Whether you work your upper and lower body on the same day comes down to how much time you have, your stamina and your motivation. While there is an optimal sequence for working muscle groups, it's not carved in stone, and it doesn't divide the upper and lower body. The only thing that really matters is allowing enough rest between workouts and making sure your workouts are balanced.
The most effective sequence for your workouts involves doing the larger muscle groups first, that is, the pecs in your chest, the lats in your middle back and the glutes, quads and hamstrings of your thighs and butt. Next come your shoulders, then your arms and calves if they weren't included in your leg exercise -- like a squat. Going out of sequence isn't the end of the world, but the idea is that working major muscles usually involves some work from smaller muscles. Tiring the smaller muscles, like your triceps, first may mean you can't do as many pushups for you pecs where the triceps assist. And your core is what helps you maintain proper form and balance. A tired core could lead to a sagging back or rounded shoulders. So it's best to do that last.
Whether you work your upper and lower body on the same or different days, the important thing is to leave 48 to 72 hours between working the same muscles. When you first start out this may not be hard to remember, since you'll be sore for at least that long, but as you get more used to exercising, you may think more is better. It's not. It's during the time between workouts that your muscles grow. In fact, if you're involved in very intense workouts, like training for a competition, you may have to take off as much as a week after the event.
Every muscle has an opposing muscle that does the opposite thing. Working one muscle, like your quads, at the expense of the opposing muscle, in this case your hamstrings, will lead to a muscle imbalance that can cause poor posture, pain and even sidelining injury. So, while you can divide your workouts any way you like -- upper body/lower body, arms and legs/larger muscles and core -- you should work opposing muscles in the same workout. If one area is significantly weaker -- meaning you can't lift as much weight or do as many reps -- you should continue to work the stronger muscles at the same resistance until your weaker opposing muscles catch up.
You should work all of the major muscle groups at least twice per week. You can go to as many as three times, but after that, the additional benefits will be negligible. If you can fit in a full-body workout that frequently, go for it. The benefit is you'll have between four and five days each week when you don't have to do anything -- well, except cardio, which you should do for at least 150 minutes a week, or 30 minutes five days per week.
- DaveyWaveyFitness: Order Matters: Switching the Sequencing of Your Workout.
- Personal Fitness Training: Theory & Practice; Mary Yoke, MS
- ACSM: ACSM Issues New Recommendations on Quantity and Quality of Exercise
- IDEA: Tapering: Science and Practice
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