Decreases in stature (shrinkage) are used to indicate exercise induced spinal loading. This study examined the effect of three running speeds on two groups of runners, one with chronic low back pain. The two groups of seven male marathon runners ran at 70%, 85%, and 100% of their marathon race pace for 30 min on separate occasions. Before and after exercise the subjects were seated for 20 min with the lumbar spine supported. Stature was measured before pre-exercise sitting, before running, after 15 min of running, after 30 min of running, and after post-exercise sitting. A stadiometer accurate to within 0.5 mm was used to record changes in stature. Results showed no differences in response to the three running regimens between the groups (P greater than 0.05). Shrinkage was greater during the first 15 min, being 3.26 (+/- 2.78) mm compared with 2.12 (+/- 1.61) mm for the second 15 min of the run (P less than 0.05). The faster the running speed the greater the resultant shrinkage. The 70%, 85%, and 100% conditions caused 3.37 (+/- 2.38), 5.10 (+/- 1.90), and 7.69 (+/- 3.69) mm of shrinkage, respectively (P less than 0.005). These results suggest that low back pain is independent of the shrinkage induced by running. Further research is required to determine the effect of longer duration runs on spinal shrinkage.