The effect of load weight on balance control during lateral box transfers

Few studies have endeavoured to measure balance control during manual material handling. This study examined the effects of load weight during a stationary manual material handling task. In total, 36 healthy participants completed 180° lateral transfer tasks of a loaded (5% of body weight) and an unloaded box. The projection of the centre of mass onto the base of support, as measured via a passive-marker 3-D motion analysis system, was used to quantify balance control. Muscle activities of lower extremity muscles were also measured. When moving the loaded box, individuals ventured ≥ 1 cm closer to the edges of the base of support and increased centre of mass movement up to 14%. In addition, muscle electromyographic activity on both sides of the shank increased. In summary, during loaded configurations, vulnerability to loss of balance was increased and individuals appeared to adapt by increasing co-contraction of the shank muscles suggesting increased ankle stiffness. STATEMENT OF RELEVANCE: Industries requiring manual material handling have a particularly high rate of injuries due to falls. This study suggests that larger load weights during lateral material handling tasks adversely affect balance control and may create a vulnerability to imbalance throughout the entire manoeuvre.     

Effects of common working postures on balance control during the stabilisation phase of transitioning to standing

Standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to assess the magnitude of this imbalance. Forty-five male participants completed three replications of conditions created by four static postures and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Body segment location and displacement of the centre of pressure (COP) were recorded using a motion capture system and two forceplates, respectively. Balance control measures were calculated during the stabilisation phase. All balance control measures were significantly affected by static posture but not duration within posture. Bending over at waist generally caused the smallest changes in balance control measures, whereas the reclined kneeling posture resulted in the largest. Findings may lead to recommendations for redesign of tasks to reduce the use of certain working postures, particularly in high-risk environments such as construction. STATEMENT OF RELEVANCE: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that the working posture chosen affects stabilisation during a transition to a standing position. Bending at the waist or squatting seems to have less of an affect on balance control measures, whereas both types of kneeling postures evaluated resulted in greater imbalance.     

Balance control during lateral load transfers over a slippery surface

Few studies have measured balance control during manual material handling, and even fewer with environmental cofactors. This study examined the effect of different surface frictions during a stationary manual material handling task. Thirty-six healthy participants completed 180° lateral transfer tasks of a load over high- and low-friction surfaces (μ = 0.86 and μ = 0.16, respectively). Balance measures, stance kinematics and lower extremity muscle activities were measured. Success during the novel slippery surface dichotomised our population, allowing us to investigate beneficial techniques to lateral load transfers over the slippery surface. Stance width reduction by 8 cm and 15° of additional external foot rotation towards the load were used to counter the imbalance created by the slippery surface. There was no clear alteration to lower extremity muscular control to adapt to a slippery surface. Changes in stance seemed to be used successfully to counter a slippery surface during lateral load transfers. STATEMENT OF RELEVANCE: Industries requiring manual material handling where slippery conditions are potentially present have a noticeable increase in injuries. This study suggests stance configuration, more so than any other measure of balance control, differentiates vulnerability to imbalance during material handling over a slippery surface.     

Effects of simulated occupational task parameters on balance

The effects of single-handed load holding, length of the base of support, and standing surface condition (narrow and wide construction beams) on balance were investigated in twenty-three healthy men between the ages of 18 and 55 years old. Balance during quiet standing was evaluated from postural sway measurements derived from center of pressure (COP) displacement. These measurements included the range or maximal displacement of the COP in the anteroposterior (AP) and mediolateral (ML) directions, the elliptical area, and mean sway velocity. Holding a load in the hand did not significantly affect postural sway measures (p > 0.05), although the effect of surface condition was significant on all COP measures (p < 0.001). Lengthening the base of support did not affect the ranges or elliptical area, but increased the mean velocity of sway (p = 0.001). Changes in the dimensional characteristics of the surface condition and length of base of support affected postural sway, possibly by requiring adjustments to balance and motor control strategies. Further research is required to determine if these changes are detrimental to maintaining balance and increase the risk of falls for workers in similar environments.     

The use of a heel-mounted accelerometer as an adjunct measure of slip distance

A human-centered measure of floor slipperiness could be useful as an adjunct to conventional tribologic measures. This paper reports on the development and evaluation of a measure of slip distance based on variables derived from the signal of a heel-mounted accelerometer. Twenty-one participants walked on a laboratory runway under several surface slipperiness conditions at three walking speeds during a protocol designed to produce a wide range of slip distances at heel strike. Analysis of variance showed significant effects of slip distance (no-slip, micro-slip and slide), walking speed (1.52, 1.78 and 2.13 m/s) and their interactions on peak forward acceleration, peak vertical acceleration and deceleration time of the heel following heel strike in 704 trials. Regression analysis of slip distance and deceleration time showed the strongest relationship with R2=0.511. Large individual variation in the strength of this relationship was observed. The heel-mounted accelerometer may have utility as an adjunct measure in the evaluation of floor slipperiness, particularly for field applications where direct measurement may not be feasible.     

Association of subjective ratings of slipperiness to heel displacement following contact with the floor

Falls precipitated by slipping are listed among the leading causes of occupational injuries. Several factors may influence the risk of slips and falls, including perception of surface conditions. The current research examined the relationship between perceptions of slipperiness and slip distance at heel strike. The investigation compared objective and subjective measures for 31 participants ranging in age from 19 to 67 years old. Subjective slipperiness ratings for several floor surface and walking velocity conditions were obtained. Small slips were not generally perceived, but a uniform slip distance threshold could not be identified due to the large variability in ratings. The Pearson correlation coefficient between slip distance and subjective ratings for slides (30-100mm) was r=-0.17. Results indicate that subjective ratings should be used cautiously as a measure of slipperiness, partially due to possible underestimation of slipperiness and the variability in perceptions.     

Estimation of forces exerted by the fingers using standardised surface electromyography from the forearm

Determination and integration of human force capabilities and requirements is an essential component of ergonomic evaluation. With regard to hand-intensive tasks, direct force measurements can be cumbersome and intrusive. Here, the use of surface electromyography (EMG) was evaluated. EMG was obtained from three standardised electrode sites on the forearms of 30 individuals. Linear regression models were generated to estimate finger force levels from normalised electromyographic measures, while forces were generated in several finger couplings. The results suggest that standardised procedures for obtaining electromyographic data and simple linear models can be used to accurately estimate finger forces during a variety of finger exertions in fixed postures, although the level of accuracy depends on the type of model. Such models begin to overcome the limitations of direct finger strength measurements of individuals.     

Effects of different physical workload parameters on mental workload and performance

The design and evaluation of an occupational task should include an assessment of mental workload, since excessive levels of mental workload can cause errors or delayed information processing. Physically demanding work that is performed concurrently with a cognitive task may impact mental workload by impairing mental processing or decreasing performance. The primary objective of this study was to determine whether there is a differential effect of various types of physical activity on both mental workload and cognitive performance. Objective and subjective assessment tools (heart rate variability and visual analog scale) were used as indicators of mental workload, while correct responses during an arithmetic task reflected levels of performance. Thirty participants (ages 18-24 years) performed a combination of tasks inducing both physical and mental workload. Type of physical effort, frequency of movement, and force exertion level were manipulated to alter the workload associated with the physical activity. Changes in subjective ratings generally corresponded to changes in both performance on the arithmetic task and objective mental workload assessment. Some discrepancies occurred at the highest physical force exertion level as participants perceived an increase in effort to maintain the same level of performance. Further research is needed to determine the force exertion threshold, beyond which the physical effort required interferes with mental workload and/or cognitive performance.

Relevance to industry

Technological advancements have increased the requirement for many workers to execute cognitive tasks concurrently with physical activity. When designing and evaluating such situations it is important to determine the interactive effects of these activities. A simple, uni-dimensional tool is suggested as a screening tool to identify situations requiring excessive or increased mental workload that many degrade performance or place additional stress on the individual.     

Bayesian assessment of overtriage and undertriage at a level I trauma centre

We analysed the trauma triage system at a specific level I trauma centre to assess rates of over- and undertriage and to support recommendations for system improvements. The triage process is designed to estimate the severity of patient injury and allocate resources accordingly, with potential errors of overestimation (overtriage) consuming excess resources and underestimation (undertriage) potentially leading to medical errors.We first modelled the overall trauma system using risk analysis methods to understand interdependencies among the actions of the participants. We interviewed six experienced trauma surgeons to obtain their expert opinion of the over- and undertriage rates occurring in the trauma centre. We then assessed actual over- and undertriage rates in a random sample of 86 trauma cases collected over a six-week period at the same centre. We employed Bayesian analysis to quantitatively combine the data with the prior probabilities derived from expert opinion in order to obtain posterior distributions. The results were estimates of overtriage and undertriage in 16.1 and 4.9% of patients, respectively.This Bayesian approach, which provides a quantitative assessment of the error rates using both case data and expert opinion, provides a rational means of obtaining a best estimate of the system's performance. The overall approach that we describe in this paper can be employed more widely to analyse complex health care delivery systems, with the objective of reduced errors, patient risk and excess costs.