A hamstring strain is caused by a rapid broad contraction or intense stretching of the hamstring muscle group, which results in high mechanical stress. This results in varying degrees of rupture within the fibers of the tendon unit. 
Hamstring strains are common in sports of a dynamic nature, such as sprint jumping contact sports, such as Australian Rules Football (AFL) American football and soccer, where rapid eccentric contractions are regular. In football, this is the most common injury. . Hamstring injuries can also occur in Recreational sports, such as water skiing and bull riding, force the knee into full extension during the injury.
The hamstrings are made up of three muscles:
- The biceps femoris
- The semitendinosus and
- The semimembranosus. 
For a more detailed look at the anatomy of the hamstrings, see the article on the hamstrings.
The cause of a hamstring muscle strain is often unknown. During the second half of the swing phase, the hamstrings are at their maximum length, when they develop maximum tension . During this phase, the hamstrings contract eccentrically to slow flexion of the hip joint and extension of the lower body legs . At this point, the muscle spindle activity of the hamstrings is at its peak. Requires a strong contraction of the hamstrings and relaxation of the quadriceps. According to “Klafs and Arnheim”, the breakdown of coordination between these opposing muscles may be Resulting in a hamstring tear . The greatest muscular tendon stretch is caused by the biceps femoris, which may cause it to be more prone to injury than the other 2 hamstrings (semimembranosus and semitendinosus) during high-speed running. 
Predisposing Factors/Risk Factors
There are various risk factors that may play a role in hamstring injuries:
- Older age
- Previous hamstring injury
- Limited hamstring flexibility
- Increased fatigue
- Poor core stability
- Strength imbalance
- Previous calf injury
- Previous substantial knee injury
- Osteitis pubis
- Increased quadriceps flexibility was inversely associated with incidence of hamstring strains in a cohort of amateur Australian Rules football players
- Players presenting certain polymorphisms of IGF2 and CCL2 (particularly their allelic form GG) may be more susceptible to serious injury and should participate in specific prevention programs
- Tight hip flexors
- Previously associated lumbar abnormalities. Kicking and abdominal strengthening exercises with straight legs have been identified as possible causes of lordosis. Anatomical reasons appear to be that the iliopsoas group is primarily involved in kicking and Straight-leg raises or straight-leg sit-ups help strengthen this muscle’. Therefore, certain physical activities and training methods that exacerbate postural deficits may also predispose athletes to injury. 
During activities such as running and kicking, the hamstrings lengthen with hip flexion and knee extension, and this lengthening can reach the muscle’s mechanical limit or lead to the accumulation of microscopic muscle damage.  Possible hamstring injury Potential uncoordinated contraction of the biceps femoris muscle secondary to dual nerve supply. 
Another debate concerns changes in the hamstrings in the muscular structure. The short head of the biceps femoris (BFS) has a longer bundle (allowing greater muscle extensibility and reducing the risk of over-stretching during eccentric contractions) and a longer bundle than Long head of biceps femoris (BFL). While the BFL presents shorter fascicles compared with the BFS, the BFS undergoes repetitive overlengthening and cumulative muscle damage.
Excessive forward pelvic tilt lengthens the hamstrings, which some research suggests may increase the risk of strain injuries. 
A hamstring strain can cause sudden onset of mild to severe pain in the back of the thigh. The impression of bursting or tearing can also be described.  Swelling and bruising may sometimes occur, but they may be delayed for several days after the injury. Rarely symptoms are numbness, tingling and Distal extremity weakness. These symptoms warrant further investigation for sciatic nerve irritation. Large hematomas or scar tissue can result from complete tears and avulsions.
Other possible symptoms:
- Loss of motion
- Decreased isometric strength
- Decreased length of the hamstrings
Hamstring strains are divided into 3 groups based on the degree of painful weakness and loss of motion.
- Grade 1 (mild): Only a few muscle fibers are damaged or broken. This rarely affects muscle strength and endurance. Pain and sensitivity usually occur the day after the injury (varies from person to person). Normal patient complains of posterior stiffness leg. The patient can walk normally. There may be slight swelling, but the knee can still bend normally.  
- Grade 2 (moderate): About half of the fibers are torn. Symptoms are acute painful swelling and mild loss of function. The patient’s walking will be affected. Pain can be reproduced by exerting precision on the hamstring muscles or bending the knee against resistance.  
- Grade 3 (severe): From more than half of the fibers ruptured to complete muscle rupture. Both the muscle belly and the tendon suffer from this injury. It causes great swelling and pain. The function of the hamstring cannot be exerted, and the muscles show Great weakness.  
When examining a patient, a physical therapist may have to differentiate between different injuries such as
- Adductor muscle strain and avulsion injury Lumbosacral referred pain syndrome Piriformis syndrome Sacroiliac dysfunction Sciatica Hamstring tendonitis and sciatic bursitis. 
- Other sources of hamstring pain may also be confused with a hamstring strain and should be considered during the examination. Specific tests and imaging are used to assess and rule out the possibility of those different pain sources. 
- Restricted sciatic nerve mobility can lead to pain in the back of the thigh, and in some cases, adverse nerve tension may be the sole source of pain without any specific muscle damage. In some cases, it can be difficult to determine whether it is the hamstrings or another muscle group such as the glutes Injured adductor muscles due to proximity (eg M. Gracilis and M. Adductor Magnus and Longus.). Imaging procedures may sometimes be needed to determine the exact location of the injury. 
- Other conditions with a similar presentation to hamstring strains include popliteus strain tendonitis at either origin of the gastrocnemius muscle, posterior cruciate ligament apophysitis-ischial tuberosity pain, lumbar spine disease, and superior tibiofibular joint injury. 
Most acute injuries can be easily detected by having the patient tell how the injury occurred. To be sure, they’ll have to do some research on the hamstring, too.
When the therapist is not sure, he can order medical imaging. This would rule out all other possibilities. 
- X-rays: One of the benefits of X-rays is that through this imaging, the cause of the pain can be distinguished. It can differentiate between muscular disorders, such as muscle strains, or bone disorders, such as stress fractures. 
- Ultrasound (US): This imaging method is heavily used because it is an inexpensive method. It’s also a great approach because it enables dynamic imaging of muscles. The disadvantage of ultrasound is that it requires a skilled and experienced clinician.  
- Magnetic resonance imaging (MRI): MRI provides a detailed view of muscle damage. But sometimes it may not be clear according to the image. If this occurs, the therapist must rely on the story the patient tells him (see Characteristics/Clinical Presentations).  
MRI studies are performed to distinguish two main groups of muscle damage: direct or indirect.
- In the group of injuries caused by indirect trauma, the classification introduces the concepts of functional and structural damage. Changes in functional muscle damage occur but there is no macroscopic evidence of fiber tearing. These lesions have a variety of causes and are divided into the following subgroups Reflecting its clinical origin, such as overload or neuromuscular disease. Structural muscle damage is those where MRI studies show macroscopic evidence of fiber tearing, ie structural damage. They are usually located at the tendon junction because these areas are biomechanically weak point.  Some studies have screened patients after hamstring injuries and they have concluded that normalization of this increased signal intensity in MRI does not appear to be necessary for a successful return to play (RTP)  .