Fitness coach Eric Roberts says 20-30 minutes of treadmill cardio before lifting depletes your energy for weights and raises injury risk. He uses a balloon analogy to explain why cardio should come after strength training, not before, if you do both the same day.
I'm gonna hop on the treadmill for 20 to 30 minutes before my workout. I'm gonna warm myself up by doing some speed walking, some running, get my heart rate up. And you think that's a proper warmup to go lift weights. But I want you to think about your muscles like a balloon. A balloon only has so much air inside of it. When you do this cardio before your workout, you're letting the air out of the balloon. So when you go to lift your weights, you have less air, and the weightlifting lets the air out of the balloon as well. So you're not able to push yourself as hard, lift as heavy, and then your results suffer, and you might even get injured because you've let all the air out, and now you're trying to work out with a flabby, soggy balloon. If you're going to do cardio and weights on the same day, it should always be done after you lift weights, not before.
