When I use it in a circuit, it is a strength movement rather than a power and we are looking for high reps and failure at around 40 seconds, rather than just a fewer numbers over that time period.
At this point in the game I am able to go for 40 seconds with 20lb weights for each of the four rounds. It is a challenge to complete about 20 reps each cycle.
Be sure to avoid a weight that makes you bend your back in an improper manner. It is possible to injure your lower back by using to great of a weight.
Now train hard and use all this outside on the rock and mountains!
piz : )