James L. Smith

The effect of load carrying and floor contaminants on slip and fall parameters

A study was conducted to provide set-up values of an important biomechanical parameter, heel velocity, for slip resistance testers. Four different floor surfaces (with or without oil contaminant) were prepared for ten subjects with each walking at a fixed velocity while carrying five different loads. Stride lengths were also measured to find whether a significant difference existed between a carrying method typically used in industry (two-handed carrying of a load in a tote box held directly in front of the body) and previously used load carrying techniques (backpack and front-back pack systems). A programmable slip resistance tester was used to measure dynamic coefficient of friction with conventional set-up values for heel velocity. The results showed that the conventional set-up value for heel velocity is valid for dry floors but is too low on oily floors because faster transfer of body weight was seen owing to decreased stride length. On oily floors, heel velocities of 60 to 140 cm/s would be recommended. An abnormal gait pattern, short stride length, was seen on oily floors or with heavy load carriage because subjects adjusted their stride length for a better stance.

Subjective assessment of floor slipperiness

Abstract Measured coefficients of friction of floor surfaces have not been shown to be consistent predictors of slipping and falling. However, human subjects appear to be able to perceive the slipperiness of a floor surface, and to modify their gait to mitigate the effects of slippery surfaces. This study investigated subjective rankings of floor surfaces (ceramic, steel, vinyl, plywood, and sandpaper) and the measured coefficients of friction. In addition, subjects reported a confidence rating on the occurence of a slip at “heel down”. The floor surfaces were selected a priori, to provide a range of slipperiness and subjects appeared to be able to reliably discriminate between those surfaces. Ceramic tile was the most frequently determined as the slippery floor with the high level of confidence ratings, CERTAIN and ALMOST CERTAIN. In other words, the confidence ratings were closely related to the subjective rankings. The higher the subjective rankings were, the higher the confidence ratings were. To prove subjective rankings by the quantitative measure, static coefficients of friction were measured and it appears that subjects were able to relate their sensations to a function of the static coefficient of friction.

Effects of age related sensory degradation on perception of floor slipperiness and associated slip parameters

A laboratory study was conducted to determine how sensory changes in elderly people affect subjective assessments of floor slipperiness, and associated friction demand characteristics and slip distance. To relate these parameters to actual slip and fall incidents, 30 subjects from two age groups (young and elderly) walked around a circular track on the slippery and non-slippery floor surfaces, while wearing a safety harness to prevent injury in case of a slip or fall. Prior to the walking experiment, the Sensory Organization Test was performed. During the experiment, subjective assessments of surface slipperiness of the floor were obtained prior to walking and after walking on the floor. Slip distance, required coefficient of friction (RCOF) and adjusted friction utilization (AFU) were assessed utilizing motion analysis and force platform systems. The results indicated that sensory changes in the elderly increased the likelihood of slips and falls more than their younger counterparts. This was due to incorrect perceptions of floor slipperiness, and uncompensated slip parameters such as slip distance and adjusted friction utilization.

Psychophysical modelling for combined manual materials-handling activities.

Most psychophysical studies in manual materials handling (MMH)are involved only with single MMH activities, i.e. lifting, lowering, carrying, holding, pushing or pulling. Very little research has been reported on the determination of operator capacities for combinations of MMH activities (e.g. lifting a box, then carrying the box, or carrying a box, then lowering the box). These kinds of combined activities are prevalent in industry and in our daily lives. The objective of this study was to utilize the psychophysical approach to examine the effects of combinations of lifting, carrying and lowering activities. Twelve male students served as subjects for the study. The capacities that were determined as the maximum acceptable workloads for a 1-h work period for four individual MMH activities—lifting from floor to knuckle height (LFK), lifting from knuckle to shoulder height (LKS), lowering from knuckle to floor height (LOW) and carrying for 3·4 m (C) —and three combined MMH activities—LFK + C, LFK + C + LKS...

A biomechanical analysis of industrial manual materials handlers

Resultant forces and torques on the joints of 11 females were studied as the subjects performed two manual materials handling tasks in their industrial environment. The subjects activities were recorded by high speed (102 frames per second) 16mm cinematography and the data analysed by a static and dynamic biomechanical model. Statistically significant differences were found between the results of the static and dynamic analyses. Slower filming rates were simulated and were found to show fewer significant differences between the static and dynamic analysis as the data sampling rate decreased. A kinematic analysis of the experienced and inexperienced lifters revealed a great deal of intra-subject variability as well as inter-subject variability indicating that the subjects varied their motion patterns as they lifted or lowered several 14 kg loads. For submaximal tasks such a variation in lifting patterns would allow the subjects to develop muscular load sharing which would help reduce localized muscle fati...

A Manual Materials Handling Study of Bag Lifting

Physiological and psychophysical research approaches were employed to determine the effects of two levels of fullness (70% and 95%) and the use of handles on bag lifting tasks. A comparison of bag lifting and box lifting was also conducted. Neither the level of bag fullness nor the availability of handles on the bag resulted in a significant change in physiological response for a 20.4 kg load. The maximum acceptable weight lifted was significantly higher when performing a bag lifting task either with handles or when the bag was 95% full. The maximum acceptable weight of lift for bag lifting tasks was higher (2.21 kg) than for box lifting tasks under the conditions of this experiment.

Psychophysical lifting capacity over extended periods

The effect of time on an individuals lifting capacity over extended periods using a psychophysical approach was studied. Twelve male subjects estimated their lifting capacity in a 25 min period, and then attempted to lift this weight for an 8 h period under varying conditions. For one experimental condition the subjects were allowed to adjust the weight, the final adjusted maximum acceptable weight of lift (MAWOL) averaged 85.4% of the original MAWOL determined in the 25 min session. The subjects also attempted to lift for an 8 h period, without any weight adjustments. All 12 subjects lasted the 8 h at 2 lifts per min, but at a frequency of 8 lifts per min only three subjects completed the eight hour lifting task. This indicates that the psychophysical approach is a valid method to measure lifting capacity across the lower lifting frequency range but overestimates the lifting capacity at the higher frequency range. Slight fluctuations were noted in heart rate and oxygen consumption which were recorded every hour.

Age related effects of transitional floor surfaces and obstruction of view on gait characteristics related to slips and falls

A laboratory study was conducted to examine gait changes between younger and older subjects as they walked across different floor surfaces. Twenty subjects participated in the experiment (five each of older and younger males and females). For half of the trials, subjects carried light loads that blocked their view of the floor surface immediately in front of them. Subjects walked on slippery (soapy water on vinyl) and stable (outdoor carpet) floor surfaces, as well as transitioning from one surface to another. Responses studied included: required coefficient of friction (RCOF), stride length (SL), and minimum toe clearance (MTC). Significant effects were found for the floor surface, load versus no load condition, and some interactions involving age (older versus younger subjects). Not all expected differences due to age were found in this experiment. The lack of significant differences between younger and older subjects could be due to the older subjects that participated in the experiment. They were volunteers at a local medical center, were in good physical shape, and were probably not typical of the population of people over 65 years of age. RELEVANCE TO INDUSTRY: Slips and falls in industry are costly safety issues in terms of human suffering as well as financial compensation. In many facilities and at home, people make transitions from one floor surface to another many times each day, while carrying loads or just walking. A better understanding of characteristics of people as they walk on slippery floor surfaces and the changes that might occur with age, will allow engineers to design better floor surfaces to reduce the incidence of slips and falls.

The use of biomechanical measures in the investigation of changes in lifting strategies over extended periods

The experiment presented here was conducted for the purpose of investigating changes in lifting patterns that occur due to the effects of learning and due to the effects of lifting over extended periods. It also illustrated the use of biomechanical measures in tracking these changes. Biomechanical measures usually have not been used to make interences about changes in lifting patterns due to such effects as practice, the number of lifts performed in a session, the frequency of lifting, etc. Six inexperienced subjects were asked to lift for an initial 4-hour session, six half-hour practice sessions, and a final 4-hour lifting session. All lifts were conducted at the rate of one lift per minute, from floor to shoulder height, with a weight chosen by the subject. Results indicated that there is a change from a leg lift to a more biomechanically stressful back lift (as inferred from greater joint center moments) due to the effects of training, but once these effects are observed they were maintained through the second 4-hour session. Furthermore, reduction in time per lift over the course of the experiment was accompanied by increases in moments at all of the joint centers, but this was more pronounced in the hip and knee joints for half of the subjects. Conclusions to be drawn from these results are that biomechanical modeling can be used under such circumstances with meaningful results and that there is a natural tendency to lift more quickly and to shift moments to the stronger muscles in the process of learning lifting patterns.

Psychophysical Modeling of Manual Materials-Handling Capacities Using Isoinertial Strength Variables

Strength testing has been used to determine a workers physical capacity as well as to screen an individual for placement in future employment. A comparison is made of the relationships between two types of strenght tests (isometric and isoinertial) and the manual materials-handling (MMH) capacities of individuals. Four MMH activities were evaluated: lifting from floor to knuckle height, lifting from knuckle to shoulder height, lowering from knuckle to floor height, and carrying for a 4.3-m distance. Psychophysical capacities were determined under three frequency conditions: one-time maximum, 1 lift/min, and 6 lifts/min. I soinertial 1.83-m maximum strength (Tl), tested on the incremental weight-lifting machine, proved to be the most highly correlated strength test to the capacities for MMH activities. Second-order polynomial regression models for MMH capacities were developed using T1as a predictor. The isoinertial strength test T1 is recommended as the most promising single screening test because of its simplicity and task similarity.