The hamstrings serve many purposes in the body. From flexing the knee and extending (and hyper-extending) the hip to providing 
power and speed, the hamstrings play an important role in proper functioning for daily activities and sports. The flexibility and strength of this group of muscles – the bicep femoris, semitendinosus and semimembranosus – also serve a major part in preventing knee injuries.
So why do hamstrings get tight? One single answer doesn’t fit each person or their particular situation or reason for having tight hamstrings, but chances are, your hamstrings are tight from one of the following issues:
Weakness
Weak hamstring muscles cause daily activities like walking to be much more difficult than they should be. Jogging, running, or sprinting with weak hamstring muscles will prove to be very difficult and your chances for injury are almost guaranteed. The over-compensation and over-exertion of these weak muscles will cause them to tighten up.
Limited ROM
A limited range of motion, or ROM, will cause tight hamstrings. Most everyday activities like sitting at a desk or walking don’t take the hamstrings through their full range of motion. As mentioned before the hamstrings are responsible for flexing the knee, or lifting the heel up toward the buttocks, and extending the hip. When you walk, you rarely lift your heel all the way up toward your buttocks. This motion is done more in running. When you sit at a desk all day or spend a lot of time driving, your hamstrings remain in a shortened position with your knees bent. This limited ROM prevents your hamstring from rarely being lengthened.
Overuse
If you workout on a regular basis, your hammys may be screaming at you to stretch them back out to their normal length. Listen to your body. If you are constantly sore, take a few days off from lifting or running to let your body recuperate.
Lack of Flexibility
Flexibility is the ability for your muscles to remain at their normal length during daily activities and during workouts or sporting events. A consistent stretching routine is the only way to achieve true sustainable flexibility and prevent injury.
A Secret Hamstring Tightener
You may fall into the category of those whose hamstring are tight due to tight hip flexors. This may be a head-scratcher, but try to visualize the following scenario:
If you are constantly stretching your hamstrings, but you can’t seem to get them from feeling tight, your hip flexors (the muscles on the front of your legs below your abs responsible for lifting your knee up toward your chest) may be to blame. The hamstrings cross the knee and hip joint. If your hip flexors are extremely tight (another downfall of sitting too much) it can cause your pelvis to anteriorly tilt, or tilt forward, subsequently lifting your hips up in the back, tightening your lower back and stretching your hamstrings in way that causing them to “hang” in a suspended form. Your hamstrings may actually be lengthened, but they feel tight because of the tightness in your back and glutes.
The trick to eliminate this secret hamstring tightener? Stretch out your hip flexors and see if your hamstring don’e relax to their normal position as your pelvis return to a neutral position.
