INTRODUCTION
With sports participation, there is an inherent risk of injury that is unavoidable (26); however, reducing the incidence of injuries is one of many priorities for strength and conditioning (S&C) coaches alongside the development of relevant physical qualities. To design an S&C program that enhances performance while reducing injury risk, a thorough needs analysis should be conducted for each athlete that is both relevant to the sport and the individual (70).
Generic movement screening systems have previously been suggested as a useful tool for identifying injury risk (43,45,68) and have been used by high-level professional organizations for the purpose of identifying athletes at risk of injury (52). However, these methods demonstrate little capacity to identify injury risk within large-scale studies with substantial sample sizes (5,53,57). This may be because of deviations from their original design, which was to establish foundational movement competency in nonspecific activities (14), along with many prospective cohort studies using sample sizes that are too small to accurately identify risk factors (49). An approach focused on examining the relevant movement patterns, which are reflective of the demands of the sport and mechanisms of injury, is likely to be required to prevent sports injuries (8). More recently, the ability to predict injury has been questioned because of the multifactorial nature and unique circumstances that accompany an injury incident (9). Nonetheless, the current body of evidence indicates that the application of well-designed interventions, encompassing resistance training, plyometrics, balance and skill training, does reduce modifiable risk factors (39,41,59,60), and the subsequent reduction of injury risk (8).
The objective of this article is to provide S&C coaches with a systems-based approach to reduce injury risk with their athletes, along with presenting an example of how this model could be applied to enhance practical application. This includes clear sequencing, whereby evidence-based screening tools are used and the information gathered from this process will then allow coaches to design injury prevention programs to target modifiable risk factors. This model is based on the work of van Mechelen et al. (79) and Finch (25). Modifications have been made for application in the setting of an S&C program. This approach, which along with effective monitoring of the training program to manage the distribution of load, has a greater potential to reduce an athlete's injury risk.
CONSIDERATIONS FOR DEVELOPING AN INJURY PREVENTION MODEL
Before the implementation of any training intervention, the S&C coach must understand the demands of the sport and the athlete's ability to cope with those requirements. Similar to the design of a performance program, a needs analysis relevant to injury risk must be conducted (70). Using this information, a testing battery can be designed and implemented to identify and reduce modifiable baseline risk factors. Figure 1 illustrates this 5-step process for the injury prevention model. After discussing each component of the 5-step process, an example will be provided as to how this model may be applied by the S&C coach.
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Figure 1.: Step process for developing injury prevention training strategies to reduce injury risk.
By applying this systems-based approach to designing and implementing an injury prevention program, S&C coaches will be able to plan effective strategies to reduce injury risk. In addition to accurate documentation of the occurrence of athlete injuries, this approach will allow for the continual development of an injury prevention program that can be refined and updated as new evidence becomes available within both the literature and the coaches' practice. It is important to note that this approach must align with the development of the athlete's physical and sports performance and should rarely be seen as a separate component. From this perspective, the injury prevention program is viewed as an inclusive element to the training process that allows the athlete to thrive in their sporting environment.
STEP 1: ESTABLISH THE RISK OF INJURY THROUGH SPORTS PARTICIPATION
The first step in identifying the injury risk associated with a sport is to examine the injury epidemiology in the literature (25). This allows coaches to establish the injury incidence for any given sport, which represents the number of injuries occurring relative to the time participating in either training or competition for the sport, divided by the number of athletes included in the data collection process (79). The major issue with this type of analysis is the manner in which injuries are reported (24). Finch (25) suggests that the following methodological limitations exist within injury surveillance research:
- The use of many injury surveys that have not been validated.
- The presence of recall bias with retrospective data collection as athletes report injuries a long time after the onset of the injury.
- The failure of studies to define sports injury and severity.
- The failure to report exposure to sport via training or competition because it relates to injury incidence.
Although recent literature has accounted for many of these issues, coaches must be sure to critically analyze the research that they use to inform their model. Importantly, research used to inform the training process should include validated measures for data collection, use of a prospective study design, documentation of the severity of injuries by identifying days missed from sport participation, and establish the athlete's exposure to training and competition. For further details and observation of consensus statements for the undertaking and examination of injury surveillance research, coaches should review the following resources (28,29,44).
Alongside such considerations, coaches should also be sure to incorporate research that is specific to the sport and their athletes' profile for the incidence data to be valid. For example, injury epidemiology research should be evaluated based on the sex (36), age (74), player position (76), level of competition (33,34), and the era in which the data were recorded because of potential rule changes or changes in sport demands (15,35). Thus, research used to inform the injury prevention model must be relevant to the athlete and their sport for successful outcomes to be achieved.
Once injury incidence has been established, coaches must also consider the severity of each injury to prioritize interventions (79). If only the incidence is considered, coaches may mistakenly prioritize the prevention of injuries that have a high frequency rate, yet lead to little time loss from training and competition. Instead, injuries that occur frequently and are associated with an increased severity should be the major focus of screening assessments and injury prevention training programs to minimize days-lost that could be used for athlete development.
Severity of injury should also be considered in relation to the nature of the injury (79), specifically as it relates to the type of tissue injured and the degree of damage (i.e., grade I versus grade III muscle strain). As muscle tissues receive a greater blood supply than ligaments, tendon, and cartilage, healing rate is expected to be more rapid (58). As such, the prevention of ligament injuries may be prioritized over muscle injuries if all other factors are equal. Severity may not only impact the time loss from sports training and competition but also the demand for treatment, cost of injury, and the potential for permanent damage that may impact the athlete's long-term health (79). All variables should be factored into the decision-making process to prioritize the development of a relevant testing battery and implementation of a targeted training intervention.
STEP 2: IDENTIFY INJURY MECHANISMS AND RISK FACTORS
MECHANISMS
Once injury incidence and severity has been identified, S&C coaches may begin to examine each injury and its accompanying mechanism. This will involve biomechanical screening of movement patterns, which is indicative of sporting situations that are characterized by increased injury risk (38). For example, in sports where anterior cruciate ligament (ACL) injuries are prevalent, activities involving rapid decelerations, such as changing direction and landing from a jump, should be assessed (38,67). Dysfunctional movement patterns displayed during these activities involving positions of heightened thigh adduction will increase the strain on the ACL (40). If possible, this should be verified with prospective cohort studies that show athletes who exhibit these aberrant movement strategies increase their risk of injury (25).
Coaches should be aware that not all injuries have a definitive mechanism. Overuse-type pathologies, such as tendinopathy, are more likely to occur through the application of an unaccustomed and excessive training load (16). Many overuse injuries do have an associated movement pattern because it relates to a specific sport. For example, patellar tendinopathy in a male volleyball player is likely related to jumping activities (50), with landing strategies using greater knee flexion increasing patella tendon loading (21). Therefore, mechanisms of injury and risk factors should be considered alongside the demands of the sport and a comprehensive monitoring program to more accurately determine the athlete's injury risk at any given time point.
RISK FACTORS
Injury risk factors can be characterized as either intrinsic or extrinsic (79). Intrinsic risk factors relate to the individual in question: age, maturation, sex, genetics, anthropometric measurements, health status, injury history, biomotor qualities (e.g., maximal strength), and training background (8,79). These factors can then be used to individualize the injury prevention model to the athlete. Extrinsic risk factors relate to environmental influences: equipment, playing surface, and weather (8,79). Risk factors can be further divided into modifiable (e.g., the athlete's maximal strength levels) and nonmodifiable (e.g., the athlete's gender) (7). Although non-modifiable risk factors are important for coaches to recognize to estimate risk, this information is not useful in the development of a test battery. However, identifying modifiable risk factors is vital to more accurately assess the athletes' risk of sustaining injuries that are relevant to their sport (8).
Importantly, coaches should not view each risk factor as an isolated variable that predisposes an athlete to injury. Instead, the complex interaction and summation of each factor should be acknowledged, which results in an athlete being more vulnerable to injury (8). For example, an athlete displaying a risk factor during a screening does not necessarily indicate the athlete will get injured. It is the exposure to the mechanism that will provide the stimulus, in conjunction with other known risk factors that make injury a possible outcome (8). As such, an athlete who presents with poor eccentric hamstring strength possesses a functional deficit that may increase the risk of hamstring strain (78). In some instances, this may not be a major concern for the S&C coach if no other risk factors are present in the athlete's profile or the athlete is not exposed to high speed running as part of their training or competition. However, if the athlete was also a male, with a previous history of hamstring strain and competes in a sport where high speed running is routinely performed (55), weak hamstring musculature may become a primary concern for the S&C coach. Coaches must therefore perform a comprehensive risk factor analysis for each individual, considering all variables that may predispose the athlete to injury.
STEP 3: DESIGN AND IMPLEMENT A VALID AND RELIABLE TEST BATTERY
In designing a test battery to identify athletes who display a greater risk of injury, coaches must select tests and procedures that are both valid and reliable (25). Examining the available literature or conducting investigations in the field would establish the reliability and validity of their testing protocols. There are currently a range of field-based tests that may be employed to screen athletes for modifiable risk factors, such as hamstring strain (27), ACL injuries (66), shoulder and elbow injuries (71), lateral ankle sprain (62), and low back pain (51). As such, a number of options are available to the S&C coach that allows for the collection of valid and reliable data for identifying functional deficits in modifiable risk factors.
Once tests that assess relevant and modifiable risk factors have been selected and performed, an athlete profile can be established. After the completion of the testing battery, the S&C coach can rank and prioritize deficiencies that may be identified, which will provide an approximate estimate of the athlete's injury risk. This can be accomplished by comparing the athlete's performance with normative data that may be obtained within the literature on similar athletic groups.
STEP 4: PRIORITIZE AND IMPLEMENT INTERVENTIONS
Research has shown the effects of targeted training programs on modifiable risk factors, with concomitant reductions in the occurrence of injury (46), including hamstring strains (4), ACL injuries (61), ankle sprains (54), shoulder pathology (3), and low back pain (17). Furthermore, well-designed injury prevention strategies have been shown to reduce the total number of injuries in sports such as soccer (43), basketball (22), and distance running (81). At this point, coaches should prioritize the goal of their interventions based on the athlete's deficits as measured in the testing battery and their estimated overall risk as identified in step 1. Coaches should also ensure that performance objectives are not neglected. From this perspective, a multifaceted approach should be adopted, discussed, and agreed with the athlete, coaching, and medical staff.
STEP 5: RETEST TO ASSESS THE EFFECTIVENESS OF THE TRAINING INTERVENTION
After a period of training, coaches should retest the athlete to assess the effectiveness of the intervention and reevaluate the athlete's injury risk. This time frame is dependent on the physical quality or skill being developed, and realistic expectations as to when adaptations should have occurred. For example, changes in ankle dorsiflexion range of motion are possible after a single exposure to a mobilization intervention (37,42); however, improvements in fundamental movement patterns and muscle architecture may take weeks to transpire (1,10). Because changes in mobility could be more immediate, retesting can take place directly after the application of a single bout of mobilization. However, if motor control or structural changes are desired, then testing sessions should be separated by a number of weeks to observe appreciable changes.
Scheduling regular opportunities to retest should be seen as an evolving process, with the injury prevention model being continually redeveloped as updated research is published. Other factors may also lead to the evolution of the injury prevention model, such as modifications to the sport itself that alter the demands placed on the athlete (i.e., rule changes) and new experiences that influence the coaches understanding of the potential injury risk. Likewise, as the athlete's level of injury risk will change based on their physical development, the model will require alterations. This may be driven by a number of factors, such as an injury being sustained, growth, and maturation or advancing age that is placing them at a greater risk for certain types of injury (i.e., hamstring strains (31)).
DESIGNING THE INJURY PREVENTION MODEL
To illustrate how the model and conceptual framework presented in this article can be applied, an example has been included in Figure 2. The selected athlete is a 23-year-old male triple jumper, competing at international level. Injury rates during the course of multiple seasons in track and field have been shown to be a key determinant in an athlete's success (69). Track and field events present athletes with a high injury risk across the course of the season, with an injury occurrence rate between 3.1 and 169.8 injuries per 100 athletes depending on the event (18). For the men's triple jump event specifically, Alonso et al. (2) recorded that 125 injuries occur per 1,000 athletes within the competitive period.
Figure 2.: Injury prevention process specific to reducing hamstring strain risk for an international male triple jumper.
Because of the nature of triple jump, acute injuries are a common occurrence when compared with distance events (20). The majority of these injuries occur in the lower extremity (2). In particular, thigh strains account for 22.8% of all injuries in jumping events (23), with hamstring strains being the most common (19). Ankle injuries are also widespread (22.7%), with sprains particularly prevalent (23). Achilles tendinopathy has also been shown to be prevalent in athletes competing in explosive events within track and field (18). Other sites of injury for jumping events are the trunk (13.6%), knee (12.1%) and foot (10.6%) (23).
Regarding the severity of injuries, the greatest time loss after injuries incurred from track and field events has been reported with hamstring strains (2,23). Other injuries in athletics that have been shown to lead to large periods away from training and competitions (>4 weeks) include Achilles tendon injuries, ankle sprains, strains at the groin and lower leg, and lower leg stress fractures (2). Based on injury incidence and severity presented here, hamstring strains, ankle sprains, and Achilles tendinopathy should be prioritized for the development of injury prevention strategies. As the available literature indicates that hamstring strains are the most common (19) and display the greatest severity (2,19), a systems-based model will be applied herein to illustrate its application.
Modifiable risk factors for hamstring strains include considerations centered on flexibility, core stability, strength, and muscle architecture (55). Although local flexibility of the hamstring musculature is poorly associated with injury risk (4), reduced hip flexor (32) and ankle mobility (31) has been identified as potentially problematic. This is because of poor hip extension and ankle dorsiflexion mobility interfering with sprint mechanics by increasing hamstring length on the contralateral extremity during the terminal swing phase leading to malalignment of the pelvis (11) and disrupting ankle proprioception (31), respectively. Likewise, the trunk musculature has also been shown to prevent excessive lengthening of the hamstrings by controlling the sagittal plane orientation of the pelvis during the terminal swing phase of sprinting (11). Deficits in core stability, therefore, have been suggested to increase the risk of hamstring injury during sprinting (55).
As the hamstring musculature are required to contract while lengthening during the terminal swing phase of sprinting (65,77), eccentric strength tests are recommended for screening. This is supported by evidence that low eccentric hamstring strength has been shown to increase the risk of developing hamstring injuries in Australian football players (64). Eccentric hamstring strength may also provide valuable information regarding the architectural make-up of the hamstrings because athletes with short fascicles in their biceps femoris have been shown to possess poor eccentric strength (78). Reduced fascicle length is hypothesized to increase hamstring strain risk, by causing an overstretching of the hamstrings while they are contracting eccentrically during the terminal swing phase (78).
As the hamstrings are both knee flexors and hip extensors, testing hamstring strength with a focus only on knee flexion is potentially problematic as it ignores their biarticular function (55). Therefore, it is suggested that hamstring strength be tested with both knee flexion–based and hip extension–based tests. This recommendation is supported by Sugiura et al. (75), who highlighted hip extensor strength as a risk factor in elite sprinters.
Figure 2 shows a sequential flow, whereby the injury risk is established and followed by identification of mechanisms and modifiable risk factors. Relevant tests have then been selected, which may be used to identify deficits in the modifiable risk factors discussed. Each of the tests selected in Figure 2 are reliable enough to recognize a functional change and are associated with injury (6,12,30,47,48,63,64,75). Although some of these tests included may require access to expensive equipment, coaches can either find more cost-effective alternatives or develop their own tests as part of their own practice. For example, if the S&C coach does not have access to an isokinetic dynamometer, the single-leg hamstring bridge may be an effective substitution to measure hip extension strength (27). However, if novel assessments are implemented and/or adapted to meet the needs of their environment, it is recommended that their reliability should be examined with their own athletes. This is to determine if a “true” change in performance can be accurately quantified, which falls outside the typical error associated with the test.
Once deficits have been identified based on the athlete's test scores, a training intervention can be designed and implemented. It is encouraged that coaches adopt evidence-based training strategies. For example, data have shown hip extensor strength (13), eccentric hamstring strength (56), core stability (73) and flexibility (80) can all be improved after targeted training programs. Figure 2 provides some general suggestions for exercises that may be used to reduce the modifiable risk factors related to hamstring strains. This approach will allow S&C coaches to develop an individualized training process for each athlete seeking to reduce identified and modifiable injury risk factors, which in turn will maximize training transfer and effectiveness.
Table 1 illustrates how such exercises may be integrated into a training program for the triple jumper who presents with poor eccentric hamstring strength during testing. This program attempts to improve multiple qualities such as explosive and maximal strength of the leg extensors to enhance sports performance while developing hamstring eccentric strength to reduce injury risk. To improve the transfer of training by developing the rate of force development of the posterior chain musculature during eccentric contractions, Table 2 shows a training program that could be incorporated in the triple jumper's weekly regime. For the interested reader, descriptions for how to perform many of these exercises are included in Sherry et al. (72).
Table 1: Example training program aimed at improving sports performance and reducing modifiable risk factors that are associated with hamstring strains
Table 2: Example program aimed at improving coordination and rate of force development of the hamstring musculature
Furthermore, if myofascial or articular restrictions were identified during testing, Table 3 provides an example program that would develop that triple jumper's mobility. This program could be prescribed as a standalone training program or integrated into a thorough warm-up before the performance of dynamic activities that ensure the athlete is prepared for the high intensity nature of training. In these examples, each program would be carefully distributed around the athlete's technical training to achieve the goal of improving sports performance. Additionally, technical, nutritional, and psychological components would also be considered as part of a multifaceted high-performance approach to injury risk reduction that would include a thorough monitoring process.
Table 3: Example mobility program aimed at reducing modifiable risk factors that are associated with hamstring strains
SUMMARY
Although S&C coaches should always strive to improve sports performance, equal efforts must be made to reduce the risk of injury for each athlete. This article provides a step-by-step, systems-based model for designing injury prevention training programs. An evidence-based approach should form the foundation and the individual athlete should also be placed at the center of the model. The current model aims to reduce modifiable risk factors through the incorporation of targeted training interventions, with the desired outcome of decreasing the athlete's overall injury risk.
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