Hamstring Strain

Position	Average % Difference PPG Change From Pre-Injury Compared to 1st Game After Injury	Average % Difference PPG Change From Pre-Injury Compared to Games 2-3 After Injury	Average % Difference PPG Change From Pre-Injury Compared to Games 4-6 After Injury
QB	-14.5%	-0.5%	-12.2%
RB	+8.6%	+57.5%	+60.2%
WR	-10.2%	+12.9%	-1.7%
TE	+24.3%	+34.2%	+37.2%

Position	Average Change in Snap% From Pre-Injury Compared to 1st Game After Injury	Average Change in Snap% From Pre-Injury Compared to Games 2-3 After Injury	Average Change in Snap% From Pre-Injury Compared to Games 4-6 After Injury
QB	+9.8%	+15.1%	-12.7%
RB	+0.3%	-2.2%	+1.9%
WR	-15.8%	-5%	-2.4%
TE	-11.5%	-8%	-4.8%

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 head), the semitendinosous, and semimembranosous.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 break for the leg when running, particularly sprinting at high speeds. The hamstrings will work eccentrically (contracting forcefully while stretching) during 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 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 loss in strength or range of motion.
• Grade II: More notable tissue damage, more pain, decreased strength and range of motion.
• Grade III: Complete tear with severe pain and loss of strength and range of motion.

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.^2,3

Cause of hamstring strains

The hamstring strain is the most common muscle injury in sports, and the incidence of injury is more frequent in the later portions of each half as players become fatigued.^1,4 Injury can occur from repetitive minor injuries that build over time or one instance of a more significant force through the muscle or tendon that is strong enough to cause injury.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 kicking.^1,4 During sprinting, the hamstring contracts eccentrically to act as a brake to decelerate the leg in preparation for contacting the ground.³ 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 vulnerable to injury.

When an athlete increases their sprinting speed from 80% of max speed to 100% of max speed, the force through the hamstring increases by 1.3x, with most of this force occurring through the biceps femoris long head muscle, explaining why it is most commonly injured out of the three hamstring muscles.^3,4 There is also some evidence that the hamstring contracts very hard when the foot first hits the ground, and this may also be when the hamstring strain occurs in some cases.⁴

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 This makes a return to playing challenging to gauge. 

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 around 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 prone to a hamstring injury. Examples would be weaknesses in the core or glutes or stiffness in the hips or ankles.¹ These can be addressed during this time.

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. The athlete will typically do isometrics for a few days to weeks and, when tolerable, can progress to concentric strengthening (muscle contracts and joint moves) and eccentrics (muscle contracts forcefully as it is stretching). Examples of concentric exercises are when you are curling your legs on the leg curl machine, and the eccentric phase is when you slowly allow the weight stack to straighten out your legs (the controlled manner is what makes the muscle contract as it is stretching).  

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.^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

Returning to running, cutting, and jumping is a significant part of returning to playing football. There is no clear consensus among rehab professionals on ideal return to running protocols, and it is dependent on how the athlete is progressing through rehab. One suggestion written by rehab professionals working in professional football recommends returning to light jogging (25-50% speed) as soon as the athlete can tolerate walking with minimal pain.³ When this light jog is tolerable, they can progress to 80% speed. When 80% is performed without pain, the athlete can slowly progress to 100% speed; however, this progression to 100% should not be rushed as over 80% speed, the force through the hamstring increases quite a bit.³ Alternative ways to progress running include using anti-gravity treadmills to reduce the effects of gravity.

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.

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).⁵ In these studies, the re-injury rate is about 17.5%, and nearly half of players show a decline in performance measured by maximal speed.⁶ 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. 1st ed. Maryland Heights, MO. Elsevier Inc.; 2010.
2. Sevensson K, Alricsson M, Eckerman M, Magounakis T, Werner S. The correlation betweehn the imaging characteristics of hamstring injury and time required before returning to sports: a literature review. J Exerc Rehabil. 2016;12(3):134-142.
3. Hickey J, Opar D, Weiss L, Heiderscheit B. Hamstring Strain Injury Rehabilitation. J Athl Train. 2022;57(2):125-135.
4. Huygaerts S, Cos F, Cohen D, et al. Mechanisms of Hamstring Strain Injury: Interactions Between Fatigue, Muscle Activation, and Function. Sports. 2020;8(65).
5. Ekstrand J, Krutsh W, Spreco A et al. Time before return to play for the most common injuries in profesional football: A 16-year follow-up of the UEFA Elite Club Injury Study. Br. J Sports Med. 2020;54(7):421-426.
6. Whiteley R, Massey A, Gabbett T, et al. Match High-Speed Running Distances Are OFten Suppressed After Return From Hamstring Strain Injury in Professional Footballers. Sports Health. 2021;13(3):290-295.