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Jul 11, 2010 
Before you start lifting heavy weights, you should always do some form of Warm up. The best way to do your warm up is to perform the exercise you are about to do, but with lighter weight. This will get your muscles loosened up and your blood flowing. Warm ups are very important because they will help prevent injury and help you perform your workout with greater success.
For an example, the following is a suggestion and how I do a warm up for the bench press…
Start out by loading the bar or dumbbell with half the weight of your maximum weight. So if you are benching 200 pounds, your warm up bench press will be 100 pounds.
10 reps is a good range for reps in any warm up set for me. The goal is to loosen up your muscles and get the blood flowing, not to fatigue your muscles.
Doing too many reps will cause growth in your muscles and fatigue it too soon, preventing you from doing your max in your workout.
Doing not enough reps will cause your muscles to remain cold and stiff, defeating the whole purpose of the warm up set. I found 10 reps to be the best rep range for my warm up. You may find 8 reps or 12 reps might fit in better with your warm ups. Experiment with different reps and see which one works out best for you.
The warm up is only meant to get you ready for the punishment and extreme intensity of your weight lifting workout. So, do only one set for the warm up and try not to rest between your warm up and the first sets of your workout. Resting too long will cause the muscle to get tight and defeat the whole purpose of the warm up.
For any warm up involving weights, you should try to duplicate the exercise you are about to do and perform that exercise with with less intensity and weight load. This will work a lot better then if you stretch before the workout as a warm up. I am sure you were told to stretch before any physical activity but when it comes to weight lifting, stretching is done after the workout as a cool down.
After an intense workout, the blood is rushing into your muscles and you will feel sore and swollen. Your sore muscles will be in need of nutrients, oxygen, and protein all of which is carried through the blood stream. Stretching will help the blood go to your sore muscles faster.
You will not want to stretch too much, but just enough so you start to feel the tension in the muscle. I like to do my cool down stretch within the first 2 minutes after my workout. The muscle is still loose at that point and waiting too long will make the muscle tight. It is better to stretch a loose muscle then a tight one, which is why in weight lifting, it is better to stretch before a workout.
Cheers