Purpose: The effectiveness of a 12-month home-exercise program on trunk muscle strength after lumbar spine fusion surgery was evaluated. Materials and methods: Three months postoperatively, 98 patients were randomized either to the exercise group (EG), with a progressive 12-month home-based exercise program, or to usual care group (UCG), with one guidance session for light home-exercises. Maximal trunk muscle strength was measured by a strain-gauge dynamometer and trunk extensor endurance was measured by Biering-Sørensen’s test at baseline and after the intervention. Results: The mean change in extension strength during the intervention was 75 N in EG and 58 N in UCG. Flexion strength improved 50 N in UCG and 45 N in EG. Trunk extension/flexion strength ratio changed from 0.90 to 1.02 in EG and from 0.98 to 1.00 in UCG. In EG, Biering-Sørensen’s test improved by 17 s, and in UCG, it improved by 24 s. No statistically significant between-group differences were found in any variables. Median exercise frequency in EG decreased from 2.5×/week during the first two intervention months to 1.7×/week during the last two intervention months. Conclusions: Twelve-month progressive exercise program was equally effective as usual care in improving trunk muscle strength. Home exercise adherence decreased, which may have influenced the strength changes.Implications for rehabilitation The 12-month home-based exercise program was equally as effective as usual care after lumbar spine fusion (LSF) in improving trunk muscle strength, however, the back-specific exercises led to better trunk muscle strength balance in exercise group only. The adherence to the home based exercise program is a challenge; therefore, different techniques could be implemented to provide purposeful support for each individual in their long-term exercising. It is important to recognize those who need more individualized rehabilitation in recovery of the spine function, while others may manage with subtle intervention after LSF.