A.M. Genaidy

A comprehensive lifting model: beyond the NIOSH lifting equation

A comprehensive lifting model (CLM) for the evaluation and design of manual tasks was developed in two stages using 11 task, personal and environmental variables. In the first stage, the model was built using the psychophysical data. In the second stage, discounting factors of various variables were tested and adjusted using the physiological and biomechanical data. Two lifting indices are proposed to evaluate lifting tasks for a group of workers (relative lifting safety index or RLSI) and for an individual worker (personal lifting safety index or PLSI).

Physiological limits in lifting.

Two different experiments were conducted to study the effects of frequency, height and load of lift on computed maximal oxygen uptake and to compare the data obtained to a standard bicycle ergometer. A progressive submaximal technique was used to predict maximal oxygen uptake. The results of the present study revealed that across all heights, an increase in either the frequency of lift or load of lift was accompanied by an increase in computed maximal oxygen uptake; however, all maximal oxygen uptake values reported showed a tendency to level off beyond the frequency of five lifts/min. The results also showed that combined arm and leg lifting recorded the highest maximal oxygen uptake followed by leg lifting then arm lifting, respectively. Maximal oxygen uptake values obtained for lifting tasks were much lower than those obtained for a bicycle ergometer. It is therefore recommended that limits for continuous work for a lifting task be based on the attainable maximal oxygen uptake for that particular task and not on a maximal oxygen uptake obtained from another task such as a bicycle ergometer test.

Biomechanical assessment of titanium and stainless steel posterior spinal constructs : Effects of absolute/relative loading and frequency on fatigue life and determination of failure modes

The goal of this study was to examine the effects of absolute/relative loads and frequency on the fatigue life of titanium and stainless steel posterior spinal constructs, and to determine the failure fracture modes. The stainless steel constructs had higher stiffness and yield strength than the titanium constructs, but the ultimate static strength was almost equal for both types of constructs. Titanium constructs, however, exhibited higher variability than the stainless steel constructs. In fatigue tests, the stainless steel constructs were significantly affected by the external load and were frequency independent. It appears from fatigue curves that 500 N can be approximated as the endurance limit for the stainless steel constructs. Titanium constructs were load-frequency dependent, and their endurance limit was somewhere between the 500 and 750 N load levels. There were no differences in performance between the stainless steel and titanium constructs at 16 Hz. At 4 Hz, titanium constructs performed as well or better than stainless steel constructs. Also, the titanium constructs resulted in better performance than the stainless steel constructs in the elastic region, and with smaller differences in the plastic region. Most of the failure modes for stainless steel constructs were in screw bending at 16 Hz with a smaller percentage of rod fractures at high loads, with a higher percentage of rod fractures observed for the stainless steel constructs at 4 Hz. Most of the failure modes for titanium constructs occurred in screw bending or fracture.

Effects of frequency and load of lift on endurance time

The purpose of the present study was to determine endurance time for manual lifting tasks which were performed over a wide range of loads (5, 10, 15, and 20 kg) and frequencies (4, 6, 8, and 10 times/min) for a lift from floor to table height. Endurance time was defined in this study as the maximum length of time during which an individual was capable of lifting a given load at a given frequency continuously. The upper limit of endurance time was set to 8 h. Eleven male subjects participated in this study, and the lifting technique utilized with the straight back-bent knees method. The results showed that endurance time was significantly reduced with an increase in frequency or load of lift. The lightest frequency-load combination (4 times/min; 5 kg) was maintained by most of the subjects for 8 h. Conversely, the average endurance time for the heaviest frequency-load combination (10 times/min; 20 kg) was about 27 minutes.

An endurance training programme for frequent manual carrying tasks.

This study was conducted with the aim of testing whether the endurance time of new employees engaged in frequent industrial carrying tasks can be significantly increased through a short training programme (2.5 weeks). Endurance time was here defined as the maximum length of time during which an individual can continuously carry a 20 kg load over a 4 m distance 8 times/min. The results showed that (1) endurance time increased by almost 100% over an 8-session training programme; (2) for a fixed workload, endurance time increased without changing job demand perception; and (3) overall body fatigue may be the limiting factor in frequent carrying tasks.

A muscular endurance training program for symmetrical and asymmetrical manual lifting tasks

The purpose of this study was to test whether the endurance limits of new employees engaged in symmetrical and asymmetrical manual lifting tasks can be significantly increased through a physical training program. Each subject participating in the training program performed a total of 16 sessions. A control group performed two sessions separated by a 4-week interval. The results of this study showed that endurance time increased by 248% for the symmetrical and 46% for the asymmetrical lifting tasks. Furthermore, the frequency of handling increased by 44% and 34% for these tasks, respectively. The control group showed no improvement in terms of endurance time and frequency of handling. The implementation of a physical training program as a tool to control overexertion injuries in industrial settings is outlined.

Strength and fatigue resistance of thoracolumbar spine implants : An experimental study of selected clinical devices

Results of strength and endurance tests of several screw/rod thoracolumbar spinal implants are presented. The devices tested were: AcroMed ISOLA, CD systems, Depuy systems, and Synergy systems. The AcroMed ISOLA devices had the greatest stiffness and strength. The Synergy and AcroMed systems had the greatest fatigue resistance. The predominant failure mode was screw bending, although several instances of rod fracture and screw fracture were observed. A number of design improvements are identified, including reducing the number of components, thereby reducing the number of failure mechanisms; avoiding notches and knurled surfaces to provide greater resistance to crack initiation; and strengthening attachments between components, particularly between pedicle screws and horizontal rods.

Physiological Guidelines for the Design of Manual Lifting and Lowering Tasks: The State of the Art

The goal of many researchers and practitioners of occupational health and safety has been to design manual materials handling (MMH) tasks so as to reduce the frequency and severity of overexertion injuries usually associated with these types of tasks. Physiological limits with respect to the manual lifting and lowering aspects of MMH have been reviewed and evaluated. The information provided by different researchers has been presented in a concise manner that is easily accessed by designers and practitioners of MMH. Recommendations for future research in determining physiological limits of individuals engaged in manual lifting and lowering tasks and other aspects of MMH activities have been provided.

Physiological and psychophysical responses to static, dynamic and combined arm tasks

The main objective of the present study was to determine the effects of arm tasks possessing varying degrees of static and dynamic components upon the physiological and psychophysical strain imposed on the human body. Heart rate and blood pressure were used as indices of cardiovascular strain, while ratings of perceived exertion and endurance time are used as indices of psychophysical strain. For each weight of load studied, static tasks recorded the highest systolic blood pressure, dyastolic pressure, and ratings of perceived exertion values followed by combined, then highly dynamic, tasks. The highly dynamic task recorded the highest endurance time followed by combined then purely static tasks. This study supports the idea that blood pressure should be incorporated as one of the monitored strain factors in setting criteria for manual materials-handling tasks. In addition, the ratings of perceived exertion can be used as an effective tool in assessing the static component of the task. Based on the results obtained from the experimental conditions studied, the weight of the load has been shown to be an important parameter in the design of arm tasks comprising varying degrees of static and dynamic components. On the other hand, the frequency of handling is not an important parameter in the design of these types of tasks.

Physiologic Responses to Static, Dynamic and Combined Work

The main objective of the present study was to determine the effects on cardiovascular stress of tasks that have varying degrees of static and dynamic components (weight holding, manual lifting and stepping). Heart rate, oxygen consumption and blood pressure were used as indices of cardiovascular stress. The results showed that oxygen consumption and heart rate responses to manual lifting are significantly lower than those of stepping and higher than those of weight holding. A low frequency lifting task evoked significantly lower systolic blood pressure than a high frequency lifting task. This study supported the idea that a physiologic fatigue criterion (PFC) based on stepping should not be applied directly to tasks such as manual lifting. In addition, a PFC for manual lifting should not be based only on oxygen consumption or heart rate; it also should incorporate systolic blood pressure as one of the monitored stress factors in setting lifting standards.