Andris Freivalds

A dynamic biomechanical evaluation of lifting maximum acceptable loads

A biomechanical evaluation of the job-related stresses imposed upon a worker is a potential means of reducing the high incidence rates of manual material handling injuries in industry. A biomechanical model consisting of seven rigid links joined at six articulations has been developed for this purpose. Using data from cinematographic analysis of lifting motions the model calculates: (1) body position from articulation angles, (2) angular velocities and accelerations, (3) inertial moments and forces, and (4) reactive moments and forces at each articulation, including the L5/S1 joint. Results indicated effects of the common task variables. Larger load and box sizes increased the rise times and peak values of both vertical ground reaction forces and predicted L5/S1 compressive forces. However, boxes with handles resulted in higher L5/S1 compressive forces than for boxes without handles. Also, in lifting the larger boxes the subjects did not sufficiently compensate with reduced box weights in order to maintain uniform L5/S1 compressive forces. Smoothed and rectified EMG of erector spinae muscles correlated significantly with L5/S1 compressive forces, while predicted and measured vertical ground reaction forces also correlated significantly, indicating the validity of the model as a tool for predicting job physical stresses.

Relationships between clinical symptom severity scales and nerve conduction measures in carpal tunnel syndrome

This study examined the severity of symptoms in carpal tunnel syndrome (CTS) in relation to nerve conduction measures of the median nerve. Clinical symptom severity and nerve conduction studies were evaluated for 64 hands with CTS in 45 patients. We found the following: (1) significant relationships identified among the clinical scales resulted in a dichotomous symptom classification scheme into primary and secondary symptoms, with the former being more specific for those symptoms usually seen in association with nerve injury; (2) there were significant relationships between symptom severity and nerve conduction abnormality; (3) the primary symptom scale correlated more strongly with the electrodiagnostic measures of nerve injury than did the secondary symptom scale. Based on these findings, we believe that these clinical scales have biological significance and reflect median nerve injury. This would support their potential utility for evaluating the outcome of CTS treatment and developing a model for exposure–severity relationship.

BIOMECHANICAL STRESSES FROM MANUAL LOAD LIFTING: A STATIC VS DYNAMIC EVALUATION.

Abstract Over the last ten years, several static biomechanical models have been developed to predict the strength capability of a population and to quantitatively identify physical acts required in highly stressful jobs. In applying static biomechanical models to a dynamic act, such as lifting, it is assumed that the effects of acceleration and momentum are negligible. A detailed dynamic biomechanical simulation of lifting psychophysically determined maximum loads showed that the compressive force at the low-back and peak task moments at various body joints were approximately two to three times greater than those based on a static biomechanical simulation. Compressive forces at the lumbosacral joint, estimated from the static biomechanical simulation, were within the “action limit,” while those forces estimated from the dynamic biomechanical simulation exceeded the “maximum permissible limit.” However, the predictions based on static biomechanical simulation were in general agreement with the psychophysic...

Evaluation of meat-hook handle shapes

Seven meat hooks including two current designs and five new designs with flat-rectangular, frustum, double frustum, and cylindrical shapes were evaluated. A two-phase study was conducted. In the first experiment, maximum pulling forces were measured by load cell to evaluate the effects of handle shapes on subjective discomfort, maximum pulling force, and muscle activity. Two pulling forces, 15 and 30 kg, were employed to a pulley mechanism to simulate pulling a beef carcass horizontally in the second experiment. FSR (force sensitive resistor) glove was used to measure the pulling forces on the meat hooks. The glove has 12 sensors that result in placement on the pulpy regions of each phalange. In addition, a biomechanical hand model was developed and applied to predict tendon forces. Double frustum shaped handles produced significantly larger maximum pulling forces and best force efficiency when normalizing forces with EMG. In terms of external forces as measured by the FSRs, the averages of finger force contributions to the total finger force were 27%, 32%, 32%, and 10% in order from index finger to little finger. The averages of phalange force contributions to the total finger force were 20.9%, 33.7% and 45.4% for the distal, middle and proximal phalanges, respectively. A Chi-square analysis indicated that the phalange force distribution for double frustum handles deviated least from the average contributions for all hooks. Double frustum handles showed the least predicted tendon forces and normalized tendon forces per unit external force. The optimality of double frustum shaped handles was also supported by the lowest discomfort ratings. Therefore, based on both empirical physiological measurements and theoretical biomechanical calculations, a double frustum handle is most efficient for pulling task, producing the least amount of tendon forces.

Backpack load limit recommendation for middle school students based on physiological and psychophysical measurements.

The load of students backpacks has raised questions over the safety and health of schoolchildren everywhere. The purpose of this study is to use electromyography (EMG), posture evaluation, heart rate, and ratings of perceived exertion and perceptions of pain to find an acceptable backpack load limit for middle school students. Twenty middle school students aged 11 to 14 (10 female and 10 male) volunteered for the study. The subjects completed two tests, standing stationary and walking on a treadmill, where they carried 5% incremental loads from 0% body mass (BM) to 20% BM. The study indicated that the Borg-CR10 ratings and trunk flexion angle for the walking trial indicated a possible load limit of 10% BM due to the non-significant difference between 0 and 10% BM and the significant difference between 10 and 15% BM.

Reaction torques and operator stress while using powered nutrunners.

Reaction torque measures of six different nutrunners were correlated with EMG measures and subjective ratings of perceived exertion for different work surface orientations, joint stiffnesses, rpm levels, air pressure levels and handle configurations. Using an underpowered tool or using a softer joint resulted in larger impulses and worse ratings. A pulse-type shut-off mechanism produced the lowest reaction torque and the best rating. Overall, ratings correlated directly with the impulse level. The subjects were good judges of stress levels, and subjective ratings were valid measures of operators stress.

Visual performance on CRT screens and hard-copy displays

Three experiments examined the effect of hard-copy print and CRT screens of different resolution/addressability ratios (RAR) on accommodation and visual search performance. Three different display modes were generated with an IBM personal computer and a Princeton Graphics Terminal: capital letters in high-RAR mode, capital letters in low-RAR mode, and capital letters in high-RAR mode programmed to simulate the low-RAR mode. Same-sized letters were also presented on hard-copy print. Experiment 1 demonstrated that accommodation to hard-copy print and high-RAR screens was more accurate than to low-RAR screens. In Experiment 2 the spatial frequency channels activated by each display were evaluated by measuring the effect of display adaptation on the contrast-sensitivity function. The results suggested that high-RAR screens and hard-copy print activated higher-frequency channels than did low-RAR screens. In Experiment 3 significantly better visual search performance was obtained for high-RAR screens and for hard-copy print than for low-RAR screens. It was concluded that screen RAR is an important variable to consider in the design of CRTs.

Evaluation of handles in a maximum gripping task

Various hook handles were tested to evaluate the effect of handle design characteristics on subjective discomfort ratings and phalange forces in a maximum gripping task. A force glove system with 12 thin force sensitive resistor (FSR) sensors was used to measure phalange forces on the hook handles. Thirty subjects (15 males and 15 females) were tested, and generally subjects preferred 30 or 37 mm (the latter for large handed males) double frustrum handles followed by 30 mm oval handles, whereas overall they showed less preference for 37 mm oval handles and 45 mm double frustrum handles. The phalange force was more related to handle shape than to handle size in this study, i.e. the individual phalange forces on oval handles were about 8% higher than those on double frustum handles. The force distributions in the maximum gripping task showed significant differences in finger and phalange forces, in the order of middle, index, ring, and little fingers and distal, middle, and proximal phalanges from the highest to the lowest forces. The findings of this study may provide guidelines for designing double frustum handles for satisfying users preference and oval handles for obtaining high phalange forces in a maximum gripping task.

EFFECTS OF GLOVES ON MAXIMUM FORCE AND THE RATE OF FORCE DEVELOPMENT IN PINCH, WRIST FLEXION AND GRIP

Abstract Gloves are used in many industrial tasks and among the most common of those tasks are finger/thumb pinch, grip, and torque production with wrist flexion. The objective of this study was to investigate these tasks by creating force development profiles and by finding the effect of gloves on maximum force and the rate of force development. The results showed that – in the particular experimental task – wearing leather gloves did not affect pinching performance despite having to overcome the pliability of the gloves and loss of tactile information. Grip force was negatively affected by gloves, both in its maximum value, and in the rate of its development (especially in the late phases of exertion). The highest rate of force development occurred in the period of 30–70% of maximum force. There was no effect of gloves on maximum torque production (contradicting findings from other studies) or the rate of its development. Relevance to industry Gloves are widely used in industry and affect the mechanics of force and torque production. Understanding the mechanics of gripping, pinching and wrist flexion is necessary in order to reduce the incidence of cumulative trauma disorders (CTD).

Ergonomic evaluation of the effects of handle types on garden tools

Abstract The objective of this study was to evaluate quantitatively the effects of handle types of garden tools on ergonomic effectiveness, user satisfaction in terms of work performance, and subjective judgment of tactile feel and control. Shoveling performance, hand grip force and electromyography (EMG), task workload, and subjective ratings of perceived exertion were measured while using three types of garden tasks/tools (shovel, rake, and hoe) with three different handle types: a conventional wood handle and two more fiberglass handle. The most effective measure for shovels was the relative efficiency of shovel performance, that is, shovel performance divided by task workload. A hollow fiberglass handle was 12% more efficient than either a wood or solid fiberglass handle. The grip force and EMG analyses showed a similar effect, with a solid or hollow fiberglass handle requiring significantly less effort than a wood handle. Subjective ratings of the perceived exertion were also lowest for a hollow fiberglass handle. A hollow fiberglass handle showed better physiological efficiency and better subjective acceptability for comfort, tactile feel, and decreased slipperiness Relevance to industry This research presents a systematic approach to the ergonomic evaluation of new materials on improved work efficiency and user satisfaction. This quantitative examination of grip force, muscular effort, and workers subjective ratings can be applied to the ergonomic evaluation of other newly developed product.