[tristramb]

You need to sprain both knees or ankles at the same time and then use one as the control.

[differentView]

Both most people favor one side of their body, so the wear and tear would already be uneven.

[jjk166]

Sprain both knees and ankles and red light treat opposite ones (ie randomly select right ankle + left knee or the opposite pair).

[Ruthalas]

You may also have trouble applying the Aspirin to only one location.

[aghilmort]

most useful comment in thread like that H.pylori ulcer discovery