I love to mix up what I do for a workout. Sometimes I do PAZAZ, other times I do all strength training but if I only have a short amount of time and I want a challenging workout I do multi- muscle exercises.
What are multi-muscle exercises? Exercises that use multiple muscle groups. (watch the above video to check out some multi muscle exercises). So when we do a squat we are using quite a few muscles in our lower body. If we hold weights in our hands and do an upward row with the weights while doing a squat we are now using muscles in both our upper and lower body. The reason multi=muscle exercises are so efficient and effective are because you are really killing 2 birds with 1 stone - working lower and upper body muscles at the same time.
When doing multi-muscle exercises you will notice an increase in your breathing rate and your pulse with speed up. This is because you are asking your body to use so many muscles at once which means you have an increased need for oxygen. So really you are killing 3 birds with 1 stone because you are not only strengthening upper and lower muscles, you are also getting a cardio workout too.
If you only have 15 minutes to workout - my recommendation "MULTI-MUSCLE"!!
I do NOT recommend multi-muslces exercises for someone who is just beginning to exercise. Proper form is critical when doing any type of exercise so it is best to master the form of just the upper body exercise and then just the lower body exercise before you put them both together.