Who are firm mattresses designed for?
People of a larger stature and preferred back sleepers are best designed for extra firm mattresses. If you're part of a heavier weight group, an extra firm mattress will evenly distribute your weight and support you properly, whereas a softer mattress could promote unhealthy spinal alignment. The same thing goes for back sleepers, where a non-conforming surface means your spine doesn't go out of place, therefore causing aches and pains.