Effect of Time on the Variability of Lifting Kinematics in a Repetitive Lifting Task

A repetitive lifting task affords the opportunity for lift-to-lift variability in the lifting strategy and kinematics which can result in changes in the stresses on the tissues of the low back. To quantify the variation of lifting kinematics over time, ten subjects were recruited to perform repetitive lifting of a load equal to 10% of body weight at a rate of six lifts/min for 30 minutes continuously. Trunk kinematic variables were measured in all three planes of motion using the Lumbar Motion Monitor. The variances of the lifting kinematics were compared for the following ten-minute intervals: 0-10 min, 10-20 min and 20-30 min. Time was found to have a significant impact on variance of the peak sagittal acceleration, with the variance seen in the first ten minutes exceeding that of the other two time intervals. The downward trend in variation of peak sagittal acceleration implies an initial adjustment as the lifters “settle in” to their preferred lifting strategy.

Variation in lifting kinematics related to individual intrinsic lumbar curvature: an investigation in healthy adults

ObjectiveLifting postures are frequently implicated in back pain. We previously related responses to a static load with intrinsic spine shape, and here we investigate the role of lumbar spine shape in lifting kinematics.MethodsThirty healthy adults (18–65 years) performedfreestyle,stoopandsquatlifts with a weighted box (6–15 kg, self-selected) while being recorded by Vicon motion capture. Internal spine shape was characterised using statistical shape modelling (SSM) from standing mid-sagittal MRIs. Associations were investigated between spine shapes quantified by SSM and peak flexion angles.ResultsTwo SSM modes described variations in overall lumbar curvature (mode 1 (M1), 55% variance) and the evenness of curvature distribution (mode 2 (M2), 12% variance). M1 was associated with greater peak pelvis (r=0.38, p=0.04) and smaller knee flexion (r=–0.40, p=0.03) angles; individuals with greater curviness preferred to lift with a stooped lifting posture. This was confirmed by analysis of those individuals with very curvy or very straight spines (|M1|>1 SD). There were no associations between peak flexion angles and mode scores instooporsquattrials (p>0.05). Peak flexion angles were positively correlated betweenfreestyleandsquattrials but not betweenfreestyleandstooporsquatandstoop, indicating that individuals adjusted knee flexion while maintaining their preferred range of lumbar flexion and that ‘squatters’ adapted better to different techniques than ‘stoopers’.ConclusionSpinal curvature affects preferred lifting styles, and individuals with curvier spines adapt more easily to different lifting techniques. Lifting tasks may need to be tailored to an individual’s lumbar spine shape.