Concentrating on one area of the body is great for building taut thighs, six-pack abs and a rock-solid chest, but how do you know if you're overdoing it?

Exercise gurus are always telling you to "feel the burn," but that burning sensation can swiftly become chronic pain if you don't exercise properly. Aimed exercise is supposed to target a specific muscle or group of muscles that has given you problems in the past, or that can help you excel at a sport you love. However, too much concentration is bad for the body.

After all, how much good will you be lying in a bed or lounging on the couch watching Judge Judy? Before you start an aimed exercise program for your legs, buttocks, abs, chest or any other part of your body, consider your starting threshold. How much pressure can you take before you crumple to the floor in pain? The exercise program you devise should never take you to that point, and you shouldn't feel too stiff to move in the morning when you wake up.

A better idea is to start small, with aimed exercise that doesn't tax you much at all. You'll start to get a feel for how much you can handle, at which point you can gradually increase your reps or weight, depending on the exercise. For example, if 25-pound weights feel light as a feather when you're doing shoulder presses, increase the weight to 35 pounds for two sessions, then go up to 40.

Also, aimed exercise is not meant to be used every single day. You shouldn't be focusing entirely on your abs seven days a week, for example, because you'll overdo it and neglect other important muscle groups. Instead, try spreading out your aimed exercise program five days a week, leaving one day for abs, one day for thighs, and so on.

Or, if you prefer, do aimed exercise on that one stubborn muscle group two days a week, then leave another three for cardio and more well-rounded exercise. The goal is to strengthen or tone a particular muscle group, while furthering your health and fitness in other areas as you progress.

If you adopt a healthy exercise routine that doesn't leave you exhausted and in pain, you'll be more likely to stick with the program for the rest of your life.

Exercises for your arms and back will become easier if you also focus on your shoulders, building strength and flexibility.

Without strong shoulder muscles, it can be difficult to properly execute even normal, everyday movements, let alone sports or other athletic activities. When your shoulders are flexible and strong, you'll have an easier time with things like lifting, pushing and pulling.

Shoulder exercises should involve all of the muscles associated with your shoulders, including the anterior deltoids, the lateral deltoids, the posterior deltoids and the supraspinatus (rotary cuff). These muscles work in unison to provide your shoulders with maximum elasticity.

One of the most popular shoulder exercises is the shoulder press, which can be executed either with dumbbells or with a barbell. Hold the bar(s) so that your hands are slightly shoulder-width apart in an overhand grip. Starting with the bar(s) at chest level, push up until your arms are nearly straight overhead, then lower them slowly again to chest height.

In a variation of the shoulder press exercise, hold a barbell behind your neck, then raise the bar slowly until your arms are nearly straight, lowering again. This focuses more on the anterior deltoids and should not be attempted without a spotter.

If you'd like to focus on the lateral deltoids, you might try the upright row in your regimen of shoulder exercises. With this movement, grasp a barbell or two dumbbells in an overhand grip, with your hands approximately shoulder-width apart. Then lift your hands until they are tucked just underneath your chin, and lower slowly back to waist-height. Your wrists will flex into a "puppy dog" position on the upswing of this movement.

And finally, for your rotary cuff, this shoulder exercise can be done either lying down or standing up, using one dumbbell at a time. When lying on your side, rest your head on your bottom hand, elbow extended above your head, and separate your legs so that the top ankle is about one foot behind the bottom ankle. Hold a dumbbell even with your hip, then slowly raise it until your hand is above your head. Lower slowly. For best results, concentrate on bringing your hand slightly behind your head on the upswing.

These shoulder exercises should make your more limber and supple in other exercises, and will increase both strength and flexibility.