HAMSTRING STRAINS
We offer readers a current, evidence based understanding of research on hamstring muscle injuries and how they can be prevented. reflecting the latest developments in the field.
Muscle injuries occur frequently; as contusion injuries in contact sports, and as strains in sports involving maximal sprints and acceleration. Hamstring injuries remain the leading cause of time‑loss muscle injury in elite football and have risen over the last decade. Since 2013, hamstring strain epidemiology in elite football has shown a sustained rise in overall burden, now constituting roughly one‑quarter of all injuries in top‑level European clubs1. Among sprinters, hamstring strains represent approximately 1/3 of all acute injuries2. Because football codes such as soccer, rugby, American football, Australian Rules football combine repeated high-speed running with frequent player-to-player contact, it is unsurprising that so many injuries involve the thigh. At the professional level, hamstring strains consistently rank as the first or second most common injury in soccer3-6, Australian Rules football7,8, rugby9,10 and American football11,12, typically accounting for one in every five to six injuries. Historical comparisons also demonstrate a gradual increase in the proportion of hamstring strains relative to other injury types such as ankle sprains when compared to data from studies from the 80s13. However, it should be noted that quadriceps strains are common in soccer5, and that muscle contusion injuries to the quadriceps muscles account for a significant proportion of all football injuries at the elite level. Hamstring strain injuries are also common in sports, where the muscles may be stretched past the usual range of movement, e.g. dancing and water skiing14.
INJURY MECHANISMS
There are two main mechanisms involved in thigh muscle injuries: direct (contusion) and indirect (distension or strain) injuries. The contusion mechanism is straightforward: The player is typically injured by a direct blow from an opponent, usually the knee hitting the lateral thigh in a tackle (a.k.a. ‘charley horse’ or ‘cork thigh’). The muscle is thereby crushed between the opponent’s kneecap and his own femur.
The hamstrings muscle group is composed of three muscles – semimembranosus, semitendinosus and biceps femoris. All of them (except the short head of the biceps) have their origin at ischial tubercle on the pelvis and they insert at the inside and outside of the lower leg right below the knee. This means that they overlap two joints – they straighten the hip joint and bend the knee joint. Muscle strains usually occur in the interface between the muscle and its tendon (the myotendinous junction), but avulsion injuries from the ischial tubercle are also seen.
There seems to be two primary strain mechanisms reported in the literature: sprinting type and stretching type injuries. It is difficult to document exactly at what time during the running cycle injuries occur15. However, since the net moment developed by the hamstrings is thought to be maximal in the late swing phase, right before heel strike, this is thought to be a vulnerable position16,17. In this instance, the hamstring muscles work eccentrically to decelerate the leg, involving high forces that can often exceed the tissues capacity15. These types of injuries commonly result in damage occurring to the biceps femoris long head. In contrast, stretching type injuries typically occur during movements involving extreme hip flexion combined with knee extension (e.g. sliding tackles or high kicking) at lower velocities, often affecting different muscles within the hamstring complex such as the semimembranosus. Recognising these different patterns of injury is important to understanding the risk profiles of different sports.
RISK FACTORS
A number of factors have been proposed to increase the risk of a hamstring strain occurring (see also Box 1);some are non-modifiable but others modifiable. Of all the researched risk factors, the strongest and most consistent variables associated with an increased risk of future hamstring strain injury are older age (particularly those in their late twenties and beyond) and a history of previous hamstring injury (especially within the last 12 months)18. These two are non-modifiable and therefore research efforts have focussed on identifying factors which may be modified by training interventions. The most researched modifiable risk factors are strength and flexibility qualities18. Running exposure and biceps femoris long head fascicle length have also been reported to have a role in hamstring strain injury incidence. Rapid spikes in high speed running load such as sudden increases in sprint distance or frequency19 have also been proposed as a potential risk factor. Qualitatively, experts from Champions League clubs have rated staff communication gaps as having a significant impact on the risk of a future injury occurring20. While lower flexibility could mean that muscle tension is at its maximum when the muscle is vulnerable close to maximum length, flexibility has limited evidence as a stand‑alone indicator of increased risk tool18. However, other studies from soccer and Australian rules football have shown low quadriceps flexibility to represent a risk factor for not only hamstrings21, but also quadriceps strains22.
Low hamstring strength would mean that the forces necessary to resist knee flexion and start hip extension during maximal sprints could surpass the tolerance of the muscle-tendon unit. Hamstring strength is often expressed as peak force during different contraction modes (eccentric, concentric or isometric), during a Nordic Hamstring Exercise (NHE – Figure 1)23 and relative to quadriceps strength as the hamstrings:quadriceps ratio. These relative strength relationships illustrate the ability of the quadriceps to generate speed and the capacity of the hamstrings to resist the resulting forces. However over the previous decade, isolated strength tests and ratios have been considered to offer limited predictive value at the individual athlete level18,24.
Shorter fascicle length in the biceps femoris long head has emerged over the past decade as a risk factor for hamstring strains25-27. Shorter fascicles are likely to have fewer sarcomeres arranged in series, which may make the muscle reach its “stretch limit” sooner, especially during eccentric contractions. It is possible that when you ask a muscle with shorter fascicles to handle the long‑length, high‑force loading of sprinting—particularly during late swing—it is more likely to experience excessive strain. When this is repeated over multiple efforts in a match or training, it can possibly lead to increased damage and a possible strain injury.
A history of previous hamstrings strains greatly increases injury risk, as documented in numerous studies28-30. Injury can cause scar tissue to form in the musculature, resulting in a less compliant area with increased risk of injury. A previous injury can also lead to reduced ROM or reduced strength, thereby indirectly affecting injury risk. Football players with a history of previous hamstring injury have a seven times higher risk of injury than healthy players— and as many as 13% can expect to suffer a new injury during one season.
Older players are at increased risk for hamstring strains, and although older players will be more likely to have a previous injury, increased age is also a risk factor independent of a history of previous injury30,31.
Other risk factors, which have been suggested but are less well studied, include race, sex, level of play, player position, improper running technique, superior running speed (peak performance), low back pain, increases or changes in the training program (particularly intense periods of training), insufficient warm up and muscle fatigue. Experts from 15 Champions League clubs have also rated “lack of high‑speed football exposure in training” and “staff communication gaps” as important risks associated with future hamstring injury risk20. Players of black or aboriginal origin sustain significantly more hamstring strains than white players30, and it has been suggested that these players may be faster runners when compared to their white counterparts, possibly because of a higher proportion of type II muscle fibres. A faster running speed will generate higher hamstring torques, which may explain the increased injury risk.
METHODS TO PREVENT HAMSTRING STRAINS
There is now strong evidence from high quality trials and large meta analyses that many hamstring strains can be effectively prevented through well designed training programs that lengthen fascicles, build eccentric strength and prepare the muscle for high speed demands.
A range of intervention studies now show that targeting the modifiable risk factors such as short fascicle length, low eccentric hamstring strength and inadequate exposure to high speed running can promote adaptations that may reduce hamstring strain risk.
Several studies indicate that low hamstring strength is a risk factor for sustaining hamstring strains32-35. EMG studies have shown that activity is highest late in the swing phase and during heel-strike, when the hamstrings work eccentrically or transfer from eccentric to concentric muscle action17,36. It is assumed that most hamstring strains occur during eccentric muscle actions37,38. It is well documented that strength training is mode specific39-44. Based on this it is highly likely that, to be specific, strength training for the hamstring muscles should be eccentric.
The Nordic Hamstring Exercise (NHE – Figure 1) has continued to be the single most evidence-supported exercise for hamstring injury prevention45. Multiple large-scale randomised controlled trials and meta-analyses demonstrate that eccentric training programs incorporating the NHE substantially reduce hamstring injury incidence, with some meta-analyses reporting risk reductions exceeding 50% when programs are implemented with adequate compliance45. The NHE produces multiple favourable adaptations, including increased eccentric hamstring strength, improved functional H:Q ratios, and increased biceps femoris long head fascicle length23,46, all of which are linked to injury risk. The magnitude of injury reduction is possibly dose dependent46 and strongly influenced by program adherence, with poor compliance limiting the possible protective effect.
Another reason to recommend the NHE as a specific tool to prevent hamstring injuries is that the program is easily implemented in a team setting. A controlled trial has also documented that if the recommended exercise prescription shown in Table 1 is followed, with a gradual increase in training load when introducing the program of Nordic hamstring lowers, players experience no delayed onset muscle soreness43. By the end of a 10-week training period, many players are able to stop the downward motion completely before touching the ground (i.e., at about 30 degrees of knee flexion), even after being pushed by his or her partner(s) at a considerable speed. When a player can reach this stage, the characteristics of the Nordic hamstring lower exercise appear to resemble the typical injury situation: eccentric muscle action, high forces, near-full-knee extension. The program has been implemented in several different sports and younger age groups, and injuries from the exercise itself have not been recorded.
Studies from Scandinavia have also shown that replacing the traditional hamstrings strength exercise used by teams – hamstring curls – with exercises to develop eccentric strength reduces the risk of hamstring strains3,47. Traditional hamstring curls have been shown to be ineffective at increasing eccentric hamstring strength among elite athletes43. Another study has also shown that using a special apparatus - the YoYo flywheel ergometer – also increases eccentric hamstring strength47. Both of these methods have been shown to prevent hamstrings strains in studies on soccer players3,47,48.
Sprint training itself may also serve as a hamstring strain mitigation strategy, particularly when progressively implemented. Controlled sprint training increases biceps femoris fascicle length49 and exposes the hamstrings to sport-specific loading patterns, potentially building tissue tolerance to the high-speed eccentric demands of competition. Combined eccentric and sprint training programs address multiple risk factors simultaneously50,51. Eccentric exercises target strength and architectural adaptations, while sprint training provides sport-specific conditioning and further fascicle length increases. This combination approach aligns with evidence that diverse exercise modalities may optimise the modification of multiple risk factors51.
The consistent finding that a history of previous injury leads to a several-fold increase in the risk of new strains has of course led to the suggestion that this is at least partly due to inadequate rehabilitation and early return to sport. A study from Swedish soccer52 has documented that a coach-controlled rehabilitation program consisting of information about risk factors for reinjury, rehabilitation principles, and a 10-step progressive rehabilitation program including return to play criteria reduced the reinjury risk by 75% for lower limb injuries in general. Although the specific effect on hamstring strains could not be assessed in this study, it seems reasonable to recommend including functional and specific rehabilitation programs and careful screening of players before return to play.
There are no intervention studies on elite athletes on the preventive effect of flexibility training on hamstring strains. However, one study on military basic trainees indicates a reduced number of lower limb overuse injuries after a period of hamstring stretching53, while another military-based study found no effect of stretching54. It should be noted that these studies were designed to examine the effect of general stretching on lower limb injuries in general, not a specific hamstring stretching program on hamstring strain risk. Questionnaire-based data on flexibility training methods collected from 30 English professional football clubs, where the stretching practices of the teams were correlated to their hamstrings strain rates, indicate that using a standard stretching protocol reduces injury risk55. Also, one study from Australian Rules football has observed a reduction in the incidence of hamstring strains with a three-component prevention program, where stretching while fatigued was one of the components56. The other factors in the program were sport-specific training drills and high-intensity anaerobic interval training. Thus, it is not possible to determine which of these factors are responsible for the observed effect. Also, the Norway-Iceland hamstring study did not show any effect of stretching, but it should be noted that teams were not randomized to the stretching program3. Therefore stretching may play a supportive role when embedded within broader multi component prevention programs that target eccentric training and high speed running exposure.
Ryan Timmins PhD
Head of Research
Aspetar Orthopaedic and Sports Medicine Hospital
Doha, Qatar
Roald Bahr MD, PhD
Professor of Sports Medicine
Director of the Aspetar Sports Injury & Illness Prevention Programme
Aspetar Orthopaedic and Sports Medicine Hospital
Doha, Qatar
Chair of the Oslo Sports Trauma Research Center at the Norwegian School of Sports Sciences
Oslo, Norway
Senior Scientific Advisor of Health, Medicine and Science in the International Olympic Committee
Lausanne, Switzerland
Contact: roald.bahr@aspetar.com
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Header Image by Franco Monsalvo (Cropped)