Good form is absolutely necessary to develop the best looking muscle shape.
Here are the 5 aspects of Good Form:
Negatives should always be slower than positives. The negative is when you are returning the weight to starting position. You are stronger in this part of the repetition so do negatives slowly.
Range of motion: Concentrate on moving the weight through the pathway where you feel a pump in the muscles you want to emphasize. This keeps emphasis on the target area.
Breathe: Inhale and exhale on each repetition, do not hold your breath. Count each repetition to yourself. Exhale as you finish each positive, inhale on the slower negative.
Rhythm: Each repetition should be rhythmic, smooth and not jerky.
Stretch between each set, at least 15 seconds.
The better your form, the more shapely your muscles will develop. Each exercise you perform in your routine is meant to isolate a specific area. If you cheat and use loose form, the effect of the exercise be- comes more global and is spread over a wider area of the body. Doing cheating barbell curls can work your lower back and deltoids more than your biceps. So use only enough weight to feel it directly in the muscle. Muscles only need so much weight to develop. Using extremely heavy weights stresses joints, tendons, and ligaments as much or more than it develops the muscles.
Everyone knows it’s easier to lower a weight than to lift it. This is due to gravity. If you lower your weights quickly you lose the muscle building effect of the negative. The negative is the eccentric part of the movement where the muscle fibers stretch back to normal. If you resist gravity by doing your negatives more slowly, you will stimulate the muscles more deeply and induce more growth. Because muscles stretch back to normal length non-uniformly you are about 40% stronger on the negative.
The positive or lifting part of an exercise has in the past been considered the most important part of an exercise movement. So body-builders did fast negatives so they could lift more on the positive phase. This did build muscle but, as I found out after training this way for some years, can lead to injury. Doing faster negatives, bouncing the weight each rep can throw the weight out of the groove and you can cause pain. Slower negatives keep the weight in the correct pathway more efficiently lessening the chance of injury.
Slower negatives make lighter weights feel heavier, so you don’t have to use a ton of weight to build muscles.
“If you want your muscles to grow, you have to do your negatives real slow.”
The positive phase of a repetition should be done with enough force to bring the weight to the end of the range of movement you want on the exercise. During the first few reps of a set, this may not mean too much effort, but for the last few reps it usually does. If all your positives are easy, as in super slow rep training with higher repetitions, you lose the effectiveness of the negative because you don’t use a heavier weight on the positive. This means your negative will naturally be lighter and build less muscle.
If you are using heavier weights that only allow a few positive reps (I don’t recommend it at this stage of the program) your positives will naturally be slower because it’s harder to push (or pull) the heavier weight. Do your positives in a controlled explosive fashion, with just enough force to go to completion, then do a slower negative.
In order to isolate a muscle you are working sometimes it is necessary to do partial repetitions. A good example of this is the front pulldown. If you extend your arms fully, locking out the elbows, you transfer the effect of the exercise from lats to deltoids. Since you are doing front pulldowns to work your lats, don’t bring your deltoids into the equation by extending your arms fully. You’ll get a better pump in your lats if you do pulldowns this way.
Success in bodybuilding depends on how good your form is. An increasingly skilled performance of training movements over time will provide improvement in your muscular development.