I know I call this ‘muscle in focus’ but the hamstrings is really a group of muscles, three to be precise. At the back of the thigh, between the hip and the knee, semimembranosus, semitendinosus and biceps femoris all have two main actions, extension of the hip and flexion of the knee. They play a crucial role in standing up, walking, running, jumping… and are the major antagonist of the quadraceps. 
They are also the most injured muscles in the body.
Hamstring muscle injury is a common athletic injury, particularly for runners, running & jumping athletes like footballers, long & high jumpers, and dancers.
The most common cause of a muscle or tendon strain is overuse, which weakens the tissue fibers. Muscles and joints may also be forced to perform movements for which they are not prepared or designed, over stretching and potentially damaging the surrounding muscle or tendon. An injury can occur from a single stressful incident, or it may gradually arise after many repetitions of a motion. 
The word “ham” is derived from the Old English ham or hom meaning the hollow or bend of the knee, from a Germanic base where it meant “crooked”. … String refers to tendons, and thus, the hamstrings are the string-like tendons felt on either side of the back of the knee.
Injuries to the hamstring group of muscles can range from a minor strain to a major rupture. A minor or grade I tear to a hamstring will most likely heal with a small amount of physical therapy, such as massage and dry needling. A major, grade III tear or full rupture could be a major disruption to an athletes sporting career and may require surgery.
Even someone at an Olympic level like sprinter Usain Bolt is not immune to injury, tearing his hamstring in the World Athletics Championships earlier this year.
How do they happen?
At the knee, these muscles act to slow down your leg as it swings forward very quickly during in preparation for footstrike. It’s this action that commonly leads to injuries because the muscle is trying to shorten and contract, but the knee is extending very quickly, which pulls on the hamstring and creates a tremendous amount of strain.
At the hip, these powerful muscles generate force just prior to the foot coming off of the ground as they extend the hip backwards. Injuries can occur at this point of the running cycle because these muscles are generating tremendous amounts of force to maintain, or increase, forward running velocity. 
What are the symptoms?
You don’t have to be an elite athlete to injure the hamstring. Over use, misuse and lack of stretching or massage can put you at risk. Even tight quadraceps can contribute to tight hamstrings.
Symptoms of a strained or torn hamstring could include sudden and severe pain during exercise, along with a snapping or popping feeling in the area, pain in the back of the thigh and lower buttock when walking, straightening the leg, or bending over; tenderness; bruising in the area. 
In severe cases, you can tear your hamstring from its point of origin on the ischial tuberosity (sit bones). This is called a hamstring origin avulsion. In extreme cases, an avulsion fracture will occur where the hamstring muscle tears the bone where it attaches off your pelvis. This happens after a sudden and forceful eccentric contraction of your hamstrings with your hip flexed, for example, when hurdling or performing the splits. When this occurs, it is necessary to undergo surgery in order to reattach the bones. 
How is it treated?
If you feel you have strained or torn a hamstring muscle, the first thing to do is R.I.C.E.R. – Rest, Ice, Compression, Elevate and Refer, as in refer to a health care practitioner.
The risk of hamstring injury can be reduced with a regular stretching program and exercises. However, a strained or torn hamstring will need soft tissue therapy, some biomechanical assessment to find the cause of the injury, heat & ice therapy and stretching. Once the pain has eased, the muscle will benefit from gentle exercise and a strengthening program.
Static hamstring stretch
Lying on your back, raise your leg straight up until you feel the tug of the muscle. Don’t over stretch or force the stretch. If you feel numbness in your foot, ease the stretch off a fraction until the numbness subsides.
Breathe deeply and hold the stretch for 30 seconds. Lower the leg and rest for 10 seconds. Repeat each leg 3 times.
If you need to support the leg, use a towel or belt looped around the arch of the foot.
To increase flexibility
Lie on the floor in a doorway or close to a corner of a wall.
Place one leg up against the wall, the other is straight on the floor and your buttock is as close as possible to the door frame or wall.
Extend the elevated leg and pull your toe own as much as possible until you feel a stretch behind your thigh.
Hold the stretch for between 10 and 30 seconds.
If you’re a particularly flexible person, but still have tight hamstrings, you could benefit from an active stretch.
Exhale, straighten the knee while lifting up on the theraband, until extended.
Lower the leg and repeat 5 times. Finish with a static stretch.
Place a foam roller on the floor and sit on the floor supporting yourself with your hands.
Place the back of one thigh over the foam roller and the other leg on the floor for support. Support your core and have proper low back posture during the exercise.