It is known that acute loading is not the only cause of
spine tissue failure. Repetitive loading can reduce the tolerance
of spine tissues to the point where previously
acceptable loads can now result in injury. Hansson
et al. (1987) applied in vitro cyclic loading to human
specimens and found that the number of cycles to failure
decreased as the relative stress of loading was increased.
Brinkmann (1988) found similar results, and both studies
determined that the relationship was highly non-linear.
Parkinson and Callaghan (2007) provide further evidence
to support the use of a non-linear weighting of forces,
especially when integrating compression forces to
approximate risk over prolonged periods of time.
3. Estimating spine tissue loading
3.1. Direct