This is one where the experiment may be flawless, but the research question is wrong.
When patients come in complaining of ankle instability they probably don't know the difference between the ankle and subtalar joint.
The instability starts when the foot is close to its relaxed stance position and not when it's at the end of range of motion, which is where the function of the ligaments matters.
If the patient complains of frequent sprains the initial problem is unlikely to be with the ligaments.
The ligaments may become a problem after several sprains, but it is not the initial problem.
Are there weak peroneal muscles or a laterally deviated STJ axis causing the frequent sprains?
What is the problem when a patient says their ankle is unstable?