When you're short of time, you may not have the luxury of being able to perform weight training and cardio separately. If you still want a workout that hits both your cardiovascular system and makes you stronger, though, what you can do is insert short bursts of cardio into your rests between weightlifting sets. This can have a positive effect on your training but also carries some drawbacks.
Rest Between Sets
Whatever your training goals, you need a rest between your weightlifting sets. How long you rest depends on what type of training you're doing, notes strength coach Jeremy DuVall on the Men's Fitness website. If you're training for muscular endurance you only need 30 to 45 seconds between sets. For muscle growth, this increases to 60 to 90 seconds, while for strength you need two to four minutes and three to five minutes for power. Rather than sitting around, talking with friends or killing time, use these rest periods for cardio.
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Increased Calorie Burn
Cardio is an effective method for burning calories. Half an hour of lifting weights burns between 90 and 133 calories, yet the same amount of time spent cycling burns 210 to 311, while running at a nine-minute-mile pace burns 330 to 488 calories. When training for weight loss this can accelerate your progress by creating a bigger calorie deficit.
From a mental standpoint, you may find you can't concentrate fully on either form of training when alternating the two. Your weightlifting performance is also likely to be impaired. This is particularly the case when performing high-intensity lower-body moves such as back squats or deadlifts. You need a full two to four minutes of rest to give your muscles time to recover. If strength is your goal, jumping on the rowing machine or elliptical between sets isn't a good idea.
For general fitness and weight loss, mixing cardio and weights can increase the challenge of your workout, make better use of time and the downsides of reduced performance won't affect your overall goals. Kinesiologist Amy Ashmore of the IDEA Health and Fitness Association recommends planning your strength and cardio supersets so they match your goals. For fat loss and improving cardio fitness, perform high-intensity intervals, such as a 30-second sprint on a stationary bike, between your weightlifting sets. Reduce the intensity and lengthen the cardio time, but keep lifting heavy to build endurance, or go really light with your cardio and use it as a way to fill time while recovering from a tough weight set.