Unstable Squatting = Dangerous Bogus

Some strength coaches like being different and advice their athletes to perform squats and similar exercises on instable grounds. A recent study by McBride et.al. found (McBride. 2010) investigated the effect of absolute and relative loading on muscle activity during stable and unstable squatting and found:

The stable trials resulted in the same or a significantly higher value for Vastus lateralis (VL), biceps femoris (BF) and erector spinae (L1) muscle activity in comparison with the unstable trials for all twelve conditions.

So if this exercise is not specific to your discipline, avoid squatting on unstable grounds at every cost. This might well keep you from being injured.