Rehab of Hamstring Strains

Picture this: you're watching a sprinter in full flight, only to see them suddenly slow down, clutching the back of their leg, their expression shifting from determination to pain. It’s a common sight in sports and often points to one thing— a hamstring strain. Hamstring strains can range from career-threatening injuries to mild sprains that take just a few weeks to heal. This post dives into everything you need to know about these injuries, from causes and symptoms to recovery and prevention.

Introduction:

Hamstring strains present the leading cause for time loss in different sports (e.g. in soccer or other high-intensity running sports such as sprinting).
Time lost from sports typically takes around 2-3 weeks, with first time injuries taking around 2 weeks and recurrent injuries (due to greater severity) taking 3+ weeks (Brooks et al., 2006).

In general, HSI can be defined as pain in the posterior thigh which causes cessation of subsequent exercise. In a hamstring strain injury (HSI) some muscle fibres of either the biceps femoris long head, semimembranosus or semitendisonsus (listed in order of injury likelyhood) get either overstretched (grade 1, mild) or torn (grade 2, moderate) or the muscle in its whole is torn (grade 3, which makes for the most severe type of injury).

Mechanism of injury:

Athletes with acute posterior thigh pain following a typical hamstring strain injury mechanism will be regarded as having a hamstring strain, until proven otherwise. Only if the mechanism is not typical, a differential diagnosis needs to confirm or rule out other pathologies.

Two different mechanisms have been suggested in the literature (Danielsson et al., 2020):

1. High speed injury: is estimated to occur at the end of swing phase of running, where the muscle elicits high eccentric (active lengthening) muscle activity. With this mechanisms, we mostly see the long head of the biceps femoris long head (BFLH) getting injured (and due to this mechanism being the most prevalent, the BFLH is the most frequently injured muscle), as this muscle gets stretched out the most and therefore high levels of muscle strain will be experienced.
   This injury makes up for 80% of all HSI (typical sprinting injury).
2. Stretching injury: when the hamstring musculature experiences a fast stretching movement (maximal hip flexion and knee extension), the muscle fibres can get overstretched. With this mechanism, we see the semimembranosus muscle that gets strained most often (making it the second most often injured hamstring muscle).
   Around 20% of all HSI are stretching type injuries (typical kicking injury).

The closer athletes get to 100% of their running velocity, the greater the forces applied to the hamstring muscles, which makes high speed running a very common injury mechanism, but high speed running injuries have shorter recovery times than the stretching type injury.

The cause or mechanism is very important to assess and should lay the groundwork for developing the exercise and prevention program, as the mechanism tells us what we need to focus on.

Managing Hamstring Strains:

When it comes to treating hamstring strains, traditional wisdom has often suggested complete rest until pain subsides. However, recent research has challenged this notion. The Hickey trial (2020) showed that training through mild pain (up to 4/10) can be beneficial, potentially leading to faster or better recovery outcomes compared to complete rest. This finding suggests that some level of pain during rehabilitation is not only tolerable but might actually be helpful.

The key takeaway is that during rehabilitation:

• Mild pain (up to 4/10) is acceptable. It can serve as a useful indicator that the targeted muscles are being properly engaged.
• Pain should not increase over time. If pain worsens, the training load should be reduced.
• Progressive loading is essential. Starting with low-intensity exercises and gradually increasing the load helps strengthen the hamstrings without over-stressing them.

The 2022 clinical practice guidelines for hamstring strains also support early initiation of hamstring strengthening, particularly with eccentric exercises, while monitoring pain tolerance.

Risk factors

Non-modifiable:

• Previous hamstring injuries. A history of hamstring strains is the strongest predictor of future injury.
• Previous ACLR. With ipsilateral hamstring autograft.

Modifiable:

• Hamstring weakness or imbalances. Especially in eccentric strength (at least in soccer players, Ribeiro-Alvares et al., 2020).
• Poor flexibility and core stability. Although evidence is mixed, addressing these areas may reduce risk. Ribeiro-Alvares and colleagues also support the physical assessment of core stability and functional movements (lower leg and core exercises of the FMS) to check all the boxes concerning modifiable risk factors and having a good grasp on your athletes condition. As a forward trunk lean during running will put the pelvis into an anteriorly tilted position which will then put more of a stretch and therefore more stress onto the hamstrings, which is also part of the reason that pelvis-control and core-stability exercises are often included in rehab, but only for those people that present with a running technique with significant forward trunk lean.
• Reduced biceps femoris long head fascicle length. Although hard to measure in practice.

HSI risk factors can all be collected at a single time point (regarding hamstring strength and fascicle length) and do not need to be assessed regularly during a season, as testing for them multiple times did not lead to superior predictive performance for identifying individuals who will suffer future HSI.

A critical review on screening test also addresses risk factors for HSI and the authors concluded, that when looking at people with decreased hamstring strength, some get injured, but others don't. As there exists a substantial overlap between people who are at risk and people who aren't, suggesting that eccentric hamstring strength is unable to accurately predict who will get injured. The accuracy, as the authors stated is no better, than flipping a coin, and they concluded by stating that "to date, there is no screening test available to predict sports injuries with adequate test properties and no intervention study providing evidence in support for screening for injury risk". Meaning we do not need to obsess over assessing athletes with high risk, but simply focus on making our athletes better athletes. Make them stronger, faster and train for more endurance, as that is probably our best bet.

Diagnosis:

Differential Diagnosis: adapted from (Hickey et al., 2022)

• Avulsion of the proximal hamstring tendon: Pain is severe and near the ischial tuberosity. A palpable deficit may be present as well as visible bruising.
• Proximal hamstring tendinopathy: Pain will not show an acute onset but gradual and will often get better during exercise (after warming up). Pain will be triggered from increased loading (especially in hip dominant movements).
• Referred pain from the lumbar spine: Forward slumped posture might be an indication for this (performing the slump-test is advised). Pain is usually felt not only on a single location as in a strain, but more diffuse.
• Adductor muscle injury: Acute onset just as in a HSI, but pain location is more medial. Mechanism can be the same (especially in kicking sports). Testing for pain in isometric adduction or abduction ROM can be helpful to differentiate it from HSI.

There may have been a popping sensation accompanying the muscle strain. If the point of maximal palpation tenderness is close to the ischial tuberosity, tendon pathology should be considered in the differential diagnosis.
A Contusion type of injury should be considered if trauma was due to direct impact onto the hamstrings.

Making the diagnosis:

As the mechanism nearly always consists of "simultaneous hip flexion and knee extension" (Danielsson et al., 2020) this is what we will bee looking for in the patient history.
If a patient presents with acute posterior thigh pain after during activity, the injury-mechanism matches the typical HSI mechanism (high speed movement in hip flexion and knee extension), there is tenderness on palpation and when the hamstrings are stretched or activated this pain is reproduced, then the diagnosis for a hamstring strain should be made (Martin et al., 2022). In people with complete tears, pain on activation will be absent, as no activation will be possible.
In another paper (Vermeulen et al., 2022), the making the clinical diagnosis of HSI included findings of localised pain during palpation, pain reproduction during isometric contraction and pain when stretching (passive straight leg raise).

For the first assessment we recommend: (the cited studies are the ones that included these methods in their diagnosis)

• Pain on palpation of the injured muscle (identifying the point of maximal tenderness) (Hickey et al., 2020; Vermeulen et al., 2022; Martin et al., 2022), to exactly locate the site of injury and check if tendon tissue is involved and assess the length of the painful area.
• Measuring the distance from the ischial tuberosity to the point of maximal palpation pain in cm is recommended, as some connection between the measured length and rehabilitation time (with greater distance indicating faster recovery) has been made (Hickey et al., 2020).
• Outcome measure: 17-item Tampa Scale of Kinesiophobia (TSK) to assess fear of movement (Hickey et al., 2020), to assess for psychological factors.
• Hamstring ROM via passive straight leg raise (Vermeulen et al., 2022) with rating of pain 0-10 and recording the ROM measured with an inclinometer, and compare that to the uninvolved side. And the maximum hip flexion with active knee extension (MHFAKE) Test also for flexibility.
• Isometric knee flexor strength (Martin et al., 2022), in the inner (90° of knee flexion) and outer range (close to 0° of knee flexion) by using a HHD and noting the peak force output, ask for 1-10 NRS. Compare it to the uninjured side or to the pre-injury value (if present).
• Asking the athlete to rate their pain NRS at the time of HSI (gives a hint on RTS time, with less pain suggesting faster recovery).
• Hip flexor flexibility and ankle dorsiflexion ROM (Hickey et al., 2022), are associated with HSI risk.
• Muscle testing with HHD for other kip and knee musculature and comparing to the uninvolved side, which may inform about exercise selection (Hickey et al., 2022)

Also ask patients about pain during daily activities. Although each one of these tests alone is not very useful to assess a patients injury standpoint, taken together, they represent a great tool to make a better picture of the athletes condition.

Prevention of hamstring strains:

Reducing the risk of re-injury needs to be a priority after a first time hamstring strain, to prevent further, unnecessary time loss from training and competition. This is especially important if an athlete presents with recurring HSI, then an even more thorough examination (complete kinetic chain) and rigorous strength training is needed.

As we have seen in the "risk factor" section, reduced hamstring strength increases risk for injury. Therefore it is only rational to include strengthening exercises in a prevention program.
One particular exercise that comes to mind is the Nordic Hamstring Exercise (NHE). The NHE is an eccentrically biased exercise, performed on the knees, with the ankles held or strapped down. During the exercise, subjects lower their upper body towards a prone position as slowly as possible.
A recent systematic review and meta-analysis found that "both high and low volume prescription can produce large-to-very large improvements in both strength (eccentric torque and eccentric force) and muscle architecture (Fascicle length and Pennation angle) over a minimum duration of 6 weeks" (Cuthbert et al., 2020).
When introducing a NHE training program, it's better to start slow, with few repetitions. Exemplary you could start performing 3 sets of 3 reps and then first increase frequency and then training session volume (up to 4 x 6-8). As the high levels of DOMS (delayed onset muscle soreness), experiences after heavy eccentric training, leads to sobering compliance levels in some athletes.
The focus needs to be on gradually increasing exercise intensity, as in traditional strength training. Maintaining a consistent training volume allows players to improve and produce force over a greater range of motion, effectively achieving desired adaptations. To address non-compliance due to severe DOMS, the evidence suggests reducing volume and keeping the training frequency consistent.

Different systematic reviews and meta-analyses exist to look at the effect of the NHE for injury prevention, in which some show significant effects of the NHE for prevention (Ishøi et al., 2020).
While another meta analysis was conducted using a different methodological approach, they found low evidence to support the use of the NHE for HSI prevention. But as they also didn't recommend against its use.
The NHE can nontheless be implemented, but reductions in HSI injury rates in the studies likely weren't due to one exercise (= the NHE), but derive from a complete training program focusing on increasing hamstring strength that also happened to include the NHE.

The 2022 clinical practice guidelines (CPG) for HSI recommend implementing the NHE for patients while also focusing on other sports-specific training, stretching and stability work, to create a comprehensive training protocol, of which the NHE makes up only a part (Martin et al., 2022). As a warm-up strategy, the FIFA 11+ program has shown to reduce HSI in football players.

The main proposed mechanism by which the NHE prevents HSI, is by reducing 2 risk factors: fascicle length and hamstring eccentric strength. As the exercise is eccentrically biased, eccentric strength will be gained and due to the lengthening under load and the longer muscle lengths achieved during the exercise, fascicle length has been shown to increase, which is associated with HSI risk.

BFlh fascicle length can be increased after an eccentrically biased training intervention for the hamstring, which can be regarded as a key mechanism by which eccentric exercises provide preventative protection against future re-injury (or at least, it's a good guess), as longer fascicle length will lead to less stain of the muscle in the stretched position (as more length is possible before the muscle reaches its limit).

We will add, that we side more with the critical review when looking at NHE for prevention. As injuries in general are simply way too complicated a topic, for there to exist "the one" exercise to significantly reduce that risk. Nonetheless, we still would encourage people to train and strengthen their hamstrings, but the evidence is simply not there to say that we can predict who is gonna have an injury and that those that perform the NHE will have a reduced risk. As most "injury risk screenings" assess either strength, flexibility, movement technique or reaction time, it is always good to include some additional exercise to at risk populations. And as NHE are easy to perform and don't have side effects (apart from maybe increasing performance), they should be included.
This intervention might not be offered only to those at "high risk" (especially those with previous HSI), but to the whole sports team.

Exercise Training:

Acute Management:

Before going into how to treat a symptomatic hamstring strain, we will quickly go into the acute management of the injury:
As we have already proposed in other blog posts that talk about acute injuries, we recommend sticking to the PEACE & LOVE protocol that was developed by Dubois and Esculier. Which emphasises to first focus on protecting the injured area for the first days (depending on how severe the injury is, but some might need crutches for the initial days if pain is too high while walking) and then slowly moving into a more active and educating approach for treating the patients. But for a more detailed explanation, check out the original post on the topic.

A patient can in theory start with the exercises the day after an injury (if training is within the patients pain-limit) but 2-3 days of rest after the injury is not the wrong choice for the not-so-highly-athletic population out there.

Long Term Management:

When deciding for an exercise training protocol and exercise selection, it's important to consider what the end-goal is (what level of performance does my patient want to get back to?) while also being informed about the injured tissue.

As hamstring strains happen during sports, the muscle group needs to be trained back to the previous, or even better, a higher performance level in order to decrease re-injury risk.
Our rehabilitation goals for athletes are returning them to sport as soon as possible while achieve the highest level of performance with minimal risk of re-injury.

In the CPG, successful interventions were shown to have adapted a training frequency of 2-3 times per week, with exercise progressions or load increased as the patients tolerated (guided by pain levels), in a 6-12 repetition range.
Additionally, we recommend choosing 2-3 hamstring specific exercises, with the bare minimum being one hip- and one knee-dominant exercise.
Eccentric loading exercises can and should be introduced early, as long as the intensity is keeping pain levels within the recommended limit.
The rehab exercises will focus on eccentric knee flexor strength and hip extensor strength at moderate to long muscle lengths.

The named exercises are adapted from Hickey et al. (2020) as all participants in the study saw increases BFLH fascicle length and isometric knee flexor strength, along with a reduced fear of movement.
In their initial rehabilitation session, participants began with bilateral hamstring bridges (elevated feet with knees slightly bent). If pain allowed (= less than a 4 out of 10), this was advanced to a unilateral bridge. They also performed 45° hip extensions on a back extension machine, later moving to unilateral 45° hip extensions (which is one hell of an exercise... definitely worth trying!). Finally, they did bilateral eccentric sliders, progressing to either unilateral eccentric sliders or Nordic hamstring curls.
After switching to unilateral movements the exercises will be further progressed by adding external loads (e.g. holding a 5 kg plate).

We know that we can regionally target muscle activation of the proximal or distal hamstring, by choosing to perform either hip- or knee-dominant exercises. The effect has been shown to be greater for the distal portion of the hamstrings. Meaning that leg-curl based exercises lead to higher distal hamstring recruitment compared to hip-dominant (e.g. deadlift) movements. This knowledge can be applied to practice in to ways (Schoenfeld et al., 2015):

1. We will first chose to overload exercises that are hip dominant for a distal injury and knee dominant for the proximal injury. In order to allow us to work as heavy as possible early on.
2. Then, once the pain has settled, we will switch the approach and focus on the exercises that will hit the exact spot of injury (as this is likely the place of greatest weakness) and focus our efforts there.

So we would start by focusing training the muscle farthest away from int site of injury, while gradually coming closer with more and more load.

The NHE will only be included after adequate strength has been acquired. But it will be seen as a rehab goal to implement the NHE into the protocol.

As HSI is often seen in sprinters, we will include hip extension exercises that prioritis the glutes, as strengthening them might take some load away from the hamstrings, leading to less fatigue and reducing injury risk. Although this is based on speculation, gluteal exercises have shown to increase sprint performance, especially the hip thrust exercise, and will be included in our training protocol.

As with everything else, the SAID principle applies here as well. Specific Adaptation to Imposed Demand means, that we need to prepare the hamstring musculature for the sports specific demands they need to handle. It is likely that this can be done via choosing from a large pool of exercises, as in a paper on expert opinion, no consensus was reached for which exercises were best to include at which rehab stage of HSI. Which means we are free to chose exercises that fit out patients needs and equipment.

Just as its important to know what the sports demands are, we need to understand which tissued (muscle or tendon) have been injured. In order to understand healing times and to create the best possible environment for healing to take place. We want to load the tissue as much as possible for it to adapt, but not too much for it to re-injure. In order to succeed with that, we need to pay attention to tissue healing times (more on that below) as well as to symptoms (pain and function). But as symptoms can subside while the tissues are not yet fully healed, symptoms should never be your sole orientation point. Combining a time-based and symptom-based approach is recommended.

Exercise progression will be made according to symptoms and symptom exacerbation. If no or minimal pain (depending on rehabilitation stage) is present, progressions can be made. The only progression where we do not want patients to experience pain is when progressing to 100% sprints, because the risk of injury increases with higher intensity exercises, so better to go safe than sorry.

Pain free movement is also recommended for a more tendinous HSI. But in all the other cases, training with a pain threshold (<4/10) may accelerate increases in isometric knee flexor strength and fascicle length compared to pain-free rehab (Hickey et al., 2020).

Higher re-injury risk persists up until 1 year after the primary injury. Making a point on why it is significant to stick to a hamstring specific training protocol for longer than the RTS time.

Apart from a heavy exercise regime, including cardiovascular exercises, on a stationary bike, is crucial for the endurance athlete, recovering from HSI. As well as upper body training, which should not be left out simply because of a lower leg injury.

Flexibility Training and Stretching for Hamstring Strain Rehabilitation

After an acute HSI, knee extension ROM is limited, mostly due to pain. But this ROM deficit usually recovers on its own, without any intervention, after the first few weeks of injury. However, if this knee extension deficit persists following HSI, then it has been shown to be a contributing factor for increased re-injury risk, and therefore needs to be addressed.

As we find it interesting as to why flexibility work might lead to reduced injury-risk in athletes, we will shortly go into the rationale of a paper from Ruan and colleagues, that proposed an underlying mechanism for this preventative intervention. In their study they used a passive static straight-leg raise for their stretching exercise, lying supine, with knee in full extension. Stretches were held for 30 seconds, repeated 4 times with 10 seconds rest between sets. Their main findings were, that stretching led to a change of peak knee flexion torque in muscle length and that Ground reaction forces were reduced in the biceps femoris. Which would mean reduced tissue strain (reduced sarcomere disruption) and therefore reduces injury risk. They also proposed that (although lacking evidence) stretching might reduce tendon stiffness which then reduces the load of the muscle, if the tendon gets stretched out more.
After static stretching of the hamstrings (SSH), there was a significant reduction in the peak ground reaction force (GRF) during the early stance phase. Correspondingly, the peak joint torque at both the hip and knee was also significantly reduced. This implies that less force was being produced in these joints during this phase. Additionally, the force production of the biceps femoris (BF) muscle was augmented at longer muscle lengths. This means that while overall force production in the joints was reduced, the biceps femoris was able to generate more force when stretched to a longer length.
In summary, the findings suggest that after hamstring stretching, there is less overall force production at the hip and knee joints during the early stance phase, which might help reduce the risk of hamstring strains (HSIs) occurring at this time.

Stretching leads to an acute increase in fascicle length, making it a useful short-term intervention for high-risk athletes, particularly during high-load or competition phases. In contrast, eccentric training should be the primary long-term strategy to prevent hamstring injuries. This approach aims to achieve sustained increases in fascicle length. For instance, static stretching before high-season competitions could be an adequate preparation strategy.
As eccentric training in low volumes, has been shown to generate increases in BF long head fascicle length. The low volume group in this study performed only 2 sets of 4 reps per week (=1 training session / week), after an initial 2 weeks of training with higher volumes (2 times / week, 4 sets of 6 reps). Indicating that 1 training session of heavy eccentric work might be enough.

Although some evidence suggests decreases in performance after static stretching, it might be a good trade off for the high-risk athlete, to decrease injury-risk for a small decreases in performance.

Running Progression

After addressing initial flexibility concerns, the next critical phase in hamstring rehabilitation is managing running progression. This section outlines the stages of a running progression, which are designed to gradually reintroduce high-speed running and minimize the risk of re-injury.

In hamstring rehabilitation, the transition to high-speed running is a crucial component. According to Hickey et al. (2022), "returning to high-speed running is arguably the most important aspect of rehabilitation." Sprinting can exert forces on the hamstrings that are approximately 50% greater than the maximal generated isometric force. Therefore, a progressive approach to running is essential to minimize the risk of re-injury.

Criteria for Starting Running

Any running progression can begin once an individual can walk pain-free. This should be done within the individual's pain tolerance, ensuring that pain does not exceed 4/10. This approach, utilized by Hickey et al. (2020), will guide our recommendations. Initially, patients can start with low-intensity jogging as long as it does not cause their pain to rise above 4/10.

• Pain does not exceed 4/10
• Able to walk pain-free

Running Speed Categories

• Jog: Up to 50% of perceived maximal running speed
• Run: Up to 70% of perceived maximal running speed
• Sprint: Up to 100% of perceived maximal running speed

Running Progression Stages

The running progression involves nine stages, each with a structured pattern of walking, jogging, running, and sprinting.

Each level should be repeated three times. Progression is only recommended when pain is less than 4/10 or the individual is pain-free.
A maximum of three levels (nine rounds) can be completed per session.
In subsequent sessions, start at the second-highest level completed in the previous session (adapted from Silder et al., 2013).

High-Speed Running and Sprinting

Before advancing to sprinting, ensure the patient is completely pain-free. This minimizes the risk of re-injury.
From the 80% mark, progression to 100% sprinting speed should occur in small increments of approximately 5% per session (Hickey et al., 2022).

Incorporating technique work at this stage is beneficial. Monitoring for exaggerated anterior pelvic tilt and integrating technique drills can enhance performance and reduce injury risk.

For example, a professional soccer player might begin their running progression with short jogging intervals and gradually increase to full sprints within four weeks. Which is a reasonable timeline.

Typical Running Session

A typical session involves three sets of four laps or eight runs, totaling approximately 700 meters. Additionally, three direction change runs add a further 120 meters, resembling the distance an elite football athlete would sprint in a professional match (Whiteley et al., 2018).

Example Progression (adapted from E3-rehab)

• Stage 1: Slow jog for 20 meters, up to 50% maximal velocity for 60 meters, then back to a slow jog for 20 meters. Progress to the next stage after completing three repetitions pain-free at 50% speed.
• Stage 2: Moderate speed for 30 meters, up to 80% maximal velocity for 50 meters, then slow down. Progress after three repetitions pain-free at 80% speed.
• Stage 3: Incremental increase to maximal sprinting speed (5% increments) over 30 meters.

Key takeaway: Progress gradually by increasing distance and intensity, while monitoring pain levels closely.

Once an athlete is beyond 70% of linear sprinting performance, change of direction training is initiated (Whiteley et al., 2018) and progression should not only commence if the athlete is pain-free (or experiences low levels of pain), but also only if he is confident in his ability to push further.

Above 40% of perceived maximum running intensity, athletes have a good ability to estimate their running effort meaningfully (Whiteley et al., 2018).

Including drills like the “triple extension jumps” and “B” drill before every set of running has been recommended in the literature for athletes, due to their focus on the late swing phase of running. As the triple extension jump is a plyometric exercise that mimics the hip, knee, and ankle extension required for powerful running strides

From a training program perspective, we recommend performing the eccentric training the day after running. As we do not want to compromise sprinting performance from muscle soreness.

Final thoughts: running progressions are like a double bladed sword, where on one side you do not want to increase running intensity too quickly, as that might increase injury risk during rehabilitation, but on the other side, progressing too cautiously will unnecessarily increase RTS-time.

In summary, adhering to a structured running progression protocol is crucial for balancing the need to avoid re-injury with the goal of returning to sport as quickly as possible. By carefully monitoring pain levels and progressively increasing running intensity, athletes can safely and effectively return to high-speed performance.

Passive interventions

Platelet-rich plasma injections appear, at best, to be a non-harmful yet ineffective treatment in accelerating RTS or mitigating the subsequent HSI risk.

In the absence of clear evidence, practitioners need to consider the potential time cost of implementing manual therapies during HSI rehabilitation against any perceived benefit of these interventions.

Return to sports:

Criteria for RTS Decision-Making:

After a sports injury, the first question asked by most athletes (and coaches) is: ‘When will I (the athlete) be able to compete again?’. The final decision to return to competition should be a shared process involving the medical staff, the coach, and the player. While social, biological, and psychological factors influence the decision, we will focus on the key objective measures:

Key Clinical Measures for RTS Decision-Making: (Van Der Horst et al., 2017)

0. Patient’s Perception of Pain:

• Pain-free clinical evaluation
• No pain on palpation or during strength and flexibility testing
• Pain-free functional performance

1. Clinical Tests and Measures:
   • Range of Motion (ROM) within 90% of the uninjured leg
   • Strength assessments using Hand-Held Dynamometry (HHD) or isokinetic testing.
   • Eccentric strength, particularly in a maximally stretched position.
   • Consider both active and passive straight leg raise tests for hamstring flexibility (both should be pain free!)
2. Functional Testing:
   • Functional tests such as hop testing cluster, sprint performance.
   • Sport-specific functional field testing.
   • Performance on field testing.
   • No pain during sprinting at 100% capacity
   • Successful completion of a progressive rehabilitation exercise program.
3. Specific Tests:
   • Askling H-test: The Askling H-test is an active hamstring flexibility test. A pain-free result correlates with a low risk of re-injury, but it can also prolong rehabilitation time. It needs to be considered for each athlete if it is worth the time.
   • Whiteley and colleagues suggested having the athlete perform repeated 30m sprints at 100% intensity as well as 40 meters of sharp direction changes, also at 100% of self-rated intensity. If these two drills are not capable of eliciting hamstring pain, then RTS is permitted (Whiteley et al., 2018)
4. Time-Based Factors:
   • Early vs. delayed lengthening exercises showed no significant difference in time to return to sport or risk of reinjury. Early lengthening group returned 8 days earlier on average, but results must be interpreted cautiously.

The above factors should be assessed prior to RTS and shortly after (after first 1-2 full training sessions), to check if the hamstrings are still doing fine.

To accelerate RTS, it's important to know when to safely implement lengthening exercises into rehab. The answer it as soon as tolerable. Literally the first day after having sustained an injury, people can start strengthening the injured tissues. In a recent RCT, they found no statistical difference in RTS if people implemented lengthening exercises (in this case the Askling diver, extender and slider) early or later during rehab. But there was a clinically relevant trend going on, favouring early introduction (Vermeulen et al., 2022), which is our recommendation. But even if an athlete is not able to tolerate such exercises, early on, there is no need for worry. Simply implement then as soon as the athlete feels ready. In the RCT of Vermeulen and colleagues, the early lengthening group had significantly greater eccentric strength at RTS, then the delayed group. Just as shown in the Hickey trial for the group that was allowed to train to progress with pain.

As a rule of thumb, the sooner a person is back to walking pain free, the earlier RTP can be initiated (Martin et al., 2022). And a previous HSI needs to be considered for the RTP decision. The CPG also mention, that a progressive exercise regimen as well as eccentric exercises should be included to reduce re-injury risk.

Ardern and colleagues defined three graded steps, that make up the RTS continuum (Ardern et al., 2016):

1. Return to participation, where an athlete undergoes exercise training during rehabilitation. Meaning that the person is active, but is not yet back in his sport.
2. Return to sport (RTS), where the athlete is back playing his or her sport, undergoing sport-specific training sessions, but not yet back at his original training level.
3. Return to performance, where the athlete is back or above his previous training and performance level, and ready to play without restrictions.

It is crucial that an athlete sticks to either these 3 proposed stages, or any other specific continuum that allows safe and progressive return to sports and performance.
To aid in the return to play decision making and weigh out the risk factors, Ardern and colleagues proposed using the Strategic Assessment of Risk and Risk Tolerance (StARRT) framework. The StARRT framework (Strategic Assessment of Risk and Risk Tolerance) is a three-step model to help evaluate the risk and readiness for returning to sport. These steps includes medical factors (e.g.: symptoms; functional tests), sport risk modifiers (e.g.: type of sport; position played) and decision modifiers (e.g.: time & season; external pressure). All of the factors assessed by the StARRT framework are relevant for the RTS decision making.

The good news is that nearly 100% of athletes return to sport after an acute HSI (apart from some individuals that experienced a complete tendon or muscle rupture) (Ardern et al., 2016). And more than 80% of all HSI compromise grade 1 and 2 hamstring strains.

There has been a suggestion to define RTP as "the moment a player has received criteria-based medical clearance and is mentally ready for full availability for match selection and/or full training" (Van Der Horst et al., 2017).

While physical readiness is critical, psychological confidence plays an equally important role in the safe return to sports.
Fear of re-injury can lead to compensatory movements that hinder a full return to peak performance. Therefore checking for psychological issues, socioeconomic status and stress levels is as important for return to sports as it is for curing pain.

Psychological readiness and confidence is crucial for save RTS. But also, some people might be confident to RTS already 2 weeks post injury, when tissues are not yet fully regenerated. In this case, we need to slow down the patients.

Time-Based Approaches to RTS:

A case needs to be made about the high re-injury rates especially in the first week and first month after RTP. And that even after an athlete "feels" like he has completely healed (because he is pain-free), we sill need to be cautious. As the the muscle tissue likely has not reached adequate healing for some individuals. But as some research on MRI findings suggests, it is not necessary to wait for full MRI tissue resolution, athletes have been shown to safely return back to competition without MRI showing full tissue healing.

So then: when are people ready for RTS?
The best way to go about this is to clear all of the above listed criteria, check them off to have the most important objective criteria settled up. Then a decision regarding safety and urgency needs to be made, which determined by time. A "probably" safe decision for RTS can be made by relying solely on the above criteria, or a "likely" safe decision by waiting at least 4 weeks before RTS. Which is the timeline suggested by a paper focusing on the physiology of muscle healing (regeneration of ruptured muscle fibers, formation of connective tissue scar... for an in depth review, check out the linked paper) , that recommends this 4-weeks healing time line, if RTP is not needed to happen ASAP but can take 1-2 weeks longer than usual. The main point being that they can only recommend a biologically safe RTP only 4 weeks after injury. Which could explain the high recurrence rate when players get allowed to RTS as early as 2 weeks post-injury. This on the other side does NOT mean that simply waiting it out for four weeks would do the job. Athletes must build strength, speed, and flexibility to handle the demands of competition. Regardless of whether the the tissue has reached its full healing capacity or not.

While of course, it is crucial to discern between less (grade I) vs more severe (grade II) injuries, that definitely require different healing times, for both grade I/II HSI, the 4 week period is recommended and can be taken as a rule of thumb when going for safety > speed. However with this approach, there are surely some people that could make a faster RTS by going the traditional way.

As waiting 4 weeks will keep some athletes from playing longer than necessary, we suggest a "time-based" approach for athletes that experienced a recurrent HSI, that are at high risk of experiencing a recurrence or where RTP is not urgent.

Key takeaway: A structured, objective assessment of strength, flexibility, and performance, combined with time-based factors and psychological readiness, is essential for making safe RTS decisions.

Outcome measures

In the Clinical Practice Guidelines the "Functional Assessment Scale for Acute Hamstring Injuries" is recommended to and before and after the rehabilitation process. Every 2 weeks, this outcome measure should be performed.

Exercise progression criteria and protocol

In this last part, we will present you with information on how to create a training program for people rehabilitating from a HSI. Here we will include hip and knee dominant exercises, while discerning if the HSI is medial or lateral, proximal or distal.

For the proximal HSI, we will start with 2 more knee dominant exercises, and 1 hip dominant movement pattern. While for the distal HSI, we will perform 2 hip dominant exercises while going slowly on the knee flexor exercises. Impacting more of the medial or lateral aspect of the hamstrings can be influenced by foot positioning (active internal rotation of the foot has been shown to increase medial hamstring rotation during the prone leg curl). While some evidence suggests that foot positioning may affect the medial hamstrings, this remains a secondary consideration and should be tailored to individual response, as evidence is still scarce.

People can start exercising the hamstrings as soon as initial pain symptoms subside and the exercises are possible to perform while experiencing pain <4 on a 1-10 scale. On the non-training days perform a running program.

Athletes must adhere to their rehabilitation protocol for optimal recovery, even if they feel ready to return sooner. Inconsistent training or skipping exercises can delay recovery and increase the risk of reinjury.

Exercise selection: (hamstring specific)

As the CPG recommend including stretching and stabilisation exercises into a complete training program, we shall do so in the provided training program at the end of the article. And creating adjunct exercises for specific sub-populations (like the forward lean group) where trunk and pelvis stabilisation exercises are needed.

Although here we discussed the exercises and techniques that are crucial to get the hamstrings ready again, during rehab, focusing on other movements such as squats, split squats, hip flexor training and whatever an athlete needs in his sports will be incorporated into the exercise regime below.

Phase 1 (early recovery)

Pick 2 of the below listed exercises and perform them slowly and without external loading. Stick to double-legged variations if possible for the exercise.

Phase 2 (mid-stage)

Pick 3 of the below listed exercises, perform the single-legged variations and add external loads if possible. Also commence training for core-stability, technique work and work the rest of the body thoroughly (perform squats, split-squats etc., everything that is non-hamstring specific can be done).

Phase 3 (late-stage/return to play)

Finally, incorporate NHE, increase loading of the chosen exercises or change them up and increase speed. Perform jumps and other plyometrics to increase hamstring loading tolerance.

Overall: Progress exercises every 1-2 weeks (or even every session) based on pain tolerance and strength improvements.

HIP DOMINANT: hip-dominant exercises focus on movements originating from the hip joint

• Supine bridges (2 legged, single legged, with slight or large knee extension)
• Supine isometric heel digs
• The "Extender"
• The "Diver"
• The "Glider"
• Deadlift variations
• Back extensions
• Hip thrust variation

KNEE DOMINANT: knee-dominant exercises focus on movements originating from the knee joint

• Nordic hamstring exercise
• Bilateral eccentric slider (progress to unilateral)
• Prone or sitting hamstring curl

Select 2-3 of the above exercises, progress them over time (5-10% per week) either by making the exercise more challenging (from 2- to 1-leg) or by adding external loads (e.g.: adding a resistance band or a 5kg plate), perform 2-3 sets with 6-15 repetitions

Example program:

First week:

Exercise: Bilateral eccentric slider
Sets: 3
Reps: 12-15
Time: 3-0-1

Exercise: Deadlifts
Sets: 2
Reps: 12-15
Time: 3-0-1

Exercise: Supine isometric heel digs bilateral
Sets: 3
Reps: 12-15
Time: 3-0-1

Second-third week:

Exercise: Unilateral eccentric slider
Sets: 3
Reps: 10-12
Time: 3-0-1

Exercise: Deadlifts (increased weight)
Sets: 3
Reps: 10-12
Time: 3-0-1

Exercise: Supine isometric heel digs unilateral
Sets: 3
Reps: 10-12
Time: 3-0-1

End-phase: 3 weeks +:

Exercise: Unilateral eccentric slider
Sets: 4
Reps: 6-8
Time: 3-0-1

Exercise: Deadlifts (increased weight)
Sets: 3
Reps: 6-8
Time: 3-0-1

Exercise: Supine isometric heel digs unilateral
Sets: 4
Reps: 6-8
Time: 3-0-1

Patient education and summary:

Given the importance of a structured rehabilitation protocol, patient education has become a pivotal component in therapeutic fields. By ensuring that athletes understand the rationale behind their treatment, adherence to the program and overall recovery outcomes can be significantly improved.
But what do we absolutely need to educate our patients on to ensure a good outcome with hamstring strains? (ps: luckily this makes for a good blog post summary):

• Training with pain (<4 / 10) is acceptable (even recommended) but is not needed for great recovery.
• Exercise is the key to recovery, and techniques like massage can help but are not enough on their own.
• Return to competition should not be considered prior to 4 weeks after the injury, due to the natural healing processes of the body.
• Return to running and sports needs to be criteria based.

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