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.

Are hybrid sit-stand postures a good compromise between sitting and standing?

Potential alternatives for conventional sitting and standing postures are hybrid sit-stand postures (i.e. perching). The purposes of this study were (i) to identify where lumbopelvic and pelvic angles deviate from sitting and standing and (ii) to use these breakpoints to define three distinct postural phases: sitting, perching, and standing, in order to examine differences in muscle activations and ground reaction forces between phases. Twenty-four participants completed 19 1-min static trials, from sitting (90°) to standing (180°), sequentially in 5°trunk-thigh angle increments. The perching phase was determined to be 145-175° for males and 160-175° for females. For both sexes, knee extensor activity was lower in standing compared to perching or sitting (p < .01). Anterior-posterior forces were the highest in perching (p < .001), requiring ∼15% of body-weight. Chair designs aimed at reducing the lower limb demands within 115-170° trunk-thigh angle may improve the feasibility of sustaining the perched posture. Practitioner summary: Individuals who develop low back pain in sitting or standing may benefit from hybrid sit-stand postures (perching), yet kinematic and kinetic changes associated with these postures have not been investigated. Perching can improve lumbar posture at a cost of increased lower limb demands, suggesting potential avenues for chair design improvement. Abbreviations: A/P: anterior-posterior; M/L: medial-lateral; LBP: low back pain; EMG: electromyography; TES: thoracic erector spinae; LES: lumbar erector spinae; VMO: vastus medialis obliquus; MVC: maximum voluntary contraction; ASIS: anterior superior iliac spine; PSIS: posterior superior iliac spine; BW: body weight; RMSE: root mean square error; SD: standard deviation; ROM: range of motion.

Acute Surgical Injury Alters the Tensile Properties of Thoracolumbar Fascia in a Porcine Model.

Recent work utilizing ultrasound imaging demonstrated that individuals with low back pain (LBP) have increased thickness and decreased mobility of the thoracolumbar fascia (TLF), an indication that the TLF may play a role in LBP. This study used a porcine injury model (microsurgically induced local injury)-shown to produce similar results to those observed in humans with LBP-to test the hypothesis that TLF mechanical properties may also be altered in patients with LBP. Perimuscular TLF tissue was harvested from the noninjured side of vertebral level L3-4 in pigs randomized into either control (n = 5) or injured (n = 5) groups. All samples were tested with a displacement-controlled biaxial testing system using the following protocol: cyclic loading/unloading and stress relaxation tests at 25%, 35%, and then 45% of their resting length. Tissue anisotropy was also explored by comparing responses to loading in longitudinal and transverse orientations. Tissues from injured pigs were found to have greater stretch-stretch ratio moduli (measure of tissue stiffness), less energy dissipation, and less stress decay compared to tissues from control pigs. Responses across these variables also depended on loading orientation. CLINICAL SIGNIFICANCE: these findings suggest that a focal TLF injury can produce impairments in tissue mechanical properties away from the injured area itself. This could contribute to some of the functional abnormalities observed in human LBP.

The effect of task type and perceived demands on postural movements during standing work.

This study investigated how task demands affect postural behaviour during standing. Twenty-four participants completed three different 12-min tasks: (1) a cognitive task that involved answering questions based on a written passage; (2) a light manual assembly task; and (3) standing quietly with no secondary task. The manual task was associated with the lowest amount of postural movement and a more static pose than the other two conditions. Specifically, postural variability of the lumbar (F = 5.8; p = 0.01) and thoracic (F = 4.2; p = 0.03) spine, and fidgets and shifts of the spine (F = 3.2; p = 0.048), were lowest in the manual task. Additionally, individuals perceiving tasks to be more demanding-regardless of task type-tended to move less (p = 0.049) than those perceiving lower demands. These findings provide important initial evidence that the type and perceived demands of standing work tasks can affect postural movement.

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.

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.

Characterization of the protective capacity of flooring systems using force-deflection profiling.

'Safety floors' aim to decrease the risk of fall-related injuries by absorbing impact energy during falls. Ironically, excessive floor deflection during walking or standing may increase fall risk. In this study we used a materials testing system to characterize the ability of a range of floors to absorb energy during simulated head and hip impacts while resisting deflection during simulated single-leg stance. We found that energy absorption for all safety floors (mean (SD)=14.8 (4.9)J) and bedside mats (25.1 (9.3)J) was 3.2- to 5.4-fold greater than the control condition (commercial carpet). While footfall deflections were not significantly different between safety floors (1.8 (0.7)mm) and the control carpet (3.7 (0.6)mm), they were significantly higher for two bedside mats. Finally, all of the safety floors, and two bedside mats, displayed 3-10 times the energy-absorption-to-deflection ratios observed for the baseline carpet. Overall, these results suggest that the safety floors we tested effectively addressed two competing demands required to reduce fall-related injury risk; namely the ability to absorb substantial impact energy without increasing footfall deflections. This study contributes to the literature suggesting that safety floors are a promising intervention for reducing fall-related injury risk in older adults.

Quantification of the trade-off between force attenuation and balance impairment in the design of compliant safety floors.

Safety floors (also known as compliant floors) may reduce the risk of fall-related injuries by attenuating impact force during falls, but are only practical if they do not negatively affect balance and mobility. In this study, we evaluated seven safety surfaces based on their ability to attenuate peak femoral neck force during simulated hip impacts, and their influence on center of pressure (COP) sway during quiet and tandem stance. Overall, we found that some safety floors can attenuate up to 33.7% of the peak femoral impact force without influencing balance. More specifically, during simulated hip impacts, force attenuation for the safety floors ranged from 18.4 (SD 4.3)% to 47.2 (3.1)%, with each floor significantly reducing peak force compared with a rigid surface. For quiet stance, only COP root mean square was affected by flooring (and increased for only two safety floors). During tandem stance, COP root mean square and mean velocity increased in the medial-lateral direction for three of the seven floors. Based on the substantial force attenuation with no concomitant effects on balance for some floors, these results support the development of clinical trials to assess the effectiveness of safety floors at reducing fall-related injuries in high-risk settings.