what?! unless you're really in top shape and have done a lot of leg conditioning and weight lifting there is NO WAY that you can do a wall sit for 30 seconds straight!

most people, even in good shape (not overweight or obese) can only do 10-15 seconds before their quads burn into a cinder

try it! its a good exercise but 30 seconds? c'mon!

also, another reason why this suggested 30 seconds duration is bad is that it will mean that most people who do try to meet the unrealistic 30 second duration will collapse down as their legs give out and this is NOT what you want to happen because risk of injury from this exercise is greatly increased if you don't get out of the position by standing up