Originally Posted by dabears

From a "scientific" standpoint on why demanding cardio is a no-no before a weightlifting session:

Glycogen in the muscles enables high exertion, and you only have so much of it in your bodily stores. It is replaced via dietary carbs for the most part. Consider this your "endurance". If you exert yourself before trying to lift heavy weights, your glycogen will pretty much be depleted, and your workout (and therefore gains) will suffer. You want to be in prime condition to go as hard and heavy as possible, and you shouldn't even THINK about being able to go for a run after. There are some days after a leg workout where I struggle to push the clutch down in my car lol

There's a lot more to this but I tried my best at a simple explanation