ACTION AND REACTION:

Muscles work in groups to produce movement. The muscle or muscle group that contracts and gets shorter (to move an arm or leg, for example) is referred to as the agonist. Its partner, the antagonist, lengthens to allow this movement. For example, when you do a biceps curl exercise (bending your elbow), the biceps is the agonist, while the triceps is the antagonist that lengthens during this motion. This reverses when you do a triceps press (straightening your bent arm): The triceps is the agonist that contracts and shortens, while the biceps lengthens.

HERE ARE SOME MAJOR MUSCLE PAIRS:

* Quadriceps and hamstrings (on the front and back of the thigh, respectively).

* Biceps and triceps (on the front and back of the upper arm).

* Tibialis anterior and gastrocnemius (on the front and back of the lower leg).

COMMON IMBALANCES:

If one muscle group is much stronger than the opposing group, this can disrupt the natural range of motion of the joint and lead to injury. For instance, if you have very strong biceps but weak triceps, this can limit your ability to fully extend your elbow. Tight, overdeveloped chest muscles and neglected back muscles can not only limit shoulder range of motion but also negatively impact your posture, resulting in a rounding of your shoulders. Tight hip flexors inhibit the opposing buttock (gluteal) muscles, which can result in destabilization of the lower back. If your lower back muscles are much weaker than your abdominal muscles, or vice versa, you're likely to have bad posture and possibly back pain.

Runners, especially sprinters, usually have highly developed quadriceps but less-developed hamstrings, which can lead to a hamstring sprain or knee injury. Similarly, women tend to have stronger quadriceps and weaker hamstrings, which is one reason why they have a higher rate of knee injuries than men.

Balanced muscles do not necessarily need to be equally strong, however. For example, the desirable strength ratio quadriceps and hamstrings is 3 to 2. That is, if you can lift 60 pounds with your quads, ideally you should be able to lift 40 pounds with your hamstrings.

Imbalances within a muscle group can also cause problems. For example, balance among the four quadriceps muscles helps with knee stability, While imbalances are associated with joint pain and cartilage damage, including arthritis.

STRIKE A BALANCE:

Many people get in a rut when they work out. When they train with weights, they tend to work on muscles that are already strong and skip the weak ones. For instance, many men focus on exercises that build chest (pectoral) muscles, such as bench presses, and thus have over-developed "pecs." But they tend to have relatively weak shoulder (rotator cuff) muscles, which then may be more prone to injury. People also often do more exercises or activities that work the muscles in front of the body, such as the quadriceps (when climbing stairs) or the biceps (when lifting packages). It's also normal to use muscles more on your dominant side.

If you belong to a health club or gym, get advice from a trainer who can evaluate your current workout and help you choose exercises -- for both strength and flexibility -- that will keep opposing muscle groups in balance. If you have an overuse injury or biomechanical problem, seek the care of a physical therapist. A well-designed strength-training program should work all the major muscle groups.