Hamstring Strain

Anatomy of a hamstring strain

The hamstring is a group of three muscles that attach from the back portion of the hip to the back portion of the knee. Therefore, the hamstring helps control motion at both the hip and knee joints. The muscles are named the biceps femoris (with two heads, long and short heads), the semitendinosus, and the semimembranosus.¹ The hamstring attaches to a part of the hip called the ischial tuberosity, which can often be the location of the injury.¹

These muscles collectively work to forcefully move the thigh bone backward (such as running or squatting) and bend the knee. Perhaps more importantly, the hamstrings function as a brake for the leg when running, particularly sprinting at high speeds. The hamstrings will work eccentrically (contracting forcefully while stretching) at the end of the swing phase of sprinting to begin to prepare the foot to contact the ground.

Injuries to the hamstring will typically occur at a region called the musculotendinous junction, where the muscle merges into the tendon or in the muscle belly itself. A challenge to recovering from hamstring injuries is that the hamstring tendons have a poor blood supply, making them slower to heal.¹ Like most soft tissue injuries, hamstring strains have a grading system to classify the severity and are listed below:^1,2

• Grade I: Very minimal tissue damage, some pain, little to no inflammation, little to no loss in strength or range of motion.
• Grade II: More notable tissue damage, more pain, muscular swelling/bruising, decreased strength and range of motion.
• Grade III: Complete tear with severe pain and loss of strength and range of motion, and significant swelling/bruising.

MRI is often used to diagnose hamstring injuries and their severity. As expected, a larger hamstring injury and an injury to the tendon that attaches to the ischial tuberosity (mentioned above) often lead to a more prolonged absence from sport.² Newer imaging studies from 2022 and 2023 suggest that tendon-involving injuries—especially proximal ones at the ischial tuberosity—are not only slower to heal but also carry a higher risk of reinjury and longer return-to-play timelines.⁴ Athletes with high-grade proximal tendon involvement may miss 6–8 weeks or more, even with optimal rehab.⁵

Recent NFL case examples like Jaxon Smith-Njigba and Keenan Allen (2022) highlight that even partial hamstring strains can cause missed games and dip in in-game usage (e.g., snap count or target share), especially if not fully healed before return.

Cause of hamstring strains

Hamstring strains are the most common muscle injury in sports, with incidence increasing late in each half of play as fatigue sets in.¹^,⁶ Injuries can result from cumulative microtrauma or a single high-force event.³

The biceps femoris long head is the most common muscle injured in a hamstring strain, and most frequently occurs during sprinting. The second most common cause of injury is the follow-through during kicking¹^,⁴. During sprinting, the hamstring contracts eccentrically to act as a brake to decelerate the leg in preparation for contacting the ground³. This puts high amounts of force through the hamstring muscle while it is in a position where it is being stretched to its maximum length. This combination of maximal contraction while being maximally stretched can lead the muscle to be vulnerable to injury.

Increasing sprinting speed from 80% to 100% of maximum increases hamstring loading by 1.3x, with the majority of this load falling on the biceps femoris long head.³^,⁶ Recent EMG and biomechanical modeling studies suggest the greatest eccentric demand on the hamstring occurs 30–50 milliseconds prior to foot contact during maximal sprinting, which may be the “hot zone” for injury onset⁶. This supports the idea that muscle overload under extreme length and velocity is a primary mechanism

Risk factors for hamstring injuries

There are two types of risk factors for injury: modifiable and non-modifiable. Modifiable factors can change via training or other lifestyle changes. Non-modifiable indicates things that are out of one’s control to change.

Modifiable: quad/hamstring imbalances, asymmetries, fatigue, nutritional deficits, poor sleep.⁷

Non-modifiable: age and prior injury are the strongest predictors.⁷

Hamstring injury recurrence rate

Hamstring injury recurrence is widely studied across many sports, interestingly, with soccer and Australian football being the most heavily studied and the NFL much less so. Different studies cite different percentages for recurrence, but the trend remains the same: the recurrence rate is high, typically in the 20-30% range.^5,6

A previous hamstring strain and other lower-body injuries, such as ankle sprain or fractures, may increase the risk for re-injury. Previous hamstring injuries, in particular, increase the risk for re-injury by 2.7x, and this re-injury usually occurs at the same part of the muscle that was injured.^3. When in the same season, the risk for re-injury increases to 5x. This makes a return to playing challenging to gauge. 

The risk factors noted above are part of the problem with hamstring injury recurrence, but we also must understand the physiological changes that happen after injury. Athletes who have had multiple hamstring injuries demonstrate decreased muscle length, strength, and speed at which the muscle can activate, all of which make the muscle more susceptible to injury.⁶

Hamstring strain rehab process

The goals for rehab following a hamstring injury are to allow the injured portion of the muscle or tendon to heal, regain the lost range of motion, prevent weakness in the hamstring and other muscles in the lower body, and regain any strength, endurance, or speed lost.¹

Initially, when the player is experiencing quite a bit of pain and loss of function, treatment is geared toward reducing pain and swelling. For example, this is achieved by relative rest, ice, compression, elevation, soft tissue work, which may include massage techniques or dry needling, and electrical stimulation for pain control.1 During this phase, it is also essential to identify factors that may have led the athlete to be prone to a hamstring injury. Examples would be weaknesses in the core or glutes, or stiffness in the hips or ankles.¹

As the athlete’s pain begins to subside, they can start to work on stretching the hamstring and progressively strengthening.¹ Strengthening begins with isometric exercises, where the muscle contracts but the joint does not move. When tolerable, the athlete will progress to concentric strengthening (muscle contracts and joint moves) and eccentrics (muscle contracts forcefully as it is stretching). A 2023 systematic review found that introducing eccentric work (e.g., Nordic curls or slider leg curls) within 1 week of injury, if pain allows, results in faster return-to-play (RTP) without increasing reinjury risk⁶. Eccentric work not only restores strength but also improves neuromuscular control of the posterior chain. Additionally, use of blood flow restriction (BFR) training is growing in early rehab phases to preserve strength while limiting joint strain, particularly helpful for tendon-based strains or athletes unable to tolerate full loads⁶

During this intermediate phase, it is critically important that the athlete continues to work on glute and core strengthening and ankle and hip mobility, if needed, and address any issues with sciatic nerve mobility.^1,3 When the glute and core muscles are weak or not working correctly, the hamstring muscles tend to compensate and overwork, which can influence injury. Some recent studies emphasize earlier use of eccentric exercises as long as the athlete can tolerate it without too much pain.³

Running progression begins when walking is pain-free. Jogging (25–50%) starts first, progressing to 80%, then finally 100% when tolerated without symptoms.³ Anti-gravity treadmills may help ease this transition. Force plate jump tests, GPS speed tracking, and isokinetic strength testing are now commonly used to monitor readiness.

Rehab Phases	Treatments	Criteria to Enter Phase
Early Phase	Rest, ice, compression, elevation, NSAIDs. Soft tissue work, dry needling, electrical stimulation (pain control). Ankle and hip mobility Glute and core strength Gentle hamstring isometrics	N/A first phase
Intermediate Phase	Soft tissue work, dry needling Ankle and hip mobility Glute and core strength Hamstring concentrics and eccentrics Hamstring stretching Initiate running program	Little to no pain with hamstring isometrics. Little to no pain with walking (for initiating running program).
Sport Specific	Full weight lifting Progress to 100% speed running Agility, plyometrics Position specific drills	No pain with hamstring eccentrics. No pain with running up to 80% speed.

Criteria to return to sport

While no consensus exists, the following criteria are commonly used to clear an athlete:

• No pain with activity
• Strength within 5% of the uninjured leg
• Symmetrical single-leg function (hops, balance, strength)
• 100% sprinting and cutting without hesitation
• No discomfort during sport-specific drills⁷

NordBord and reactive strength testing can identify subtle deficits. A 2023 study found >90% eccentric strength symmetry significantly reduces reinjury risk.⁸

In fantasy, players may be active but still limited, especially WRs or DBs. In 2022, Julio Jones and Keenan Allen both returned to play but were eased back in with limited usage.

Time missed

There are limited studies on NFL football players’ time missed, re-injury rates, and performance disparities from pre-injury compared to post-injury. However, there are numerous studies on this data for professional European soccer. In European soccer, players average missing about 18 days due to a hamstring injury (equivalent to 2-3 weeks of NFL season).⁷ In these studies, the re-injury rate is about 17.5% but can be seen in the 20-30% range, and nearly half of the players show a decline in performance measured by maximal speed.⁸ While soccer and football are two different sports, this data may translate to football, as we all know how pesky hamstring injuries can be. 

References

1. Sueki D, Brechter J. Orthopedic Rehabilitation Clinical Advisor. Elsevier Inc.; 2010.
2. Sevensson K, Alricsson M, et al. J Exerc Rehabil. 2016;12(3):134–142.
3. Hickey J, Opar D, et al. J Athl Train. 2022;57(2):125–135.
4. Elabd A, Elabd O. B Int J PT. 2023;1(1):10–18.
5. Jankaew A, Chen J, et al. Sports Health. 2022;15(4).
6. Huygaerts S, Cos F, Cohen D, et al. Sports. 2020;8(65).
7. Whiteley R, Massey A, et al. Sports Health. 2021;13(3):290–295.
8. Bourne MN, Williams MD, et al. Br J Sports Med. 2022;56(11):630–637.
9. Ekstrand J, Krutsch W, Spreco A, et al. Br J Sports Med. 2020;54(7):421–426.