In this blog I wanted to bring special attention to the importance of horizontal deceleration ability to speed performance and injury-risk reduction in multi-directional speed (MDS) sports. This post essentially builds on a previous blog written by Alistair McBurnie where he summarised the applied implications from a recent publication (written by the Science of MDS team) titled ‘Deceleration Training in Team Sports: Another Potential ‘Vaccine’ for Sports-Related Injury’ (McBurnie et al., 2022). Similar to the previous blog, this post will continue to build on discussions around why horizontal deceleration ability may be able to ‘kill two birds with one stone’ with regards to enhancing MDS performance and injury resilience.
First…What is Horizontal Deceleration Ability?
From a purely mechanical perspective deceleration is defined as decreasing velocity with respect to time (Winter et al., 2016). It is also important to acknowledge that deceleration is a vector quantity, therefore in accordance with Newton’s laws of motion is directly proportional to the direction of force applied (Winter et al., 2016). Therefore, to manipulate the rate of horizontal deceleration, and subsequently horizontal momentum, an athlete must adjust either the magnitude or duration of force (i.e., impulse) applied in the horizontal direction (Winter et al., 2016; Winter & Fowler, 2009). It is important to stress that the optimisation of braking impulse requires a high level of technical ability (Harper et al., 2022). Therefore, horizontal deceleration should be regarded as a skill where athletes capable of generating a greater horizontal component of the ground reaction force vector will have superior horizontal deceleration performance.
Another notable and unique feature of horizontal deceleration (that will be highlighted later) is the very high impact forces and loading rates imposed when braking hard. Therefore, a critical requirement when braking is the necessity to be able to skilfully attenuate and distribute these forces throughout the muscles and connective tissue structures of the lower limbs. Accordingly, an athlete’s horizontal deceleration ability should consider not only the ability to rapidly reduce momentum, but also the ability to attenuate and distribute the high mechanical forces that are associated with braking. Based on these considerations, we recently proposed that horizontal deceleration ability should be defined as:
“a player’s ability to proficiently reduce whole body momentum, within the constraints, and in accordance with the specific objectives of the task (i.e., braking force control), whilst skilfully attenuating and distributing the forces associated with braking (i.e., braking force attenuation)” (Harper et al., 2022).
This definition highlights two key components that are illustrated in figure 1, including Braking Force Control and Braking Force Attenuation.
These two key components of horizontal deceleration ability align with the implications for MDS performance enhancement and injury-risk reduction. In brief, braking force control requires the athlete to manipulate their centre of mass (COM) posterior to the centre of pressure (COP) to ensure anterior foot placement and the required orientation of the braking force. Whereas, braking force attenuation requires muscles and connective tissue structures to attenuate and distribute forces throughout the lower-limbs to help reduce soft-tissue damage and neuromuscular fatigue that can result from repeated intense horizontal decelerations (Harper & Kiely, 2018).
Horizontal Deceleration Demands in Multi-Directional Sports Competition
An important starting point for any sports science and medicine practitioner is to understand the demands of the sport, with specific attention given to what the athlete maybe exposed to during competitive match play. Probably the first reported data of horizontal deceleration demands during competition was by Bloomfield and colleagues (2007) in male English Premier League (EPL) soccer players using an observational video notational approach that became known as the ‘Bloomfield Movement Classification’. They observed that EPL soccer players performed on average nine decelerations following high-speed running and sprinting actions every 15 minutes, meaning they would total approximately 54 decelerations per match, with the majority (77%) being performed after a sprint. Furthermore, they also observed that 72% of these decelerations were less than 1s in duration, with almost all (96%) less than 2s. More recent data using global positioning system (GPS) tracking devices illustrates that high-intensity decelerations (< -2.5 m.s-2) could be more frequent than equivalent intensity accelerations in many MDS sports, including: soccer, rugby sevens, rugby league, rugby union, hockey and Australian football (Harper et al., 2019). In summary, horizontal decelerations are highly frequent in MDS sports, and as such can contribute to teams winning official competitive matches (Rhodes et al., 2021), and to creating and preventing goal scoring opportunities (Martínez-Hernández et al., 2022).
Unfortunately, systematic video evidence from competitive match play in a variety of MDS sports (e.g., soccer, basketball, American football, Australian football) also confirms that rapid horizontal decelerations are one of the most common situational patterns associated with major injuries like anterior cruciate ligament (ACL) rupture (Cochrane et al., 2007; Della Villa et al., 2020; Lucarno et al., 2021; Schultz et al., 2021; Waldén et al., 2015). For example, in soccer it has been reported that between 33-66% of male (Della Villa et al., 2020; Waldén et al., 2015) and 58% of female (Lucarno et al., 2021) players sustain ACL rupture during defensive pressing when performing a rapid horizontal deceleration from high movement speeds (Figure 2).
Furthermore, whilst hamstring strain injuries are often associated with accelerating and high-speed running, recent systematic video data from rugby union (Kerin et al., 2022) and soccer (Gronwald et al., 2022) report decelerating and braking to contribute to 18% and 35% of all hamstring strain injuries in professional players, respectively. This is likely associated with deceleration and braking strategies where the knee is extending upon a forward rotating trunk imposing increased eccentric ‘stretch-related’ forces to the hamstrings (particularly biceps femoris) (Gronwald et al., 2022; Kerin et al., 2022). This braking strategy is often observed in other MDS sports (e.g., American football) when performing rapid horizontal decelerations, and is therefore likely to be a mechanism of hamstring strain injuries in this and other MDS sports also. Therefore, in addition to high speed running, deceleration field-based exercise training exercises may also be an important tool for helping to mitigate the high frequency of hamstring injuries in MDS sports with a particular emphasis on trunk control in the sagittal plane (Gronwald et al., 2022).
Why is Horizontal Deceleration a Critical Link to Both Speed Performance and Injury Risk Reduction?
The most likely explanation is the potential to generate very high forces in short time periods. For example, when compared to initial acceleration and maximal velocity sprinting, the forces in the initial braking steps when decelerating from high movement speeds have been reported to be around 2.7 to 1.3 times greater in magnitude, respectively (Figure 3) (Bezodis et al., 2008; Verheul et al., 2021). As illustrated in figure 3, high impact forces and loading rates occur during the first 10 to 40% of stance and must be rapidly attenuated and distributed over very short time periods (<50 ms). This is pertinent to note as ACL injuries to tend to occur in < 50 ms.
From a performance perspective such high forces permit very rapid changes in velocity across very short time frames and distances. When compared to maximal acceleration, Varley et al. (2012) reported a 17% greater rate of change in velocity when performing a maximal deceleration from high running speeds. Similarly, during a 180° change of direction task requiring significant deceleration, average rates of change in velocity have been reported to be up to 70 to 75% greater during the deceleration (-4.43 to -6.82 m.s-2) compared to the acceleration (2.55 to 4 m.s-2) phase, due to much shorter time frames in which changes in velocity occur (Zamparo et al., 2019). These trends have also been reported in soccer match play, with all positional roles producing higher magnitudes of decelerations (-5.7 to -6.3 m.s-2) compared to accelerations (4.4 to 4.7 m.s-2) (Oliva-Lozano et al., 2020). Collectively, these findings highlight the importance of players possessing a greater deceleration ‘reserve’, with the requirement for greater deceleration capacity as horizontal acceleration and top speed capabilities increase (i.e., they need to be able to generate greater braking impulse to reduce higher forward momentums). Figure 4 illustrates the performance implications of a greater deceleration reserve by comparing a player from a pre- to post-deceleration training period or when comparing between two athletes with different maximum deceleration reserve capacities.
From an injury-risk perspective the high forces associated with braking when decelerating, coupled with a high frequency of repetitions in match play, can impose high physiological and biomechanical demands on lower limb tissue structures (i.e., muscle, tendons, ligaments, bone) (McBurnie et al., 2022). In an article titled “Damaging Nature of Decelerations: Do we Adequately Prepare Players” Harper and Kiely (2018) proposed that these characteristics could lead to an increased risk of tissue damage and injury in athletes participating in MDS sports (Figure 5). This risk is further heightened when braking during deceleration due to the necessity for fast velocity eccentric muscle contractions (i.e., active lengthening of muscle fascicles) which can be particularly sensitive to muscle damage and subsequent declines in force producing capacities (Chapman et al., 2006). Accordingly, Harper and Kiely (2018) proposed that the risk of tissue damage and neuromuscular fatigue could be reduced by improving the performers deceleration skill along with the specific strength capacities than underpin greater deceleration ability (Figure 5). This aligns with proposed frameworks for understanding causes of tissue damage and injury in athletes (Kalkhoven et al., 2020), but places a specific focus on deceleration activities due to the high magnitude of stress (i.e., force per unit area) and strain (i.e., amount of deformation or length change) this activity can impose on lower limb tissue structures. This also brings us back to the importance of the deceleration reserve concept because athletes with greater deceleration capacity are likely to have greater tissue load bearing capacities that enable them to tolerate and generate higher braking forces. This is particularly important for withstanding high acute forces that may surpass tissue material strength capacities resulting in tissue structural failure (i.e., macro-damage). Or, in cases of chronic repetitive high impact braking forces which could result in tissue material fatigue that progressively lowers the strength baring capacity of tissues (i.e., micro-damage) resulting in increased chance of acute or overuse injury associated with decelerating frequently.
Horizontal decelerations are highly frequent in team sports requiring athletes to generate and tolerate high impact forces and loading rates repetitively. Horizontal deceleration ability considers both the athlete’s capacity to reduce momentum, whilst skilfully attenuating and distributing the forces associated with braking. Increasing the athlete’s deceleration reserve capacity may not only help to enhance match performance capabilities but could also help to increase injury resilience to the high impact forces associated with decelerating; therefore ‘killing two birds with one stone’. This has important implications for sports science and medicine practitioners tasked with preparing MDS athletes for the most demanding actions of match play and for optimally returning athletes to sports performance following injury.
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