Academic journal article Human Factors

Self-Selected Manual Lifting Technique: Functional Consequences of the Interjoint Coordination

Academic journal article Human Factors

Self-Selected Manual Lifting Technique: Functional Consequences of the Interjoint Coordination

Article excerpt

The pattern of movement self-selected by 39 subjects to lift light loads from 9 cm above the ground is described in kinematic and electromyographic terms. Hamstring length changes were estimated from hip and knee angular kinematics. Subjects adopted a posture at the start of the lift intermediate between stoop and full-squat postures. A consistent coordination between knee, hip, and lumbar vertebral joints during lifting was described through calculation of the relative phase between adjacent joints and found to be exaggerated with increases in load mass. During the early phase of lifting, knee extension leads hip extension, which in turn leads extension of the lumbar vertebral joints. Early in the lifting movement, when load acceleration is greatest, the erectores spinae are thus relatively long and shortening slowly. Both of these factors produce greater back extensor strength. Rapid hamstring shortening is also delayed, which enhances their strength, and coactivation of the monoarticular knee extensors and biarticular hamstrings observed early in the lifting movement suggested that the knee extensors contribute to hip extension through a tendinous action of the hamstrings.


Manual lifting is consistently linked with a high incidence of occupational injuries (e.g., Jensen, 1988). It seems reasonable to suggest that the damaging physical effects of lifting might be partly a function of the postures adopted throughout the lifting movement and the patterns of joint movement involved.

Lifting techniques have been defined in terms of the posture adopted just before a load is lifted. It has been proposed (e.g., Davis and Troup, 1965; Trafimow, Schipplein, Novak, and Andersson, 1993) that, although intermediate postures are possible, the postures adopted to lift loads from a low level may be characterized in terms of two extremes. One is described as a stoop in which the knee joints are almost fully extended and the hip joints and vertebral column are flexed to reach the load. The second, described as a squat, is one in which the knee joints are fully flexed and the trunk is held in as vertical a posture as possible. The latter posture has been traditionally recommended as least likely to lead to injury (e.g., Bendix and Eid, 1983; Davis, 1959, 1967) and is still taught as such (e.g., McCauley, 1990).

Research results do not support such a uniform recommendation, however. Several authors (e.g., Garg, Sharma, Chaffin, and Schmidler, 1983; Noone and Mazumdar, 1992; Park and Chaffin, 1974) have cautioned that a technique involving a vertical trunk and fully flexed knees may not always minimize joint forces because the full squat may involve keeping the load farther away from the body than with other techniques. Other authors (e.g., Ayoub and Mital, 1989, p. 268; Parnianpour, Bejjani, and Pavlidis, 1987) have suggested that a single best method may not exist for any particular task. It has also been suggested (Ayoub and Mital, 1989, p. 51; Garg and Saxena, 1979) that the technique spontaneously adopted by the subject may actually be the least likely to lead to injury.

This last proposition -- that a freely chosen lifting technique might be preferable in terms of injury prevention -- raises the question of what postures and patterns of joint movement are adopted to lift low-lying loads when no prescriptive instructions are given. Self-selected lifting techniques have not been systematically described, although a number of opinions have been expressed.

Some authors (e.g., Troup, 1979) postulate that no natural lifting technique is identifiable. Others (e.g., Gagnon and Smyth, 1991, 1992; Genaidy and Asfour, 1989; Park, 1973; Park and Chaffin, 1974; Trafimow et al., 1993) have concluded that when no prescriptive instruction is provided, the posture adopted to lift a load involves a moderate range of knee flexion and an inclined trunk -- that is, a posture intermediate between the extremes of a stoop and a full squat with a vertical trunk. …

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