The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
7. Sitting (a man is to meditate), on account of the possibility.
As meditations connected with members of sacrificial action depend on action, we need not raise the question whether they are to be carried on in a sitting, or any other posture. The same holds good in the case of perfect intuition, since knowledge depends on its object only. With regard to all other meditations, on the other hand, the author of the Sûtras raises the question whether they may be undertaken indifferently by a person standing, sitting, or lying down, or only by a person sitting.
The pûrvapakshin here maintains that as meditation is something mental there can be no restriction as to the attitude of the body.--No, the author of the Sûtras rejoins; 'Sitting' only a man is to meditate.--Why?--'On account of the possibility.' By meditation we understand the lengthened carrying on of an identical train of thought; and of this a man is capable neither when going nor when running, since the act of going and so on tends to distract the mind.
[paragraph continues] The mind of a standing man, again, is directed on maintaining the body in an erect position, and therefore incapable of reflection on any subtle matter. A man lying down, finally, is unawares overcome by slumber. A sitting person, on the other hand, may easily avoid these several untoward occurrences, and is therefore in a position to carry on meditations.