STUDIES OF SPINAL SHRINKAGE TO EVALUATE LOW-BACK LOADING IN THE WORKPLACE

Measurement of spinal shrinkage (stadiometry) has been suggested to be a convenient measure of low-back load in workplace settings. This rep ort documents three separate experiments that collectively form a cent ral theme: Is the measurement of spinal shrinkage a suitable assessmen t technique to quantify the cumulative effect of loading on the low ba ck 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 n o statistically significant differences in stature change found betwee n either the sitting or standing posture for any condition suggesting that nearly all height changes occur in the spine. The second experime nt 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 e xperiment, subjects performed work-rest cycles consisting of periods o f sitting and lifting, and repeated over two days, to examine the reco very phenomenon. No specific pattern emerged owing to unpredictable su bject variability. The first general observation obtained from the res ults of all three experiments is that the response of subjects to a wi de 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 approa ch appears to be that it is one of the few available techniques to ass ess cumulative loading for both isometric postures, prolonged sitting, repeated tasks and responds to the positive adaptive changes that occ ur from periods of rest. However, it would appear that more quantifica tion of the relationships that modulate spinal shrinkage are required to account for the variance in stature measurements.