Trekking poles aren't designed to be a substitute for bones and leg muscles. Yes, you can get a little extra push up or braking going down out of them when ascending or descending, but just as ski poles are an aid and supplement for turning, trekking poles are meant to be an aid for balance by being an extra point of contact. They aren't like Canadian (forearm) crutches which are designed to support the full body weight of a person. If you're putting anything close to even half your body weight on them while descending, you're not using them for their designed purpose.