Chapter

Summary

Low-back pain is a major cause of sickness and disability in industrialized countries. The aetiology of low-back pain is poorly understood. Numerous epidemiological studies have come up with a large number of individual, physical and psychosocial factors that are correlated to the development of low-back pain. Although multicausality of low-back pain is not rejected, recent reviews on epidemiologic research conclude that indicators of mechanical low-back loading, are most consistently related to the development of low-back pain. These factors are lifting, twisting and bending movements and whole body vibration. In addition, it is argued that higher risk estimates for physical loading are produced in research that uses more accurate measurements of mechanical low-back loading (Burdorf and Sorock, 1997; Ferguson and Marras, 1997; NIOSH, 1997).

The main aim of this thesis was to improve mechanical modeling in order to open up new perspectives for biomechanical research of lifting activities. In addition, the effect of asymmetry in lifting movements on low-back loading was investigated. Furthermore, some aspects of the tuning of the lifting movement to the object to be lifted were investigated in terms of the control of the body center of mass (COM) and of the initial lifting forces.

In chapter 2 the performance of two linked segment models was compared in terms of whole body mechanics and in terms of net loading of the lumbo-sacral joint. In one model mechanical properties of body segments were estimated proportional to a population average (Looze et al., 1992b). The other model, using a large number of individual anthropometric measures, estimated mechanical properties of body segments by individual geometrical representations (Yeadon, 1990a). It turned out that the reliability of the models was dependent on the lifting technique that was used. Therefore, neither of the two models can be regarded as the better one.

Human equilibrium in lifting movements as well as many other activities, is characterized by relatively small fluctuations of the ground reaction force vector around the COM. Consequently, research on equilibrium control, using whole body mechanics, is strongly dependent on precise estimation of the COM. In chapter 3 an optimization procedure was presented to improve estimates of the body center of mass (COM) trajectory during (lifting) movements. It was shown that considerable improvement of the estimated COM trajectory can be obtained. This opens new perspectives for research on human balance control.

In chapter 4 a whole body 3-D linked segment model was developed and validated. In line with the validation of a 2-D linked segment model in Looze et al. (1992b) the validation was accomplished by comparing the torques at the lumbo-sacral joint that resulted from a top-down and a bottom-up analysis. In addition, the measured ground reaction force was compared to the sum of the segment masses times acceleration plus gravity, summed over all body segments. It was concluded that the internal validity of the model was quite satisfactory. In chapter 5 the 3-D model was applied to lifting movements ranging from 0 to 90^o asymmetry. In contrast to previous studies (Kromodihardjo and Mital, 1987; Plamondon et al., 1995) a consistent increase in lateral flexing and twisting torque was found with increasing asymmetry. Even with 10^o of lifting asymmetry, subjects did not prevent asymmetric low-back loading by twisting their pelvis far enough in the direction of the object to be lifted. Asymmetric low-back loading increases the need of co-contractions and will thus results in increased (compressive) loading of the lumbar spine (Lavender et al., 1992; Marras and Mirka, 1992). In addition, the stress in the intervertebral disc is likely to be distributed over a reduced number of annular fibres in asymmetric trunk movements (Ueno, 1987). The adverse effects of asymmetrical lifting are furthermore underlined by epidemiologic studies, showing an increased risk of developing low-back pain (Marras et al., 1995) and of acute herniation of the intervertebral disc (Kelsey et al., 1984) if occupational lifting movements are regularly asymmetrical. In chapter 6 the results of a 2-D and a 3-D linked segment model were compared in symmetrical and asymmetrical lifting movements. It was shown that the calculated extending torques with a 2-D model are prone to fast increasing errors when lifting movements are increasingly asymmetrical.

Combining the results of chapter 4, 5 and 6, it emerges that (1) asymmetrical lifting should receive more attention in ergonomics research and (2) a 3-D model, with an accuracy comparable to the model developed in this thesis, is a prerequisite for such research.

Chapter 7 and 8 were dedicated to two factors that could contribute to the stability of the subject during lifting and to the mechanical loading of the low-back as well. Those factors are the control of the initial lifting forces and the control of the COM position during lifting. To get more insight into these factors, gravitational forces were reduced to values close to zero. To this aim, four subjects performed two series of seven lifting movements while they were exposed to microgravity during parabolic flights. In chapter 7 it was investigated whether the elevated initial lifting effort in lifting a large box compared to lifting a small one of equal weight, holds under microgravity conditions. This elevated lifting effort is associated with the size-weight illusion, meaning that for two objects of equal mass but different volume, subjects consistently report the larger object to feel lighter (Charpentier, 1891). It was shown that subjects, repeatedly lifting two boxes of different volume but equal mass, persistently applied more lifting forces in the larger box compared to the small one, under normal gravity as well as microgravity. In comparison, an overall fast reduction of lifting forces was found upon entering weightlessness. This suggests that the persisting elevated initial effort (and the associated size-weight illusion) may be related a persisting upward scaling of the force component necessary to accelerate the object rather than the force component necessary to overcome the weight of the object. Possibly, conscious knowledge of object mass as well as experience in previous lifts results in adaption of the weight-related lifting force but not in adaptation of the force, necessary to accelerate the object.

In chapter 8 it was demonstrated that the COM is shifted backward under microgravity but recovers partly after several parabola’s. Lower limb joint angles during lifting differ between microgravity and normal gravity conditions. Recovery of joint angles towards the angles found under normal gravity, mainly occurred where this recovery contributed to a recovery of the COM position. It was argued that control of the horizontal COM position has at least two functions. One of them, equilibrium control, becomes redundant under microgravity. The other function is to generate, together with the ground reaction force vector, an external moment that is adequate for the rotational body movement. This function is still required under microgravity and may be the reason for the backward shift of the COM. The initial "overshoot" of this backward shift could be caused by disturbed sensory information.