SELF-SELECTED MANUAL LIFTING TECHNIQUE - FUNCTIONAL CONSEQUENCES OF THE INTERJOINT COORDINATION

The pattern of movement self-selected by 39 subjects to lift light loa ds from 9 cm above the ground is described in kinematic and electromyo graphic terms. Hamstring length changes were estimated from hip and kn ee 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 lif ting was described through calculation of the relative phase between a djacent joints and found to be exaggerated with increases in load mass . During the early phase of lifting, knee extension leads hip extensio n, which in turn leads extension of the lumbar vertebral joints. Early in the lifting movement, when load acceleration is greatest, the erec tores spinae are thus relatively long and shortening slowly. Both of t hese factors produce greater back extensor strength. Rapid hamstring s hortening is also delayed, which enhances their strength, and coactiva tion of the monoarticular knee extensors and biarticular hamstrings ob served early in the lifting movement suggested that the knee extensors contribute to hip extension through a tendinous action of the hamstri ngs.