Your upper legs consist of two opposing muscle groups: hamstrings (sometimes jokingly referred to as "hammies") and quadriceps, or “quads.” Together they move your knee and hip joints and stabilize your knee. However, your hamstrings tend to be weaker than your quads, a situation that increases your risk of injury.

Anatomy of Quadriceps and Hamstrings

The quadriceps and hamstrings are the large, dominant muscles in your thigh. The biceps femoris, semitendinosus and semimembranosus muscles make up your hamstrings and are located on the back of your thigh. Your front thigh muscles, or quads, are the rectus femoris, vastus medialis, vastus intermedius and vastus lateralis.

Your hamstrings extend your hip and bend or flex your knee, whereas your quads flex your hip and extend your knee. Both muscle groups stabilize your knee, especially during weight-bearing activities.

Read more: Things to Do to Heal Sore Hamstrings

Prevalence of Hamstring Imbalance

The normal hamstring to quadriceps strength ratio is between 50 and 80 percent, with 100 percent being equal strength, according to the Journal of Athletic Training. This means the hamstrings are usually weaker than your quads. The strength imbalance between your hamstrings and quads is partly due to your quads being larger and used more frequently during daily activities; tight quad muscles can also contribute to imbalance.

Women typically have lower hamstring and quadriceps strength ratios compared to men. The Journal of Strength and Conditioning Research reported that female athletes may have an even lower hamstring to quadriceps ratio because of dominant quads, although this can vary, depending on the sport pursued. Age does not appear to impact the hamstring to quadriceps ratio; instead, strength decreases equally in both muscle groups with aging.

Imbalance Increases Risk of Injury

A strength imbalance between your hamstrings and quads increases your risk of injuries, such as muscle strains and ligament sprains. This is especially true for women, who are up to six times more likely to suffer a knee injury compared to men. The risk is even greater for women athletes, who are more likely to be quad-dominant.

When you contract your quads to extend your knee, for example, your hamstrings act as the antagonist to control the movement and stabilize your knee. If your hamstrings are weak, the contraction of your quads and knee extension may be too forceful, causing damage to your joints, muscles or ligaments. Weaker muscles also fatigue more quickly and therefore may result in greater strength imbalances and further injury.

Exercises That Bring Balance

Targeted exercises help increase your hamstring strength and improve your hamstring-to-quadriceps strength ratio, reducing your risk of injury. Hamstring exercises include resisted knee curls, bridges, hip extensions and the Nordic hamstring curls.

Perform the Nordic hamstring curl by kneeling on both of your knees with someone holding your ankles. Slowly lower your torso toward the floor using your hamstrings. For an exercise that is easier and doesn't require assistance from another person, try the sliding hamstring curl. Hamstring curls can also be done with machine assistance. Finally, stability exercises — such as single-leg balance, single-leg squats and lunges — are also important in improving coordination between your hamstrings and quads.

Read more: Quadriceps Exercises Using Your Body Weight