One of the basic pieces of furniture, a chair is a type of seat. Its primary features are two pieces of a durable material, attached as back and seat to one another at a 90° or slightly greater angle, with usually the four corners of the horizontal seat attached in turn to four legs—or other parts of the seat's underside attached to three legs or to a shaft about which a four-arm turnstile on rollers can turn—strong enough to support the weight of a person who sits on the seat (usually wide and broad enough to hold the lower body from the buttocks almost to the knees) and leans against the vertical back (usually high and wide enough to support the back to the shoulder blades). The legs are typically high enough for the seated person's thighs and knees to form a 90° or lesser angle.
It depends on his real ability and the extent of it. Every Muslim is responsible to perform his worship in the proper way as far as he can. If there is a real inability, then what ever is possible for him is obligatory on him. If he can not stand at all, then he can pray while sitting, but if he can stand for a while then unable to continue standing, then he has to stand for Takbiratul Ehraam and what ever he can, then when he feels unable to continue, he can sit. If he can sit and pray, he can not then pray lying on bed unless he reaches to a stage that he can not sit. Standing is must as far as it is possible even part of it. Same with sitting for those who are unable to stand, then lying on bed for those who can not sit either at all, so they sit, or some times, so they sit according to their present ability.
Rokoo' and Sojood is performed also according to the extent of the ability.