Lydia Willgress, writing in The Independent, states that the key considerations are the hardness of the material, which influences the comfort and support they provide; size and weight; and appearance.These considerations led to the development of yoga bricks in other materials, and to the marketing of hollow wooden blocks, which are lighter but more costly and less durable.[5][1] Manufacturers such as Manduka make recycled foam blocks, offering the combination of a light and strong prop with a low ecological impact.[10] Yin Yoga (founded c. 1975) uses props such as blocks to help get sensation into whichever area is of concern; the purpose may be to increase stress in an area, to reduce stress where it is not wanted, to make some poses accessible to the practitioner, to provide enough support to allow the muscles to let go, and to make poses more comfortable, permitting them to be held for longer.[11] Restorative Yoga (founded c. 2007) uses blocks extensively, for example arranging them under a bolster to create a ramp or to raise a part of the body.