['Is eight weeks of high-intensity strengthening of the isolated lumbar extensors more effective than low-intensity strengthening or no strengthening? Are any gains maintained 16 weeks later?', 'Randomised, three-arm trial with concealed allocation, assessor blinding, and intention-to-treat-analysis. Participants in the waiting list control group were randomised again, after the first 8 weeks, to either the high-intensity or the low-intensity strengthening program.', 'Sixty-five army personnel with non-specific chronic low back pain.', 'The high-intensity training group received 10 sessions of 15 to 20 repetitions for the isolated lumbar extensor muscles. The low-intensity training group received a nonprogressive, low-intensity resistance protocol.', 'Primary outcomes were global perceived effect and disability. Secondary outcomes were health-related quality of life, fear of movement/(re-)injury, and isometric lumbar extensor muscle strength. Measures were taken before and after the training and 16 weeks later.', 'At eight weeks, SF-36 overall score was on average 7% (95% CI 1 to 13) greater in the high-intensity training group compared with the low-intensity training group and the waiting list control group, and self-assessed decrease of back symptoms was on average 39% (95% CI 14 to 64) greater in the high-intensity training group compared with the waiting list control group. There was no difference in improvement between the groups for any other outcome at 8 weeks or 24 weeks.'] Is a high-intensity lumbar extensor strengthening program little better than a low-intensity program or a waiting list control group for chronic low back pain : a randomised clinical trial?