Injury

Author: Dr Damian Harper (Founder of Human Braking Performance).  In my previous blog I outlined why deceleration and braking have such significant implications for sports performance and injury-risk reduction for athletes participating in multi-directional sports. I highlighted that: Intense decelerations are highly frequent in match-play, and The braking steps associated with decelerating are characterised by a unique ground reaction force profile imposing high impact peaks and loading rates across very short time periods (I will go into more detail of these mechanical demands in the next post). It is therefore vitally important that athletes involved in multi directional sports are well prepared to tolerate these forces to reduce the potential risk of tissue damage and/or injury that could result from repeated exposure to these actions. To achieve this aim, myself, and John Kiely (@simplysportssci) highlighted in our editorial titled: ‘The damaging nature of decelerations: Are we adequately preparing players?’ that deceleration ability could be a critical mediator moderating the athlete’s risk to tissue damage/injury and could also help athletes maintain their performance of high-intensity activities that are integral to match play such as sprinting and rapid changes of direction. In this blog I want to highlight what horizontal deceleration ability is. First let’s cover some important scientific principles. From a purely mechanical perspective deceleration is defined by decreasing velocity with respect to time and is fundamental to decreasing whole body momentum (mass x velocity). In accordance with Newtonian’s laws of motion deceleration is directly proportional to the direction of force applied. Therefore, to manipulate the rate of horizontal deceleration an athlete must adjust either the magnitude or duration of force (i.e., impulse) applied in the horizontal direction. It is important to highlight here that the optimisation of braking impulse requires a high level of technical ability. 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 deceleration performance!! Another notable feature of decelerating, as we have already highlighted, is the necessity to generate very high ground reaction forces. 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: 1) Braking Force Control and 2) Braking Force Attenuation, both of which are illustrated in the figure below:  As can be seen in the figure above braking force control requires the athlete to position the centre of mass posterior to the lead foot braking limb to ensure anterior foot placement and the required orientation of the braking force. This can be seen with a negative shin angle and forces directed opposite to the direction of motion. It must be stressed that the precise positioning of the lead limb braking foot requires a complex sequence of muscle activation and de-activation strategies to ensure optimal co-ordination between the trailing and lead foot braking limbs. A lower vertical and more posterior centre of mass position are also key to dynamic stabilisation and helping to maintain the centre of mass behind the lead limb braking foot, thereby prolonging the time in which horizontal braking forces can be applied (i.e., greater braking impulse and thus greater reduction of momentum, reflecting the impulse-momentum relationship). Another key aspect of braking force control reflected in our definition of horizontal deceleration ability is the requirement to decelerate, within the constraints, and in accordance with the objectives of the task. This reflects the perceptual demands of decelerating during competitive match play, whereby players are required to make rapid braking decisions based on a dynamic, emerging environment that takes into consideration their teammates and opponent’s actions.  The braking force attenuation component of deceleration ability can be considered critical for helping to reduce soft-tissue damage and neuromuscular fatigue resulting from repeated intense horizontal decelerations that can impose high force eccentric (i.e., active muscle lengthening) braking and pseudo-isometric muscle actions. The figure illustrating braking force attenuation also highlights the potential critical role of tendons acting as force (power) attenuators upon ground contact. We often view tendons for their role in power amplification to enhance power output in jumping and running actions, however, the lengthening of the tendon (i.e., tendon compliance/elasticity) when performing intense braking actions also serves a vital function in helping to attenuate peak forces and rate of active lengthening of the muscle fascicles (Roberts & Konow, 2013). Therefore, tendons can help to protect muscles from damage when performing intense horizontal decelerations. As such, increasing the capacity of the muscle-tendon unit to withstand high eccentric braking forces logically serves to enhance deceleration ability and mitigate injury risk.  So, to conclude this post the definition of horizontal deceleration ability helps us to conceptualise the importance of this skill for both sports performance and injury-risk reduction. Both braking force control and braking force attenuation are key components underpinning horizontal deceleration ability, both of which will interact to govern how much braking impulse the athlete can generate. On this note, I like to say, “an athlete will not speed up what they can’t slow down”!! ─ Improving horizontal deceleration ability is key for our athlete’s performance, health and wellbeing. Hope you enjoyed the read, please share your thoughts in the comments box below, or on social media platforms. Thanks, Damian Harper – Founder of Human Braking Performance References Harper, D.J., & Kiely J. (2018). Damaging nature of decelerations: Do we adequately