Strength training exercises should be done at least twice a week but can be done up to four times per week as long as you rest that muscle group at least one day in between. The rest phase is very important to strength builidng, since this is the time the muscle heals and builds itself back up becoming stronger in the process. Women don't need to worry that they will start bulking up like men do. Because men have more testosterone, they tend to bulk up more with strength training.
There are more benefits to strength training than simply increased strength. We all know that cardio is good for weight loss and burning calories and fat. But strength training plays an important role.
1) Strength training provides a more-lasting boost to BMR by altering your body's composition. Muscle at rest burns more calories than fat at rest. That's why men enjoy a naturally higher BMR than women, as they tend to have more muscle mass.
While it’s true that a pound of muscle burns more calories than a pound of fat — about seven to 10 calories a day versus two calories — most people don’t put on enough muscle to notice a difference to their weight loss.
Building muscle is very difficult for most individuals because it requires heavy weight workouts and a higher intake of calories. Average fitness enthusiasts who are working out to gain muscle may add about four to five pounds of lean mass, which will burn an additional 28 to 50 calories a day. Over the course of a year, that adds up to 3 to 5 pounds of weight loss added to your efforts at no extra effort. Not bad, really.
2) Strength training does burn calories.
You burn calories during strength training, and your body continues to burn calories after strength training. More calories are used to make and maintain muscle than fat, and that can really jumpstart a weight loss plan. Exercise longer and / or exercise more intensely and burn calories at a faster rate. Standing exercises and exercises that use more than one muscle will burn more calories than most exercises that target one muscle while sitting on gym equipment.
3) Strength training preserves muscle mass during weight loss (weight will be lost from muscle and fat during weight loss). Also, adults lose between five and seven pounds of muscle every decade after age 20. Decreased muscle mass can be one of the main reasons metabolism slows with age. Disuse can cause people to drop as much as 10 pounds of muscle between the ages of 30 and 50. Losing that much muscle means you'll burn 350 to 500 fewer calories a day. (Slowing metabolism = weight gain. At 40, you could be burning 100 to 300 fewer calories a day than you did at 30. That can translate into a 10-25-pound weight gain in a year).
4) Strength training boosts energy levels and improves your mood.
Strength training has been shown to be a great antidepressant and to help you sleep better (due to increased levels of the neurotransmitter,serotonin), and to improve your overall quality of life. Strength training may also elevate your level of endorphins (natural opiates produced by the brain) with high intensity interval training, as it does with high intensity aerobic exercise - which will make you feel great.
Trying to get some cardio benefit from your strength training time?
Remember, it won't do you any good in the long run if you injure yourself. Simply speeding up your moves may cause injury and it reduces the benefit to your muscles that a full range of motion provides.
Instead, try full-body / double-duty moves that combine upper and lower body work, for example, rather than focus on one muscle at a time. You'll work more muscles in a more functional way and that mimics how we use our bodies in everyday life, which is why it provides more functional benefits.
Also, instead of resting between sets, try circuit training. Move from one exercise to another with no break in between. I usually do a series of 2 - 4 exercises in a row, and then repeat that group before moving on to another series of 2 - 4 exercises. Not all of my exercises are full-body or double-duty, though many are.
Another option is to alternate high intensity interval training with your strength training exercises. After 3 sets of squats with weights (can double duty adding overhead press), try skipping rope or doing jumping jacks as fast as you can for 1 minute. Then move to another strength training exercise.
Remember, form is important, not only to protect your body from injury, but also to get the most out of your workout.
There are many online sources of specific exercises. Try a few, see what you enjoy. If you don't enjoy something, chances are you won't stick with it. So find what works for you and keep changing things up now and then to keep your interest and create more challenge to your body.