We studied the temporal relationship between pain and activity in patients with acute or chronic low back pain. Our hypothesis was that activity exacerbates acute pain, but not chronic pain. To test this hypothesis, we concurrently measured activity and pain using continual electronic recording, and analyzed the data using the cross-correlation function.
After obtaining IRB approval and patient consent, we studied fifteen patients with acute low back pain and fifteen patients with chronic low back pain over 3 weeks. The activity levels were collected automatically using a wrist accelerometer, and were sampled every 1-minute. The pain levels were recorded semi-automatically using a computerized pocket-sized diary, every 90 minutes. The patients were prompted to enter a number between 0 and 10, where 0 is no pain, and 10 is the worst possible pain. Patients were allowed to enter additional measurements as often as they wanted.
The first seven and the last seven of the daily pain and activity time series from each patient were then analyzed using the cross-correlation function at various time lags between -60 and +60 minutes. The null hypothesis was tested by R to Z transformation, followed by a one-sample t-test against an expected Z score of zero.
We found that during the first seven measurement periods of acute low back pain, there was a significant (p<0.01) degree of cross-correlation between activity and pain. As these patients improved and reported less pain, the relationship between activity and pain disappeared. There was no such relationship at any point in time among the patients with chronic low back pain. These results confirmed our hypothesis in this small sample of patients.