When someone said “a bad result is still a result”, he or she was not aware of what people might do to invoke bad results on purpose. Apple Watch, one of the best in the business will surely work on your ankle if your manage to keep it there still using one of the ankle straps. But the question is how accurately can it measure and assess your vitals at that position. To answer your question, it probably won’t work desirably on your ankle, but will still work. But, it is suggested to wear it on the wrist because it is a smartwatch first and the rest of the features come later.