Option 1: upright sitting
This is probably the posture you think of as "good" posture. The defining feature of this option is thatthe trunk is upright.
A key component of upright sitting is that the feetcan comfortably rest on a surface, whether the flooror a footstool. This position also makes it easy toadjust posture within the chair (fidget) and changeposture to get out of the chair.
It's also important the arms hang down from the shoulders vertically with elbows by the trunk, unless the forearms are supported on the work surface. Holding unsupported arms forwardrequires the muscles connecting the shoulder and neck to work harder. This often results inmuscle fatigue and discomfort.
The head should be looking straight ahead or a little downwards. Looking upwards wouldincrease tension in the neck and likely lead to discomfort.
This posture is useful for common office tasks such as working on a desktop computer.
Option 2: forward sitting
The defining feature of this posture is that the trunk is angled forward, and the arms arerested on the work surface. Allowing the thigh to point down at an angle may make it easier tomaintain an inward curve in your lower back, which is suggested to reduce low back stress.
For a time, special chairs were developed to enable the thigh to be angled downwards, andusually had a feature to block the knees, stopping the person sliding off the angled seat base.
By perching on the front of an ordinary chair and resting your elbows on the work surface, youcan use this posture to provide variety in sitting. This posture is useful for tasks such asdrawing or handwriting on a flat work surface, either with paper or a touch screen device.
Option 3: reclined sitting
The defining feature of the third option is the trunk is angled backward, supported by thechair's backrest. Back muscle activity is lowest in this posture, as some of the upper bodyweight is taken by the chair.
This position may reduce the risk of fatigue in the back muscles and resultant discomfort. Butsitting like this for hours each day may result in the back muscles being more vulnerable tofatigue in the future.
This posture is useful for meetings and phone conversations. But it doesn't work well forhandwriting or using a computer as the arms need to be held forwards for these things, requiring neck and shoulder muscle activity likely to result in discomfort.