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Human muscular and postural responses in unstable load lifting
1Department of Industrial Management, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China. yhlee@im.ntust.edu.tw
Spine
|September 11, 2002
Summary
Handling unstable loads significantly increases lifting time and muscular effort compared to stable loads. The body compensates for sudden load shifts by adjusting posture and increasing muscle activation to maintain stability.
Area of Science:
- Biomechanics
- Occupational Health
- Ergonomics
Background:
- Manual material handling (MMH) research often overlooks unstable loads.
- Accurate load weight assessment is crucial but not always available.
- Unexpected load shifts pose a significant workplace risk.
Purpose of the Study:
- Investigate muscular and postural responses during unstable load lifting.
- Quantify the workplace risks associated with unstable loads.
Main Methods:
- 12 subjects performed lifting tasks with time-variant (18kg box with 12 (kg.m)/s impact) and stable loads.
- Varied load-shift conditions (rolling, stable), foot placements (wide, narrow), and lifting techniques (stoop, squat).
- Measured normalized electromyography, joint angles, and center of pressure path length (COP-length).
Main Results:
- Unstable loads increased lifting time, COP-length, and muscular contraction levels.
- Subjects exhibited greater elbow extension, shoulder/trunk flexion, and knee flexion (approx. 5°) during unstable lifts.
- Erector spinae contraction was 12-25% higher for unstable loads compared to stable loads.
Conclusions:
- The central nervous system actively detects and responds to load-shift perturbations.
- Joint stabilization, particularly near the perturbation site, is a key compensatory strategy.
- Findings highlight the increased physical demand and risk associated with handling unstable loads.