Quality of Quantity - WHY YOU'RE STUCK
Most of us stick true to the idea that quality is better than quantity in many aspects of our lives...
WHY do we do the opposite when we step into the gym? I have seen very very few people in the gym that exert the amount of concentration and focus it takes to maximize your efforts in the gym and in the cases that do, they can typically do half the amount of "reps" as their counterparts and acheive greater results. Below I have broken down the three portions of a reptition that apply to every rep and how you can maximize each one.
Improve your training.
ECCENTRIC is the "muscle lengthening" part of the rep where you're controlling/lowering the weight. In a bicep curl you're lowering the weight. In a squat you're lowering the weight. In a lat pull down you're controlling the weight back to the top. Regardless, you're stronger eccentrically than you are isometrically and concentrically which allows for some cool "overload" strategies with emphasized eccentric training. Worth mentioning, the eccentric is where most of the muscle damage (the good kind) happens. Issue is, most people speed through the eccentric and/or cut the range short. If you really want to get the most out of your reps you need to go slow and controlled and "own" the eccentric. No cheating.
ISOMETRIC is the "non-moving" part of the rep where your muscles are contracting but not generating movement. This could be for the brief moment in between the eccentric & concentric. Or there are some training methods which specifically focus on isometric contractions for time. Gymnastics, for example, has tons of isometric holds (think of the iron cross and flag pole). Lots of benefits to be had with isometrics when used properly.
CONCENTRIC is the "muscle shortening" part of the rep where the muscle is physically contracting. In a bicep curl it's raising the weight. In a squat it's standing up with the weight. In a lat pull down it's pulling the weight to your chest. Regardless, this is the portion of the rep most people cheat on and use too much momentum. Remember, the goal is NOT to move as much weight as possible. It's to control as much weight as you can through a full range of motion. Use your muscles -- not momentum -- and you'll get wayyyy more bang for your buck.
Is one part of the rep better than the others? No. They're all important and should all be utilized within your training.