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Normal response to large posteroanterior lumbar loads--a case study approach
1School of Exercise and Sport Science, University of Sydney, Australia. m.lee@cchs.usyd.edu.au
Summary
Spinal posteroanterior (PA) loading force-displacement relationships remain approximately linear up to 275 N in subjects without low back pain. Stiffness increases moderately with higher loads, suggesting linearity extends beyond the commonly used 100 N threshold.
Area of Science:
- Biomechanics
- Spinal Mechanics
- Musculoskeletal Research
Background:
- Spinal stiffness is often characterized by the force-displacement relationship under posteroanterior (PA) loading.
- Current assessments typically use forces between 30 N and 100 N.
- Manual spinal assessment may involve forces up to 300 N, raising questions about linearity at higher loads.
Purpose of the Study:
- To investigate the linearity of the spinal force-displacement relationship beyond 100 N.
- To determine if the relationship remains linear up to 275 N of PA loading.
Main Methods:
- A single-case study design with replication was employed.
- Two male subjects with no recent history of low back pain participated.
- A mechanical device applied PA loads up to 275 N to the L3 vertebra, measuring force-displacement responses.
Main Results:
- Force-displacement responses were similar between subjects and showed approximate linearity up to 275 N.
- A slight nonlinearity was observed, leading to a moderate increase in stiffness with increasing load.
- Stiffness calculated up to 275 N was approximately 25% greater than that calculated up to 100 N.
Conclusions:
- The spinal force-displacement relationship under PA loading is approximately linear up to 275 N in asymptomatic individuals.
- While largely linear, a trend of increasing stiffness with higher forces was noted.
- Findings suggest current clinical assessment ranges may underestimate spinal stiffness at higher force levels.