Reliability of repetitive dynamic strengths as a screening tool for manual lifting tasks

Test-retest reliability of isometric and isoinertial testing in symmetric and asymmetric lifting

The aim of this study was to evaluate test-retest reliability of a dynamometer in measuring lifting strength or force parameters under several combinations of ergonomic factors. Thirteen healthy participants were tested on peak force (PF), related variables and isometric strength (IS) twice, at intervals of 3 months. Correlation coefficients for all parameters in the sagittal plane were 0.60-0.85. Coefficients of variations (CVs) of methodology error for PF in the sagittal plane were 6.2-6.9%. Correlation coefficients and CVs for IS at 90 degrees to the lateral plane were 0.51-0.54 and 16.6-17.9%, respectively. In paired t-tests of the parameters under all conditions, there was a significant difference between test and retest. In the test and retest, ratings of perceived exertions for the low back and the right arm in isometric lifting were significantly higher than those in dynamic lifting. It was concluded that the test-retest reliability of dynamic forces in the dynamometer was high. The peak force in the sagittal plane was considered reliable. In isometric lifting, isometric strength in the sagittal plane seemed reliable, while that at right angles to the lateral plane was considered to be less reliable.

Weight Limits for Foldaway Wheelchairs

Designers have neglected the issue of portability in wheelchair design.

Isokinetic pull strengths in the vertical plane: effects of speed and arm angle

The significance of knowing dynamic (isokinetic) strengths of individuals and the impact of selected individual and work-related factors on these strengths have been addressed in previous works. This follow-up study investigated the effects of pulling speed and angle of the preferred arm on isokinetic strengths of males and females in a vertical plane. The results indicated that speed of pulling has a major influence on strength exertion capabilities of people. Isokinetic strength exertions are maximum in the vertical plane but are smaller than isometric strength exertions in the vertical plane. As expected, men exerted greater force than females. Force exertions in the sitting and standing postures were not appreciably different from each other. RELEVANCE: Knowledge of strength exertion capabilities of individuals is necessary for designing work controls, and in assessing rehabilitative effects of physical therapy programmes. How the speed of motions and orientations of the arm influence strength can assist in designing work which is within the capability of the working population.

Determination of gross weight limit for foldaway powered wheelchairs through isometric and psychophysical strength simulations

This study concerns the maximum weight of a powered wheelchair for transfer to and from the trunk of a car by individuals with limited exercise endurance. The loading and unloading of a folded wheelchair from the trunk of a medium size American car was simulated. Both static and dynamic simulations were carried out. The isometric (static) and psychophysical (dynamic) strengths of eight adult males while simulating the loading/unloading activities were measured. The results indicated that: (1) the maximum weight of a powered foldaway wheelchair that individuals can load in the car and unload from the car is much lower than the weight of most commercially available powered wheelchairs, and (2) static simulation of the folded wheelchair loading and unloading activity provides wheelchair weight ceiling that is easily exceeded during the dynamic simulation of the activity. The upper limit on the weight of a foldaway powered wheelchair is provided and implications of this limit on future foldaway powered wheelchair designs are briefly discussed. In addition, the user gender and age effects on the foldaway powered wheelchair weight ceiling are also discussed.

Test-retest variation in lifting capacity and indices of subject effort

Tests of lifting capacity ideally produce consistent outcomes from one test to the next. Physiological indices of effort, such as heart rates, coefficients of force variation, and isokinetic force-distance curve variability, should also remain constant. This study examined the test-retest reliability of isometric, isokinetic and isoinertial maximal lifting tests in pain-free, well-motivated subjects. First, isokinetic and isoinertial maximum lifting forces and weights were quite consistent between test sessions, while isometric forces increased slightly in the second session. Second, heart rates did not vary significantly between test sessions of isokinetic and isoinertial modes. However, other indices of effort, including force variances, force-weight differences and ratios, and force-weight curve variability, varied significantly between test sessions. Test-retest variations in these non-cardiac effort indices imply that these indices may not reliably distinguish maximal from submaximal lifting efforts even in normal subjects.

RELEVANCE

Efforts to reduce occupational low-back injuries and disability have focused recently on measurements of lifting capacity. Accurate and reliable estimation of subject effort is critical to the validity of such measurements. This study demonstrates a need for reconsideration of previously reported indices of lifting effort.

Isokinetic pull-up strength profiles of men and women in different working posture

This paper presents isokinetic (dynamic) strength profiles of males and females, engaged in infrequent force exertion, in 15 different working postures. The postures investigated ranged from kneeling to standing and involved both one-handed and two-handed exertions. The results indicated that both males and females exert maximum pull-up force with two hands in the stooped posture. Least force was exerted with the preferred hand at reach distance, in the sagittal plane (arm fully extended), in the sitting posture. As expected, large differences existed in the force exertion capability of males and females.

Boundary line between the strength and endurance regions in manual lifting

Previous work by Mital et al. showed that repetitive dynamic strength (RDS) of individuals is a better predictor of psychophysically acceptable weight (PAW) than either the maximal dynamic strength (MDS) or maximal static strengths (MSSs). It was also pointed out that somewhere between a frequency of one lift per minute and a frequency of three lifts per minute, the orientation of lifting tasks changes and muscular strengths no longer remain the limiting factor. This paper reports the results of a follow-up study conducted to pinpoint the lifting frequency just beyond which this transition takes place. The results indicated that a significant correlation existed between RDS and PAW for lifting frequencies up to 1·5 lifts per minute. Beyond 1·5 lifts per minute, no significant correlation between the two measures was found. We therefore concluded that lifting tasks performed every 40 seconds, or less frequently, are strength oriented. For those tasks which require lifting more frequently than once every 40 seconds, individual's strengths are not a limiting factor.

Human strengths and occupational safety

This paper presents a review of relatively recent North American literature concerning human strengths. It briefly covers basic strength definitions, measurement and prediction of strengths, and the role of strengths in employee screening and job matching procedures. In addition to discussing variations in strength with body posture and duration of exertion, a brief reflection on torque exertion capabilities of individuals with common hand tools is also included.