In my previous two blogs I discussed the intricacies of horizontal deceleration and why it is the most mechanically demanding task in multi-directional sports. Despite its clear importance to sports performance and injury-risk reduction I often highlight how overlooked and misunderstood this quality has been in comparison to other speed qualities such as horizontal acceleration and maximal velocity sprint running. One of the main reasons for this has been due to the difficulty to reliably assess horizontal deceleration in an applied field-based environment in comparison to horizontal acceleration and maximal velocity sprint sprinting, where surrogates of performance can simply be attained using split-times across different distances. Therefore, research and applied practices in the past have been dominated by evaluating the more easily measurable components of performance that rely predominantly on the generation of propulsive forces (i.e., horizontal acceleration and maximum velocity sprinting). Subsequently, the generation of knowledge about the determinants of performance and the most effective training methods have all been heavily biased towards getting athletes faster (i.e., greater horizontal acceleration and maximal velocity sprinting capabilities). So, based on this we can perhaps conclude that we are getting our athletes faster without understanding if they have the required deceleration capabilities to reduce higher movement speeds (i.e., whole body momentum)! Obtaining a greater understanding of our athlete’s horizontal deceleration capabilities has huge potential implications for performance enhancement (Harper, McBurnie, et al., 2022), injury-risk reduction (McBurnie et al., 2022) and for effectively informing return-to-sport rehabilitation protocols (Wolfe et al., 2023). Therefore, an important question that coaches, sports scientist, and medical practitioners should consider is “how are we assessing our athlete’s horizontal deceleration capabilities?”.
Assessing Horizontal Deceleration: A 4-Step Process
Step 1: Select a Horizontal Deceleration Test
More recently there has been greater research devoted to the development of protocols that could be used in an applied-field based environment to reliably profile an athlete’s horizontal deceleration capabilities. These tests can be divided into: 1) acceleration-deceleration ability test (known as an ADA test) or 2) change of direction (COD) test that requires significant deceleration prior to turning (i.e., typically > 90º) and re-accelerating (Eriksrud et al., 2022; Hader et al., 2015; Kaneko et al., 2019) (figure 1). For the ADA test there are two options: 1) commence deceleration at a pre-set distance before performing a back-pedal (Harper, Morin, et al., 2020) or 2) decelerate and stop at a pre-set distance (Graham-Smith et al., 2018).
With each of the deceleration testing protocols illustrated in figure 1 it is also important to consider the sprint distance that will be selected prior to decelerating. Significantly greater deceleration demands have been reported when comparing a 10m to 20m ADA test (table 1), with a high percentage of athletes reported to have above average deceleration performance on one ADA test but not the other (Philipp et al., 2023). This is also likely the case with COD tests where athletes may also adopt different braking strategies dependent on the deceleration distance and number of braking steps needed prior to turning (Dos’Santos et al., 2021; DosʼSantos et al., 2017; Nedergaard et al., 2014). Therefore, careful consideration should be given to the distance and speed that will be attained prior to performing deceleration. A thorough horizontal deceleration profile may require deceleration to be performed from both lower and higher sprinting speeds given the potentially unique demands and physical qualities associated with braking and decelerating from higher and lower movement speeds (e.g., different postures, stride characteristics and ground contact times).
Step 2: Selecting a Measurement Device
Probably the most important factor to consider is what measurement device you are going to use to measure deceleration. To obtain an accurate measurement of deceleration it is important to be able to precisely identify when the athlete reaches their peak velocity, which is essentially the start of the deceleration phase. This is perhaps synonymous to measuring your athlete’s horizontal acceleration capabilities and accurately identifying when the athlete starts to accelerate. To accurately identify the start of deceleration practitioner’s need to choose a measurement device that can capture the athlete’s instantaneous velocity throughout the entire deceleration test. Examples of these devices include radar, laser, global positioning systems (GPS), motorised resistance, light detection and ranging (LiDAR) technology and high-speed cameras with option for automatic detection of key deceleration metrics using artificial intelligence (AI). Figure 2 shows an athlete commencing an ADA test with instantaneous velocity measured using the Stalker ATS II radar device (Applied Concepts, Inc., Dallas, TX, USA) that samples at 47 frames per second.
Step 3: Analysing the Data
Once you have collected the data with your measurement device you should end up with an instantaneous velocity-time profile of your athlete. Figure 3 illustrates an example of a velocity-time profile obtained from an athlete during an ADA test with deceleration commencing at a pre-set distance of 20 m. The time point immediately following maximum velocity (Vmax) is used to define the start of the deceleration phase with the end of the deceleration phase defined using the lowest velocity (VLow). The deceleration phase can then be further sub-divided into early (DECEarly) and late (DECLate) horizontal deceleration sub-phases by using the time point associated with 50% of maximum velocity (50% Vmax).
In addition to the velocity-time profile, figure 4 also displays instantaneous acceleration and deceleration data calculated from the velocity-time data throughout the ADA test. To calculate instantaneous horizontal deceleration the following equation is used to calculate deceleration (m.s-2) between each time point:
Where v is the velocity, t is the time, f is the final velocity or time, and i is the initial velocity or time.
Using the same analysis procedures applied to a 180º COD test, figure 4 illustrates an example velocity-time profile obtained from a modified 505 test using a motorised resistance device (1080 sprint; 1080 Motion, Lidingö, Sweden) capturing at 333Hz. Velocity measurements using this device during a modified 505 test have been reported to be valid when compared to high-speed video cameras (Eriksrud et al., 2022). As can be seen in figure 4, the deceleration phase (highlighted in red) can be determined during the period prior to the turn when the athlete must decelerate their momentum following an initial acceleration before turning and re-accelerating.
Key Horizontal Deceleration Metrics
Using the velocity-time profile obtained from either an ADA or COD test there are several metrics that can be used to evaluate your athlete’s horizontal deceleration ability. Table 2 gives a definition of these metrics and their importance for sports performance and injury-risk.
Average deceleration is the stand-out deceleration capacity ‘performance’ measure. It provides an indication of how well the athlete can maintain high deceleration values across the entire deceleration phase, or if using a pure mechanical definition how quickly they can reduce their velocity with respect to time. The average deceleration measured during the early deceleration phase also seems to be a critical horizontal deceleration capacity performance measure because it signifies how quickly the athlete can decelerate within the initial braking steps of commencing deceleration. For example, the early horizontal deceleration phase seems to have greater association with overall deceleration performance, when compared to the late horizontal deceleration sub-phase (Harper, Cohen, et al., 2022). This most likely highlights the importance of being able to generate greater horizontally orientated braking forces in the initial braking steps, when moving at higher speeds, and when lower ground contact times are apparent (i.e., the ability to generate high braking impulse). Conversely, athletes who do not have the capacity to generate or tolerate high braking impulses in the early horizontal deceleration sub-phase are likely to prolong this phase, and/or, require decelerating more during the late horizontal deceleration sub-phase when whole body movement speed is lower and ground contact times are longer (i.e., longer time available to generate braking forces). The early-to-late horizontal deceleration ratio can be used to provide some indication to what deceleration strategy the athlete is using. For example, a value closer to 1 would seem to suggest a more balanced deceleration strategy or that the athlete can generate higher deceleration within the early horizontal deceleration sub-phase relative to the late horizontal deceleration sub-phase.
What is maximum deceleration telling us?
Maximum deceleration is a single time point within the whole deceleration phase and in table 2 is referred to as peak deceleration. For example, with radar technology that samples at ~50Hz (i.e., 50 frames per second) this time point is represented by the deceleration that occurs in just 0.02 seconds. Data from Harper et al. (2022) shows that the maximum deceleration value is significantly associated with the late deceleration phase (r = 0.89). Whilst this value may represent the peak mechanical demands occurring during each deceleration manoeuvre, practitioners should be aware that this value may not be a good representation of an individuals overall horizontal deceleration performance and in fact could have some implications for injury-risk particularly if the athlete is adopting a ‘bang on the brakes’ strategy due to not having sufficient physical capabilities to brake earlier when at higher speeds within the early deceleration phase.
Deceleration distance and time-to-stop
The horizontal deceleration distance and time-to-stop are important metrics to look at alongside the athlete’s horizontal deceleration performance values. However, many practitioners still do not appreciate that deceleration is a multi-step task and can occur over relatively long distances when examined relative to the acceleration distance in any given task. For example, based on data reported by Graham-Smith et al. (2018) a 15 m sprint to stop could require around 6.61 m to decelerate, which would be around 44% of the 15 m distance preceding the COD step during a traditional 505 COD test! Obviously, from a performance perspective it would be advantageous to be able to reduce the distance or time-to-stop from any given movement speed. For example, during COD manoeuvres the athlete would be able to accelerate for longer before braking, resulting in faster overall COD performance times (Dos’Santos et al., 2021).
An additional metric that has recently been suggested could be a ‘missing-link’ particularly for rehabilitation purposes is the deceleration index (Wolfe et al., 2023). The deceleration index is a measure of the rate at which an object slows down relative to its ability to accelerate and can be calculated by dividing the deceleration time by acceleration time. The authors suggest that this metric provides an indication of how well an individual can control their movements and reduce the risk of injury. Therefore, an athlete with a low deceleration index would be suggestive of low control and higher risk of injury. This is a promising metric, since a high percentage of non-contact anterior cruciate ligament (ACL) injuries are associated with defensive pressing when a player may demonstrate poor decision making by approaching an opponent at high speed with reduced ability to decelerate and change the intended action (Gokeler et al., 2023). Further research is needed to investigate the importance of this metric for both performance, injury-risk reduction and rehabilitation purposes and determine how best to calculate this metric since the time of the acceleration and deceleration phases can be disproportionate dependant on the task being performed.
Example Player Braking Performance Profile
Figure 5 illustrates an athletes braking performance profile using an adapting version of the total score of athleticism proposed by Turner (2019) to provide a more holistic indication of an athletes overall braking performance capabilities. In addition to the athletes horizontal braking performance that was evaluated using the 20m ADA test, a vertical countermovement jump (CMJ) is also used to provide some further insights to specific neuromuscular qualities that may underpin horizontal deceleration ability (Harper, Cohen, et al., 2020). The athlete presented in figure 5 recorded the highest peak velocity and momentum prior to commencing deceleration and was therefore ranked 1st in the group. Despite also having some of the best horizontal deceleration metrics, the distance-to-stop and time-to-stop was ranked bottom of this group highlighting the influence peak approach velocity/momentum, braking strategy and anthropometric characteristics can have on these variables.
The vertical CMJ metrics provide some interesting observations. Despite this athlete having some of the best horizontal deceleration performance metrics, it is clear this athlete could benefit from improving performance in the eccentric (downward) phase of the CMJ. By improving performance in these eccentric phase metrics it is possible that this could further help enhance horizontal deceleration performance and maybe help reduce the deceleration distance and time-to-stop. For example, training exercises that enhance fast eccentric-braking capabilities may be selected to reduce eccentric duration and enhance eccentric peak force and eccentric-deceleration RFD (Harper, McBurnie, et al., 2022). It is important to note that the vertical braking performance profile could also be further supplemented using drop jump neuromuscular performance qualities, since some of these metrics have also be shown to be associated with horizontal deceleration ability (Harper, Cohen, et al., 2022). Importantly, future research is needed to establish the effectiveness of training interventions that target horizontal and vertical braking performance capabilities and ultimately how these interventions improve horizontal deceleration performance capabilities.
Other Horizontal Deceleration Testing Considerations
Alongside the whole-body instantaneous velocity-time data described in this article, practitioners may want to simultaneously explore capturing other kinematic data through high-speed video that will enable evaluation of joint kinematics and foot-to-foot spatial-temporal characteristics (e.g., ground contact times, air time, step length, step frequency, touch down distance, deceleration attained per foot strike etc). Figure 6 displays an example of how ground contract time (GCT) and air time (AT) can be used to calculate a braking index (GCT/AT) to obtain further insights to the unique demands and performance of different braking steps within the early and late deceleration phase. As can be seen in figure 6 the braking steps during the early deceleration phase have substantially shorter ground contact times than the late deceleration phase requiring the athlete to generate braking forces in much shorter time periods (i.e., requiring generation of rapid rate of braking force). This is represented by a much lower braking index (i.e., 0.94) than that observed in the late deceleration phase (2.23). As highlighted earlier in the article, these unique braking characteristics associated with different deceleration subphases likely require unique physical qualities to be developed.
Advancements in technologies (e.g., inertial measurement units, foot force/pressure sensing insoles) are now also providing opportunity to evaluate foot-to-foot ground reaction forces that are generated when braking during horizontal deceleration manoeuvres. Such information could prove highly valuable to help inform and evaluate deceleration and return-to-sport training programmes, whilst adding more in-depth insights into the mechanical demands of braking during horizontal decelerations.
Summary and Implications
Whilst horizontal decelerations are crucial to performance in multi-directional sports, they can also expose athletes to high risk of injury and tissue damage. Despite this athlete profiling has been heavily biased towards understanding an athlete’s acceleration and maximum velocity sprinting capabilities, without understanding the athlete’s horizontal deceleration performance capabilities. This can be likened to taking your car for an MOT, checking the engine is operating fine, but having no checks or criteria to ensure the brakes are operating well!
Excitingly, new developments in measurement technologies and AI now provide practitioners with a much wider variety of options to accurately profile their athlete’s horizontal deceleration performance capabilities in an applied field-based setting. This information could help to identify and monitor the athlete’s horizontal deceleration performance capabilities and enable specific training prescription to target enhancements in horizontal deceleration performance, whilst helping to reduce risk of injury and tissue damage, thus making our athletes more robust to repeated intense horizontal deceleration activities. Future performance diagnostics should look to profile an athletes horizontal deceleration performance capabilities alongside their acceleration and top speed capabilities.
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