Whenever you do any type of strength-training exercise, you're really asking two different parts of your muscle to participate. The "positive" phase of the exercise, which takes place while the muscle is lifting a weight (either lifting the dumbbell in a bicep curl or elevating the bar in a bench press), is called concentric contraction. The end of that motion is the point at which your arms are close to your chest (in the curl) or fully extended (in the case of the bench press).
The other part is the eccentric contraction, which is termed the "negative" phase of the exercise. This is the phase that calls
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upon muscle resistance as you slowly return the weight to its starting position.
Interestingly enough, it's just as important, if not more important and more physiologically demanding, to let the weight slowly return to the original position in the negative phase as it is to push or pull the weight, as the case may be, in the positive phase. Returning the weight slowly and with resistance on every repetition is important, because this is the phase that promotes greater flow of red blood cells to your muscles, which in turn builds greater strength.
Of course, another way you'll know your form is correct, in addition to using mirrors to check yourself as you exercise, is that you'll feel warmth, some fatigue, and a "burning" feeling at the end of each set for each muscle group. If you don't get this sensation, review your form and pay greater attention to what you see in the mirrors. They're telling you something. Your form may be poor, and if so, you may not be allowing a full complement of blood to reach your target muscles.
Though getting tired is not a sure sign that you're doing an exercise correctly, either feeling lactic acid accumulate in your muscles or not experiencing fatigue is usually is a sure sign that you're not doing it right.
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Bodybuilding requires commitment. It is a totally different lifestyle that entails letting go of old habits and adopting new ones. You cannot go into bodybuilding and be half- hearted about it. It is a test of strength, self-discipline and willpower. Start only when you are sure you can commit time, effort and energy. Learn tips like this one and 99 more.