Abdominal exercises are prescribed for both the prevention and treatment of low back injury. However, these exercises sometimes appear to have hazardous effects on the lumbar spine. The purpose of this study was to identify quantitatively abdominal exercises that optimize the challenge to the abdominal muscles (rectus abdominis, external oblique, internal oblique) but impose minimal load penalty to the lumbar spine. Nine volunteers performed 12 different abdominal exercises. For a given task the maximum abdominal muscle EMG value was divided by the maximum compression value, resulting in an abdominal challenge versus spinal compression cost index. In general, the partial curl-ups generated the highest muscle challenge-to-spine cost indices. However, those exercises that generated the best challenge-to-cost indices did not necessarily record the lowest compression levels along with the highest EMG activations. No single exercise was found that optimally trained all of the abdominal muscles while at the same time incurring minimal intervertebral joint loads. It was concluded that a variety of selected abdominal exercises are required to sufficiently challenge all of the abdominal muscles and that these exercises will-differ to best meet the different training objectives of individuals.
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.
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