The hamstrings consist of three main muscles -- biceps femoris, semitendinosus and semimembranosus -- extending from your sit-bones to your knees. These muscles are responsible for bending your knees. If they’re strong, you can jump high, run fast and accelerate with explosive power. With well-developed hamstrings, you can maintain good posture and prevent leg injuries.
Because your hamstrings cross the knee and hip joints, they work to bend your knees and draw your hips backwards. When executing explosive movement, these muscles play an important part in shifting the load from your knees to your hips. They also contribute to your ability to absorb the shock of movements involving high velocity or force. While your hamstrings contribute to functional motion, such as walking, they help you to achieve speed, power and agility in many sports. For example, a sprinter’s performance pivots on strong hamstrings.
During the course of a contraction, your hamstrings shorten and lengthen. While the concentric phase, or shortening, allows you to bend your knee, the eccentric phase, or lengthening, of the muscle can help you to control the deceleration of your body or legs in motion. For example, when you’re running downhill, the lengthening of your hamstrings helps you to control the speed of the descent. The ability to properly decelerate lowers the amount of pressure on the joints in your lower body and prevents injury.
Posture and Alignment
Strong hamstrings work to stabilize your hips and keep your spine properly aligned. Envision your skeleton system as a connected chain. If one link moves out of alignment, problems ripple throughout the chain. If your hamstrings are weak and tight, they’ll tug on your hips, tipping them forward, and compromise functional movement. A swayback posture can result in which your lower back arches and shoulders round. Your hamstrings also keep your knee and surrounding connective tissue in alignment.
Since your muscles operate in pairs, the development of each muscle in this complementary relationship must balance. If one muscle is stronger than the other, it can lead to injury. For many athletes, the muscles in front of their legs, or quads, are more powerful than the hamstrings. Because weak hamstrings limit both range of motion and explosive power, this imbalance affects an athlete’s ability to jump, run, land and quickly change direction. It also makes an athlete vulnerable to injury, such as ligament tears. In particular, female athletes depend more on their quads to stabilize their knees.