Title: Exploring the effects of task design on the flexion-relaxation phenomenon in the lumbar spine during repetitive lifting
Thesis Supervisors: Dr. Kayla Fewster
Committee members: Dr. Jean-Sébastien (co-supervisor), Dr. Michael Hunt
Defence Chair: Dr. Romeo Chua
Abstract:
Repetitive forward flexion of the lumbar spine is involved with occupations like manual materials handling. Repetitive lumbar flexion superseding 80% of healthy range of motion, has been accepted as a risk factor for low back disorders (LBD) and low back pain (LBP). Evidence suggests that passive tissues in the lumbar spine are essential to maintaining spine function and preventing injury. At near-maximal lumbar flexion, the passive tissues in the lumbar spine produce sufficient tension and counter-moment to the upper body where the lumbar erector spinae musculature enter a period of myoelectric silence. This neuromechanical event is termed flexion-relaxation (FR).
Throughout bouts of repetitive lifting, the lumbar angle at which FR onset occurs will typically increase. Therefore, change in FR onset angle has been used as a metric to quantify changes in passive tissues as a surrogate measure for creep deformation and increase in injury risk. High lifting load and lift frequency are factors that cause changes in passive tissue mechanics during repetitive lifting tasks. The goal of this thesis was to investigate the effect of task design on the FR onset angle in the lumbar spine in two external load-matched tasks. Condition 1 involved 75 cycles of lifting 13 kg at a rate of 1 lift per minute while Condition 2 involved 150 cycles of lifting 6.5 kg at a rate of 2 lifts per minute. 20 participants (10 male, 10 female) were included in this investigation.
Males showed greater changes in FR onset angle relative to females in Condition 2. Similar trends are seen in changes in peak lumbar flexion angle during lifting, but these differences were not found to be significant. Condition 1 imposed greater peak L5/S1 compression force and shear force in comparison to Condition 2. However, the reduced peak loading in Condition 2 was not enough to compensate for the increased frequency of lifts in Condition 2, which led to participants cumulatively experiencing greater peak loading across the task in comparison to Condition 1. Peak L5/S1 shear forces were greater in males than females in Condition 1, but not in Condition 2.
These findings suggest task design within repetitive lifting tasks may be sensitive to sex differences and task parameters of external load-matched task designs may have an influence on low back injury risk via changes in passive tissue mechanics.