Hamstrings

Overall, what is a hamstring injury and how does it occur ? 

Hamstring strain or muscle damage occurs when the hamstrings are working to slow down a run. In this case, their purpose is to decelerate the extension of the knee by developing tension but at the same time lengthening, representing a significant eccentric contraction. However, they can also occur when players sprint or change direction. They switch from eccentric operation, to decelerate the knee extension at the end of the swing, to concentric operation, to become an active extensor of the hip joint(1)(2)(3). It has been suggested that it is during this rapid change from eccentric to concentric function that the muscle is most vulnerable to injury (4). Slow healing may be associated with this condition and persistent discomfort or apprehension may accompany the return to sport, representing a complex problem for staff, coaches and the player himself (5). It should also be noted that the relapse rate for hamstring injuries is 12-31%,((6)(7)(8)(1)) and is often the result of an early return to play and incomplete rehabilitation (9)(10). The risk factors, advice on physiotherapy management and notions of prevention of this injury will be discussed in this article. 

Anatomically, which muscles are hamstrings ?

The hamstring muscle complex is a bi-articular muscle group that functions by flexing the knee and extending the hip. In everyday movements, hip and knee flexion occur together, with opposite effects on hamstring length (7).

This group consists of 3 muscles:

• Biceps femoris (2 chiefs): the largest muscle and the most prone to injury è The long chief inserts on the ischial tuberosity the short chief at the distal half of the femoral line. They join to end at the head of the fibula, all with expansions on the lateral condyle of the tibia. 
  
• The semitendinosus : Starts from the ischial tuberosity and ends on the crow’s feet (upper and medial side of the tibia) close to the gracilis and the sartorius. 
  
• The semimembranosus : also originates from the ischial tuberosity and ends outside the semitendinosus, on the tibia and fibula via 3 tendons (one direct, one reflex and one recurrent). It also has slight attachments to the meniscal wall. 

What are the risk factors ? 

Muscle ratios : 

One of the most scientifically recognised risk factors is a significant muscle imbalance. The hamstring/quadriceps strength ratio should be around 60% with less than 10% difference tolerated between the two legs. If it is lower than this value, the ratio highlights ischios that are too weak. If the quadriceps is disproportionately dominant, anteversion of the pelvis may occur. This biomechanical change may be responsible for a higher initial tension in the hamstrings but also for all the soft tissues around the knee and hip, considerably increasing the risk of injury. In addition, low IJs can also be considered risk factors for ACL rupture as they must prevent the tibia from moving too far forward to maintain proper knee physiology. 

It has been shown that players without any strength imbalances in pre-season had an injury frequency of 4.1% (Group A) and this rate increased significantly to 16.5% in players with untreated muscle imbalances (Group B) (5). These results reflect the limitation of muscle strength as a function of training. Indeed, at a certain exercise intensity, the player exceeds the mechanical limits tolerated by the muscle unit, justifying the analysis of force imbalance as a factor leading to hamstring injury. Playing football with force imbalances between the quadriceps and hamstrings thus increases the risk of injury fourfold. By normalising the force profile the frequency dropped to 5.7%, showing an equally significant improvement. 

Flexibility : 

A commonly accepted risk factor for hamstring injuries is inadequate extensibility in the posterior compartment of the thigh (11)(12). This is because the lack of elasticity in the muscles limits optimal and efficient movement and increases the risk of injury. This poor flexibility can be addressed by incorporating stretching. These can play a role in preventing hamstring strains by improving force absorption for a given length of muscle, thus making the muscle more resistant to stretch injury (13)(14). 

Fatigue : 

Wood et al have also looked at fatigue. Indeed, this is also one of the most important factors to watch out for, as a significantly higher number of competition injuries occurred towards the end of the two halves. This reflects the involvement of fatigue in hamstring injuries (1). It has also been shown in another study that muscles are injured at the same length of extension whether they are fatigued or not. However, fatigued muscles are able to absorb less energy before reaching the degree of stretch that causes injury, thereby directly increasing the occurrence of injury (7).

Other factors :

In addition, it has been stated that a combination of abnormalities have been reported to increase the risk of hamstring strain, this includes lack of strength, flexibility, insufficient warm-up or simply fatigue which significantly reduces the healthy response of the fibres to the stresses encountered (9).

What physiotherapy treatment ?

• Phase 1 : acute : in the first few days: rest / ice / compression / elevation / cardiovascular activation as soon as possible (arm exercises only) to control bleeding, minimise inflammation and pain. As far as anti-inflammatory drugs are concerned, according to the literature it would be good to start treatment only 3 days after the injury and not directly. This is because they interfere with cell chemotaxis (the migration of cells according to the environment in which they are found) which is necessary for the repair and remodelling of the regenerating muscle. Once inflammation and pain are reduced, early movement exercises come into play to prevent or reduce adhesion in the connective tissue. Active knee flexion and extension exercises can be performed during treatment with ice. It is important that the exercises are painless to prevent re-injury during rehabilitation.
  
• Phase 2: In the sub-acute phase (after the signs of inflammation which are swelling, warmth, redness and pain) it is important to continue muscle action to avoid atrophy and promote healing. Light isometric strength exercises should be followed by regular concentric exercises. The latter can be started at this stage when the athlete has achieved a full range of motion without pain. The common recommendation at this stage is a submaximal isometric contraction at multiple joint angles. If the athlete experiences pain, the intensity should be reassessed and readjusted…
  
• Phase 3: Remodelling : This is a long phase, lasting up to 6 weeks in the case of a severe injury, which occurs in order to prevent the hamstring muscle from becoming less flexible and regenerating optimally after the injury. Hamstring stretching should begin during this phase as loss of flexibility is a characteristic of hamstring strains. It is probably due to pain, inflammation and scarring in the connective tissue. In their study, Malliaropoulos et al (15) investigated the role of stretching in rehabilitation. They showed that the group that stretched much more regularly recovered their range of motion more quickly and had a shorter rehabilitation period (3). During this remodelling phase, it is also advisable to start eccentric strengthening, which requires more force than concentric strengthening. This mode of contraction is also interesting in order to achieve a good alignment of the fibres and avoid the formation of recalcitrant adhesions during rehabilitation. It is essential to start this strengthening only when the muscle is well regenerated in order to avoid a new lesion induced by too strong constraints.
  
• Phase 4 : Functional : This phase allows the recovery of pre-injury values of strength and flexibility with the aim of resuming sports without injury. At the same time, the resumption of running is done progressively, leading to intensive sprints and changes of direction. The best indicator of readiness to return to play is participation in specific sports activities without pain or discomfort. Returning to competition before this period may result in recurrent or more severe injury.
  
• Phase 5 : return to competition : For the amateur athlete after a muscle strain, it is worth pushing the timeframe for return to competition to at least 4 weeks depending on the severity of the strain, however one should not be afraid to push this to several months until the injury is fully treated. Without a suitable delay, recurrence of the injury is almost inevitable. Feelings on hill sprints are a good indicator of the healing of the injury. In general, it is important, when returning to competition, to keep the player informed of his feelings and to make him aware of the need to continue strengthening, stretching and all the notions that will allow him to prevent a new injury. 

How can we effectively prevent these injuries ? 

To prevent hamstring injury, there are many factors that can be influenced during training sessions. 

Flexibility : 

As explained above with the study by Malliaropoulos et al (15), stretching plays a key role in the proper healing of injuries but also in injury prevention. The study by Hartig et al (16) compared the effects of a stretching programme over 13 weeks. They showed that flexibility increased significantly in the intervention group compared to the control group. The number of injuries was also significantly lower in the intervention group (incidence rate of 16.7% versus 29.1%). Thus, incorporating a stretching routine throughout the season but also during pre-season physical preparation can significantly reduce the risk of this type of injury. 

Strength : 

Regarding strength, several studies have examined the influence of muscle strengthening in relation to the frequency of hamstring injuries. For example, the study by Mjølsnes et al (17) showed that it is possible to significantly increase the eccentric torque (force exerted as a function of the angle of movement) in well-trained football players during a 10-week training programme focusing on eccentric exercises. At the same time, it was shown that good eccentric and isokinetic strength training led to a significant reduction in the subjective intensity of discomfort and the avoidance of re-injury of the hamstring (5). Having highlighted the effects of effective strengthening, it seems interesting to link these studies with that of Askling et al (18). These researchers set up a strengthening programme, targeting the hamstrings in concentric and eccentric, for 10 weeks, also comparing it to a control group. The researchers then observed the frequency of injuries in the players followed over several months. The study showed an increase in strength (concentric and eccentric), an increase in maximum running speed and a decrease in the number of injuries in the training group compared to the control group. However, an experiment with 30 participants may pose limitations to the study and therefore it would be interesting to repeat this protocol on a larger scale. Other studies show the interest of eccentric strengthening in the outer track because it is often this part that is affected during the end of the range of motion. 

But in this case, when should this strengthening be carried out? Before or after training ? 

The study by Lovell et al (22) tried to answer this question. Although improvements in strength were felt almost equally in both groups, it seems more risky to perform this strengthening before a session as fatigue may increase the likelihood of muscle damage during the training afterwards. 

Warm up : 

A progressive warm-up that includes eccentric contractions is essential in the prevention of hamstring injuries. It seems important to incorporate a few repetitions of nordic hamstring for example, acceleration followed by rapid braking but still respecting the continuity of the warm up. This advice should be applied even more for people who have already had injuries of this type before in order to increase the level of muscle activation before preparing for a match or training session. 

A perennial favourite for hamstring injury prevention :

Arguably the most popular hamstring exercise, Nordic Hamstring has been ranked in the top five injury prevention exercises by practitioners (19). Chronic NHE programmes have been shown to reduce hamstring injuries by 57% to 72% in amateur and professional footballers (20)-(21). It is suitable for the indications outlined above as it focuses on eccentric loading of the muscle and also works on trunk sheathing in parallel. 

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