You don't have as much power because your body automatically scales down to prevent you from injuring yourself. In addition, performing an exercise wrong doesn'

You don't have as much power because your body automatically scales down to prevent you from injuring yourself. In addition, performing an exercise wrong doesn't mean that you automatically get injured, just that your chances of hurting yourself go up. If you're lucky, you're never going to break anything - this also depends on the kind of exercise you're doing and on its intensity. As for the max-effort thing, the Valsalva maneuver can be performed at different degrees of intensity. It's not like every time you do it you have veins popping in your eyeballs, and I don't expect those untrained men referenced in the article to do it very strongly. It is correlated with the effort you're doing and on how much you're consciously compressing the air. But yes, longer effort with less snow per shovel would definitely help.