Hamstring

Semitendinosus: 

• The semitendinosus attaches to the ischial tuberosity proximally (along with other hamstring muscles).  
• Distally, it attaches to the anteromedial surface of the proximal tibia, just below the medial condyle (as a part of the pes anserinus, along with sartorius and gracilis). 

Semimembranosus: 

• The semimembranosus muscle has its proximal attachment at the ischial tuberosity (along with other hamstring muscles). 
• It attaches to the posterior surface of the medial tibial condyle. 

BLOOD SUPPLY: 

Arterial supply: 

The hamstring muscles receive arterial supply from 

• Inferior gluteal artery 
• Deep femoral artery 
• Popliteal artery 

The Inferior Gluteal Artery (a branch of the Internal Iliac Artery) mainly supplies the proximal one-third of the muscle group, near the ischial attachment. 

This provides a relatively poor blood supply to the proximal tendon. This is one of the reasons why the proximal hamstring injuries have slow healing. 

The Deep femoral artery (profunda femoris artery) mainly supplies the middle one-third (muscle belly), providing rich vascularization to this area. 

The popliteal artery (genicular branches) mainly supplies the lower one-third of the hamstring muscles. This also ensures good vascular supply to the distal tendons. 

Venous drainage: 

The deep femoral vein and the inferior gluteal vein, which drain into the femoral vein. Distal parts are also drained by the popliteal veins. 

Nerve Innervation: 

Nerve innervation for the hamstring muscle group is from the Sciatic nerve (L5-S2). All the hamstring muscles are innervated by the tibial component of the sciatic nerve, except for the short head of the biceps femoris, which is supplied by the fibular component of the sciatic nerve. 

Where are the hamstrings located?

The hamstring group of muscles is located in the back of the thigh, running from the lower part of the pelvis to the leg. The tendons of the hamstring muscles are large and palpable as thick cords at the back of the thigh. 

The biceps femoris (both long and short heads) is located laterally. The tendon of this muscle is felt on the outer aspect of the back of the thigh.

The semitendinosus and semimembranosus are both located on the medial aspect and are palpable in the inner aspect of the back of the thigh. 

This difference in location (biceps femoris – lateral; semitendinosus and semimembranosus – medial) allows these muscles to rotate the tibia when the knee is flexed.

What are the hamstrings made of?

Hamstring muscles are made of 

• Skeletal muscle fibers, which contract and generate force, 
• Connective tissues that provide structural integrity,
• Tendons that connect the muscle to the bones (proximal hamstring tendon, pes anserine tendon). 

Types of Muscle Fibers in Hamstrings: 

The Hamstring muscles contain a mix of all three types of skeletal muscle fibers: slow oxidative (Type I), fast oxidative glycolytic (Type IIa), and fast glycolytic (Type IIx) fibers. However, the proportion of these fibers can vary depending on genetics, as well as the fitness level and the demands of everyday activities. 

The proportion of muscle fiber types can be modified to some extent by regular exercise training. The type and intensity of training determine the shifts in muscle fiber composition that occur over time. Endurance activities: More type I fibers, Power and strength activities: more type II fibers. 

Tendons of the Hamstring muscles:

The proximal hamstring tendon is the common tendon shared by the long head of the biceps femoris, semitendinosus, and semimembranosus. It attaches to the ischial tuberosity and is commonly injured in athletes, especially sprinters, as well as in avulsion fractures. 

The semitendinosus tendon combines with the sartorius and gracilis to form the pes anserine tendon distally, and it is also a preferred graft for ACL reconstruction surgery.

How are your hamstring muscles structured?

Hamstring muscles are structured to be a long and strong muscle group with a common proximal attachment site at the ischial tuberosity and two distinctive distal tendon attachments.  

All hamstring muscles (except the short head of the biceps) are two-joint muscles, crossing both hip and knee joints. This makes them a crucial muscle group in the lower limb functions, but also more prone to injury, especially during rapid stretch–shortening movements (like sprinting). 

Sometimes, anatomists claim that the short head of the biceps femoris is not a true hamstring, as it differs from the other hamstring muscles in various aspects. 

• It does not attach to the ischial tuberosity. 
• It receives nerve innervation from the fibular component of the sciatic nerve, while the others are inervated by the tibial component. 
• It is a single-joint muscle, crossing only the knee joint, while the others are two-joint muscles crossing both the hip and knee.

Function: 

All three muscles work together primarily to produce knee flexion and hip extension. 

Concentric (shortening) action: 

Standard mover action:(distal attachment moving towards proximal) 

At the Knee joint: 

• The Hamstring muscles flex the leg at the knee joint (bending your leg towards your thighs).  
• When the knee is flexed, hamstring muscles can rotate the tibia relative to the femur. The biceps femoris (lateral hamstring) causes Lateral or External rotation of the tibia, while the semitendinosus and semimembranosus (medial hamstrings) cause medial or internal rotation of the tibia.

At the Hip joint: 

• Hamstring muscles all together extend the thigh at the hip joint (moving the thigh backward). 
• Additionally, they rotate the thigh at the hip joint.  The biceps femoris produces external rotation of the hip, whereas the semitendinosus and semimembranosus produce internal rotation of the hip. 

Since the hamstring muscles are Biarticular (Two-joint muscles), they cannot produce knee flexion effectively when the hip is extended. This is because of the active insufficiency of the hamstrings, which is the inability to generate maximum force or a full range of motion in both the joints they cross. 

Reverse mover action:(proximal attachment moving towards distal)

At the Knee joint: 

• Hamstring muscles can flex the thigh at the knee joint (Nordic curl- while bringing the body up from the curl). Apart from such special circumstances, the reverse mover action of the hamstring is not usually seen in everyday functional activities. 

At the Hip joint: 

• Hamstring muscles cause posterior pelvic tilt at the hip joint.   

Eccentric (lengthening) action: 

• All three of the hamstring muscles control and decelerate the extension of the knee joint.  
• They control and slow down the flexion at the hip joint. 
• Additionally, they also contribute to the controlled anterior pelvic tilt.  

The eccentric function of the hamstrings is very important for limiting excessive anterior translation of the tibia during knee extension, assisting the ACL ligament. 

During knee extension, the quadriceps exert a strong pull on the tibia, causing anterior translation. ACL will prevent the tibia from going into excessive anterior translation. At the same time, the hamstrings muscles will contract eccentrically to control knee extension and by pulling the tibia posteriorly, thereby reducing strain on the ACL. 

When the hip flexes and the knee extends at the same time, the hamstrings are lengthened eccentrically to control these movements. However, because of passive insufficiency, they cannot effectively stretch across both joints simultaneously. This limitation makes them highly vulnerable to injury during explosive actions such as sprinting or kicking, where they may fail to absorb the high forces and control the rapid hip flexion and knee extension.

Isometric function: 

• The hamstring muscles work together isometrically to stabilize the knee joint and hip joint during various lower limb activities. 
• At the distal attachment, the semitendinosus, along with gracilis and sartorius (pes anserine muscles), provides medial stability to the knee joint. 

Axis and plane:

The muscle fibers of all hamstring muscles create movement in an oblique plane. 

The Biceps femoris (lateral hamstring) runs postero-laterally across the hip joint as well as the knee joint, so it will produce hip extension along with lateral rotation at the hip joint and produce flexion and lateral tibia rotation at the knee joint. 

The semitendinosus and semimembranosus (medial hamstrings) run postero-medially across the hip and knee joints. These will produce hip extension and medial rotation as well as flexion and medial rotation at the knee joint. 

Muscle fiber arrangement and force production: 

There are several types of muscle fiber arrangement, such as pennate, parallel, convergent, circular, and spiral. Of these, the hamstring muscles have pennate and parallel arrangements. 

Pennate – muscle fibers attach obliquely to the tendon (like a feather).

Parallel – muscle fibers run more in line with the tendon.

Pennation is the arrangement of muscle fibers at an angle to the tendon, allowing an increased number of muscle fibers within a given cross-sectional area, resulting in increased force production. 

But this reduces the overall shortening ability of the muscle, resulting in a reduced range of motion. 

Parallel arrangement of muscle fibers parallel to the long axis of the tendon. This arrangement allows for a greater range of motion and faster contraction. However, this reduces the force production capacity because of the smaller cross-sectional area.

Muscle fiber arrangement of the hamstring muscle group: 

• Biceps femoris – unipennate muscle fibers, 
• Semitendinosus –  parallel muscle fibers 
• Semimembranosus- bipennate muscle fibers (especially in the distal portion) 

The pennation of the biceps femoris and semimembranosus helps in greater force production. But the short fascicles of these pennate muscles reduce their ability to produce speed and to lengthen effectively. This makes them (especially the biceps femoris) prone to strain injuries during sprinting, because they can’t handle rapid lengthening. 

On the other hand, the parallel arrangement of the semitendinosus helps with faster contraction, a better range of motion, and good velocity control, but it does not produce as much force as the other two muscles. 

This varied muscle fiber arrangement significantly contributes to the role of the hamstring group in explosive movements, such as sprinting, running, and jumping. 

What is the purpose of your Hamstring muscles?

The hamstring muscle group has the primary purpose of producing hip extension and knee flexion. Because of that, it plays a crucial role in various lower limb functions, from everyday activity to athletic performance. 

Gait control: 

• During the gait cycle, hamstrings work mainly at the end of the swing phase, generating force for hip extension as well as controlling knee extension.
• During the heel strike phase, hamstrings work along with ACL in preventing the excessive anterior translation of the tibia, thereby serving as the dynamic stabilizer of the knee joint. 

Sprinting: 

• Eccentric action of the hamstrings is not only helpful in gait, but especially important during sprinting. 
• When the hip is flexed and the knee is extended, the hamstrings lengthen under tension to control and slow down the limb. 
• Additionally, they take off the load on the ACL ligament and prepare the limb for an effective and safe ground contact. 

Pelvic and hip control: 

• It provides dynamic stability to the hip joint and allows the limb to decelerate, reducing the impact during heel strike. 
• It also helps maintain a stable pelvis by resisting the excessive anterior pelvic tilt.  

Common injuries: 

Some of the most common hamstring-related injuries include:

• Hamstring strain (muscle tear)
• Hamstring tendinopathy 
• Tendon rupture
• Avulsion fracture
• Hamstring contusion 

Hamstring Strain: 

A hamstring strain is the most common injury that involves overstretching or tearing of the hamstring muscle fibers, usually during explosive activities that demand rapid lengthening fop the hamstrings.

It usually involves the biceps femoris (the pennated muscle having short muscle fibers and high eccentric load). 

• Cause: Eccentric overload during sudden acceleration, deceleration, or explosive movements like sprinting, jumping, or kicking.

• Symptoms: Sharp pain, swelling, muscle tightness, reduced strength, reduced range of motion, reduced sprint ability, and possible bruising.

• Hamstring muscle strain can range from mild overstretch to severe muscle stretch:
  • Grade I: Mild overstretch, minimal fiber damage
  • Grade II: Partial muscle tear
  • Grade III: Complete muscle rupture

Hamstring Tendinopathy: 

It is a chronic overuse injury at the hamstring tendon insertion, often involving the proximal attachment at the ischial tuberosity (proximal hamstring tendinopathy). It is commonly seen in runners, cyclists, and people who sit for long hours.

• Causes: Because of higher load and relatively less blood supply, this becomes prone to injury and slower healing.

• Symptoms: Deep buttock pain which worsens with running, sitting, or stretching hamstrings, tenderness at the ischial tuberosity.

Hamstring tendon rupture 

It is the complete rupture of the hamstring tendon, commonly occurring in the proximal tendon attachment at the ischial tuberosity. 

• Causes: Sudden explosive load on the tendon during rapid hip flexion and knee extension (as in sprinting, kicking, etc). Those who have tendinopathy are more prone to tendon rupture. 
• Symptoms: Sudden severe pain, audible “pop”, bruising, swelling, difficulty walking, and a gap may be felt at the tendon origin. 
• Partial rupture can be managed by conservative management, whereas complete rupture needs surgical management. 

Avulsion fracture 

An avulsion fracture occurs when the hamstring tendon pulls off a piece of bone from the ischial tuberosity during explosive movements. 

• Causes: Explosive movements (sprinting, hurdling, dancing, water skiing). It usually happens in adolescents, as their growth plates are weak. It can also occur in older adults with weak bones (osteoporosis).

• Symptoms: Sudden severe pain in the buttock or posterior thigh, inability to continue activity, bruising/swelling, and local tenderness at the ischial tuberosity.
• It often requires surgical repair if the bone fragment is displaced.

Hamstring Contusion (Bruise)

Contusion refers to the blunt injury caused by a direct blow to the back of the thigh (e.g., in contact sports), which causes bleeding within the muscle. It is usually less severe than strains but can limit function for days to weeks.

• Causes: Direct blow to the back of the thigh during contact sports, falls, tackles, martial arts, etc. 

• Symptoms: pain, swelling, stiffness, discolouration, and sometimes hematoma formation. In case of severe swelling, it can result in compartment syndrome.

Is a hamstring injury serious?

It depends on the type and severity of the injury. In the case of tendinopathy, even though it is not serious, it can be long-lasting with slow recovery and impact a person’s lifestyle. 

A mild hamstring strain or mild contusion may not be serious injuries with relatively quick recovery, whereas a severe hamstring strain, an avulsion fracture, tendon rupture, and severe contusion are serious injuries and take a longer time to heal. 

Mild strain or contusion may heal faster, and the person might recover within 1-3 weeks of rest and rehab. 

Severe hamstring strain may take 9-12 weeks to get back to sports. Tendon rupture and avulsion fractures need surgical repair followed by proper rehab. The contusion, if very severe, can even lead to compartment syndrome and myositis ossificans. 

What are the common injuries that affect the hamstrings?

Some of the most common hamstring-related injuries include:

• Hamstring strain (muscle fiber tear)
• Hamstring tendinopathy (chronic overuse injury)
• Tendon rupture (complete tear of the tendon)
• Avulsion fracture (tendon pulls and breaks a fragment of bone)
• Hamstring contusion (bruise due to direct blunt trauma)

What are the symptoms of Hamstring muscle injuries?

The symptoms of hamstring injuries depend on the type of injury, its severity, and duration. But some symptoms are common with hamstring injuries, which include

• Pain in the back of the thigh or buttock 
• Tenderness over the ischial tuberosity or the hamstring muscles
• Swelling in acute strains and contusions 
• Bruising or discolouration over the buttocks and back of the thigh
• Muscle tightness attributed to protective spasm 
• Reduced range of motion of the hip and knee joints, especially hip extension and knee flexion.
• Reduced muscle strength and flexibility 
• Limping or difficulty in performing day-to-day activities that involve bending the knee joint. 
• Visible deformity or palpable gap in the case of tendon or muscle ruptures, and also in avulsion fractures. 
• Popping or snapping sound during injury 

What are the risk factors for Hamstring muscle injuries?

Understanding the risk factors is crucial in preventing the incidence and recurrence of hamstring injuries. Risk factors of hamstring injuries include: 

• Age: As we age, there will be a decrease in tissue elasticity, which increases the susceptibility to injury. 
• Muscle weakness: reduced strength in the hamstrings puts them at risk of injury as they have to work against the strong quadriceps muscles. 
• Reduced flexibility: Injury occurs due to the high eccentric demand being placed on the hamstrings, especially during explosive movements that involve hip flexion and knee extension. 
• Muscle structure: The hamstring muscles, especially the biceps femoris, are pennate and cross two joints. So the speed of contraction and muscle lengthening capacity is diminished, which reduces the muscle’s ability to decelerate the limb during a high impact. 
• Overloading: A sudden increase in training volume or repetitive overloading more than the muscle’s capacity can result in muscle injury. 

Demand > Capacity→ injury

• Previous injury: A history of hamstring injury without proper rehabilitation is a risk factor for reinjury. 

• Inadequate recovery: Allowing rest and recovery is very crucial for muscular adaptation. Overtraining the hamstring muscle without allowing for recovery is a risk factor for injury. Recovery includes rest, adequate sleep, nutrition, and hydration. 

• Activity related: Sprinting, kicking movements, especially in acceleration and deceleration phases.Sports with explosive actions, such as football, rugby, basketball, and athletics. Sudden changes in direction or kicking movements, as in football, basketball, etc. 

What does a hamstring injury feel like?

A person with acute hamstring injuries feels a sudden, sharp pain and tenderness, especially in the back of the thighs and buttocks. Some might see swelling, bruising, or discolouration in the injured area. 

On the other hand, with chronic injuries like the proximal hamstring tendinopathy, it feels like a deep, dull, nagging ache, especially when sitting, climbing stairs, or running.

What happens if the hamstrings are weak? 

Hamstring weakness results in poor movement, function, which impacts everyday life and increases injury risk. 

Reduced Movement & Function

• Reduced knee stability:  As a dynamic stabilizer of the knee joint, hamstrings prevent excessive anterior translation of the tibia, reducing the load for the ACL.Hamstring weakness results in reduced knee joint stability during movements, especially in sports. 
• Poor hip extension: As glutes and hamstrings help extend the hips during running, jumping, and climbing. Weak hamstrings reduce explosive power in these movements.
• Altered gait and running efficiency: Weak hamstrings cannot help with deceleration of the limb during the heel strike, and cannot efficiently help in the push-off phase. 
• Inefficient movement mechanics: When the quadriceps muscle group overpowers the hamstrings, it leads to inefficient mechanics.

Injury Risk

Weak hamstrings increase the risk of injuries in and around the hip and knee joints, which include the following:

• Hamstring strains/tears: weak hamstrings fatigue quickly and fail to absorb sudden forces, resulting in strain or even a tear. 
• ACL injury: As mentioned earlier, hamstring muscles support the ACL by counteracting the strong pull of the quadriceps. So, when hamstrings are weak, it increases the load placed on the ACL, resulting in ACL injuries, especially in athletes. 

• Lower back pain: Hamstrings maintain pelvic stability by preventing anterior pelvic tilt. When they are weak, other muscles (glutes, erector spinae) may overcompensate, and this serves as a contributing factor for low back pain.
• Knee pain (patellofemoral issues): Imbalance between quadriceps and hamstrings changes knee tracking and results in knee pain.

Everyday Life Impact

Hamstring weakness impacts a person’s everyday life. The person may experience the following: 

• Difficulty with climbing stairs, standing up quickly, running for a bus, or bending forward.

• Fatigue and tightness in the back of the thigh during walking or prolonged sitting.
• Reduced ability to control pelvic tilt, leading to slouched or unstable positions.

How to heal a hamstring fast?

Proximal hamstring attachment is vulnerable to repeated injury, and it is also the site of slow healing. This is mainly attributed to poor blood supply to the proximal part. 

The healing process in hamstring injuries has no shortcuts. But adequate rest, medical assistance, and proper rehabilitation can help with better outcomes in hamstring injuries. 

Conservative management of acute injuries with the PEACE and LOVE protocol is shown to be helpful. 

PEACE protocol- Immediately after injury,

• Protect by limiting activities that aggravate pain to prevent any further injury
• Elevate the injured area
• Avoid anti-inflammatory agents, as they may slow down the healing process
• Compression with an elastic bandage to help with swelling
• Educate and reassure the patient, explain the natural healing process, and avoid unnecessary rest or imaging unless severe.

LOVE protocol- Recovery and rehabilitation phase

• Load optimally– controlled loading stimulates tissue repair and strengthens the muscles. 
• Optimism – positive thoughts aid in recovery and reduce the fear of re-injury.
• Vascularization– Including light aerobic activity, increases blood flow and promotes faster healing.
• Exercise to restore mobility, strength, and functional ability. Start with a gentle range of motion exercises and slowly progress to strengthening exercises, which include isometrics initially, and progress to eccentrics like Nordic curls. 

How to avoid hamstring injury? 

As the hamstring muscle is prone to injury, it is better to prevent than to cure.

Key points to be considered to avoid hamstring injuries: 

• Strengthening- Improving muscle strength, especially the eccentric strength, is crucial to avoid hamstring injury. Eccentric hamstring training,  such as Nordic curls, improves tolerance to rapid lengthening forces during sprinting and kicking.
• Improving Mobility: The better the mobility and flexibility, the lower the injury risk. 
• Gradual progression of load: prefer gradual load progression and avoid sudden increase in intensity, frequency in weight training, and sprinting intensity and distance. Always remember, when the demand exceeds the muscle’s capacity, it results in injury. Always optimize your load to gradually build the tissue capacity.  
• Core strength: Strong glutes, deep core, and lower back muscles help efficiently transmit the forces and stabilize the pelvis and lower limb, reducing excessive strain on the hamstrings during running.
• Proper warm-up: Warm-up prepares the joints and muscles for an activity. It enhances neuromuscular coordination and movement efficiency. Inadequate warm-up can make the muscle contract without preparation, reducing force generation and leading to injury.  
• Allow adequate recovery time: Adequate rest, sleep, hydration, and nutrition with a good amount of protein for muscle repair are crucial for preventing injuries. Avoid training on fatigued muscles. Most of the hamstring injuries occur late in a game or at the end of a workout, when muscles are fatigued and cannot efficiently generate force.