PubMed · 7763161
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P J Potter. 1995. Stretching exercises.. https://doi.org/10.1016/s0003-9993(95)80516-8
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Many problems of the lumbar spine that cause pain are attributed to instability. The ligamentous spine (without muscles) is unstable at very low compressive loads. This study examined the hypothesis that instability of the lumbar spine is prevented under normal circumstances by the stiffness of spinal musculature, without active responses from the neuromuscular control system. The effect of muscle activity (force and stiffness) on the stability of the lumbar spine was analyzed for maximum voluntary extension efforts with different spinal postures in the sagittal plane. The analysis included realistic three-dimensional representation of the muscular anatomy with muscles crossing several motion segments. The stiffness of motion segments was represented using in vitro measured properties. Under a range of conditions with maximum extension effort, active muscle stiffness was required to prevent the lumbar spine from buckling. The dimensionless value of the muscle stiffness parameter q as a function of activation and length had to be greater than a critical value in the range of 3.7-4.7 in order to stabilize the spine. Experimentally determined values of q ranged from 0.5 to 42. These analyses demonstrate how changes in motion segment stiffness, muscle activation strategy, or muscle stiffness (due to degenerative changes, injuries, fatigue, and so on) might lead to spinal instability and "self-injury."
This paper reports dynamic viscoelastic and steady shear measurements of aqueous solutions of two commercial hydroxyethyl ethers of potato starch, allowing us to define three different viscoelastic regions, depending on the polymer concentrations. The fluid-like zone (I) corresponds to a homogeneous solution, free of associations between chains, where the linear viscoelastic model can be applied; the fluid-gel transition zone (II) is associated with the rheology of complex systems which possess intermolecular specific interactions; and the gel-like zone (III) involves a network formation giving rise to a gel. The reversibility of the hydrogels was studied by means of steady/dynamic transient experiments allowing breakdown of the gel network and analysis of rehealing. Comparing the elastic moduli of the starch derivative gels leads us to assume that the functionality depends on the capacity to form hydrogen-bonding associations between polymer chains.
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