The Roman chair places you flat on a table that supports the legs and stomach. The upper body is free and unsupported. The feet are strapped in. The back muscles are engaged by lifting the head and trunk into extension.
Studies at the Musculoskeletal Research Lab in Syracuse, New York have looked at this. Scientists know that the low back muscles are weak in patients with low back pain. They also get tired easily. In fact, EMG studies show that these muscles give up before they’ve even contracted fully.
When about 55 per cent of the muscle fibers are contracting, other muscles start firing to help. The low back muscle actually “derecuits” its own fibers and calls in help from the hip and leg muscles. This could account for your soreness. When you are working your back muscles beyond 55 per cent of their maximum, other muscles start to help or even take over.