For years we believed the normal range of motion of the subtalar joint was 10 degrees of eversion and 20 degrees of inversion from subtalar joint neutral. I used to religiously draw the bisections down the back of the leg and calcaneus and measure this angle.
After a while I came to the realisation that:
1. Many studies were showing that the measurements were not reliable
2. Once I got the number (assuming that I was actually reliable!), I did nothing with the number
3. 20 and 10 were probably not the normal range of motion
What is the normal range of motion? For straight ahead walking on a hard surface, we probably only need 4 degrees of eversion and no inversion. If we want to turn a corner, we going to need more. If you going to play tennis, you going to need a huge amount more. I have seen a slow motion video of a high jump foot plant and take off and the STJ/rearfoot complex everted (pronated), something like 50-60 degrees.
What is the normal range? There isn't one because it activity dependent. I suggest its simply a matter of checking the range and noting if its adequate or inadequate for the activity of the person – that’s all that is needed. What is the normal range came up in this thread on Podiatry Arena on foot pronation.