Originally Posted by DraftyAiresMum
Running is not nearly as high impact as jumping is. When running, the weight of the horse is more or less distributed evenly over all four legs. When jumping, at one point or another, the hind legs and then the forelegs bear the entire weight of the horse and rider. Add to this that, in addition to bearing the entire weight of the horse and rider, the forelegs bear the concussive force of that weight crashing down and forward onto them, this can lead to a horse with less than optimal conformation breaking down much more quickly.
Out on the trails, he'd be fine occasionally jumping a 12-18" log or what-have-you. You said nothing about trails in your original post, so we automatically assumed that when you said "jumping," you meant competitive jumping, like hunters or show jumpers.
Thanks for the info, but do you think occasional 2nd field foxhunting would be too hard on his knees? I go out about 3 times a season and since its second field, there would be a very limited amount of jumps. Usually under 5.