There are few factors as important to arena performance as the footing your horse performs on. If the surface is something like concrete or hard-packed clay then that will lead to unnecessary impacts on the horse's legs and joints. Just like when people run or walk on a hard-packed surface all day, this impact can quickly lead to muscle pain at best, and joint damage at worst. On the other hand footing that's too soft, like deep sand or loose soil, will lead to a loss of traction. That means the horse will have to work extra hard to get traction, and that could lead to faster exhaustion, a loss of performance, and an increased heart rate.