Only losing fat from around your thighs will define your quads. And that’s done primarily through dietary measures.
You could do countless leg extensions, but without the right dietary measures you’ll never have defined thighs. On the other hand, you could define your thighs without ever doing a single leg extension, provided you adopt the right dietary measures for long enough.
Most typical bodybuilders struggle to have sufficient mass in their quads. Leg extensions aren’t going to add any mass of significance, if any at all, to the quads of typical hard-gaining bodybuilders.
If you make the barbell squat your sole quads exercise, and provided you perform it correctly, you’ll do way more for your quads than leg extensions ever can, no matter how good a leg extension machine you use, how many sets you perform on it, or how precisely you perform those sets.
If you can’t do the conventional barbell squat effectively, try the parallel-grip deadlift. The parallel-grip deadlift has proven to be a godsend for many bodybuilders who don’t have the leverages to squat well.
Most bodybuilders who can’t perform the barbell squat well, use the leg press. And many who do have the leverages to squat well, but prefer a less taxing exercise, perform the leg press.
A good leg press machine, used properly, produces a good exercise that’s much more effective for building muscle than the leg extension, but it’s not a match for a properly performed barbell squat or parallel-grip deadlift.
Pulling the bar to behind your head. This is an unnatural, awkward motion. It causes the head to crane forward to some degree, puts the shoulders in a poor position, and makes it more difficult than it should be to fully retract your shoulder blades in order to fully involve your lats and other back musculature.
Do the exercise to the front. That’s much more comfortable, and makes it much easier to fully retract your shoulder blades so that you more readily work your lats properly. (Extend your neck just sufficiently so that you don’t strike your head with the bar.)
And if you use a supinated grip about shoulder-width, or a little closer, that may enable you to train your lats better still, for improved muscle-building results. The supinated hold enables you to have a greater range of motion than a pronated hold.
Yes, provided it’s strong enough, and that the attachment you use between the resistance and the belt—strap, chain, or rope, for example—is strong enough. Up to about 50 pounds or 25 kilos should be fine, but beyond that I recommend you use a proper dipping belt made from thick leather and with a purpose-made chain attached to it. A padded area at the inside rear of the belt provides increased comfort.
But very heavy weights are tricky to load on a belt, and the very heavy load pulling down on the spine during the exercise may be harmful, and are best avoided. A shoulder harness would be a safer option, or a dip machine.
For the conventional parallel bar dip, if you don’t have a proper dipping belt, or even a proper lifting belt, there are alternatives for adding small weights. One of them is to wear any strong, leather belt normally used for trousers, and put a dumbbell inside it through having the ’bell vertical and the belt across the handle. Another option is to use a piece of strong rope to attach a dumbbell or plates to that belt. Let the resistance hang at the front of your thighs. If that’s uncomfortable, try suspending it from the rear.
Strength permitting, add weight slowly and in small increments. To work from one fixed-weight dumbbell to the next, attach one or more small discs, or use an adjustable dumbbell. Alternatively, use individual weight plates, with a belt fitted through them before being buckled.
Done properly, the parallel bar dip is an excellent exercise—one of the very best for the torso.
But don’t use an excessive range of motion. Never exaggerate the depth of motion.
For most bodybuilders, the safe full range of motion is where the rear arm is parallel with the floor, or a little beneath that point. Some bodybuilders can safely dip below the parallel position, whereas others need to stop a little above parallel.
Never descend on a deflated chest. Inhale before you descend, then exhale during the ascent.
Going into the bottom position of the dip on a deflated chest increases the risk of injury to the rib cage, so keep your chest full during the descent and early part of the ascent.
With the machine dip, a slight change in torso or wrist position can produce significant improvement in comfort, and the range of movement can be easily controlled. In addition, because resistance starts at little or nothing, the machine unit can be used by trainees who don’t yet have sufficient strength to do regular parallel bar dips.
For everything you need to know about exercise program design, and exercise technique, please see here.