Begin using the backrest only after you become fully comfortable in the seat. Your fitness for healthy sitting will improve and as it does, you will find yourself using the seat in a more central position. Some men have difficulty in adjusting. This is made easier by having the seat tilted down at the front to the maximum. Also, when sitting down, sliding back onto the seat helps to position the male parts more comfortably, the same as horse riders.
Some people experience tightness in thighs and hips.
Again your body will adjust. The position with your knees apart greatly reduces the chance of hip joint disease at a later age. Tight muscles, tendons and ligaments will soften and relax over time, which is desirable.