Why are hamstring injuries so difficult to treat?
Ross Hauser, MD, Caring Medical Florida
David N. Woznica, MD, Caring Medical Regenerative Medicine Clinics, Oak Park, Illinois
Danielle R. Steilen-Matias, MMS, PA-C, Caring Medical Regenerative Medicine Clinics, Oak Park, Illinois
In this article, we will examine the difficulties and frustrations athletes suffer from acute and chronic hamstring problems.
We will often see patients who come in with chronic hamstring injuries who have a sense of bewilderment that some of the “treatments” and “remedies” that they were trying to get themselves back to running were, in fact, causing them more pain and derailing their ability to get back to their sport quicker. This would include a heavy dose of stretching. For many athletes, stretching is a vital part of their condition and “rehabilitate.” For some athletes, they were very wrong.
“I thought stretching was the best thing for my hamstrings . . . it turned out to be the worst thing I could do.”
For years when an athlete pulled up in a race or during a game and grabbed the back of their leg, the first thing the training staff or teammates would do is throw the athlete to the ground, grab an ankle and start stretching their leg. The research below will tell you this is not a good thing. Nor is the continued stretching of a leg to the point of pain. If you are reading this article, it is likely that you have suffered from chronic hamstring problems and have been advised to stretch them out, or, you have been advised NOT to stretch them out. It is also likely that it was hard for you not to stretch because you have been told your whole sports career to stretch. For some of you, your stretching, you thought, was providing a benefit and you went a little more aggressive in your stretching. You wound up making things worse.
You are not alone, overstretching is one of the things athletes will do when they have a hamstring injury.
You are not alone, overstretching is one of the things athletes will do when they have a hamstring injury. When these patients come into our clinics we talk about treatments that may help muscles, stretching can help, HOWEVER, stretching may be disastrous for a tendon that is already overstretched. Amazingly enough, some patients, when told this will ask, “what is a tendon?” Immediately this tells us the patient may not have been given good care.
This is why you are “tight,” all the time
A tendon, we explain, is the elastic stretchy soft tissue that holds your muscle to the bone. When that tendon is stretched out, it loses its strength, your muscle compensates for lost strength by staying in spasm, “being tight all the time.” “Why does it do that?” the patient will ask? “Because,” we suggest, “the muscle is trying to protect the tendon so it can heal.”
The longer your hamstring problems go on, the more damaged the tendon gets. In other words, the tendon, which transmits force from muscle to bone to propel and provide locomotion and strength, is not doing its job because it is hurt. Your muscle is in spasms because it is not only trying to do its own job as a muscle, it is trying to do the tendon’s job too. The muscle gets easily tired and fatigued. It rolls itself up in a knot to try to regain strength and spams to tell you that it needs rest. Below we will talk about treatments that tighten, repair, and restore strength to the damaged tendons of the semitendinosus, semimembranosus, and biceps femoris muscles of the hamstring.
Tight hamstring muscle syndrome
The difference between spasming hamstrings, tight hamstrings, and tight hamstring muscle syndrome is that tight hamstring muscle syndrome or simply hamstring syndrome is a complaint among some athletes. Athletes may experience gluteal sciatic pain (a pain in the bottom of the butt) from an inflammation of the sciatic nerve at the insertion site of hamstring muscles to ischial tuberosity.
Tight hamstring muscle syndrome can also be a postural abnormality seen with spondylolisthesis in athletes, which is the dislocation of a vertebra and may be referred to as disc related tight hamstring syndrome. Symptoms may include a pinching of the nerve that extends to the hamstring muscle because of the dislocation of the vertebrae.
Grading the different types of hamstring injuries
Hamstring injuries are a hamstring strain/pull, a partial tear, or a complete tear. A diagnosis of chronic hamstring problems may include Tight hamstring muscle syndrome or simply Hamstring syndrome.
- Grade 1 hamstring tears are considered the mildest injury.
- Grade 2 hamstring tear and Grade 3 hamstring tear injuries may take months to heal if they respond at all to conservative treatments. In grade 3 tear the muscle can completely pull away from the bone at the tendon attachment.
For someone with a chronic hamstring problem, telling them that hamstring muscle syndrome or a “pulled hamstring” can be difficult to treat would come as no surprise.
Why are hamstring injuries so difficult to treat? Are doctors using ineffective treatments?
Doctors from Northwestern University Feinberg School of Medicine recently suggested in the medical journal Orthopedics (1) that the typically difficult to treat hamstring patients may have already been given several traditional conservative treatments, including physiotherapy and nonsteroidal anti-inflammatory drugs, which are both considered inconsistent treatments. Corticosteroid injections are frequently recommended and have demonstrated limited success but can adversely affect local tissues and the detrimental long-term effect of cortisone has been well documented.
You are stuck in Hamstring Syndrome problems because of treatments that do not work.
In the Journal of Strength and Conditioning Research, (2) University and medical researchers in Taiwan wrote that medical treatment of hamstring muscle syndrome typically involves at least part of the RICE protocol, rest, ice, compression, and elevation. Although some of these treatment components do bring some initial pain relief from hamstring injuries. Research evidence shows that “ice application or topical cooling does not enhance and appears to delay the return to normal of muscle damage markers and subjective fatigue feeling after eccentric exercise.”
In other words, RICE hinders the healing of muscle tissue. Even the inventor of the RICE protocol is now saying both ice applications and rest delay muscle healing.
The dangers of ice therapy have been noted for decades, in a landmark 1992 study from Duke University published in the Journal of Athletic Training, (3) ice applications were shown to cause muscle damage, because of the dangers of freezing tissue. This danger could also extend to nerve damage.
Back to the problems stretching causes. Stretching does not prevent hamstring injuries, stretching may cause them
- Dutch researchers at the University Medical Center in Utrecht released their findings in early 2017 (4) suggesting that in elite soccer players, although hamstring flexibility is thought to play a major role in preventing hamstring injuries, there is no significant relationship between hamstring flexibility and hamstring injuries.
- Stretching does NOT prevent hamstring injuries.
Is it time to retire flexibility as part of sport’s training?
James L Nuzzo, Ph.D of the University of New South Wales, Sydney wrote this May 2020 opinion in the medical journal Sports Medicine.(5)
“Flexibility refers to the intrinsic properties of body tissues that determine maximal joint range of motion without causing injury. For many years, flexibility has been classified by the American College of Sports Medicine as a major component of physical fitness. The notion flexibility is important for fitness has also led to the idea static stretching should be prescribed to improve flexibility.
The current paper proposes flexibility be retired as a major component of physical fitness, and consequently, stretching be de-emphasized as a standard component of exercise prescriptions for most populations. First, I show flexibility has little predictive or concurrent validity with health and performance outcomes (e.g., mortality, falls, occupational performance) in apparently healthy individuals, particularly when viewed in light of the other major components of fitness (i.e., body composition, cardiovascular endurance, muscle endurance, muscle strength). Second, I explain that if flexibility requires improvement, this does not necessitate a prescription of stretching in most populations. Flexibility can be maintained or improved by exercise modalities that cause more robust health benefits than stretching (e.g., resistance training).”
More confusion about stretching. Researchers unsure if flexibility causes or prevents injuries in sprinters during the end of races
Dr. Nuzzo was not the first to point out a realistic argument that flexibility, stretching, be “retired,” from essential sports training. No wonder patients are frustrated and confused by what stretching can and cannot do for their hamstring injuries. Here is a study that even the researchers admit is confusing.
Researchers at the Beijing Sport University wanted to help sprinters from pulling up with injuries in the last portion of their races. So they measured and tested the hamstring flexibility of runners competing as sprinters. They published their research in the Journal of Sport and Health Science.(6)
Here is a summary:
- The researchers examined “the effect of hamstring flexibility on the peak hamstring muscle strains in sprinting, until now, remained unknown, which limited our understanding of risk factors of hamstring muscle strain injury (hamstring injury). . . . this study was aimed to examine the relationship between hamstring flexibility and peak hamstring muscle strains in sprinting.”
So hamstring flexibility is an unknown factor in hamstring injury. The researchers then wanted to see if hamstring flexibility prevented hamstring injury, at least in sprinters.
What did they find? The researchers are not sure. Here is what they said:
- The relationships of hamstring flexibility and peak hamstring muscle strains found in this study are limited and only suggest that individuals with good hamstring flexibility may have lower peak hamstring muscle strains compared to those with poor flexibility, BUT does not necessarily mean that improving hamstring flexibility would result in a decrease in peak hamstring muscle strains in sprinting for a given individual.
- ACTUALLY, the researchers COULD NOT DISPROVE their own previous research and that of others that suggested FLEXIBILITY IS A RISK FACTOR for hamstring injury in sprinting as previous studies showed.
WHAT? Are stretching and flexibility good or bad?
- The results are inconclusive.
- This leads to the idea that overstretched tendons may be THE factor that is being overlooked in all this muscle research.
Does hamstring injury preventative training put your knee at greater risk for injury?
Doctors at the University of Health and Sports in Berlin published these findings in the journal Medical Hypotheses. (7)
- Both hamstring and ACL injuries are among the most typical injuries, particularly in a change of direction and high speed running sports. They’re also difficult to treat.
- In the past few years, sports medicine practitioners and exercise scientists have mainly been focusing on the development and implementation of preventive programs in order to reduce the number of lower limb injuries (Hamstring and ACL), mainly by improving knee alignment. (We call this creating knee stability. The problem of joint instability in the cause of hamstring injuries is discussed below).
- A number of studies have been able to prove the success of training interventions which are mainly addressing sensorimotor (balance training, such as on a wobble board) abilities and plyometric (speed-strength training – rapid jumping up and down for example) activities in preventing hamstring and ACL injuries.
- DESPITE THIS: The number of non-contact hamstring injuries has nevertheless been on the rise, particularly in sports like soccer and football.
- The researchers introduce the hypothesis that some training interventions have a massive influence on the activation patterns in the targeted muscle group, and on the biceps femoris in particular. THEY CAUSE INCREASED INJURY RISK.
- WHAT DOES THIS MEAN? It means that while your hamstring prevention program can help prevent strains, it may also cause them by creating an imbalance in the biceps femoris. As the biceps femoris attaches at the hip and the knee, when strength training focuses on the hip area it pulls at the knee area attachment. Equally, when the knee is is subjected to training, the biceps femoris pulls on its own hip attachments. What are the attachments in jeopardy? TENDONS.
Hamstring Muscle Syndrome, Buttock Pain, and High Hamstring Tendinopathy – Focus on strengthening the tendons may be the answer to muscle strains and hamstring problems
From a regenerative medicine perspective, the preferred long-term solution is to strengthen the ischial tuberosity, a point of attachment for the hamstring muscles with regenerative soft-tissue treatments such as Prolotherapy and Platelet Rich Plasma Therapy. These injection treatments stimulate repair of the buttock pain and posterior thigh pain. One way they do so is by strengthening the tendon attachments of the semitendinosus, semimembranosus, biceps femoris muscles of the hamstring.
Prolotherapy involves the injection of a small amount of dextrose, a simple sugar solution into multiple painful ligament and tendon insertions (enthesis), pain trigger points, as well as into the adjacent joint spaces to induce healing of the injured structures. Dozens of research studies have documented Prolotherapy’s effectiveness in treating chronic joint pain.
- Danielle R. Steilen-Matias, MMS, PA-C, describes the pelvic anatomy and how these various structures in the pelvis can cause Ischial tuberosity pain.
- The Ischial tuberosity, the bony prominences at the base of the pelvis, or commonly, the “sit bones” or the “seat bones,” are just that. The bones you sit on. This is a suspected pain origin when a patient complains of “pain in the butt.”
- A patient will often complain of problems with tight hamstrings as well. They report that they do a lot of stretching, they “foam roll,” they have had cortisone shots, the pain returns.
- There are varying reasons for pain in this area.
- The patient’s pain is in fact related to the hamstring attachment at the ischial tuberosity.
- (Image of Ischial tuberosity) If this hamstring tendon attachment is damaged, the patient will suffer pain when they put their whole body weight on it to sit.
- Sometimes a patient will come in with a chronic Ischial tuberosity pain that doesn’t get better with anything they’ve tried because the pain is coming from elsewhere. It could be from the sacroiliac joints. When there is sacroiliac joint instability, this can tug on the hamstrings. This is why hamstring treatments may fail. The hamstrings are under constant stress from the sacroiliac joint. To fix the hamstring problems, the sacroiliac joint instability must be addressed. We have seen this more commonly in runners. This is why patients will tell us that they “pull their hamstring all the time, then I rest. Then I pull it again.” If hamstring treatments are not helping, an evaluation of the hip may reveal a labral tear or hip instability that is also pulling on the hamstring tendon attachment and injuring it.
- Part of the workup when you come to Caring Medical with complaints of butt pain or a diagnosis of ischial tuberosity pain syndrome or impingement is an examination of all these areas, the hip, the sacroiliac joint, the low back, the groin area.
- Treatment: Ischial tuberosity pain syndrome is a degenerative condition. We have to first stop the causes of the damage and then we can begin to repair the area and get these tissues to regenerate and be strong. We use Prolotherapy injections, this is an injection of simple dextrose or sugar. Sometimes we may use the platelets in your own blood, this is better known as Platelet Rich Plasma Therapy or PRP. It usually takes 4 to 6 treatments. These treatments are described in more detail below.
Finding and treating tendon injuries may be the answer to your hamstring problems
Is it your tendons? The answer is yes as attested to by the pain deep in the buttocks or upper back-side of the thigh reported by some patients and confirmed by physical examination as tendon injury near the origin of the hamstring muscle off of the ischial tuberosity.
That is the name of the bone which bears body weight in the seated position. It is an injury we see in mid to long-distance runners, yoga practitioners, and occasionally non-athletic individuals who have been injured by improper chiropractic adjustments or physical therapy.
- Occasionally the damage to the tendon in chronic cases can be severe enough to cause irritation of the nearby sciatic nerve, thus producing back pain symptoms of referred pain down the entire back of the leg which is more typically associated with herniated lumbar discs.
The pain is worse when the patient is seated on a hard surface, and after running, especially at faster speeds. There can be pain at rest in severe cases.
- The bent knee stretch test is applied for the diagnosis of this condition with the patient lying supine. The hip and knee are first maximally flexed towards the patient’s chest, and then the knee is extended gradually. Pain at the origin of the hamstring on that side which reproduces the patient’s pain is diagnostic.
Additionally, a diagnosis for injection treatments such as Prolotherapy relies on direct thumb pressure over the proximal (center) hamstring tendon in the buttocks, the so-called “jump sign” to reproduce the pain, localize the injury, and determine the best treatment. Direct firm pressure on affected areas causes the patient to “jump” because of the accentuation of the pain.
Finally, injection of a local anesthetic solution into the tendon-bone junction (enthesis) of the affected hamstring tendon can cinch the diagnosis by immediately eradicating all evidence of pain at rest or with motion or sitting.
In summary, there are several other conditions that can produce a similar symptom complex of high hamstring tendinopathy.
- Herniated lumbar discs and sacroiliac ligament strain both refer pain into the deep buttock area.
- Spasm of the piriformis muscle is another common condition in athletes and, in older individuals, metastatic cancer to the bone and pelvic stress fracture must also be considered.
Fortunately, all of these conditions except for the latter two will respond to appropriately applied Prolotherapy, but making the differential diagnosis is critical in order to achieve the most rapid response. When necessary, X-rays, bone scan, or MRI imaging is ordered.
Prolotherapy injections can offer the fastest route to recovery from high hamstring tendinopathy. The only possible danger from prolo for this disorder is the proximity of the hamstring tendons to the sciatic nerve, but proper experience and technique will always avert any problems in this regard. Should the nerve ever be touched by the slowly advancing prolo needle (a precautionary measure on the prolotherapist’s part), there will be a quick electrical sensation down the leg. As the needle is withdrawn or redirected, any discomfort will be mild and temporary.
Demonstration of the treatment
The Prolotherapy treatment begins at 1:37
In this video Ross Hauser, MD. demonstrates an Ischial tuberosity Prolotherapy treatment.
- The patient in this video has been given a Topical analgesic and is comfortable during the treatment. The treatment is done quickly.
- It is a long needle because the injections have to reach the Ischial tuberosity bone.
- This treats the attachments of the semitendinosus, semimembranosus, biceps femoris muscles of the hamstring, and the sacrotuberous ligaments.
- In a typical treatment, all these attachments are treated.
Platelet Rich Plasma Therapy and Prolotherapy treatments to repair damaged tendons and heal hamstring injuries
Many people reading this article may have been told about Platelet Rich Plasma therapy. They may have heard very good things, they may have heard some not good things. The not good things usually are confined to the “it will not work for you,” suggestion they are given.
We are going to challenge that statement with some independent research and the 27+ years of experience we have in offering patients treatments for their hamstring and sports-related injury problems.
- Platelet Rich Plasma Therapy draws out your own blood into a vial. Your blood is then “spun,” in a centrifuge to isolate out the components that heal injuries. These would be the anti-inflammatory and growth factors found in the blood platelets. This “Platelet Rich,” solution is then reintroduced, via injection, into the areas causing pain and weakness.
All of the athletes in our case series with hamstring or patellar tendinopathy were fully able to return to sport.
An October 2019 study from Northwestern University and the University of Chicago published in the Journal of Orthopaedics (8) presented 15 cases in which PRP injection was used for either patellar or hamstring tendinopathy in varsity collegiate athletes. All of the athletes in our case series with hamstring or patellar tendinopathy were fully able to return to sport. Three of the athletes with patellar tendinopathy were referred for surgery, while none of the athletes with hamstring strain underwent a subsequent surgical procedure.
Hamstrings and the ischial tuberosity
Doctors from the University of Colorado School of Medicine and the Southern California Hip Institute published research in the Muscles, Tendons, Ligament Journal which cited the research surrounding the use of Platelet Rich Plasma Injections for hamstring tendinopathy.(9) Here is the summary:
- Most chronic hamstring injuries result from tendinosis at the muscle origin site, the ischial tuberosity.
- Tendinosis is ligament damage that no longer is inflamed. This is not a good thing. Inflammation means that the body is still trying to heal damage. When the inflammation stops, the body has given up.
- Platelet rich plasma has been shown to have positive effects on tendon healing.
- Platelet rich plasma is increasingly being used to treat various conditions within orthopaedics, including tendinopathies.
- The majority of the currently available reports demonstrate improved healing and reduced pain when using PRP to treat various tendinopathies
- The results of our study indicate that ultrasound-guided platelet rich plasma injection at the ischial tuberosity for chronic recalcitrant hamstring tendinitis is an effective reliable treatment modality when traditional conservative treatment fails.
Time to return to sport was found to be significantly better in the PRP group
In a study from the British Journal of Sports Medicine, (10) doctors compared Platelet Rich Plasma Therapy, Platelet Poor Plasma Therapy (as an anti-inflammatory treatment), and no injection in professional athletes with MRI-positive hamstring injuries.
- All patients underwent a standardized rehabilitation program. Time to return to sport was found to be significantly better in the PRP group (mean 21 days) compared with the PPP group (mean 27 days). No significant difference in re-injury rate was noted between the three groups at 2 months or 6 months following intervention.
Chronic proximal hamstring tendinopathy and a single PRP injection
- In another study led by the University of Colorado Hospital Department of Orthopedic Surgery, (11) doctors examined 18 patients with chronic proximal hamstring tendinopathy who received a single PRP injection by ultrasound guidance. Patients had chronic hamstring pain symptoms for an average of 32.6 months prior to their injection and all patients had attempted other non-surgical treatments such as cortisone injections and physical therapy prior to injection. Based on a visual analog scale (VAS) for pain, patients had an average improvement in pain of 63% at 6 months following PRP injection.
In recent research that we cited at the beginning of this article, doctors at Northwestern University (1) looked at 18 patients, 12 females and 6 males – average age 42.6 who had pain that lasted an average of 32 months.
- The patients received one PRP treatment. Six months after the injection, 10/18 patients had 80% or greater improvement.
Overall, the average improvement was 63% with one patient – little to no improvement. The only documented side effect was post-injection discomfort that resolved within seventy-two hours.
This was ONE Injection – please see our article on choosing a PRP doctor, we discuss the idea of single injection PRP treatments.
These results could not be replicated long-term in cortisone groups. In a study from Stanford University, “Fifty percent of patients had symptomatic improvement lasting longer than 1 month after percutaneous corticosteroid injection, and 24% of patients had symptom relief for more than 6 months.” (12) Compare that to the PRP treated patients at Northwestern who had 80% relief.
In other words, the typical conservative treatment of cortisone is not a guaranteed cure for this hamstring injury.
What these researchers found is what Prolotherapy doctors have long known: a better approach to a hamstring injury is to strengthen the fibro-osseus junction of the ischial tuberosity with Prolotherapy. The ischial tuberosity bears body weight in the seated position and is a point of attachment for the hamstring muscles. In the above research, Platelet Rich Plasma injection was used at the “muscle origin” the fibro-osseus junction.
“Platelet-rich plasma vs. steroid injections for hamstring injury-is there really a choice?”
The above header is actually the title of an April 2019 paper in the journal Skeletal Radiology (13). It comes from a team of researchers at The University of Texas Health Science Center at Houston. The goal of this research? “To assess the effectiveness of pain relief in patients with grade 2 proximal hamstring injury, treated with platelet-rich plasma (PRP) or corticosteroid injection, by using the primary outcome of visual analog scale (VAS) at 1 week and 4 weeks of follow-up.” So what were the results?
- Among 56 patients, 32 received PRP and 24 received steroid injections with ages from 13 to 75 years old.
- At one week post-injection follow-up, 23 patients (71.9%) from the PRP group and 11 patients (45.8%) from the steroid group showed a positive response. The positive response rate in the PRP group was higher than the steroid group
- At 4 weeks post-injection, 23 patients (71.9%) from the PRP group and 13 patients (54.2%) from the steroid group showed a positive response with no statistical significance.
- “The PRP group had shown more favorable response compared to steroid group at 1 week post-injection, which suggests that PRP therapy can be considered as a conservative treatment choice for grade 2 proximal hamstring injuries with better short-term pain relief based on limited pilot data.”
PRP and dextrose Prolotherapy as a combination treatment
Although PRP is a type of Prolotherapy, we do not use PRP as a solution by itself. We offer PRP therapy with dextrose Prolotherapy treatments. The main reason is that dextrose Prolotherapy works on stabilizing injured structures by helping to repair damaged and elongated ligaments. The ligaments hold the bones to the bones whereas the tendons hold the muscle to the bone. When you stabilize bone-to-bone motion by way of the ligaments, you stabilize the bone to muscle motion and help prevent hamstring problems.
A multi-national team of researchers including those from Rutgers University, Virginia College of Osteopathic Medicine, and the University Regensburg Medical Centre in Germany tested the effects of Prolotherapy on tenocytes repair (tendon cells). Published in the journal Clinical Orthopaedics and Related Research, (14) what the team was looking for was how did Prolotherapy injections change the immune system’s response to a difficult-to-heal tendon injury.
These are the highlights:
- Prolotherapy injections changed the cellular metabolic activity to a healing, regenerative environment in the tendon cells.
- Prolotherapy activated RNA expression. The healing phase of soft tissue injury starts spontaneously after the tendon injury. Healing occurs in three phases: inflammation, proliferation, and maturation. RNA expression is the communication changes in genes (remember the gene expression from above) that coordinates the beginning and end of these three cycles of healing and injury repair process.
- Activated Protein secretion – the process of rebuilding. For a fascinating look at this subject please see our article on Extracellular matrix in osteoarthritis and joint healing.
- Cell migration. The ability of healing cells to get to the site of an injury, and the denial of damaging inflammatory factors from reaching the same site.
In our own published research, we reported in the Clinical medicine insights. Arthritis and musculoskeletal disorders, (15) we reported that the consensus is growing regarding the effectiveness of dextrose Prolotherapy as an alternative to surgery for patients with chronic tendinopathy who have persistent pain despite appropriate rehabilitative exercise.
Do you have a question about hamstring injuries and repair? Get help and information from Caring Medical
1 Wetzel RJ, Patel RM, Terry MA. Platelet-rich Plasma as an Effective Treatment for Proximal Hamstring Injuries. Orthopedics. 2013 Jan 1;36(1):e64-70. doi: 10.3928/01477447-20121217-20. [Google Scholar]
2Tseng CY, Lee JP, Tsai YS, Lee SD, Kao CL, Liu TC, Lai C, Harris MB, Kuo CH. Topical cooling (icing) delays recovery from eccentric exercise-induced muscle damage. J Strength Cond Res.2013;27(5):1354-61. [Google Scholar]
3 Malone T, Engelhardt D, Kirkpatrick J, Bassett F. Nerve injury in athletes caused by cryotherapy. J Athl Train. 1992; 27(3): 235–237. [Google Scholar]
4 van Doormaal MC, van der Horst N, Backx FJ, Smits DW, Huisstede BM. No Relationship Between Hamstring Flexibility and Hamstring Injuries in Male Amateur Soccer Players. Am J Sports Med. 2017 Jan;45(1):121-126. [Google Scholar]
5 Nuzzo JL. The Case for Retiring Flexibility as a Major Component of Physical Fitness. Sports Med. 2020;50(5):853-870. doi:10.1007/s40279-019-01248-w [Google Scholar]
6 Wan X, Qu F, Garrett WE, Liu H, Yu B. The effect of hamstring flexibility on peak hamstring muscle strain in sprinting. Journal of Sport and Health Science. 2017 Sep 1;6(3):283-9. [Google Scholar]
7 Ertelt T, Gronwald T. M. biceps femoris–A wolf in sheep’s clothing: The downside of a lower limb injury prevention training. Medical Hypotheses. 2017 Nov 1;109:119-25. [Google Scholar]
8 Lee KY, Baker HP, Hanaoka CM, Tjong VK, Terry MA. Treatment of patellar and hamstring tendinopathy with platelet-rich plasma in varsity collegiate athletes: A case series. J Orthop. 2019;18:91-94. Published 2019 Oct 31. doi:10.1016/j.jor.2019.10.007 [Google Scholar]
9 Fader RR, et al. Platelet-rich plasma treatment improves outcomes for chronic proximal hamstring injuries in an athletic population. Muscles, Ligaments and Tendons Journal. 2014;4(4):461–466. [Google Scholar]
10 Hamilton B, Tol JL, Almusa E, Boukarroum S, Eirale C, Farooq A, Whiteley R, Chalabi H6. Platelet-rich plasma does not enhance return to play in hamstring injuries: a randomised controlled trial. Br J Sports Med. 2015 Jul;49(14):943-50. [Google Scholar]
11 Kraeutler MJ, Garabekyan T, Mei-Dan O. The use of platelet-rich plasma to augment conservative and surgical treatment of hip and pelvic disorders. Muscles, Ligaments and Tendons Journal. 2016;6(3):410-419. [Google Scholar]
12 Zissen MH, Wallace G, Stevens KJ, Fredericson M, Beaulieu CF. High hamstring tendinopathy: MRI and ultrasound imaging and therapeutic efficacy of percutaneous corticosteroid injection. AJR Am J Roentgenol. 2010 Oct;195(4):993-8. [Google Scholar]
13 Park PY, Cai C, Bawa P, Kumaravel M. Platelet-rich plasma vs. steroid injections for hamstring injury—is there really a choice? Skeletal Radiology. 2019 Apr 1;48(4):577-82. [Google Scholar]
14 Ekwueme EC, Mohiuddin M, Yarborough JA, Brolinson PG, Docheva D, Fernandes HA, Freeman JW. Prolotherapy induces an inflammatory response in human tenocytes in vitro. Clinical Orthopaedics and Related Research. 2017 Aug 1;475(8):2117-27. [Google Scholar]
15 Hauser RA, Lackner JB, Steilen-Matias D, Harris DK. A Systematic Review of Dextrose Prolotherapy for Chronic Musculoskeletal Pain. Clin Med Insights Arthritis Musculoskelet Disord. 2016;9:139-59. Published 2016 Jul 7. doi:10.4137/CMAMD.S39160 [Google Scholar]