Toward memory-based human motion simulation: development and validation of a motion modification algorithm

Computer simulation of human motions helps test hypotheses on human motion planning and fosters timely and high-quality human-machine/environment interaction design. The current study introduces a novel simulation approach termed memory-based motion simulation (MBMS), and presents its key element "motion modification" (MoM) algorithm. The proposed approach implements a computational model inspired by the generalized motor program (GMP) theory. Operationally, when a novel motion scenario is submitted to the MBMS system, its motion database is searched to find relevant existing motions. The selected motions, referred to as "root motions", most likely do not meet exactly the novel motion scenario, and therefore, they need to be modified by the MoM algorithm. This algorithm derives a parametric representation of possible variants of a root motion in a GMP-like manner, and adjusts the parameter values such that the new modified motion satisfies the novel motion scenario, while retaining the root motion's overall angular movement pattern and inter-joint coordination. An evaluation of the prediction capability of the algorithm, using both seated upper body reaching and whole-body load-transfer motions, indicated that the algorithm can accurately predict various human motions with errors comparable to the inherent variability in human motions when repeated under identical task conditions.     

Effect of handle height on lower-back loading in cart pushing and pulling

This paper presents results of a study conducted to estimate lower back loadings in cart pushing and pulling. Experiments were conducted in the laboratory using a cart. Six subjects with different weights (ranging from 50 to 80 kg) were tested for three different pushing and pulling forces (98, 196 and 294 newtons), three different heights of exertion (660, 1090 and 1520 mm high) and two different moving speeds (1.8 and 3.6 km/h). It was found that, in general, pushing a cart results in lesser lower-back loading than pulling. Subject body weight affected the lower-back loadings more significantly in pulling (50% increase as body weight increased from 50 kg to 80 kg) than in pushing (25% increase). Handle height of 1090 mm was found to be better than other handle heights in pushing while 1520 mm handle height was better for pulling in reducing lower-back loadings.     

A Biomechanical Evaluation of Two Methods of Manual Load Lifting

Since the middle 1930's it has been advocated that all lifting should be carried out with the back near vertical and with the knees bent. The basis for this recommendation has been that it shifts the stresses on the body from the low-back to the legs. Under certain conditions this recommendation is invalid. In this paper Chaffin and Baker's biomechanical lifting model is extended by considering the erector spinae muscles and by considering load accelerations. Mathematical modeling shows that forces on the erector spinae muscles and the lumbosacral disc can be as much as 50% higher when using the recommended “straight back, bent knees” method when compared to the common “stooped back” method. The straight back method is recommended only for lifts when the object is initially close to the spine.     

Biomechanical Stresses From Manual Load Lifting: A Static vs Dynamic Evaluation

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 psychophysical weight limits in determining the degree of risk involved with a given act of lifting. The resulting trade-offs between the static and dynamic biomechanical simulations of lifting are discussed. Also, maximum voluntary isometric lifting strengths are compared with the maximum weights acceptable to the subjects.     

Evaluation of workers exposed to elemental mercury using quantitative tests of tremor and neuromuscular functions

Workers exposed to metallic mercury vapor were subjects for tremor, EMG, and psychomotor tests. Regression analysis revealed statistically significant trends in these test results related to workers' urine mercury histories. Effects were subclinical, functionally insignificant and most associated with those workers whose urine mercury had exceeded 0.5 mg/L in the previous year. In agreement with previous reports, effects were reversible upon reduction of mercury exposure.     

A Computer-Assisted Manual Work-Design Model

This article describes the development of computerized human simulation models to assist in designing or analyzing manual jobs. The philosophy of computer assisted job design is discussed. Two existing models from The University of Michigan are then presented. One model predicts the normal time to perform a manual task, and the second model predicts the reach capability and preferred body position of the working population when reaching to specific locations in the work-place. Other models that are being developed to predict learning rates, human force capabilities, and physical fatigue rates are briefly described.     

A Biomechanical Model for Analysis of Symmetric Sagittal Plane Lifting

A computerized model of the human body is developed. It assumes the skeletal muscle system to be analogous to a series of eight solid links representing the major body segments which are articulated in a manner corresponding with selected body joints. It is shown by empirical tests that the model can be used to predict the maximum strengths of persons asked to lift objects while assuming different postures. It is proposed that this type of model represents a rational method for investigating the many variables that affect the physical strength capability of any individual required to perform materials-handling activities.