While not moving, a human can be in one of the following main positions.
- standing; requires sufficient headroom, e.g. it is not possible in a regular car; one can stand freely or lean against a wall, a pole, etc.;
- sitting; requires a more or less horizontal structure, e.g. a chair or the ground; special ways of sitting are with the legs horizontal, and in an inclined seat; while on a chair the shins are usually vertical, on the ground the shins may be crossed in an "Indian-style" sitting (with a name that has questionable political correctness), or the shins may go horizontally underneath the thigh in a seiza.
- lying; requires sufficient space in one direction; usually done on a bed;
- on all fours
It is also possible to classify these main positions into further detail, as well as to consider intermediate forms.
For sitting and lying softness and cleanliness are relevant factors. Sometimes paper, cardboard or cloth is used when sitting or lying on the ground, a dirty bench, etc. Sitting or lying in the grass or on a sandy beach is comfortably soft.
For sleeping and sexual activities one often lies down. For most activities which do not involve moving, sitting is usually preferred, e.g. reading, watching television, or using a computer; this also applies for moving in a vehicle.
Standing and squatting is mainly done when there are not enough seats, e.g. in a public transport vehicle, a train station, a bus stop, a waiting room; whether people will sit depends on the availabilty of other places to sit (including enough space on the floor), how inventive one is, how conventional, how dirty these places are, how dirty one is willing to become, and whether paper etc. is available to sit on (these things also apply when there are seats, but dirty).
Availability of seats is sometimes somewhat subjective, e.g. whether an additional person fits on a bench. This depends also on shyness and feelings about proximity.
Standing in a moving vehicle is less stable than sitting and usually requires holding on to something to absorb accelerations (going faster and slower and making turns); for this poles and/or handles are often fitted. Squatting may be difficult because of being too unstable, unless it is possible to lean against something.
- Sitting Pretty: Ten easy steps to good posture (http://www.chirotx.com/books/reviews/sitting_pretty/index.htm)