Spontaneous Transitions in Repetitive Lifting and Lowering

Robin Burgess-Limerick, Bruce Abernethy, & John Shemmell

Department of Human Movement Studies,
The University of Queensland, Brisbane, Australia

This paper reports spontaneous transitions between different patterns of coordination in repetitive lifting and lowering movement as a consequence of gradual changes in lifting height. Angular displacement of ankle, knee, hip, and lumbar vertebral joints were estimated from three dimensional coordinates of infra-red emitted diodes placed on appropriate landmarks while six participants performed 12 trials of a task in which a light load was repetitively raised from a shelf to a constant waist height, then lowered back to the shelf. A ramp protocol was employed for each trial such that the shelf was stepped once per cycle between 10 and 50 cm, in 40 equal steps. That is, each trial consisted of 40 cycles of lifting and lowering in which the shelf height varied continuously in 1 cm increments each cycle, either increasing from 10 cm to 50 cm (ascending trials) or decreasing from 50 cm to 10 cm (descending trials). The frequency of lifting and lowering was self-selected by the participants.

Participants first performed 6 trials in which the lifting technique was self selected while the direction of each trial alternated between ascending and descending. No instructions were provided regarding the lifting technique which should be employed. The remaining 6 trials continued to alternate between ascending and descending directions. For the three ascending trials the participants were instructed to begin the trial using a squat technique (involving significant knee flexion), while for the descending trials the participants were instructed to begin the trial using a stoop technique (involving minimal knee flexion). Participants were instructed to maintain the specified techniques for as long as they remained comfortable.

Two qualitatively different patterns of coordination were adopted when the lifting technique was self-selected: a squat technique in which the ankle, knee, hip and lumbar vertebral joints exhibited in phase coordination (simultaneous ankle plantar-flexion and knee, hip, and lumbar vertebral extension), and a stoop technique in which the range of movement of the knee was reduced and the coordination of the ankle was 180 out of phase with respect to the other joints (ie., ankle plantar-flexion was accompanied by knee, hip and lumbar vertebral flexion). Five participants exhibited transitions from in-phase to out-of-phase coordination during ascending trials and from out-of-phase to in-phase coordination during descending trials in both self-selected and imposed trials. The remaining participant adopted in phase coordination for all self-selected trials, and remained in the imposed technique for all imposed trials. Hysteresis was suggested in that transition occurred more frequently during descending trials, and the average shelf height at which transitions occurred was higher in the ascending trials than the descending trials.