Hamstring Sprain / Tendinopathy
Hamstrings are a group of muscles present at the back of thigh. They extend from the sit bones in pelvis to just below the knee joint and play an important an important role is daily activities such as walking & running. The names of the individual three muscles included in hamstrings are semimembranosus, biceps femoris and the semitendinosus. These, work in opposition to the muscles in the front of the thigh (quadriceps) and the two group of muscles together stabilize movements of the knee and pelvis.

INCIDENCE
Hamstring injuries account for approximately 12–16% of all injuries in athletes. They are seen more commonly in sports that involve sprinting, acceleration, deceleration, rapid change in direction and jumping such as football, basketball, rugby and baseball. Runners, ballet dancers and older adults who do a lot of walking are also at increased risk. Reinjury rates are high and generally require more time away from the field. Although any injury for a sportsperson is painful, this one can be quite frustrating for both the sufferer and the treating physician.

SYMPTOMS
Symptoms vary depending on the severity of the problem. When the upper part of hamstrings is involved the most common symptom is deep buttock pain or irritation at the back of thigh (minor sprains) associated with tightness or cramping sensation. Pain can radiate down the back of thigh and is generally aggravated by physical activity such as walking, running uphill, high speed or long distance running, leaning forwards, squats and sitting on a firm surface for long duration. Morning increase in severity of pain is not uncommon in this condition. In the early stages pain may reduce after warm up and then recur after activity. This changes with time with the pain persisting throughout the day.

Partial or complete tear of hamstrings may present with severe stabbing pain, bruising with inability to weight bear or walk. Sciatic nerve is present close by and its irritation can cause pain to radiate further down the leg. In severe cases the tendon may completely tear away, often taking a piece of bone with it and this is addressed as avulsion injury.

RISK FACTORS & BIOMECHANICS
Muscle attaches to the bone with the help of a special type of tissue called tendon. To simplify, you can look at these as ropes tying the muscles to the bones. Injuries can involve the muscles or the area where the muscle transforms into the tendon (myotendinous junction) or the tendon itself. Generally the closer the injury is to the pelvis / sit bones the longer it takes to heal. Of the three muscles, biceps femoris is the most commonly injured one.

The hamstrings cross two joints and help to bend the knee and move the hip backwards (extension). They play an important role in propelling the body forwards as we move. Hamstring injury may occur by high speed mechanisms such as running or low speed mechanisms such as stretching. Sudden loading of muscle while it is stretched as while kicking a football is also a common injury mechanism.

The risk factors most consistently associated with hamstring muscle strain-type injuries are age, previous hamstring injury and quadriceps peak torque. As mentioned previously hamstrings and quadriceps oppose each another and it is not uncommon to see imbalance between the two groups of muscles with the latter being stronger. This is expressed as low hamstring to quad ratio which essentially means weaker hamstrings. Weak hamstrings can quickly turn into tight hamstrings and require hamstring to work harder which tires them easily. Tight, tired and weak muscles are predisposed to injuries. When the hamstring are injured, other nearby body muscles are called into action such as those in lower back and hip predisposing them also to injuries/ pain.

Risk factors foe hamstring injury can be classifies into modifiable and non-modifiable ones.

Modifiable risk factors

Volume of training and rapid variations
Muscle fatigue
Weak hamstrings
Repeated overloading with insufficient warm-up
Over striding during running or abruptly changing direction
Lower back, core and pelvis weakness or trunk instability
Prolonged sitting (work, cycling etc)
Biomechanical issues such as unequal leg length
Non-Modifiable risk factors

Previous hamstring injuries – most consistent risk factor with two to six times increased risk of recurrence. Most repeat injuries occur within two months of return to the sport but the risk remains elevated up to three times for an year after initial injury.
Age – teens and young adults are more likely to experience hamstring injuries as muscles do not tend to grow at the same speed as bones. Aging adults are also at a higher risk possibly due to reduced muscle cross-sectional area.
Genetics (collagen types)
HAMSTRING INJURY GRADES
Most hamstring injuries occur in the thick, central part of the muscle or where the muscle fibres join tendon. Muscle strains are graded from 1 to 3 depending on their severity

Grade 1 – or hamstring pull is most minor form and usually heals readily. Most patients with this are able to walk easily although may notice pain at the back of the leg after prolonged or quick walking.

Grade 2 – this is associated with more pain (often shooting type) and patients may struggle to walk / limp.

Grade 3 – represents more marked muscle tears including complete tears which present with more severe pain, swelling and difficulty weight bearing. These may require several months of rehabilitation.