This might be a matter of having seats specially built by Otto Bock or similar. Generally, Volvos have good seats. What is important are long squabs that are tilt-adjustable to support the thighs (many cars don't offer squab tilt), seat height adjustment and adjustable lumbar support. It is important to recline the seat slightly so that part of the weight of the driver's torso is supported by the backrest of the seat rather than fed directly down into the base of the driver's spine. To get all of these features you usually have to go for the high-end spec models, but may then encounter the problem that these have large wheels and hard-riding low profile tyres. Air suspension as 'Airmatic' in the Mercedes Benz C-Class helps enormously.