Introduction
Injuries in football exert negative impacts on team performance1 and club finance.2 Hamstring
strain injuries (HSI) are consistently the most prevalent time loss injury in football3
and as such are an important issue in football medicine. Despite an ever-increasing
wealth of information emerging on the aetiology of HSIs, their incidence in football
is increasing.3 This could be explained by the increased intensity and physical demands
of football match play over the last decade4; but practitioners should also question
their approach to injury prevention and physical preparation of players, an area where
research is lacking.5–8
There is an evidence-based strategy to prevent HSIs, however, there appears to be
a large disconnect between this available evidence and its adoption in elite football.5
6 For example, the Nordic hamstring exercise (NHE) programme has been shown to effectively
reduce HSIs by 65%–70%.9 10 But recent surveys of elite European Champions League
(n=32) and Norwegian Premier League (n=18) teams showed that only 11% of these teams
fully adopt the programme.11
Within elite football, there is a large disconnect between evidence-based practice
and the actual interventions used by science and medical teams to prevent injuries.12
In line with an emerging body of evidence-based research, we believe that preventing
HSIs in elite football requires a holistic approach, which considers multiple risk
factors and their inter-relations, implemented effectively.6–8 13 In designing our
injury prevention programme, we combine both the theory and the available evidence
on HSI risk factors and intervention studies, as well as considering the context surrounding
the player when designing our overall injury prevention approach.
Important considerations when designing an injury prevention programme in elite football
Why do HSIs happen? Risk factor analysis and programme planning
To prevent HSIs, it is important to understand why they occur, so that an appropriate
intervention can be developed, targeted at the specific risk factors. Importantly,
there is not a clear understanding of why HSIs happen. Although there is a wealth
of research examining HSI risk factors, there is a lack of evidence reporting consistent
prospective risk factors for HSIs,14–16 and also a lack of research examining how
the supposed risk factors interact with one another.7 17 Despite the difficulties
of studying the interactions of multiple risk factors, it appears that risk factors
do not operate in isolation, but instead function as a complex web of determinants.17
As such, our first approach when designing an HSI prevention programme is to consider
all the possible risk factors which may be implicated, blending theory, evidence and
anecdotal experiences (see figure 1).
Figure 1
A diagram of factors to consider in hamstring muscle injury prevention. CV, cardiovascular.
Table 1 depicts the approach of our practitioners when considering the contents of
the programme. It contains the list of risk factors linked in some way to HSIs, either
through prospective or retrospective research studies, or from anecdotal evidence
from within our group. Clearly, when designing a programme, one cannot specifically
target every possible risk factor or assign equal weight to all risk factors. Instead,
it is essential to rank the factors in order of importance considering both specific
and general risk. This is by no means an exhaustive list, nor do we suggest that any
of the risk factors displayed are permanently associated with the table category (eg,
acute:chronic workload ratio (ACWR) could in some cases be moved to any of the other
categories depending on the overall factors presented by the player). In our experience,
those risk factors presented in the ‘Recognised but does not inform normal practice’
category are explored in more complex/detailed cases of preventing HSI. Multidisciplinary
team-mediated player discussion in preseason serves to inform an individual’s programme
design and resource allocation.
Table 1
Risk factors linked to hamstring strain injuries, either through prospective, retrospective
or anecdotal evidence. The table shows the typical manner in which our team would
classify the risk factors associated with hamstring strain injury
Important
Semi-important (implement where possible)
Recognised but does not inform normal practice
Specific risk factors
General risk factors
Previous hamstring injury14–16
ACWR (all parameters)48–51
BFlh muscle/tendon architecture83–85
Wisdom teeth/dental hygiene86
Hamstring eccentric strength (fresh)30 32 33
Lumbopelvic hip stability15 62–66
Glute dominant hip extension pattern
TMJ dysfunction
Weekly speed exposure44
Functional strength72 73
SIJ kinematics
Hamstring fatigue resistance79 82 87
ACWR (HSR/vHSR)52
Muscle fascicle length30
Psychosocial factors57
Tib/Fib joint
Individual recovery rates/strategies (sleep, rest, nutrition, massage, cryotherapy,
hydrotherapy)
Environmental factors (eg, playing surface, travel, location of match, footwear, and
so on)
Time between games88
Movement quality
Previous recent injury (any)
Aerobic fitness74
ACWR, acute:chronic workload ratio; BFlh, biceps femoris long head; Fib, fibula; HSR,
high-speed running; SIJ, sacroiliac joint; Tib, tibia; TMJ, temporomandibular joint;
vHSR, very high-speed running.
Gain key stakeholder buy-in
Successful preparation and acting on player-related recommendations in professional
football are highly dependent on ‘buy in’ from key decision makers (coaches, players
and chief executive officers).18 Coaches exert influence on injury risk,19 and as
such, any injury prevention strategy needs to have their support. The Sports Medicine
and Science department should seek to understand the expectations that coaches and
management staff have of them; and medical and performance strands should explore
how the coaching team prefer to receive information (eg, chunking information, succinct
messages, diagrammatic methods). Furthermore, the support staff implementing an injury
prevention strategy should be mindful of how important education and communication
is.20 The fundamental principles which may be obvious to the practitioner should be
put into the context of each of the audiences they are dealing with to aid in an understanding
of the process. With open and honest discussion and two-way communication, there will
be an opportunity to feed into the injury prevention process and ensure it is the
best fit for the individual.
Target the interventions at the individual player
We believe an essential element of HSI prevention programmes is the individualisation
of the programme, targeted at the individual’s injury risk profile. Players are typically
screened by the Sports Medicine and Science department for health evaluations, musculoskeletal
evaluations, athleticism (eg, strength, speed, power and agility), psychological profile,
injury history, neuromuscular control/movement quality and hamstring strength, typically
at the start of each season. This information, in conjunction with appropriate ongoing
injury reporting, screening and monitoring processes, provides the necessary information
to create individualised training approaches to reduce player injury risk by targeting
specific risk factors relevant to the players’ profile,21 while also serving as benchmark
criteria for return to play.
Implement HSI prevention work as part of a complete injury prevention programme
It is important that any HSI prevention programme be implemented as part of a complete
injury prevention and performance programme. HSIs account for 12% of the total injuries
sustained,22 and as such are the most common injury in elite football. However, they
should also put in context alongside other injuries (which constitute the remaining
88%). Thus, recognising the interplay between general risk factors for all injuries
and maximising the crossover of interventions is important in programme design.
Our five-point strategy to HSI prevention in elite football
Below, we present our five-point HSI strategy, which contains research evidence as
well as clinical experience in a truly evidence-based approach, generalisable to all
elite-level teams. Additionally, this can be scaled appropriately to inform HSI prevention
at all levels.
Strengthen the hamstring muscles
Injuries to the myofascial/muscle tendon junction of the hamstrings are thought to
occur when forces in excess of the mechanical limits of the tissue cause mechanical
disruption.23–25 As such, a key prevention strategy to prevent HSIs should be to increase
the capacity of the tissue, thereby raising the safety threshold for injury. The use
of eccentric hamstring strength training is the most widely researched and recommended
evidence-based strategy for HSI prevention and has been shown to significantly reduce
the risk of primary and secondary HSIs (65%–85%).9 10 26 Eccentric strength training
also results in alterations of muscle architecture, specifically elongation of the
biceps femoris long head (BFlh) fascicle length (16%–34% after 5–10 weeks of eccentric
strength training),27 28 although not always.29 Professional players with short BFlh
fascicles (<10.6 cm) were at fourfold greater risk of HSI than players with longer
fascicles.30 HSI risk was reduced by 75% for every 0.5 cm increase in fascicle length.30
It is hypothesised that shorter fascicles, with fewer in series sarcomeres, may be
more susceptible to being overstretched and having damage elicited via powerful eccentric
actions, typical of the terminal swing phase of high-speed running (HSR).30 31
Despite clear evidence for a role of eccentric strength training in HSI prevention,
the actual evidence concerning the role of ‘strength’ in HSI risk is equivocal.14
30 32–35 It is not clear whether it is changes in strength per se or fascicle length
following eccentric strength training which supports the reduction in injury risk.
Discrepancies between the role of strength and HSI could be explained partly by differences
in constraints of the tasks (strength typically assessed under isolated conditions,
eg, isokinetic dynamometry32 33 vs injuries occurring during high-speed, high-force
and highly complex tasks, eg, sprinting34 35). Possible factors which need to be considered
when designing a strength programme include:
Hip extensor torques in sprint running exceed that of the knee flexor torques.36 The
hamstring muscles contribute simultaneously to knee flexor and hip extensor torque.
Weakness in hip extension strength was identified as a prospective risk factor for
HSI in elite-level sprinters.37
Consideration needs to be given to all parts of the force-velocity curve in regard
to its potential for athletic development and injury reduction. Indeed, force-velocity
profiling has become more common in this environment in order to address areas of
potential improvement in an individual as well as exercise selection adopted by the
practitioner. Deficits in explosive strength would limit the ability to express maximal
strength during sprinting. Indeed, previously injured hamstrings expressed deficits
of 30% in eccentric rate of force development on return to sport after HSI.38
Poor intermuscular and intramuscular coordination can result in insufficient expression
of isolated strength functionally. Suboptimal strength/activation of the stabiliser
muscles can result in compensatory recruitment of the agonists and antagonists for
dynamic joint stability (eg, to act as global stabilisers). This would result in compromised
force output.39 40 There must also be an acknowledgement of the continuum of complexity
in regard to the coordination and activation patterns involved in actions like sprinting/ball
striking versus the isolated strengthening that takes place in the gym. While both
have their place, the progressions and constraints placed on the athlete are key to
minimising the occurrence of HSI.
In terms of our approach to hamstring muscle strengthening, we, like many teams in
elite football, value the role of eccentric strength training,41 but do not adopt
the full NHE programme.11 Instead, we use a more holistic approach to hamstring muscle
strengthening,13 one which recognises the need for optimal hamstring function (maximal
and explosive strength across the force-velocity profile, balancing the focus on isometrics42
and eccentric and concentric actions). Although isometrics are perhaps less understood/researched
than eccentrics, practice-based evidence has seen a big shift towards this in recent
years. Additionally, it is important to recognise the need for both knee flexor and
hip extensor strength; the similar morphological adaptations (hamstring eccentric
strength and fascicle length changes) which accompany both knee dominant and hip dominant
hamstring strengthening exercises27; similar adaptations to both low/high-volume NHE43;
and the need for specificity in training.39 We focus primarily on the use of running
(HSR, high speed acceleration and attaining peak running speed), given its high specificity
(and possible performance benefits), the large hip and knee joint torques36 and biomechanical
symmetries to HSI mechanism and reduced barriers to its implementation. Admittedly,
there is limited research into hamstring adaptations to sprint running (vs commonly
assessed NHE), but recent research indicates that regularly achieving peak or near-peak
running speeds in training is associated with a lower risk of HSI.44 As such, we aim
to expose all players to within 95% of maximum speed one to two times per week. This
programme is supported by gym-based individualised strengthening programmes (balancing
the combination of hip and knee dominant and isolated and compound exercises) depending
on player and coach buy-in and current playing status (eg, additional gym-based hamstring
eccentrics are not sensible during congested match play in regular starting players).
Also, injury history and the player profile support the programme design. We specifically
look for players who have poor eccentric hamstring strength (<350 N)30 or a right
to left asymmetry in eccentric strength (typically >10% although we state this with
caution as there are individuals who naturally appear to have a greater deficit than
this up to 25%45 46), assessed using the NordBord. We currently use fatigue resistance
(single leg bridge to capacity test47) primarily for return to play monitoring.
Optimise the training balance: carefully monitor players and ensure optimal recovery
strategies
Perhaps the most important element of preventing injuries in elite football lies in
optimally managing player load. There is emerging evidence that heightened ACWR which
accompanies congested match play is one of the key factors leading to increased injury
risk.48–51 Exposing players to large and rapid increases in HSR distances above their
two yearly session average was shown to increase the odds of HSI. In particular, elevated
HSR in the week before injury demonstrated a 6.4 times increased risk of HSI.52 Previous
research by our group indicated higher accumulated and acute workloads were associated
with a greater injury risk, but progressive increases in chronic workload may develop
the players’ physical tolerance to higher acute loads and resilience to injury risk.49
Other researchers have also found high training loads to be protective in elite team
sport.51 In terms of ACWR, it appears that HSR may be more predictive of HSI risk52
and exposing players to large and rapid increases in training above what they are
habitually used to increases the odds of sustaining an HSI. A high ACWR may be in
part due to a low chronic training (eg, low denominator) or congested match play (eg,
typical of the winter periods or when involved in European or domestic cup match play).
In terms of our approach to managing players’ training workloads, we monitor the ACWR
using Global Positioning System technology (and TRACAB for match loads), limiting
exposure to high ACWR (<1.4, although this appears to be largely individual), particularly
in HSR (>5.5 m/s) and very HSR (vHSR; >7 m/s). This should be achieved by progressively
increasing player training load (avoiding spikes in acute loading) to achieve long-term
moderate to high chronic training loads, which aim to prepare the players for the
equivalent of three matches worth of load per week (figure 2). Thus, during fixture
congested periods, the players have a greater tolerance and robustness for the heightened
loads. Additionally, while the load increases are progressive, they involve week-to-week
fluctuations, involving deloading periods for adequate rest and recovery, as well
as periods of overloading to produce adaptation53 (figure 2). Within this, we acknowledge
that consistently high loads may lead to other stress-related injuries and that an
individual’s physiological response to spikes in ACWR may vary; thus, experience in
managing individual responses should still be considered, as should individual banding
of speed thresholds to better reflect energy system output.
Figure 2
Graphical representation of the progressive increase of a player’s chronic high-speed
running over the course of an 8-week intervention. The player begins with a chronic
load of 2000 m representing a two-match load and is trained with the goal of progressively
increasing his chronic training load, adopting a structured periodisation model to
fluctuate the weekly training demands to achieve a chronic high-speed running of three
matches equivalent per week. The current programme used a 2-week overload and 1-week
deload maintaining the acute:chronic workload ratio below 1.4 (peak 1.35) and deload
week remaining above 0.8 (lowest 0.82) to prevent acute heightened risk of injury. A,
acute; C, chronic; HSR, high-speed running.
Football training typically mimics the physical intensity and movement patterns of
match play, but can often result in limited exposure to HSR/vHSR due to the use of
smaller areas. Match play represents the largest stimulus in terms of HSR and thus
balancing training to reflect match play HSR load is important. Anecdotally, those
players often at risk of HSI are regular non-starters or players returning from injury,
who are suddenly required for a period of congested match play. In these players,
there is a need to individualise their training, closely monitoring their HSR and
peak running speeds in training and providing top-ups to maintain sufficient acute
and chronic workloads in the absence of first team match play. These top-ups may take
the form of additional running during or after training, use of additional training
sessions (eg, match day+1, or additional sessions in the afternoon, for example) and/or
organised matches (eg, training/development team matches). Additionally, substitutes
who play little or no minutes complete HSR or speed exposure sessions (specific to
the individual needs) immediately after game, to allow them to maintain the same schedule
as the playing squad. Although this is an area which we need to gain a better understanding
of, greater knowledge of an individual’s typical demands in training/match play may
help us keep them appropriately conditioned. Individualised speed zones based off
maximum speed can help ensure the player is monitored relative to their own physical
capabilities/energy systems, and thus their outputs are not over/underestimated relative
to them. During periods of congested match play, it is essential to have optimal strategies
in place to accelerate recovery (eg, hydrotherapy, nutrition, rest/sleep, cryotherapy,
cold water immersion and soft tissue massage), and limit possible residual strength
loss into the next match.54–56
An additional important element of managing player load is to recognise this is not
just physical, but can also be psychological/emotional. Psychosocial factors (ie,
stressful life events) may contribute to injury risk.57 Elite football can be highly
stressful, and as such recognising psychological load may also be important. Therefore,
load monitoring strategies need to encompass all forms of load (eg, wellness questionnaires,
sleep, ratings of perceived exertions, training and match loads). Additional in-depth
analysis (eg, assessment of hamstring isometric strength after match play)58 should
be undertaken with high-risk players (eg, older and/or previously injured players14–16),
which can support an optimal load management strategy and possible early warning system.
Implement a lumbopelvic hip stability programme
A widely held belief in elite sport is that that lumbopelvic hip stability training
is important in the prevention of HSI. However, its use has recently been questioned
due to a very small amount of evidence to support this.59 However, based on the available
theory, positive anecdotal experience and a link with other injuries,60 we promote
its inclusion as part of a holistic approach to preventing HSIs. For instance, the
pelvis is responsible for load transfer between the lower limbs and the spine.61 Aberrant
pelvic motion, specifically anterior pelvic tilt, or trunk motion such as lateral
trunk tilt during sprint running is thought to increase HSI risk.62–64 Leg-length
asymmetries are associated with innominate bone rotation64 and an asymmetry greater
than 1.8 cm is linked with a fourfold heightened risk of HSI.65 An uncontrolled anterior
pelvic tilt during sprint running can elicit substantial stretch on the hamstring
muscles, thereby increasing tension on the BFlh in terminal swing phase.66 Hip flexor
tightness is a prospective risk factor for HSIs15 67 and is associated with an anterior
pelvic tilt in sprint running, increased stretch on the hamstring muscle at late-swing
phase and also associated with inhibition of the gluteus maximus, via reciprocal muscle
inhibition and subsequent synergistic dominance of the hamstring muscles.68 69 Hip
extension torques can exceed double that of knee flexion torques during sprinting,36
and so reduced strength or activation of the gluteus maximus (both identified as prospective
risk factors for HSI)37 70 would elevate the demands placed on the hamstring muscles.
It appears that altered lumbopelvic stability, alignment and coordination can both
limit the expression of hamstring strength at longer muscle lengths (eg, terminal
swing phase) and simultaneously elevate their demands.
Comerford and Mottram71 offer a framework for the assessment and retraining of uncontrolled
movement, when applied to the lumbopelvic hip complex. This offers a logical system-based
approach which can assist the practitioner in establishing the site and direction
of uncontrolled movement in both low/high-load situations. Practitioners should consider
the training dichotomy between (1) the efficiency of local stabilising muscles, tonic
in nature with slow motor units, which are used for non-fatiguing functional tasks,
controlling translation and contributing to postural control, versus (2) high threshold,
global stabilising muscles, whose predominant fast motor units are phasic in nature
and capable of generating high force to limit or decelerate range of motion.68 Many
methods exist for training stability within the lumbopelvic hip complex. Floor-based
exercises, Pilates adjuncts or machines may be convenient methods to isolate movements,
challenge postural awareness and ability to dissociate, and build muscle capacity;
and all have a component part in our practice of HSI prevention. A key element of
these isolated exercises is to ensure optimal recruitment of the target muscles, recognising
the possibility of primary agonist inhibition and subsequent synergistic dominance.69
Yet the law of specificity should be respected to ensure transference to athletic
tasks (see point 5).
Develop players’ physical conditioning
There is evidence indicating an important role of player’s physical conditioning (eg,
general lower body compound strength and cardiovascular (CV) fitness) in reducing
risk of injury. Across a range of sports, those with superior physical qualities are
more robust to injury.72–74 Additionally, the implementation of strength training
has been shown to reduce injury risk.75 Elevated ACWR is associated with injury,48–51
but those athletes who are stronger73 and have superior CV fitness74 are better able
to tolerate elevated ACWR.
Furthermore, lower body compound strength is essential for sports performance during
explosive tasks such as sprint running and jumping,76 while sufficient CV fitness
is essential to cope with the movement demands without adverse fatigue and its associated
negative consequences (eg, compromised player biomechanics and neuromuscular control/capacity to
produce force).77 78 It is also known that fatigued muscle has reduced ability to
absorb energy before reaching the amount of stretch that results in injury.79 Players
will undergo on-field testing at various stages of the season. Their performance to
exhaustion on the Yo-Yo Intermittent Recovery Level 1 test will provide useful information
as to the players’ aerobic/anaerobic capacity. The same test can be repeated at a
submaximal level to a predetermined point on the test to provide the practitioner
with useful data in regard to the players’ heart rate (HR) response and subsequent
HR recovery.80
Our programme consists of a general strength and conditioning approach for both performance
optimisation and injury risk reduction. Often training and match play provide a sufficient
stimulus for CV adaptation/maintenance. However, it is important to monitor players’
HR responses to training and where necessary provide additional off-feet conditioning
(eg, we incorporate hypoxic interval-based high-intensity session on the bike/cross-trainer,
achieving set targets of HR). Off-feet conditioning is also important for the load
compromised player (eg, less robust players with high risk of injury or extensive
injury history or the player returning from injury, and as such typically having a
lower chronic training load) in reducing injury risk. We also use this approach with
target red zone (>85% max HR) minutes, training impulse, or sessions that match the
output of the training group/high-intensity periods of the game to replicate the energy
system demands.
Incorporate a focus on movement quality
We have a philosophy of assessing/improving the fundamental movement patterns (squat,
step lunge, hinge, jump, land). Such movements need to be performed in different directions,
speeds and planes to improve athletic movement and reduce the risk of potential injury.81
Practising athletic movement challenges coordination, timing and neuromuscular control,
and is associated with an increased ability to express and apply strength functionally
(eg, in sporting-type movements such as running).39 40 82 Programmes incorporating
athletic movement with good technique and repetition may enhance neuromuscular control
and sequencing. Specific drills which break down components of the running action,
focusing on a stable pelvic position, and upper and lower limb position during marching,
skipping and running tasks form an important component of our conditioning continuum.
These qualities are trained in a variety of ways as part of on-field warm-ups, physical
development sessions in the gym and preactivation sessions prior to regular football/skill-based
training. A graded approach which establishes good technique at slower speeds and
simple tasks establishes a basic competence which supports the development of drill
speed and complexity, further challenging neuromuscular development of strength, control
and sequencing within athletic tasks, which we feel supports both performance enhancement
and injury reduction.
Anecdotally, these drills naturally assist the development of linear speed and acceleration
programmes and seem of value improving pelvic and hip position, as well as upper and
lower limb position, particularly at speed. It should be noted however that they are
biased towards sagittal plane horizontal movement, and that field-based sports which
are inherently multidirectional in nature require support in this approach with movement
quality drills which develop jumping, landing, deceleration and change of direction.
Conclusion
This paper presents our five-point strategy for preventing HSIs. We believe that high
HSI rates are in part due to the recommendation of a simple single component approach
to a complex multifactorial problem. We suggest from our learning experiences that
there is a need for a holistic approach that translates existing knowledge on HSI
injury risk and applies this to a football context. However, it is important to recognise
the contextual factors which can influence the adoption, implementation and sustained
use of such a programme. We advise those in football when designing their HSI prevention
strategy to gain key stakeholder buy-in, consider HSIs as part of an overall prevention
strategy and treat the individual player where possible. We do not seek to discredit
the use of any one particular programme in HSI prevention; we suggest rather than
implement a single modal intervention, we encourage a holistic approach and consider
the complex interaction of multiple risk factors in HSI alongside our five-point strategy.
This approach is not wholly evidence based, but we believe this represents a judicious
approach to HSI prevention, which builds on our shared anecdotal experience and remains
evidence informed. Our goal is to share our approach as we believe it can form part
of the journey towards the current best practice approach to HSI prevention at both
elite and other levels of football.