I absolutely love this article:
There's more to workout intensity than simply moving fast or lifting heavy weight
June 11, 2012
If you’re doing intervals and your heart rate reaches the max range fast and slows down slowly, again that likely means you are not very fit. The faster your heart rate goes down, the fitter you are. If you experience lactic acid build up (that burning sensation in your muscles) at a lower heart rate, it means your muscles don't have enough oxygen.
When you are well trained, you are able to get a lot of work done. If you’re performing high resistance cardio (running fast), and your heart rate stays pretty low, you are fit. The fitter you are, the lower the heart rate at any given speed. On the other hand, if you can talk without being out of breath during your cardio; it does not qualify as an efficient workout.
Now, over to weight training. What qualifies as being intense? If your mind is drifting away to some tropical island, you are not working hard enough. Lifting intensely means you aren’t concentrating on anything else other than the weight and how it is moving. And an intense heavy workout does not mean moving the weight using lousy form! The heavier the weight, the more dangerous it is, so you have to be extremely cautious.
It doesn’t matter how much weight you use if you don’t know how to do it properly. Place any amount of weight at the right angle, and it can be very heavy. It's not the weight in itself that is heavy per say; it's what you are doing with the weight that makes the difference.
If you squat with a weight using bad form, for example, sure you’ll still build up your squat ... but you want to stimulate the right muscles, not just move the bar. Intense heavy training builds muscle, but so does sloppy form. You can cheat and still build muscle. However, the muscle built may not look as great as if you paid attention to details. (You’re also putting yourself at risk for injury.) It's not just lifting weights, it's also feeling the weight so it hits the right area. And ladies, if you want a tight round booty, you have to squat – properly.
Another example: when you are performing rowing movements for your back, you’re a lot stronger when you let your upper traps do most of the work. That does not give you a nice looking back, though! Lifting weights is not a "from a to b" movement.
Pull ups are the ultimate exercise for back development, but there’s more to this movement than simply getting your head above the bar. Check your posture. Are your shoulders hunched forward? Do your traps shrug? Are your arms in front of your body? Those are not "real" pull ups.
You can train to just lift some heavy weight, add a bit of muscle and likely injury yourself, or you can train smart. You make the choice. There are plenty of resources out there to help make sure you are doing it right, including right here on the Muscle & Fitness HERS website, so do your homework if you want the best results.