Another pillar of the prayer is standing, for those who are able.
The Prophet (may the peace and blessings of Allāh be upon him) used to stand in both the obligatory and voluntary prayers, fulfilling the command of Allah the Exalted:
"...Stand before Allaah, devoutly obedient." [The Qur'an: al-Baqarah, 2:238]
As for those who are unable to stand, due to illness or infirmity, they are permitted to sit, either on the floor, or on a chair. However, if a person is able to stand for even a part of the prayer, they should do so.
As for the voluntary prayers, it is permitted to sit, whether there is a need to do so or not. However, the one who is sitting without an excuse in the voluntary prayers only gets half of the reward.
'Abdullah ibn 'Amr said that he was told that the Messenger of Allah (blessings and peace of Allah be upon him) said:
"A man's prayer sitting is half a prayer." [Muslim: 735]