Our structured survey revealed that the majority of premier-league teams surveyed (124; 95%) used a continuum approach to guide RTP following hamstring injury using a combination of clinical, functional and psychological criteria. Clinical criteria were most common at RTRun and RTTrain, while functional criteria were consistently assessed across all phases. Across the later phases of the RTP continuum, greater focus was placed on the assessment of psychological readiness. Eighty percent of clubs adopted a shared decision-making process with at least two people involved at any one phase. Despite myriad of challenges being perceived to influence decision-making, teams often met the criteria that they set to progress through the RTP continuum.
RTP Continuum in Premier League Football Teams Worldwide
Based on our sample of premier-league teams worldwide, the majority (124; 95%) assessed criteria over a continuum to guide RTP following hamstring injury. Of 124 teams, 102 (78%) reported assessing criteria at the four specified phases; RTRun, RTTrain, RTPlay and RTPerf. Of the remaining 29 teams, 22 (17%) implemented a criteria-based approach at RTRun, RTTrain and RTPlay, but not RTPerf. Unfortunately, the teams did not provide sufficient details for us to confidently report why this was the case; however, of the minimal feedback we did receive, it was specified that they believed the RTPlay phase should be where the player is also considered to be back to full performance.
Seven (5%) teams did not follow a RTP continuum and did not explain why. Our findings provide preliminary support (at least in our sample) that general research recommendations and practice align in that the majority of team practitioners view RTP from the point of injury until at least returning to play and most through until returning to desired performance. Our RTP continuum differs from the one specified in the 2016 consensus statement. In particular, we had specified an additional phase early in rehabilitation (RTRun). Football (and sport in general) and research are constantly evolving, and the application of a continuum framework within and between sports may need to be adapted to the specific needs of those monitoring and controlling the overall RTP process. Therefore, models such as the RTP continuum may need to be adaptable to suit these needs and research should consider this also.
Criteria were Widely Used to Guide RTP but Highly Varied Across Premier League Teams
Team practitioners used a combination of clinical, functional and psychological criteria to guide RTP following a hamstring muscle injury. Multifactorial and criteria-based rehabilitation programmes are advocated in research to support RTP decision-making [26,27,28]. Such a criteria-based decision approach provides practitioners with an individualized approach to RTP that integrates quantifiable assessment (objective and subjective) to systematically progress rehabilitation. Criteria-based approaches may reduce re-injury risk and improve player performance and availability of footballers [26, 29]. In our survey, we asked respondents to specify their top three most important criteria used at each of the RTP phases (Table 2) with the aim of uncovering some consistently used criteria, metrics and thresholds that could inform current practice and guide future research.
Criteria to Progress to RTRun
While over seven different criteria were represented at this phase, absence of pain and hamstring strength were the two most frequently reported top three criteria used to inform progression to RTRun. Absence of pain (reported frequency; 1st–57%, 2nd–21%, 3rd–27%) aligns with perceptions previously presented in the research literature [7, 8, 30]. Within our survey, emphasis appeared to be placed on the absence of pain during clinical evaluation (e.g. on palpation, or strength and flexibility tests) and/or following functional performance testing (e.g. running mechanic drills, low-moderate speed running) which is similar to the RTP Delphi survey of football experts by van der Horst and colleagues . In a recent systematic review  of criteria used to inform rehabilitation progression and RTP clearance following hamstring strain injury, it was highlighted that progression was typically only permitted within pain-free limits. The presence of localized discomfort on palpation following return-to-play may increase the risk of hamstring re-injury in athletes . Remaining pain free during rehabilitation has also been challenged with the suggestion that it may unnecessarily prolong rehabilitation, thereby increasing the injury burden . Additionally, athletes’ subjective ratings of pain poorly quantify progress within rehabilitation following hamstring injury . Therefore, there does not appear to be any clear and confident recommendations on the role of ‘absence of pain’ prior to RTRun or in general throughout RTP process.
Relative to other recorded criteria, hamstring strength was also more frequently reported by practitioners as a top three criteria at RTRun (reported frequency; 1st–17%, 2nd 40% and 3rd–24%). There is an important consideration with strength, however, which was identified in the Delphi surveys of van der Horst and colleagues  and Zambaldi and colleagues , in which ‘strength’ can encompass a variety of types and evaluations (e.g. eccentric, isometric, imbalance between legs and within legs). Yet what specific components of strength should inform RTP progression remain unclear. In the Zambaldi et al.  consensus, it was agreed that full hamstring strength is essential to for a safe RTP. However, in contrast, the experts in the Delphi survey of van der Horst and colleagues  did not reach consensus, with experts unable to agree if eccentric strength should be used as a criterion, although they did agree that other contraction types should not be used as criteria for RTP. Unfortunately, our survey respondents did not provide sufficient information on the types of hamstring strength they tested as criteria. In 2014, Tol and colleagues  showed that normalisation of isokinetic strength was not necessary for successful hamstring RTP in professional footballers, while a 2017 systematic review  recommended the opposite: that hamstring strength could be a useful criterion during hamstring RTP. However, the systematic review was not specific to professional football only and specificity of population is arguably necessary. Since then, scientific studies (e.g. cohort studies) are building that question the utility of hamstring strength and specifically isokinetic cut-values as progression criteria for hamstring RTP [34,35,36]. However, it should be noted that these studies are concerned with the RTPlay phase and to our knowledge no studies have investigated the role of strength prior to returning to high-speed running.
Criteria to progress from RTRun to RTTrain
To inform progression to RTTrain, despite a variety of top three criteria being reported, training load (reported frequency; 1st–39%, 2nd–25% and 3rd–20%) and hamstring strength (1st–22%, 2nd–29%, and 3rd–18%), were the most frequently reported criteria by practitioners. Hamstring strength was discussed in the previous section. The higher reported frequency of training load monitoring is consistent with the perceptions of medical practitioners in UEFA Champions League  and FIFA national teams  where training load was highlighted as one of the top criteria for injury prevention. It is currently unclear how training load relates to re-injury risk or specifically, muscle/hamstring re-injury, if at all. While only expert opinion, it has been recommended to maintain ‘high control’ over running loads (and speeds) during this rehabilitation phase with particular consideration given to the progression of speed and player characteristics, e.g. position, style of play . We discuss training load as to how it might relate to the RTP in the following RTTrain to RTPlay phase.
Criteria to Progress from RTTrain to RTPlay
To inform RTPlay decision-making, training load was again a criteria more frequently considered by practitioners (1st–41%, 2nd–38% and 3rd–14%). Existing RTP recommendations advocate achieving GPS benchmarks based on player/position-specific match metrics (e.g. max speed, high-speed running distance, sprint number) are important to ensuring readiness to RTPlay [7, 8]. Stares and colleagues  recently reported that longer RTPlay (to progressively develop greater weekly and total training loads) was associated with reduced risk of re-injury in Australian rules footballers. Specifically, achieving running loads above peak values prior to the injury resulted in an extra ~ 10 days (31.6 ± 10.8 days vs. 21.6 ± 2.5 days) missed. We should be aware that the time to progress through RTP phases is an ongoing risk assessment whereby an extra 10 days missed could be the difference between two to three matches (in elite contemporary football) and potentially up to nine points.
The finding that performance/sport specific field testing was one of the more frequently reported criteria at this phase was not surprising (1st–24%, 2nd–18% and 3rd–14%). This criterion should theoretically allow practitioners to assess the player’s readiness to load the injured muscle as required during progression to activities with higher demands as seen at RTTrain and RTPlay. Performance during on-field testing was considered to be a ‘vital’ criteria in determining RTP clearance by the football experts . A carefully planned RTP programme that addresses all aspects of the game may be important for restoring functional performance levels while minimizing the risk of re-injury [26, 39]. However, further research is needed to validate functional tests to guide RTPlay decisions.
Criteria to Determine When Players Have Returned to Performance
While the majority of premier league teams followed a RTP continuum approach, RTPerf was the one phase that 21% teams highlighted that they did not follow with anecdotal feedback suggesting that they believed players should be back to desired performance levels upon RTPlay. Defining what represents the desired performance level is important and to our knowledge this has not yet been achieved in the research literature. The criteria for RTPerf proposed in the 2016 consensus statement  stated that this phase may be categorized by personal best performance or expected growth as it relates to performance. In the professional football setting this likely refers to match-related metrics related to physical, technical, tactical and cognitive qualities.
As with RTTrain and RTPlay, training load was one of the most frequently reported criteria (1st–33%, 2nd–21%, 3rd–15%), yet little is currently known about training load and RTPerf. Given that the majority of a starting player’s in-season loading is derived from match play (i.e. typically 2 games/week), the inability to maintain training load throughout rehabilitation has been suggested as a risk factor for re-injury and may contribute to the high rate of ‘early’ recurrences (< 2 months) observed following RTPlay [40, 41]. Normalization of training loads comparable to the team was not achieved until after RTPlay in Australian rules football , while footballers returning to play were at increased risk of subsequent injury for up to 12 weeks . Accordingly, extending player monitoring/observation beyond RTPlay may represent an interesting aspect to assess during the RTPerf phase, as recommended by Stares et al.  to not only ensure pre-injury performance benchmarks are being achieved but also as a tertiary-level injury prevention strategy. However, this represents only one preliminary study and in a different sport.
Other Considerations Regarding Criteria
Psychological criteria were highlighted in the global criteria used by team practitioners (Fig. 1) and specified as important to consider in the research literature [44,45,46,47] as well as the previous Delphi surveys conducted in elite football [7, 8]. Psychological readiness was infrequently reported by practitioners. In view of the modifiable nature of psychological factors/traits, it has been recommended in research that psychological factors should to be assessed from the time of injury . While limited in football, expression of positive psychological responses across rehabilitation (e.g. higher motivation, low fear of re-injury) has been associated with successful return-to-sport (i.e. RTPlay in our study) outcomes within a variety of different athletic populations [44, 49, 50]. Few practitioners specified which psychological readiness tool they used (if they used any formal evaluation). This may be due to a lack of well-validated instruments to measure psychological readiness and may explain the relatively low accumulated points. Research is urgently needed to validate and evaluate the effectiveness of psychological readiness questionnaires for professional footballers.
What does RTP Decision-Making Look Like in Practice?
A shared decision-making approach was used by 80% of premier-league teams surveyed. This is an encouraging finding as low-quality internal communication may be associated with (re)injury rates and reduced player availability [17, 51, 52]. Only eight (6%) teams reported using isolated decision-making across all continuum phases. Eighteen (14%) teams used a combination of isolated and shared approaches to guide rehabilitation progression.
Medical staff (club doctors and physiotherapists) were most frequently consulted throughout the decision-making process. Traditionally regarded as the gatekeepers of the RTP decision, medical staff clearly hold a prominent role within the decision-making practices of clubs. In 96 teams (73%), medical staff were the lead practitioner responsible for the RTP programme. Across each phase of the RTP continuum, ≥ 87% of teams consulted with at least one medical practitioner (Table 3).
While medical staff involvement in decision-making across all RTP continuum phases was reported by both medical and science practitioners surveyed (Table 3), their perceptions as to how other stakeholder groups are involved in decision-making throughout RTP differed. Specifically, medical staff reported less involvement of science and coaching staff across all phases of the continuum and for players at RTTrain and RTPlay compared to when science staff answered the survey. We cannot answer why this is, as potential bias for respondents placing greater emphasis on the involvement of their own discipline should then have also been evident in the responses of science staff, yet this was not the case. Our results raise an important question about how staff are actually involved in the RTP continuum process. Despite an initial encouraging finding that the RTP decision-making is shared among stakeholders, the inconsistency found in the composition raises some potential concerns about the specific dynamics of the communication among staff.
Achieving the Criteria Set Across the RTP Continuum
Premature RTP has been suggested as a possible risk factor for re-injury [41, 53,54,55]. Throughout the RTP continuum, surveyed practitioners highlighted encountering various challenges capable of influencing their decision-making (Table 4). When progressing through the RTP continuum following hamstring injury, team practitioners reported that there were occasions when the player did not meet all of criteria set (Fig. 2). However, these occasions were not common. Typically, teams met the criteria they set ≥ 90% of the time, yet, the variations demonstrate the reality of the practical setting where it is not possible to achieve this all of the time.
Each injury case must be assessed individually, based on a risk assessment. Accordingly, the risk associated with accelerating a player’s RTP to ensure availability for a decisive fixture may be more readily accepted in the case of the key 1st team player as opposed to the promising youth team prospect—who might be afforded a longer RTP timeframe to reduce reinjury risk. While surveyed teams predominantly displayed a high degree of success in achieving criteria, this finding reflects only one muscle-group (hamstring). Therefore, we do not know if this is representative of rehabilitation across other muscle-groups or injury types.
An inherent limitation of survey-based research is its lack of external validity owing to low response rates. One hundred and thirty-one (42%) of 310 invited teams completed the survey. Accordingly, caution should be exercised when interpreting or generalizing these results, as the extent to which they characterise the perceptions and practices of the non-responding teams is unclear. How these findings extend to other levels of competition (professional vs. amateur), genders, different age groups (senior-level vs. academy-level) and other muscle-groups or injury-types is also unknown and warrants consideration in future research. Representing current opinion (level 5 evidence), we acknowledge our findings may change with emerging evidence and paradigm shifts. Therefore, the perceptions and practices of practitioners should be re-evaluated in the future, based on new research recommendations. While sampled clubs appear to display a high degree of success in meeting their outlined criteria, a perceived limitation (although not a specific focus of our survey) could be that we did not ask practitioners to elaborate on instances where RTP was accelerated without achieving criteria. It is not known if, in these instances, re-injury occurrences predominantly occurred. We also acknowledge that survey responses correspond only to the perceptions and practices of science and medical practitioners responsible for the return-to-play programme. It is possible that responses could vary according to the position of the stakeholder surveyed while the perceptions of other key stakeholders’ groups involved in decision-making (e.g. managers, players) were not considered. We could not compare cultural differences as participating clubs from different confederations/leagues were not equally represented. Further investigation adopting techniques capable of facilitating a more comprehensive picture (e.g. qualitative focus groups, individual interviews etc) of how specific metrics and thresholds inform return-to-play decision-making is required.