If you do any kind of strength training, which we recommend, or regularly play a sport, you should know about muscle balance. A key aspect of muscle balance is the fact that each muscle group has an opposing group with which it works, and it’s important to strengthen both groups to keep them in balance. Such workouts can help improve physical performance and reduce the risk of injury. Well-balanced muscles are also essential to good posture.
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).
- Gastrocnemius and tibialis anterior (on the front and back of the lower leg).
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 example, 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 a 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 women have a higher rate of knee injuries than men.
Because muscles must work not just by shortening when they contract (concentric action) but also when lengthening (eccentric action), strength training exercises should include both components. A study in the Journal of Orthopaedic and Sports Physical Therapy in 2010 found that people with an imbalance between the hamstring eccentric strength and quadriceps concentric strength have a four times increased risk of hamstring injury.
Balanced muscles are not necessarily equally strong. For example, the desirable strength ratio for 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 given muscle group can also cause problems. For example, balance among the four quadricepsmuscles helps with knee stability, while imbalances are associated with joint pain and cartilage damage, including osteoarthritis.
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 focus on exercises that build chest (pectoral) muscles, such as bench presses, and thus have overdeveloped “pecs.” But they tend to have relatively weak shoulder (rotator cuff) muscles, which then may be more prone to injury. People often do more exercises or daily activities that work the muscles in the front of the body, such as the quadriceps (when climbing stairs) or the biceps (when lifting heavy packages). It’s also normal to use the muscles more on your dominant side.
If you belong to a health club or gym, get advice from a personal trainer who can evaluate your current workout and help you choose exercises—for strength and flexibility—that will keep opposing muscle groups in balance. Or if you have an overuse injury or biomechanical problem, seek the care of a physical therapist or athletic trainer. A well-designed strength-training program should work all the major muscle groups.
Also see The Power of Strength Training for general advice about strength training.