Strain of the facet joint capsule during rotation and translation range-of-motion tests: an in vitro porcine model as a human surrogate

BACKGROUND CONTEXT

Prior data about the modulating effects of lumbar spine posture on facet capsule strains are limited to small joint deviations. Knowledge of facet capsule strain during rotational and translational intervertebral joint motion (ie, large joint deviations) under physiological loading could be useful as it may help explain why visually normal lumbar spinal joints become painful.

PURPOSE

This study quantified the strain tensor of the facet capsule during rotation and translation range-of-motion tests.

STUDY DESIGN/SETTING

Strain was calculated in isolated porcine functional spinal units. Following a preload, each specimen underwent a flexion/extension rotation (F/E) followed by an anterior/posterior translation (A/P) range-of-motion test while under a 300 N compression load.

METHODS

Twenty porcine spinal units (10 C3-C4, 10 C5-C6) were tested. Joint flexion/extension was imposed by applying a ±8 Nm moment at a rate of 0.5°/s, and translation was facilitated by loading the caudal vertebra with a ±400 N shear force at a rate of 0.2 mm/s. Points were drawn on the exposed capsule surface and their coordinates were optically tracked throughout each test. Strain was calculated as the displacement of the point configuration with respect to the configuration in a neutral joint position.

RESULTS

Compared to a neutral posture, superior-inferior strain increased and decreased systematically during flexion and extension, respectively. Posterior displacement of the caudal vertebra by more than 1.3 mm was associated with negative strains, which was significantly lower than the +4.6% strain observed during anterior displacement (p≥.199). The shear strain associated with anterior translation was, on average, -1.1% compared to a neutral joint posture.

CONCLUSIONS

These results demonstrate that there is a combination of strain types within the facet capsule when spinal units are rotated and translated. The strains documented in this study did not reach the thresholds associated with nociception.

CLINICAL RELEVANCE

The magnitude of flexion-extension rotation and anterior-translation may glean insight into the facet capsule deformation response under low compression (300 N) loading scenarios. Further, intervertebral joint motion alone, even under low compression loading, does not appear to initiate a clinically relevant pain response in the lumbar facet capsule of a nondegenerated spinal joint.

Monocular 3D Sway Tracking for Assessing Postural Instability in Cerebral Hypoperfusion During Quiet Standing

Postural instability is prevalent in aging and neurodegenerative disease, decreasing quality of life and independence. Quantitatively monitoring balance control is important for assessing treatment efficacy and rehabilitation progress. However, existing technologies for assessing postural sway are complex and expensive, limiting their widespread utility. Here, we propose a monocular imaging system capable of assessing sub-millimeter 3D sway dynamics during quiet standing. Two anatomical targets with known feature geometries were placed on the lumbar and shoulder. Upper and lower trunk 3D kinematic motion were automatically assessed from a set of 2D frames through geometric feature tracking and an inverse motion model. Sway was tracked in 3D and compared between control and hypoperfusion conditions in 14 healthy young adults. The proposed system demonstrated high agreement with a commercial motion capture system (error [Formula: see text], [-0.52, 0.52]). Between-condition differences in sway dynamics were observed in anterior-posterior sway during early and mid stance, and medial-lateral sway during mid stance commensurate with decreased cerebral perfusion, followed by recovered sway dynamics during late stance with cerebral perfusion recovery. This inexpensive single-camera system enables quantitative 3D sway monitoring for assessing neuromuscular balance control in weakly constrained environments.

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.

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.

The Flooring for Injury Prevention (FLIP) Study of compliant flooring for the prevention of fall-related injuries in long-term care: A randomized trial

BACKGROUND

Fall-related injuries exert an enormous health burden on older adults in long-term care (LTC). Softer landing surfaces, such as those provided by low-stiffness "compliant" flooring, may prevent fall-related injuries by decreasing the forces applied to the body during fall impact. Our primary objective was to assess the clinical effectiveness of compliant flooring at preventing serious fall-related injuries among LTC residents.

METHODS AND FINDINGS

The Flooring for Injury Prevention (FLIP) Study was a 4-year, randomized superiority trial in 150 single-occupancy resident rooms at a single Canadian LTC site. In April 2013, resident rooms were block randomized (1:1) to installation of intervention compliant flooring (2.54 cm SmartCells) or rigid control flooring (2.54 cm plywood) covered with identical hospital-grade vinyl. The primary outcome was serious fall-related injury over 4 years that required an emergency department visit or hospital admission and a treatment procedure or diagnostic evaluation in hospital. Secondary outcomes included minor fall-related injury, any fall-related injury, falls, and fracture. Outcomes were ascertained by blinded assessors between September 1, 2013 and August 31, 2017 and analyzed by intention to treat. Adverse outcomes were not assessed. During follow-up, 184 residents occupied 74 intervention rooms, and 173 residents occupied 76 control rooms. Residents were 64.3% female with mean (SD) baseline age 81.7 (9.5) years (range 51.1 to 104.6 years), body mass index 25.9 (7.7) kg/m2, and follow-up 1.64 (1.39) years. 1,907 falls were reported; 23 intervention residents experienced 38 serious injuries (from 29 falls in 22 rooms), while 23 control residents experienced 47 serious injuries (from 34 falls in 23 rooms). Compliant flooring did not affect odds of ≥1 serious fall-related injury (12.5% intervention versus 13.3% control, odds ratio [OR]: 0.98, 95% CI: 0.52 to 1.84, p = 0.950) or ≥2 serious fall-related injuries (5.4% versus 7.5%, OR: 0.74, 95% CI: 0.31 to 1.75, p = 0.500). Compliant flooring did not affect rate of serious fall-related injuries (0.362 versus 0.422 per 1,000 bed nights, rate ratio [RR]: 1.04, 95% CI: 0.45 to 2.39, p = 0.925; 0.038 versus 0.053 per fall, RR: 0.81, 95% CI: 0.38 to 1.71, p = 0.560), rate of falls with ≥1 serious fall-related injury (0.276 versus 0.303 per 1,000 bed nights, RR: 0.97, 95% CI: 0.52 to 1.79, p = 0.920), or time to first serious fall-related injury (0.237 versus 0.257, hazard ratio [HR]: 0.92, 95% CI: 0.52 to 1.62, p = 0.760). Compliant flooring did not affect any secondary outcome in this study. Study limitations included the following: findings were specific to 2.54 cm SmartCells compliant flooring installed in LTC resident rooms, standard fall and injury prevention interventions were in use throughout the study and may have influenced the observed effect of compliant flooring, and challenges with concussion detection in LTC residents may have prevented estimation of the effect of compliant flooring on fall-related concussions.

CONCLUSIONS

In contrast to results from previous retrospective and nonrandomized studies, this study found that compliant flooring underneath hospital-grade vinyl was not effective at preventing serious fall-related injuries in LTC. Future studies are needed to identify effective methods for preventing fall-related injuries in LTC.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01618786.

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 (mass_max). Grip strength was the only worker characteristic significantly associated with mass_max (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.

Older females in the workforce - the effects of age on psychophysical estimates of maximum acceptable lifting loads

The number of older workers in the workforce is increasing substantially, and advanced age is associated with factors that could influence musculoskeletal injury risk and work capacity. This study's goals were to test whether psychophysical estimates of maximum acceptable weight of lift (lift_max) differed between younger and older workers, and to examine potential explanatory factors. Twenty-four female workers (half 50 + years; half 20-32 years) self-adjusted a box's mass to their perceived lift_max during four lifting tasks. Older workers' lift_max values were significantly lower (by approximately 24%) than their younger counterparts. There were no age-related differences in resting heart rate, or peak joint angles and final heart rate during the lifting trials. However, the older group demonstrated lower grip strength (by 24%), and lower heart rate reserve during the trials (by 18%). These results question whether current maximum acceptable lifting weights based on psychophysical information are appropriately protective for female workers greater than 50 years of age. Practitioner Summary: This psychophysical study demonstrated that older female workers (aged 50-63 years) selected maximum acceptable lift masses that were (on average) 24% lower than younger workers (aged 20-32 years), which corresponded with lower grip strength and heart rate reserve. Current maximum acceptable lifting weights based on psychophysical information may not protect female workers greater than 50 years of age.

Compliant flooring to prevent fall-related injuries in older adults: A scoping review of biomechanical efficacy, clinical effectiveness, cost-effectiveness, and workplace safety

Background

Compliant flooring, broadly defined as flooring systems or floor coverings with some level of shock absorbency, may reduce the incidence and severity of fall-related injuries in older adults; however, a lack of synthesized evidence may be limiting widespread uptake.

Methods

Informed by the Arksey and O’Malley framework and guided by a Research Advisory Panel of knowledge users, we conducted a scoping review to answer: what is presented about the biomechanical efficacy, clinical effectiveness, cost-effectiveness, and workplace safety associated with compliant flooring systems that aim to prevent fall-related injuries in healthcare settings? We searched academic and grey literature databases. Any record that discussed a compliant flooring system and at least one of biomechanical efficacy, clinical effectiveness, cost-effectiveness, or workplace safety was eligible for inclusion. Two independent reviewers screened and abstracted records, charted data, and summarized results.

Results

After screening 3611 titles and abstracts and 166 full-text articles, we included 84 records plus 56 companion (supplementary) reports. Biomechanical efficacy records (n = 50) demonstrate compliant flooring can reduce fall-related impact forces with minimal effects on standing and walking balance. Clinical effectiveness records (n = 20) suggest that compliant flooring may reduce injuries, but may increase risk for falls. Preliminary evidence suggests that compliant flooring may be a cost-effective strategy (n = 12), but may also result in increased physical demands for healthcare workers (n = 17).

Conclusions

In summary, compliant flooring is a promising strategy for preventing fall-related injuries from a biomechanical perspective. Additional research is warranted to confirm whether compliant flooring (i) prevents fall-related injuries in real-world settings, (ii) is a cost-effective intervention strategy, and (iii) can be installed without negatively impacting workplace safety. Avenues for future research are provided, which will help to determine whether compliant flooring is recommended in healthcare environments.

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 (COP_Net) 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 (Correl_LR) 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

Correl_LR values did not differ across postures (AP: p = 0.395; ML: p = 0.647). Further, there were no main effects of time on Correl_LR (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 Correl_LR 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.

Study protocol for the Flooring for Injury Prevention (FLIP) Study: a randomised controlled trial in long-term care

BACKGROUND

A promising strategy for reducing the incidence and severity of fall-related injuries in long-term care (LTC) is to decrease the ground surface stiffness, and the subsequent forces applied to the body parts at impact, through installation of compliant flooring that does not substantially affect balance or mobility. Definitive evidence of the effects of compliant flooring on fall-related injuries in LTC is lacking. The Flooring for Injury Prevention (FLIP) Study is designed to address this gap.

METHODS

The FLIP Study is a 4-year, parallel-group, 2-arm, randomised controlled superiority trial of flooring in 150 resident rooms at a LTC site. The primary objective is to determine whether compliant flooring reduces serious fall-related injuries relative to control flooring. Intervention (2.54 cm SmartCells compliant; 74 rooms) and control (2.54 cm plywood; 76 rooms) floorings were installed over the top of existing concrete floors and covered with identical 2.00 mm vinyl. The primary outcome is serious fall-related injury, defined as any impact-related injury due to a fall in a study room that results in Emergency Department visit or hospital admission. Secondary outcomes include minor fall-related injury, any fall-related injury, falls, number of fallers, fractures, and healthcare utilisation and costs for serious fall-related injuries. Randomisation of study rooms, and residents in rooms, was stratified by residential unit, and flooring assignments were concealed. Outcome ascertainment began September 2013.

DISCUSSION

Results from the FLIP Study will provide evidence about the effects of compliant flooring on fall-related injuries in LTC and will guide development of safer environments for vulnerable older adults.

TRIAL REGISTRATION NUMBER

NCT01618786.

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 effects of body mass index and sex on impact force and effective pelvic stiffness during simulated lateral falls

BACKGROUND

The incidence of hip fractures is highest for underweight females with low body mass index (BMI). However, it is unknown how these factors influence impact dynamics during in-vivo lateral hip impacts.We used a pelvis release paradigm to compare: (1) absolute and normalized forces applied to the femur-pelvis system across sex and BMI groups; (2) the force-prediction accuracy of vibration-based versus force-deflection-based estimates of effective pelvic stiffness; and (3) effective pelvic stiffness between BMI and sex groups.

METHODS

Twenty-eight persons participated (7 low-BMI females, 7 low-BMI males, 7 high-BMI females, 7 high-BMI males,with BMI criteria of <22.5 and >28 for low- and high-BMI groups respectively). The participant's pelvis was released from heights of 0 to 5 cm. A force plate measured impact loads, while a motion capture system measured pelvic deflection.

FINDINGS

Peak impact forces were 22.6% higher, while normalized peak forces were 31.2% lower, for high- compared to low-BMI participants. Accuracy of peak force predictions improved by 25% for the force-deflection versus the vibration-based stiffness estimation method. Effective pelvic stiffness was greater for males than females, but no significant differences were observed between BMI groups.

INTERPRETATION

This study adds to clinical understanding of the effects of sex and BMI on impact dynamics during falls on the hip, and raises questions about the biomechanical mechanisms underlying the protective role of high BMI on hip fracture risk. Understanding the relationship between impact mechanics and faller characteristics should lead to more effective prevention of hip fractures.

Energy absorption during impact on the proximal femur is affected by body mass index and flooring surface

Impact mechanics theory suggests that peak loads should decrease with increase in system energy absorption. In light of the reduced hip fracture risk for persons with high body mass index (BMI) and for falls on soft surfaces, the purpose of this study was to characterize the effects of participant BMI, gender, and flooring surface on system energy absorption during lateral falls on the hip with human volunteers. Twenty university-aged participants completed the study with five men and five women in both low BMI (<22.5 kg/m(2)) and high BMI (>27.5 kg/m(2)) groups. Participants underwent lateral pelvis release experiments from a height of 5 cm onto two common floors and four safety floors mounted on a force plate. A motion-capture system measured pelvic deflection. The energy absorbed during the initial compressive phase of impact was calculated as the area under the force-deflection curve. System energy absorption was (on average) 3-fold greater for high compared to low BMI participants, but no effects of gender were observed. Even after normalizing for body mass, high BMI participants absorbed 1.8-fold more energy per unit mass. Additionally, three of four safety floors demonstrated significantly increased energy absorption compared to a baseline resilient-rolled-sheeting system (% increases ranging from 20.7 to 28.3). Peak system deflection was larger for high BMI persons and for impacts on several safety floors. This study indicates that energy absorption may be a common mechanism underlying the reduced risk of hip fracture for persons with high BMI and for those who fall on soft surfaces.

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).

Novel safety floors do not influence early compensatory balance reactions in older adults

Novel safety flooring systems are a promising approach for reducing fall-related injuries in seniors, as they have been demonstrated to substantially reduce impact severity during falls, while minimally impairing balance control in community-dwelling older women. This pilot study aimed to characterize the potential effects of flooring conditions on dynamic balance control in retirement home-dwellers with more limited mobility. A tether-release paradigm was used to simulate a trip-type perturbation in 15 seniors across five flooring surfaces (three novel safety floors and one carpet compared to institutional-grade resilient rolled-sheeting). Kinetic and kinematic data tracked the displacement profiles of the underfoot centre-of-pressure and whole-body centre-of-mass, which were used to characterize compensatory balance reactions. Difference tests (ANOVA) found that the onset of the compensatory balance reaction was not associated with floor condition, nor were the timing and magnitude of peak centre-of-pressure excursion (minimum margin of safety) and velocity. Accordingly, the minimum margin of safety of the centre-of-mass was not significantly different across floors. Equivalence tests supported these findings. This study provides evidence that the carpet and novel safety floors tested do not negatively influence characteristics of initial dynamic balance responses following a lean-and-release perturbation compared to an institutional-grade resilient rolled-sheeting surface. In combination with reports of substantial force attenuative properties during fall-related impacts, these findings support the promise of novel safety floors as a biomechanically effective strategy for reducing fall-related injuries.

The body configuration at step contact critically determines the successfulness of balance recovery in response to large backward perturbations

The mechanical efficiency of stepping to recover balance can be expressed by a biomechanical model that includes the trunk inclination angle and the angle of the leg at the instant of stepping-foot contact. The aim of the present study was to test the hypothesis that this model would accurately predict the successfulness of recovery attempts (recovery vs. falls) following large backward perturbations. Ten young participants were exposed to a series of 12 very large postural perturbations in the backward direction by means of a support-surface translation. At the instant of stepping-foot contact, we calculated the trunk inclination angle and the angle of the stepping leg with the vertical. Reaction time, step duration, step velocity and step length were also determined. A logistic regression analysis revealed that the model with leg and trunk inclination angles accurately predicted successful recovery, with a more forward tilted trunk and a further backward positioned leg increasing the probability of success. The set of spatiotemporal step variables was significantly less predictive. Over the course of the experiment, participants gradually became more successful in recovering balance, which coincided with an increase in leg but not in trunk angles. In conclusion, the body configuration at the instant of first stepping-foot contact accurately predicted successful balance recovery after a backward postural perturbation. Given the observation that participants improved their performance by increasing their leg angles, which suggests that it may be easier to improve this variable, compared to the trunk angle, by exercise interventions.