Studies of spinal shrinkage to evaluate low-back loading in the workplace

Abstract

Measurement of spinal shrinkage (stadiometry) has been suggested to be a convenient measure of low-back load in workplace settings. This report documents three separate experiments that collectively form a central theme: Is the measurement of spinal shrinkage a suitable assessment technique to quantify the cumulative effect of loading on the low back given the many sources for variability in the signal? A stadiometer was fabricated to measure both sitting and standing height. The first experiment was to compare sitting with standing stature changes over time in an attempt to locate the major site of shrinkage. There were no statistically significant differences in stature change found between either the sitting or standing posture for any condition suggesting that nearly all height changes occur in the spine. The second experiment compared the cumulative effects from static load holding to dynamic load lifting. Some subjects experienced more shrinkage in the static task while others experienced more in the dynamic task. In the third experiment, subjects performed work-rest cycles consisting of periods of sitting and lifting, and repeated over two days, to examine the recovery phenomenon. No specific pattern emerged owing to unpredictable subject variability. The first general observation obtained from the results of all three experiments is that the response of subjects to a wide array of activities appears to be variable both within each subject and over repeated exposures to identical conditions on different days. While subject variability (and perhaps biological variability) is a liability, it may be feasible to develop load time integrals for load exposure in the future, since the asset of the spinal shrinkage approach appears to be that it is one of the few available techniques to assess cumulative loading for both isometric postures, prolonged sitting, repeated tasks and responds to the positive adaptive changes that occur from periods of rest. However, it would appear that more quantification of the relationships that modulate spinal shrinkage are required to account for the variance in stature measurements.

Prolonged standing as a precursor for the development of low back discomfort: An investigation of possible mechanisms

“…This altered loading of the passive structures may have been one potential source of the reported discomfort. Further, stadiometry research has shown that prolonged standing results in disc height loss [20,21]. This likely also occurred in the current study, which creates several possible pain generating pathways.…”

Biomechanics of Changes in Lumbar Posture in Static Lifting

“…To resist only slightly smaller net external moments, the smaller active force in kyphotic postures was, however, compensated by larger contributions from passive components (Table 3; Figure 7). The moment-carrying role of the ligamentous spine was much greater in kyphotic postures (27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44) than that in lordotic postures (Figure 7). The contribution of passive muscle forces in kyphotic postures was also crucial (Table 3; Figure 7), a role that has been neglected in previous works when suggesting that in kyphotic postures the subjects hang on their ligaments 22 or the spine is without much muscular support.…”

“…Evidently, an improved assessment of risk of injury depends on a more accurate estimation of the load partitioning in the human trunk in forward flexion tasks. Spinal loads have indirectly been estimated by measurement of intradiscal pressure, 30,31 load on spinal instruments, 32,33 spinal shrinkage, 34,35 and EMG activity of trunk muscles. 36,37 Because of the limitations in foregoing methods, biomechanical models have been recognized as indispensable tools for evaluation of spinal loads and system stability in various occupational and athletic activities.…”

Sitting versus standing: Does the intradiscal pressure cause disc degeneration or low back pain?

“…Stadiometry of different postures has a small effect size, but relatively large variation between subjects and between trials for the same subject (Althoff et al, 1992;McGill et al, 1996;Lievseth and Drerup, 1997;van Deursen et al, 2005). The load-cell fixator study was a series of single cases, with (Nachemson and Morris, 1964;Nachemson, 1965;Nachemson and Elfstrom, 1970;Okushima, 1970;Sato et al, 1999;Wilke et al, 1999), from L3-4 and L4-5 intervertebral discs.…”