Goal: You want to lose weight.
If weight loss is your goal, weights before cardio is your best bet. For an American Council on Exercise study, researchers from Western State Colorado University explored the optimal order of exercises in a workout. After 24 training sessions, each in a different exercise order, researchers found that doing weights first led to a more elevated heart rate, which means a bigger calorie burn for your cardio buck. All while having more energy to lift heavier weights, which increases your metabolic lean muscle mass.
Goal: You want to look lean.
Whether you're looking to show off your abs and seriously strong arms in a bathing suit on the beach or on a bikini competition stage, split your cardio and strength training to separate days. Why? "In the pursuit of a lean and athletic physique, you need to be able to perform at both types of exercise at 100 percent," says Michael Berg, a certified personal trainer based in New York City. "When you split up the focus of an individual workout session, you split the amount of energy you can dedicate to each activity." However, if you do an all-out weight training session on Monday and then an interval cardio session on Tuesday, your body is able to recover overnight so it can perform at its peak during the Tuesday workout. If you must do a training mash-up, Berg suggests doing weights first, then cardio.
Goal: You're training for a race.
Whether you're cycling a century or running a half-marathon, do your cardio first. Research published in the Journal of Strength & Conditioning Research found that whichever muscle you worked out first saw the most benefit. In this case, if you're cycling or running first, the muscles needed to excel for a faster race will be stronger. Basically, order your workout by prioritizing your goal.
Goal: You just want to be healthy.
When your goal isn't about peak performance or maximum weight loss, you have a lot of leeway. In fact, when exercise is moderate—say working out three times a week doing both strength training and cardio—exercise order doesn't affect your results, according to a study published in European Journal of Applied Physiology. Which means if you prefer to hit the weight room before your spin class, go right ahead. After all, the best workout is the one you actually do.