Most of the studies published so far about lumbar fusion surgery have been interested in surgical procedure itself or in comparison of conservative or operative treatment. Less information is available about long-term outcome and exercise programs for operated patients. Therefore the investigators have developed "Spine fusion register", which include data on surgery procedure and outcome (pain, disability, back muscle function, quality of life, working capacity, reoperations and complications etc.) before and after surgery up to 5 years (started in January 1st, 2008). The main purpose of the present study is to analyze the significance of 12-month strength and aerobic training for outcome of the lumbar fusion patients compared to usual care. Patients (N=100) with lumbar fusion because of isthmic or degenerative olisthesis will be will be stratified by catchments area and randomized into combined strength and aerobic training group or into the control group.