Posture refers to the pose of the body. It can be defined by the position of all the anatomical segments relative to each other and with respect to gravity. The relative orientation of body segments can also be described by the angles of the joints. Body posture is strictly speaking a static phenomenon but we can also consider the concept more loosely during dynamic movement, e.g. that body posture is generally (standing) upright when we are walking, or that it is generally sitting when we are riding a normal bicycle - or lying down when we are riding a reclined bicycle.
Figure P.1.4 [awaiting upload] Rodin's The Thinker in contemplative pose. Source: Wikimedia Commons, 2020.
The main body posture types are:
One could also think of postures which are a mix of these, for example kneeling or squatting is a mix of sitting and standing, reclining a mix between lying and sitting, and lounging an ambiguous low-energy posture somewhere between lying, sitting, and standing and most likely involving some external support like a sofa or a table to uphold it.
Subtypes within the main postures include for example variation in the position of the limbs, e.g. legs may be parallel or crossed or one bend more than the other when sitting on a chair or the arms and hands may be in different positions such as holding a mobile phone to one ear. When standing, our arms may also be hanging down by the side of the body, or they may be horizontal if we are working at a standing desk, or pointing upwards when we assume the winning pose. The exact characterisation of any given posture is provided by the measurement of the position of all body segments, relative to gravity. We can measure segment angles with modern accelerometers that are sensitive to gravitational acceleration (not time-varying).
Postures is generally considered a subset of physical behaviour types, with another subset being dynamic activity types. Hence postures feature prominently in behaviour type classification methods.
Postures differ in their metabolic profile and require different levels of energy expenditure. For example, standing expends more energy than sitting due to the continuous contraction of muscles in the legs and around the spine (the postural muscles) to keep the body upright and in balance. The slight movement back and forth shifting the weight to back and front of the feet which is typically observed when a person is freely standing up is known as the postural sway. Other postures characterised by a postural sway element is standing on one leg or doing a handstand. The definition of sedentary behaviour includes a postural element; the person must be sitting or lying down (but awake) as well as expending only very low levels of energy.