BRINGING SCIENCE TO TREATMENT

Demands of Sport: Part V – Braking new ground

The role of deceleration on injury etiology and prevention continues to gather the attention of sports medicine practitioners. Understanding the deceleration demands of sport is essential in ensuring optimal prevention and rehabilitation of injuries. Helen Bayne uncovers the assessment of deceleration and provides practical tools for practitioners to improve the management of athletes.

Rafael Nadal (ESP) chases down a drop shot during a victory over Carlos Alcaraz (ESP) in their semifinal match in the BNP Paribas Open at the Indian Wells Tennis Garden. Mandatory Credit: Jayne Kamin-Oncea-USA TODAY Sports

Deceleration is the rate of change in velocity or, in other words, how quickly the athlete reduces their velocity with respect to time. This occurs any time an athlete decreases their running speed. A recent meta-analysis of 15 studies showed that high-intensity decelerations (defined as >2.5m/s2) occur more frequently than accelerations that exceed the same threshold in soccer, hockey, Australian football, and rugby (league, union, and sevens)(1). The ability to decelerate is an essential attribute of team sports athletes as they need to be able to evade opposition players, execute strategic plays, and cut or turn while sprinting.

When running forwards in a straight line, slowing down represents horizontal deceleration of the body’s center of mass. To decelerate, athletes must apply an external force in the opposite direction to the direction of travel – the horizontal component of the ground reaction force vector is known as the braking force. The biomechanical demands of sudden deceleration are greater than acceleration, as peak ground reaction force and loading rate are higher. Furthermore, the change in velocity occurs more rapidly(2). An athlete’s momentum (mass x velocity) decreases when decelerating. If the aim is to reduce speed in a short amount of time, a greater initial momentum will require greater braking forces to slow the athlete down. So, while attaining high running speed ability is emphasized in athlete preparation, if deceleration capacity is lacking, athletes will unlikely reach top-end speed during match play because they would intuitively avoid reaching speeds from which they cannot stop or change direction safely and effectively. Deceleration ability may therefore be a limiting factor in an athlete’s game speed potential.

Assessing deceleration ability

Practitioners have typically overlooked assessing an athlete’s deceleration ability until recently. However, in the past few years, several researchers have published deceleration tests that involve a sprint acceleration phase from a stationary start followed by a deceleration phase. For example, the “acceleration-deceleration ability (ADA) test” requires an athlete to accelerate maximally for 20-m and then stop as quickly as possible (see figure 1) (3). To avoid athletes decelerating too soon and ensuring they reach their maximum speed at the 20-m mark, practitioners should perform a separate linear sprint test before performing the ADA test. Practitioners must then ensure that the time achieved at 20-m in the ADA test is within 5% of the 20-m split in the linear sprint test. Next, practitioners use the time- and distance-to-stop to quantify deceleration performance. Practitioners can acquire the necessary split times using timing gates or video cameras. Additionally, using a radar or laser to acquire instantaneous velocity allows further analysis. The deceleration phase may be divided into subphases based on the timing of certain thresholds – for example, early and late deceleration, defined as the periods before and after 50% of maximum velocity. Practitioners calculate the instantaneous acceleration from the velocity-time data. Furthermore, the kinetic variables (average horizontal braking force, power, and impulse) may then be estimated using Newton’s laws of motion and the mathematical relationships between these variables.

An alternative test prescribes the start and stop point, allowing the athlete to self-select the maximum speed they will reach and the relative distribution of the acceleration/deceleration periods(4). Practitioners use laser or radar to measure velocity and identify the point at which maximum speed occurs. Next, they calculate the distance from here to the designated stopping point. A limitation of this test is that shorter prescribed distances do not allow athletes to reach maximum speed before decelerating. For example, athletes achieved  90% of the linear 30-m sprint test in a 20-m trial, ~70% in a 10-m trial, and 55% in a 5-m trial. However, a regular assessment of an athlete’s percentage of maximum speed that they can attain in this type of test could provide valuable insights into changes in deceleration ability.

Figure 1: Acceleration-deceleration ability test(3)

Vpeak is the maximum velocity and defines the start of the deceleration phase; 50% Vpeak separates early and late deceleration; Vlow is the lowest velocity and represents the end of the deceleration phase; HDECEarly is the early deceleration phase, and HDEClate is the late deceleration phase.

Another method examines the difference between the time taken to complete a 15-m linear sprint test and the equivalent distance approach phase of the 505 test (where an athlete sprints 15 m, turns 180°, and sprints back in the opposite direction), which has been termed the “deceleration deficit” (see figure 2)(5). This method is relatively simple to implement; however, it is limited to an indirect assessment of deceleration ability as it does not specifically measure the deceleration portion of the task.

Figure 2: The 505 test

Physical attributes for effective braking

To effectively decelerate from high running speeds, coordinated organization of the body and the physical capacity to produce the necessary forces are needed. Hip and knee flexion and ankle dorsiflexion occur at very high angular velocities during the braking steps(6,7,8). This demands strong eccentric action of the hip extensors, knee extensors, and ankle plantarflexors. Knee extensor strength in concentric and eccentric contraction modes has been associated with superior deceleration ability (measured using the ADA test), highlighting the importance of this muscle group in particular(9). Furthermore, team sport athletes with better horizontal deceleration ability have greater reactive strength index (RSI) and force production during the concentric phase of a drop jump(10).

Therefore, training to enhance horizontal deceleration ability should include a range of strength interventions. Isolated single joint lower body training at various speeds focusing on concentric and eccentric actions is an important foundation, particularly for the quadriceps. The program should also include exercises that induce fast eccentric load, such as a drop squat, and plyometric exercises that include the subsequent concentric phase, such as the drop jump. Progressions may add external load using dumbbells or a hex bar. Practitioners can increase task complexity and specificity using field drills that require an athlete to run and then decelerate and stop before a given target, manipulating the intensity by varying the approach distance and speed.

Implications for injury risk mitigation and rehabilitation

The force production capability of muscle is higher when lengthening than shortening and increases with increasing speed during eccentric actions. This, in part, may explain why athletes can brake at a greater rate than they can accelerate. Nevertheless, a sudden deceleration from a high running speed is a biomechanically demanding task. High-intensity decelerations occur frequently in team sports, and they are likely contributors to neuromuscular fatigue and tissue microtrauma. This may precipitate injury through compromised movement patterns or cumulative tissue damage. Furthermore, the physical attributes needed for effective linear horizontal deceleration also underpin single-leg landings and change of direction motions where acute non-contact injuries frequently occur. Therefore, developing the physical capacity to execute sudden decelerations and tolerate repeated braking efforts is vital for athlete preparation and essential for players returning to sport after lower limb injury.

References

  1. Sports 2019;49:1923-1947
  2. Sports 2021; 20:571-582
  3. Sports Biomech. 2020; DOI: 10.1080/14763141.2020.1792968
  4. ISBS Conference Proceedings Archive. 2018; https://commons.nmu.edu/isbs/vol36/iss1/3
  5. J Strength Cond Res. 2020; DOI: 10.1519/JSC.0000000000003856
  6. J Electromyograph Kinesiol. 2016; 30:36054
  7. J Strength Cond Res. 2020; 34:1285-1296
  8. J Sports Sci. 2014; 32:1897-1905
  9. J Strength Cond Res. 2018; 35:465-472
  10. Eur J Sport Sci. 2021; DOI: 10.1080/17461391.2021.1930195
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