The seat supports the upper legs and the buttock region. The seat base can be of a sling or rigid type. The hammock or sling type base (Fig. 2) is the least expensive and folds easily with the chair. This is appropriate if a person is only using the chair on an occasional basis. If an individual is spending a great proportion of the day in a wheelchair a firmer base is needed. On the seat base a cushion will be placed. Every wheelchair should be fitted with a removable cushion for pressure distribution, shock absorption, and positioning. The types of cushions are innumerable but are most often foam, gel, air, or combination cushions. A proper cushion helps to decrease pressure by distributing it over as great an area of the person’s legs and gluteal region as possible. For example, if the seat depth is too short it will decrease the weight that can be carried under the thighs and could cause dangerous increases in pressure in the gluteal areas leading to skin breakdown. It is generally recommended that users have one to two inches of space between the cushion edge and the back of the knees. On the other hand, if a cushion is too long it can create pressure behind the knees and interfere with the legs when used to assist in propelling the wheelchair. Sometimes cutouts or contours are made to the cushion so that a person can use the legs to assist in propelling.
The seat height can be lowered for people who can use their feet to move their wheelchair. Foam cushions can be contoured. For example, in a person with thigh spasms, a central hump or pummel can help keep the legs properly positioned. People with poor sensation are at even higher risk for pressure damage to the skin. Air or gel cushions may be recommended in such people to further reduce pressure. The cushion cover is important too. It should be removable for laundering, and if incontinence is a problem waterproof or water-resistant covers may be desirable. The dimensions of the compressed cushion will affect many of the wheelchair dimensions, such as backrest and arm rest heights, and thus the decision on the type of cushion should be made early.