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i think the only potential issues with using a treadmill is that the path of your knee may not be optimal for full ROM practice at the ankle. you have to make sure it moves slightly to the outside to prevent your instep from collapsing. but could you walk like that? not sure. also as you walk, your heel comes up which may also prevent full ROM. on a moving surface, you may not be able to pause long enough to get the benefit of the full ROM before the heel comes up.