we used to use them in the 70’s for cross country and soccer endurance training. It turns out they put terrible stress on all the bones in the ankle area. Remember those red hand weights designed for running that were so popular in the 80’s ? the idea was you would run a lot faster after training with them; my experience was that I got better at the extra effort needed to carry the weights but that didn’t translate into speed. Tempo runs once a week have been giving me faster runs, swims and rides. I think holding an 80% heart rate for extended periods in training has increased my aerobic threshold. It’s hard though. I’m putting off my tempo run right now by writing this !