Research shows that people who are physically inactive can lose as much as 3%-5% of their muscle mass per decade after the age of 30. However, resistance training along with a healthy diet, you'll protect your muscle & burn more fat instead. Most importantly, resistance training can decrease the risk of heart disease, improve blood pressure & cholesterol levels.
First off, I'll repeat what you've probably heard many times: "Muscle burns fat". But what does that mean? Well, muscle doesn't exactly burn fat but more accurately muscle raises your Resting Metabolic Rate (RMR).
Adipose tissue (i.e. fat) takes no energy to sit on your body, that's why once it's there it will stay there until you exert enough energy to start using it as your energy source. Skeletal muscle tissue is called "active tissue" because it requires energy to maintain itself. To just sit on your body, each pound of muscle on your body uses about 30-60 calories per day.
THE AFTER BURN That hour of cardio was great to burn that stored energy, but when you're done on the cardio machine you're done burning calories. Weight training, on the other hand, keeps your metabolism at an elevated energy use rate for about an hour after you're done. Another bonus to weight training!
WEIGHT WORKOUT: HIT IT HEAVY Muscle tissue growth is only stimulated when pressure is applied to it. If you use light weights and do rep after rep, your muscle will never have the stress applied to it that it needs to respond too. This means that although you eat cleaner and are on a reduced-calorie diet, your muscles will not grow.
Many dieters lighten up on their weight because they feel heavy is needed only during a bulking phase, and female dieters especially don't want to lift heavy for fear of getting bigger rather than smaller. These are myths to say the least.
Women should not shy away from heavier weights because we do not have enough testosterone to get the physique of a bodybuilder.
Lower rep/heavy weight workouts burn more calories during the workout because of greater exertion, and will ensure you will not lose an ounce of precious fat-burning muscle.
The most common resistance exercises are performed using body weight, free weights & machines. It is recommended that strength training be performed a minimum of two-nonconsecutive days each week, with one set of 8-12 repetitions of 8-10 different exercises that target the major muscle groups. Machine based exercises are safer to use for someone who is new to fitness & starting a strength training program. Here are a few examples of resistance training using weight machines.