If you have been thinking about weight training, you need to separate bodybuilding facts from fiction.
1. 12 Repetitions are a must do
Not really. Most weight training programs extol 12 repetitions as a hard and fast rule for gaining muscle. The truth is, this approach actually denies important muscle groups of enough tension for effective muscle gain over the life of your training program. High tension, caused by use of heavy weights, provides tension so the muscle group being worked actually grows in size. This leads to noticeable gains in strength. Using the 12 rep rule boosts muscle size by generating tension on tissues around muscle fibers. The payoff is greater strength and endurance. 
The standard prescription of eight to 12 repetitions provides a balance but only will get you to a level where you can esaily handle the weight. At that point, tension is no longer provided. In other words you peak and do not generate greater tension levels necessary for muscle growth and even greater strength and endurance. So, what to do?
The answer for you may be heavier weights and lesser reps. This steps up the level of tension and you get bulky muscles in the process. You may end up looking like Arnold. On the other hand, if you just want to lose some fat and tone your muscles, the key is less weight and more reps. Use just enough weight to feel tension and go for more reps. Listen to your body and adjust the number of reps and the weight to where you get your heart rate elevated but not to the pointing of grunting red-faced.
2. 3 Set rule with the 12 rep rule
Apply this correctly and there's nothing wrong with three sets. A set, in case you don't know, is doing 12 reps 3 times in a row. Each set of 12 reps is one set. And, the number of sets you perform should be based on your goals and not on a hard and fast rule that's been around for 50 years. Your body is unique. Listen to it. A good rule of thumb is, the more repetitions you do on an exercise, the fewer sets you should do, and vice versa. This keeps the tension provided by the total number of repetitions at a manageable level. But, you decide the right amount of tension, not the rules.
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3. Three to four exercises per muscle group
Wrong and a waste of you gym dues. You will spread yourself too thin and not accomplish the tension you need for each muscle group. Instead, focus on parts of the body over several days. In other words, work your upper body one day, your abs the next day, your lower body the next and just wash, rinse, and repeat. This gives muscle groups 24 to 72 hours for repair and rejuvenation.
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