A. I hear that question all the time and for good reason. In an ideal world, you would do your weight training and your cardio on two separate days—or at least space them apart in the morning and evening. Many of us, however, don’t have that luxury, so the answer to the before-or-after question depends on the type of fitness results you want.
The two different types of physical activity—weight training and cardio— impact our muscle cells in different ways. Lifting weights activates genes that are normally dormant within a muscle cell, but the cellular changes that your body requires to adapt to resistance exercise are different from the changes your body needs for cardio- vascular, or endurance exercise. So as it turns out, when you do both types of exercise in the same workout, the two stimuli want to cancel each other out. The result is a little bit of improvement in both strength and endurance, but not as much as if you did only one type of exercise.
So how can you time them in the same workout session? If your goal is to build muscle, do a cardio workout first. A study looking at the hormonal response to doing cardio before or after weights found that in men, anabolic hormones like testosterone remained elevated longer when weight lifting was done after cardio. There is some evidence that this applies to women as well.
If you’re looking to build endurance more than muscle mass, consider doing the opposite: Another study looked at the different genes and signaling pathways that were activated by switching the order of weights and cardio. Essentially, the final type of exercise you end your workout session with has a greater adaptive effect. Doing cardio last suppressed the anabolic effect of weight lifting to some extent, and it increased protein breakdown more. Doing weights last allowed those pathways involved in protein synthesis and muscle growth to remain active longer.
