Bryan's response:
Your back is sore because your form was wrong. If your form was right, not only would your stomach be sore (instead of your back); but you probably wouldn't have been able to do 150 of them!
Much of exercising is ensuring that you maintain proper form. Doing so will not only get you better results more quickly; but it will also help keep you from injury. Follow these 7 rules to maintain proper form on any exercise, and watch the results!
The "core" is the group of muscles that comprise the abdomen. They include 4 muscle groups: the rectus abdominis, the internal and external obliques, and the tranversus abdominis.
You must always keep them engaged or "tight" when performing any exercise, as this gives you the stability required to do the exercise correctly. Imagine sucking your belly button in to the back of your spine to get an idea of how it feels. As you develop your abdominals, you will learn more and more how to engage all of these muscle groups.
When you lose "core integrity", you begin to use different muscles than you want to perform the exercise. In your situation, your loss of core integrity most likely led to an arch of the back which caused the muscle strain you are describing. I recommend you ice, rest, and stretch gently...
You must always maintain the proper range of motion. This means that you must allow for a full concentric and eccentric motion. For example, on a leg extension machine, this would mean that you fully extend your leg without hyperextending the knees, of course; and then allow the legs to bend all the way until the weight stack is about one inch from touching, which would result in a rest. Apply the same principle to all your exercises.
In the example of the rev. crunch, put your hands under your hips, cradling your back on the floor, and engage the core to keep it pressed against the ground. Then slowly extend the legs until you feel a slight pull in the stomach and you feel like you would lose core integrity if going any further (back would begin to arch). From there, pull back slowly to the beginning of the rep.
Always maintain a 3-4 second pace on the eccentric and the concentric phase of motion. So, when you go "up", you go through the proper range of motion in a smooth 3-4 seconds.
Follow this with a one second muscle flex (of whatever muscle you are working).
And then a 3-4 second "downward" motion. At the "bottom", wait for a second, allowing the bar (or whatever) to become perfectly still, so as to not carry any momentum. Begin again.
You must always isolate the joints above and below the muscle group being worked.
In the case of a Bicep curl,for example, you must isolate the shoulder (above) and the elbow (below). This means you carry out the ROM at the proper speed while ensuring that you do not move the elbow or the shoulder at all! Easier said than done!
Always lift a weight that becomes challenging at rep # 10. You should feel as if reps # 11-12 are difficult to complete. If you feel like you could have done 15-20 with proper form (see above), then you should increase the weight accordingly. If you are unable to complete the 12 with proper form, then you must lower the weight.
Large muscle groups: 8-12 sets of 12 reps.
Large muscles are: Chest, back, quads (thighs), hamstrings
Small muscle groups: 6-10 sets of 12 reps.
Small muscles are: biceps, triceps, shoulders (front, side, and rear delts, and traps), calves.
So, for example, you can do 2 exercises at 5 sets each to reach a total of 10 for the large muscle group; or you could do 3 exercises at 4 sets each, or 4 exercises at 3 sets each. It really doesn't matter as long as you have the muscle working with proper form for this number of reps.
Remember, muscle growth is all about time under tension!
Much of the exercise isn't how much weight you can lift, but how much you can lift properly. Constantly checking, tweaking, and perfecting your Form will allow you to get more results, faster. It's all about time under tension (for muscle growth), and if you follow the above tips, you should be able to perfect your form easily. Just remember, you can always do it better and tighter. It's a constant battle, but it does get easier (and it is rewarding!)