[Paper Review] From Stoop to Squat: A comprehensive analysis of lumbar loading among different lifting styles

Michael von Arx, Melanie Liechti+4 more|arXiv (Cornell University)|Sep 1, 2021

Musculoskeletal pain and rehabilitation43 references67 citations

TL;DR

This study uses subject-specific motion capture-driven musculoskeletal modeling to compare lumbar spinal loads during freestyle, squat, and stoop lifting in 30 healthy individuals. Stoop lifting produced significantly lower compressive and total lumbar loads (−0.3 to −1.0 BW) than squat or freestyle lifting, though shear loads were higher in upper lumbar segments; lifting speed and lumbar lordosis range of motion also significantly influenced loading patterns.

ABSTRACT

Lifting up objects from the floor has been identified as a risk factor for low back pain, whereby a flexed spine during lifting is often associated with producing higher loads in the lumbar spine. Even though recent biomechanical studies challenge these assumptions, conclusive evidence is still lacking. This study therefore aimed at comparing lumbar loads among different lifting styles using a comprehensive state-of-the-art motion capture-driven musculoskeletal modeling approach. Thirty healthy pain-free individuals were enrolled in this study and asked to repetitively lift a 15 kg-box by applying 1) a freestyle, 2) a squat and 3) a stoop lifting technique. Whole-body kinematics were recorded using an optical motion capture system and used to drive a full-body musculoskeletal model including a detailed thoracolumbar spine. Compressive, shear and total loads were calculated based on a static optimization approach and expressed as factor body weight (BW). In addition, lumbar lordosis angles and total lifting time were calculated. All parameters were compared among the lifting styles using a repeated measures design. For all lumbar segments, stoop lifting showed significantly lower compressive and total loads (-0.3 to -1.0BW) when compared to freestyle and squat lifting. Stoop lifting produced higher shear loads (+0.1 to +0.8BW) in the segments T12/L1 to L4/L5, but lower loads in L5/S1 (-0.2 to -0.4BW). Peak compressive and total loads during squat lifting occurred approximately 30% earlier in the lifting cycle compared to stoop lifting. Stoop lifting showed larger lumbar lordosis range of motion (35.9+/-10.1{\deg}) than freestyle (24.2+/-7.3{\deg}) and squat (25.1+/-8.2{\deg}) lifting. Lifting time differed significantly with freestyle being executed the fastest (4.6+/-0.7s), followed by squat (4.9+/-0.7s) and stoop (5.9+/-1.1s).

Motivation & Objective

• To investigate whether different lifting styles (freestyle, squat, stoop) produce significantly different lumbar spinal loads in healthy individuals.
• To examine the influence of lifting speed and lumbar lordosis range of motion on spinal loading during object lifting.
• To challenge the conventional recommendation of squat lifting as the safest technique by quantitatively comparing loads across styles using continuous, time-resolved data.
• To evaluate whether the commonly held belief that a flexed spine increases injury risk is supported by biomechanical evidence under controlled conditions.
• To provide evidence-based insights for reevaluating current lifting guidelines, particularly for single-repetition, moderate-weight tasks.

Proposed method

• Employed a 16-camera optical motion capture system to record whole-body kinematics during repetitive lifting tasks.
• Used subject-specific musculoskeletal modeling with a fully articulated thoracolumbar spine based on external back profiles from skin marker data.
• Applied static optimization to compute continuous time-series data for compressive, anterior-posterior shear, and resultant total spinal loads.
• Expressed all loads as multiples of body weight (BW) for standardization and comparison across participants.
• Utilized Statistical Parametric Mapping (SPM) to analyze continuous load data over time, identifying statistically significant differences across lifting styles.
• Measured lumbar lordosis angles and total lifting duration to contextualize load patterns and assess movement strategy differences.

Experimental results

Research questions

• RQ1 How do compressive, shear, and total spinal loads differ between freestyle, squat, and stoop lifting styles during a single repetition?
• RQ2 At what point in the lifting cycle do peak loads occur for each lifting style, and how does this relate to load magnitude?
• RQ3 How does lifting speed (duration) influence the magnitude and temporal pattern of lumbar spinal loading?
• RQ4 To what extent does lumbar lordosis range of motion vary across lifting styles, and how is it related to load distribution?
• RQ5 Are there segment-specific differences in load patterns (e.g., T12/L1 vs. L5/S1), and if so, what explains them?

Key findings

• Stoop lifting produced significantly lower compressive (−0.3 to −1.0 BW) and total (−0.3 to −1.0 BW) lumbar loads compared to freestyle and squat lifting across all lumbar segments.
• Stoop lifting resulted in higher anterior-posterior shear loads (+0.1 to +0.8 BW) in the upper lumbar spine (T12/L1 to L4/L5), but lower shear loads (−0.2 to −0.4 BW) at the L5/S1 segment compared to squat lifting.
• Peak compressive and total loads occurred approximately 30% earlier in the lifting cycle during squat lifting than during stoop lifting, indicating a faster onset of high load.
• Lifting duration varied significantly: freestyle (4.6±0.7s) was fastest, followed by squat (4.9±0.7s), and stoop (5.9±1.1s) was slowest, suggesting speed may influence load magnitude.
• Lumbar lordosis range of motion was greatest during stoop lifting (35.9±10.1°), followed by freestyle (24.2±7.3°) and squat (25.1±8.2°), indicating greater flexion during stoop despite instructions to maintain a neutral spine.
• Spinal loads consistently increased toward the caudal end of the lumbar spine, with differences between lifting styles being most pronounced in the upper lumbar segments.