We studied the temporal relationship between pain and activity in patients with 
acute or chronic low back pain. We studied 15 patients with acute low back pain 
and 15 patients with chronic low back pain over 3 wk. The activity levels were 
collected automatically using a wrist accelerometer and were sampled every 
minute. The pain levels were recorded at least every 90 min using a pocket-sized 
electronic diary. The time series from each patient were then analyzed using the 
cross-correlation function at various time offsets. We found that during the 
first 7 days of acute low back pain, there was a significant (P < 0.01) degree 
of cross-correlation between activity and pain. On average, pain followed 
activity by approximately 30 min. As these patients improved and reported less 
pain, the relationship between activity and pain disappeared. There was no such 
relationship at any point among the patients with chronic low back pain.