Hamstring Conditioning Strategies for Injury Risk Reduction and Performance

Hamstring strain injuries (HSI) can account for 20% of the absence days caused by injury within team sports (2, 7-9), with an incidence rates currently increasing (9). A primary mechanism for a hamstring strain injury has been proposed to be related to the lengthening action and the strain placed upon the hamstrings during the late swing phase of sprint running (30). The hamstrings also have a meaningful role in athletic performance and dynamic activity (6, 22-24), as a primary driver of athletic tasks, such as jumping and accelerating due to their importance for knee flexion and hip extension (23). Given their dual importance for injury mitigation and athletic performance, there is a high priority to train the hamstrings with both considerations accounted for, which would require the performance of both gym-based conditioning strategies and field-based strategies.

Gym based Conditioning Strategies

To optimally prepare the hamstrings, gym-based strategies should be very holistic in their approach using a variety of methodologies in a planned and sequential fashion (26). Planning the training for the hamstrings should involve an understanding of force and velocity characteristics of exercises, an appreciation of the kinesiology and biarticular nature of the hamstrings, in addition to the prescription of hip and knee dominant based tasks.

The planning and implementation of high-force, slow velocity exercises (Table 1) and high rate of force development (RFD) exercises (Table 2), to elicit adaptations in rapid force should be done for simultaneous and individualised adaptations. Eccentric training methods, such as the Nordic hamstring exercise (NHE), have received the greatest amount of attention in research and practise, despite having mixed adoption rates within team sports, potentially due to having a poor reputation from coaches for causing delayed onset muscle soreness and lack of task specificity (2, 7, 8). However, eccentric exercises specifically the NHE has been reported to be the most effective method of reducing HSI events within team sport athletes (27, 31), with the addition of external load or even bodyweight alone if a break point is still achieved.

Implementing eccentric exercises, which have been found to promote positive adaptations in muscle architecture and hamstring strength, produce the greatest forces at slow velocities. This contrasts the commonly referenced force-velocity curve (Figure 1), this is a misunderstanding of the original work by A.V. Hill (14) where the force velocity curve only represents the action a single muscle fibre not an entire muscle. Hence, slow velocities result in the greatest forces being achieved, for example in the nordic hamstring exercise or isokinetics, where the ability to apply high forces is limited with higher velocities constraints. Moreover, it is likely then that the adaptation of slow velocity tasks which enable the greatest forces to be realised will be driven by time under tension. Supra-maximal eccentric exercises, such as the loaded nordic hamstring exercise or supra-maximal Romanian deadlifts (RDL), do require sufficient levels of athlete readiness prior to completing including them within the training, with a history of performing conventional exercises to ensure the repeated bout effect and other structures are appropriately conditioned. This is crucial as loading the NHE within weaker athletes will result inhibition during the exercise and reduced forces. Moreover, a supra-maximal RDL prior to development of sufficient strength and control of associated structures could result in poor performance and potential injury. It should also be noted if we are progressing with supra-maximal eccentric loading, while continuing to minimise time under tension the prescribed volume should still be minimal, with small dosages of hamstring exercises proven to be effective.

As the hamstrings are required to produce rapid forces in addition to high peak forces, gym-based strategies could also include high RFD exercises. These exercises are commonly isometric in nature; but that doesn’t always mean static. Rapid dynamic exercises requiring the ability for the hamstrings to produce a high force to decelerate the body or resistance either during knee extension or hip flexion tasks, can be accomplished using a variety of equipment. For example, rapid lower-limb catches require a rapid unweighting from a shortened position followed by a rapid braking in a lengthened position. Similarly, oscillatory variations require the ability to rapidly contract and relax, although under low forces could encourage some positive adaptations in neuromuscular control.

Although there is contention around the potential of an isometric vs eccentric action with regards to the muscle action occurring during the terminal swing phase of running, it is possible that both are occurring during different phases of the swing phase, or even within different parts of the muscle during the same action. Moreover, isometric exercises can provide the rapid force stimulus that may not be achievable with eccentrics and slow velocity tasks. Static isometric tasks can split into two types, commonly known as “overcoming” (i.e., pushing / pulling) where the aim is push / pull against an immovable object or “yielding” (i.e., holding) where the aim to hold a set position. Overcoming isometric tasks can result in the highest forces being achieved but prescribing them in cluster format, with the cue to be “as fast and as hard as possible” can result in high rates of force development. Yielding isometrics exercises although unlikely to promote the high rates of force development, they can be used as an entry exercise into hamstring conditioning.

Periodising hamstring gym training

As with the development of any physical quality within team sports there will be periods where you are looking to maintain a physical quality while stimulating others, which will fluctuate through the season both on a microscale (i.e., within each micro-cycle) and a large macroscale (i.e. across mesocycles within a season taking in different phases of a season).

Starting with macroscale plan, the three key phases of a team sport season; off season, which is now arguably a transition phase between seasons, pre-season, where the practitioners focus is on preparing athletes for the upcoming season. The final all-encompassing focus for a practitioner is in-season. Within each of these phases the training prescription should differ (Figure 2), within the off-season an early focus should be on physical restoration while also providing an opportunity for anatomical adaptation which could be achieved with low-moderate force exercises for both the knee and hip, performed at a slow velocity without the competing demands of training or match play. It is worth noting towards the end of this transition phase as they lead into pre-season, the introduction of exercises emphasising RFD at a low-moderate intensity could be prescribed this will begin to prepare athletes for the increasing demands of pre-season (especially running and locomotive demands such as high-speed running).

The goal of pre-season is to prepare athletes for the coming season with regards to physical performance and injury prevention, this includes providing adaptations that maximise high force production via the inclusion of high force, slow velocity exercises and high force, high RFD exercises. As these two factors are likely important to reduce likely mechanisms on hamstring strain injury and the performance of performance tasks, increases in force (especially eccentric capabilities) will also have a protective effect to muscle soreness through the repeated stimulation resulting in the repeated bout effect (18) and reducing the potential for muscle damage (25). As the athletes will be performing more field-based training as part of pre-season to be able to cope with the potential volumes of work likely to be performed in matches during the season, it is also important to implement field-based conditioning strategies. This will include the implementation of exercises to optimise sprint performance that may be able to reduce the risk of injury incidence, while also monitoring the volumes of work performed. However, it is important to note with the latter that this is only to monitor excess changes in the performance of work, not to reduce the work entirely and as physical preparation practitioners we need to be aware that we need to develop robust athletes. Importantly, this requires progressive overload and not an over reliance of maximal intensities or volumes of work performed, especially within pre-season.

During the in-season phase, all these strategies will need to be implemented (i.e., mixed methods approach) and fluctuated to ensure appropriate progression via the stimulation of physical qualities and the maintenance of others, while maximising progressive overload throughout. It is also important to note that the fluctuation of these strategies will also likely be related to the expected volumes of work players may be exposed to, for instance during periods of fixture congestion (2 games in 7 days/3 games in 10 days) or when players are on international duty (4). International training camps and fixtures commonly expose players to greater physical demands than experienced at the domestic level, although this is team (both domestic and international) and athlete dependent. Within women’s elite football significant increases in sprint distances and high intensity running are observed within international versus domestic league matches (1), highlighting that what players are conditioned for (i.e. domestic league matches) may not be sufficient for the international competition. Moreover, playing standard also impacts the physical demands players experience, within rugby league NRL players are exposed to increased demands in comparison to lower level competition (10). To optimally prepare athletes for these potential increases in physical demands, it would be suggested that small increases in domestic club training load prior to international selection, when players return to perform normative training load levels at the domestic club training will act as a reduction in load and intensity and potentially resulting in a supercompensation effect.

Field based Conditioning Strategies

As the mechanism of hamstring strain injuries is on the field, it makes sense to have a strategy within the field to maximise performance and minimise injury risk.  Field based conditioning strategies will fall into three foci: technical skill, performance output, and monitoring (Figure 3). This will ensure all areas of risk and performance appropriately covered, with figure 3 detailing what is incorporated within each focus.

Technical skill development by the inclusion of sprint specific drills (i.e. marches, skips, hops etc.) has come to the forefront of many practitioners’ processes with regards to physical development and are commonly dosed within field-based warmups and gym-based sessions (12, 13). The development of these underpinning characteristics can increase sprinting performance, improve sprint efficiency, and decrease risk of injury. One of options of when to prescribe exercises for athletes to improve the technical characteristics can be achieved as part of warm up, as part of the ramping process of intensity where whole body or joint movement velocities, muscle-tendon unit changes and ground-contact times can be progressively loaded. Another option with the use of technical skills is to use in conjunction with monitoring practises where inclusion of sprint specific drills can offer a velocity constraint to avoid maximal velocities when required but still allows for the development of technical qualities (this will be discussed further with monitoring). It is worth noting, however, that with improvements in sprint performance, athletes need to be physically prepared to perform at these higher intensities as higher running velocities require the hamstrings to generate a greater force to decelerate the shank or they could be exposed to increased risk of injury so developing through accumulation and intensification could be useful in the improvement process when using these skips and drills.

With regards to the technical characteristics identified within Figure 3 (3), these can be sub-categorised into spatial temporal characteristics with stride length and stride frequency. The key goal here would be to avoid ‘over-striding’ with its link to injury and creating braking forces, thus slowing the athlete, and thus trying to optimise both to increase sprint performance (3). Another core technical component is control, for example, trunk control, whereby avoiding excessive lean (sagittal and frontal plane) or flexion, which can alter tissue lengths and associated lever arms that can increase injury risk and decrease the transfer of force (3). Moreover, team sport athletes should also have adequate control to be able to dissociate between the upper and lower body, specifically to support appropriate scanning to enable execution of sport specific skills, such as passing and receiving, particularly during high velocity running efforts. Similarly, lumbo-pelvic control is also required to maximise the hamstring and pelvis interaction with loading and avoiding anterior pelvic tilt, which can increase tissue strain (3). During the swing phase, there is the potential for ‘back-side’ mechanics to alter the stretch and strain of muscles within the lower body, while also not being an efficient way to the transfer and complete the subsequent ground contact (3). However, the ability for athlete to understand the potential role of backside vs front side mechanics is important as there will be instances where they could be incorporated to great effect, for instance if trying to avoid a tap tackle in rugby back-side mechanics whereby there is a quicker retraction of the heel could be beneficial (12). Hence, developing an athlete’s ability to tolerate the tissues loads associated with both ‘back-’ and ‘front-side’ mechanics while understanding how to differentiate and blend between them is important.

Exposure to maximal effort sprinting is crucial element in the physical preparation of team sport athletes for several reasons. Firstly, maximal effort sprinting can be used to compliment resistance-based strategies to stimulate an adaptation in the modifiable risk factors (eccentric hamstring strength and muscle architecture) while providing a strong performance stimulus to lead to adaptations in sprinting and jumping performance (26). Secondly, consistent weekly exposure has been observed to be a method to reduce the risk of hamstring strain injury incidence, this could be related to the repeated bout effect as protective mechanism to muscle damage (16, 17). Finally, exposure to maximal sprint efforts over varying distances and which may even include resisted loads both of which can have a positive impact on the mechanical outputs during the sprint improving performance and reducing the risk of injury, including the sprint force-velocity profile (5, 6, 15, 19-21, 28, 29). Mechanical insights derived from kinematic data have recently been identified as a potential risk factor for hamstring strain injury, which is also impacted by a previous hamstring strain injury. Therefore, providing a positive stimulus to improve higher rates of horizontal acceleration relative to an individual’s velocity profile, via the inclusion of sprint efforts, could be seen as an essential feature within training (15, 20, 21, 29). As consistent maximal sprint efforts within training can have the benefit of improving performance, decreasing injury risk, and providing a protective stimulus to muscle damage, however, we also need to consider how much volume we are exposing our players too (i.e. monitoring).

Continual monitoring of the volume and frequency of sprint efforts is an essential part of physical preparation coach’s role when working with team sport athletes. It is also getting easier with the amount of wearable and tracking technologies available to practitioners, but it is also getting increasingly overwhelming with large amounts of data available. Practically, coaches need to consider the volume of high-speed work being performed (i.e., sprint distance) and frequency of sprint efforts, this can be done within a single training session, across micro-cycles and mesocycles.

Within a single session there may be an expected distance to be covered or frequency of sprints required, this will be dictated by the amount of work performed within the micro-cycle up to that point and the plan for the subsequent days of the micro-cycle. This could fluctuate depending on fixture congestion, planned over-reaching or deload of physical qualities and the planned tactical and technical training sessions within the micro-cycle, which highlights the co-creation of planned training session volumes and intensities with the sports coaches to developing a key monitoring strategy. Tracking technologies can be used in this situation and the availability of live GPS collection can be extremely useful in training situations; however, there needs to be considerations from the fitness perspective not to constrain players but allow the overload of volume and intensity when appropriate.

 Across a micro-cycle there are constant fluctuations in the volumes and intensities of work performed, with the potential for periods of fixture congestion, the requirement for periods higher volumes and intensities in certain periods to stimulate an adaptive response and the opposite for periods of restitution to allow for supercompensation. Additionally, when players are selected for international training camps or fixtures, similar considerations need to be made to optimally prepare athletes to go to camps athletes should be exposed to higher intensities prior to departing which will ensure they can cope with the expected greater physical demands, although this should be considered on an individual basis (domestic team, international level and current training load). This physically prepares athletes to be at a reduced risk of injury and enhanced performance at the international level and on return to the club domestic level where there will be a reduction in load and intensity which could result in a supercompensation effect to ultimately enhance performance.

A periodised approach to physical preparation requires a systematic development using a planned blueprint that aims to develop or maintain physical qualities simultaneously. The idea of developing all physical qualities simultaneously is commonly attributed with novice athletes. With the higher the performance level the greater the focus on that physical quality requires, although a minimum volume is required of other stimuli to ensure the maintenance of physical qualities (i.e., the premise of emphasis modelling as suggested by Professor Greg Haff). The systematic development of physical performance requires progression and variation in the volumes and intensities to allow for the enhancement of physical performance through modulation of fitness and fatigue parameters, improvement of injury risk factors and continued development of physical performance characteristics without plateauing (Figure 5).

Summary

Physical performance and injury risk need to be considered as one and the same, not merely siloed off into different aspects of training. Hence, there is a need for robust strategies that can be applied in resistance-based setting and field. The strategies explored can provide a highly effective and robust process to enhance athletic performance and reduce the risk of HSI within team sport athletes. The prescriptions and practises advised can be easily adopted into training and as part of the physical performance coach’s role, especially when working within the multi-disciplinary team (i.e. sports coach or coaches) to ensure optimal preparedness. As practitioners we also need to prioritise the development of physical qualities and not be too constrained or focused on the need for recovery, which would lead to potential detraining in modifiable risk factors of hamstring strain injury and decreases in performance by not stimulating the physical qualities frequently enough.

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