If the number of days you can work out each week is constrained or if you value cardio training enough to do it 5+ days a week, you probably have to do both your lifting and your cardio exercises on the same days. It’s not ideal, but you can maximize the benefits of your workout if you do weights before cardio.
As PictureFit explains in the video above, the WHY largely comes down to glycogen. Glycogen is how your body stores glucose which it metabolizes quickly for energy. Both lifting and cardio require glycogen, but your lifting routine will suffer most from a lack of glycogen stores because lifting requires quick burst of energy whereas cardio is more of a slow burn. Because cardio’s glycogen burn is slow and steady you can train your body to perform cardio with low glycogen levels, there’s actually evidence out there that this may even increase your performance over time.
Another factor at play is the body’s production of the enzyme mTOR which is essential for muscle growth. Your body produces the enzyme when you lift weights, but AMPK, an enzyme produced during low-intensity exercise, like cardio, inhibits mTOR. If you do cardio first, AMPK can decrease the amount of mTOR you produce when you lift. Basically, cardio first = less gains.
The last factor to consider is fatigue. Cardio leaves your body fatigued which compromises your muscle’s ability to perform. Your heart will also be worn out, which affects nutrient and oxygen delivery to the muscles. Cardio first can also lead to mental and CNS fatigue which can mean danger when you’re lifting heavy stuff.
I’ve found over the years that cardio first is a no-brainer for me. If I ride first I have nothing left for weight lifting, but if I lift first I find I can function on the bike and get in a good workout just fine. It takes a week or two to get your body used to the idea, but once it’s adjusted it works great. There’s no going back once you do weights before cardio.