_Clinical Biomeqhayks 1991; 6: 88-96 Reduction in anterior shear forces on the b/L5 disc by the lumbar musculature J R Potvin MSC R W Norman Pho S M McGill PhD Occupational Biomechanics Laboratories, University of Waterloo, Ontario, Canada Summary The purpose of this study was to assess the possible role of muscles in offsetting the anterior shear forces caused by the load and upper body mass and their accelerations that act on the WLs intervertebral joint during dynamic squat lifts. Fifteen males lifted five loads from 5.8 to 32.4 kg. Anterior shear forces estimated to be acting on the lumber spine, based on model output, ranged from 492 N at 5.8 kg to 736 N at 32.4 kg. However, the peak shear force that’had to be supported by the facets and possibly the disc remained relatively constant at approximately 200 N. regardless of the load mass. The posteriorly directed fascicles of the lumbar portions of the iliocostalis lumborum and longissimus thoracis muscles increased their force output, as estimated from an EMG driven model, in proportion to the anterior load shear force demands, thereby sharing the load on the intervertebral joint. It appears that the combination of anatomical design and neural control of the musculature leads to a situation where the resultant shear force on the joint can be maintained at a relatively constant and safe level in the types of lifts studied. This ‘safety’ mechanism is useful only with the preservation of lordosis during lifting, when the muscles must provide the majority of the support moment. Relevance This paper is relevant to understanding low back injury mechanisms, how people may reduce shear loads on lumbar joints by appropriately activating the back extensor musculature, and why it is important to represent the musculature in biomechanical models as accurately as possible. Key words: Lumbar region, dynamic biomechanical model, shear forces, muscle forces Introduction There has been considerable interest for many years in improving understanding of the aetiology of low back injury. The general agreement that the major cause of injury is mechanical overload or overuse of the tissues of the lumbar spine has led to considerable effort to develop methods, usually biomechanical models, to determine the sizes of the forces to which these tissues are subjected during lifting and other load handling Received: I June 1990 Accepted: 26 October 1990 Corrkpondence and reprint requests IO: J R Potvin MSc, Department of Kinesioloev. Universitv of Waterloo. Waterloo. Ontario, Canada N2L3Gl --’ - @ 1991 Butterworth-Heinemann Ltd 0268~0033/91/020088-09 tasks. In parallel, research has been done to determine the load tolerance of some lumbar tissues. The estimation of the sizes and effects of compression forces on intervertebral discs has been of particular interest to many researchers. There is a substantial amount of data on the force tolerance of discs in this mode of loading’. However, much less attention has been given to adequately determine the sizes and the effects of shear forces supported by the structures of the intervertebral joint. In normal human load handling movements, the anterior shear forces produced by the weight of the upper body are routinely in the order of 400 N. This can be illustrated by an example where an 80 kg person has bent his or her upper body forward to a static, horizontal position. Since the weight of the upper body above the pelvis is about 50% of the total body weight,