is suggested to reflect a neural strategy that individuals adopt to maintain postural stability when cognitive resources are divided between 2 concurrent tasks.

Age-related changes in the control of perturbation-evoked and voluntary arm movements.

OBJECTIVE: This study examined how handrail location predictability affects perturbation-evoked arm responses in young and older adults and whether age-related changes in perturbation-evoked arm responses are specific to mechanisms associated with reactive postural control. METHODS: Young and older adults reached for a handrail in response to a support surface translation (perturbation-evoked) or to a visual cue (voluntary). For both movement tasks, the handrail location was made predictable or unpredictable to the participant. Electromyographic (EMG) activity and kinematics of the reaching arm were recorded to quantify the arm response. RESULTS: Posterior deltoid EMG activity during perturbation-evoked and voluntary movements were delayed by 15-74 ms (p<0.001) and 16% smaller (p=0.024) when the handrail was in an unpredictable compared to a predictable location. While ageing resulted in a 12-16 ms delayed initiation of EMG activity during perturbation-evoked reaching (p=0.003), the effects of handrail predictability and movement task did not interact with age. CONCLUSIONS: Age-related differences in perturbation-evoked arm responses are independent of both handrail location predictability and movement task. SIGNIFICANCE: Age-related differences in perturbation-evoked arm responses cannot be solely attributed to declines in reactive postural control. Rather, ageing leads to a deterioration of neural mechanisms common to both perturbation-evoked and voluntary arm movements.