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

Abstract

Objective: Lifting 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.

Methods: Thirty healthy adults (18-65 years) performed freestyle, stoop and squat lifts 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.

Results: Two 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 in stoop or squat trials (p>0.05). Peak flexion angles were positively correlated between freestyle and squat trials but not between freestyle and stoop or squat and stoop, indicating that individuals adjusted knee flexion while maintaining their preferred range of lumbar flexion and that 'squatters' adapted better to different techniques than 'stoopers'.

Conclusion: Spinal 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.

Keywords: Lumbar spine; biomechanics; kinematic analysis; lifting; positional MRI.

Figure 1

Average lumbar spine shape (mean) in standing identified by shape modelling (n=30) and the first two modes of variation (mode 1 and mode 2). The total (L1–S1) and intersegmental angles demonstrate the amount and distribution of curvature within each mode when separately varied by ±2 SD.

Figure 2

Mean flexion angles during symmetrical lifting, beginning from unloaded upright standing and finishing standing with a weighted box in the hands. Grouped by mode 1 (curviness) score into curvy_ext (>1 SD from mean) and straight_ext (<–1 SD from mean) spine shape groups.