In a previous post, we introduced technical frameworks to aid the coaching of agility actions and provided a brief overview of this concept. Although agility may be a global concept, the classification of sub-components (e.g., perceptual and decision-making factors, and change of direction speed) allows us to break down and isolate the specific cognitive, biomechanical, technical, and physical requirements of each task, enabling a more precise application of targeted training methods. With reference to the physical component, practitioners may employ a wide variety of training methods that aim to develop key underpinning strength- and power-related characteristics (i.e., reactive strength, rate of force development, peak force). These are fundamental physical qualities which are necessary to achieve high and rapid magnitudes of braking and propulsive impulse, thus change in momentum, which are central to changing direction and the performance of agility actions. These physical qualities have been shown to underpin key multi-directional speed components, such as deceleration capacity, acceleration, and top-end speed capabilities, which may provide an athlete with a wide range of physical affordances, thereby increasing their likelihood of out-maneuvering their opponents and closing/creating time and space to bring about positive opportunities for their team. The following article will discuss the physical capacities which characterize the components of COD when assessing, training, and monitoring progress. This may allow a reverse-engineered approach based on the requirements of the task, and determine underpinning technique, biomechanical and physical factors that contribute to performance. This will be based on our technical framework to aid the coaching of agility actions along with the initial work of other authors (Clarke et al., 2018; Fox, 2018; Nimphius et al., 2017; Young et al., 2015).
Biomechanics and Types of Directional Changes
A COD is defined as “a reorientation and change in the path of travel of the whole-body COM towards a new intended direction” (David et al., 2018; Wyatt et al., 2019), and the ability to change direction rapidly is an important action associated with successful performance in multidirectional sports (Bourgeois et al., 2017; Havens and Sigward, 2015a).
A COD action can be split into four phases, and these phases being key in the determination of exercise selection for physical development:
- Initiation (approach)
- Preparation (deceleration)
- Execution (foot plant)
- Follow-through (re-acceleration)
As outlined in our previous posts, changing direction is a multi-step action. It is worth acknowledging that the preliminary deceleration and redirection requirements during directional changes will be governed by the approach velocity, intended COD angle, sporting scenario (i.e., pre-planned, offensive, or defensive agility), and the athletes’ physical capacity (neuromuscular control and ability to rapidly produce force) (Dos’Santos et al., 2018). Several reviews outlining the biomechanical differences between COD techniques have been published (Dos’ Santos et al., 2022, 2019), thus a brief overview of the COD biomechanical demands is presented in Figure 1.
The foot plant is preceded by preliminary deceleration (preparation), explaining why COD is a multi-step action given the differences in approach velocities and ground contact times for a given task. Indeed, substantial braking in the steps prior to the foot plant have been shown prior to 90° (Havens and Sigward, 2015b), 135° (Nedergaard et al., 2014), and 180° COD (Graham-Smith et al., 2009; Jones et al., 2016). Furthermore, the preliminary deceleration and the resultant strategy adopted could be self-regulated and limited, and impacted by their physical capacity to produce the most efficient movement possible (Jones et al., 2017). Given the sub-phases of COD outlined above, and the distinct biomechanical differences of the different phases (i.e., sprint during approach vs deceleration during preparation) it is suggested that specific muscle strength qualities have roles in specific phases of a change in direction. Specifically, eccentric strength of the knee extensors to reduce velocity in the initial direction of approach (preliminary deceleration), isometric strength during amortization (final step), and concentric strength during propulsion to help reaccelerate (Spiteri et al., 2014). Yet, the role and primary emphasis of muscle strength qualities is likely dependent on the demands of the task (i.e., velocity dominance for force / sharp redirection dominance). Therefore, biomechanical factors such as velocity of entry and angle of COD have clear implications for differences in physical requirements for the different ranges of COD performance required in sport and thus should be acknowledged when designing physical preparation programmes to best prepare for the COD demands of sport (Figure 1).
Physical Capacities Underpinning Change of Direction
Initiation (‘Approach’)
The aim of the approach phase is to achieve the highest controllable velocity while encouraging visual scanning. The velocity-angle trade-off would also infer that approaching at lower velocities will make it easier to perform an evasive and sharper directional change to create separation and increase tackle evasion success (i.e., tackled from an opponent(s)) during agility actions (Dos’ Santos et al., 2019). Classically, approach velocity (whole-body running speed) attained during this phase is a result of the interaction between stride length and stride frequency. In fact, stride length and stride frequency are multifactorial outcomes associated with velocity, both primarily affected by force production during ground contact. Therefore, directly intending to increase stride length (over-striding) or stride frequency (reduced ground contact time) will likely lead to sub-optimal mechanics and misguided training practices. Rather, it is likely beneficial to focus on key variables associated with ground force characteristics during ground contact, notably horizontal and vertical impulse (i.e., magnitude and orientation of force). The hip extensors (hamstrings and glutes) appear to help facilitate this orientation of force. It is therefore likely that such connections exist between the ability of the hamstring muscles to produce horizontally oriented force during ground contact and their ability to produce high levels of eccentric force, specifically at the end of the swing phase (combined hip flexion and knee extension) (Morin et al., 2015; Edouard et al., 2018) where the biceps femoris and semitendinosus muscles reach their greatest length (Yu et al., 2008; Schache et al., 2012). Thus, hamstring muscles play a major role in high-speed running and the approach phase (Morin et al., 2015). The hamstrings and glutes (hip extensors) have a key role here in facilitating a high early rate of force development, without excessive braking force. As the glutes are shortening throughout ground contact, rate of force development rather than maximum force is a key quality for development. This is characterized by a longer ground contact time during acceleration (~0.2 s) versus those observed at maximum velocity (~0.1 s) (Wild et al., 2011; Yu et al., 2016). Thus, the approach phase may be influenced by fast reactive strength through use of the stretch-shorten cycle with each foot contact and linear sprint speed in this case.
Preparation (‘Deceleration’)
The preparation phase requires a reduction in velocity and / or to optimally position the body for the plant step (speed-accuracy trade-off), thus, the penultimate foot contact (and possibly antepenultimate foot contact and steps prior) is required to 1) effectively position the whole-body for push-off in the execution phase, and 2) reduce whole-body momentum (brake) prior to push-off (execution). The role of these preparatory steps has been described in a previous post. Based on the literature, it appears that for CODs ≤45°, preparatory braking requirements and forces are limited, with velocity maintenance a key priority. Yet, greater braking impulse (regardless of COD angle) will help lower ground reaction force and knee-joint loads and facilitate effective push-off during the execution phase. Also, anterior cruciate ligament injuries are shown to occur ≤50 ms; thus, reducing momentum is critical because there is limited time for any postural adjustments. However, as the COD angle increases 45-60°, the preparatory steps play more of a role in braking, and therefore more deceleration is required. Due to the often suddenly imposed nature of CODs involving high braking requirements, the greater impact peaks and loading rates observed (e.g., tall-thin impulse profile) (Verheul et al., 2021) during the ground contacts of the early-preparation steps (i.e., antepenultimate foot contact) suggests a greater rate of change in velocity and the expression of forces over even more abbreviated timeframes to reduce whole-body momentum. Eccentric overload training of the hamstrings led to substantial improvements in contact time, time spent braking, and braking force during 45-60° COD tasks (de Hoyo et al., 2016) suggesting eccentric strength to be an underpinning factor of efficient preliminary deceleration (McBurnie et al., 2022). Results from our work corroborate this finding, with eccentrically (knee flexor and extensor) stronger female soccer players shown to approach faster and display greater reductions in velocity and braking forces during 180° turns, whereas eccentrically stronger (knee flexor and extensor) soccer players were able to maintain velocity, attain higher minimum speeds, and tolerate greater loads during a 90° cutting task. Furthermore, eccentric knee flexor strength helps to generate hip extensor moment to help maintain trunk position (reduce trunk flexion) in transition to the ‘plant’ step and assist with knee joint stability (co-contraction). As such, high eccentric strength may provide athletes with a higher threshold of braking capabilities; yet, during deceleration, high levels of eccentric forces also need to be expressed rapidly over large ranges of motion. As such, the combined need for high peak eccentric force and a high rate of eccentric force development will be critical for sustaining rapid reductions in whole-body momentum alongside the maintenance of neuromuscular control and stability of the knee joint when decelerating over rapid time frames. Additionally, maintaining trunk position and avoiding excessive trunk flexion at an extended knee posture is critical to mitigate loading to the hamstrings and avoid a stretch-load hamstring injury mechanism. This may likely be due to the lower center of mass position through greater hip and knee flexion to reduce velocity and prepare for optimal position for the ‘plant’ step (execution). It is therefore likely that such connections exist between eccentric strength (knee flexors and extensors and hip extensors) and preliminary deceleration. Specifically, the more aggressive the COD angle and distance and velocity of the approach, eccentric strength will be of greater importance.
Execution (‘Plant’ Step)
The aim of the execution phase is to perform the directional change safely and efficiently. Thus, the foot is placed laterally (i.e., side-step cutting), medially (i.e., crossover cutting) or in front of the body (i.e., turning) to facilitate braking and propulsion into the opposite direction. During side-step cutting, wide lateral leg plants are performed to generate greater medio-lateral ground reaction force for acceleration into the new intended direction, whilst during turns the foot is placed ahead of the center of mass as a mode of dual-support to reduce loading in the ‘plant’ step and help facilitate re-acceleration out of the turn. Ground contact times of the ‘plant’ step vary according to COD task and Figure 1 highlights that the ability to produce rapid and high magnitudes of net force, thus impulse is critical for facilitating braking and propulsion. There is a clear trend of increased ground contact time as the COD angle increases; highlighting the angle (and approach velocity) of the COD will result in differing braking and propulsive requirement of the ‘plant’ step. During 45° cuts the deceleration and braking requirements are limited (Dos’Santos et al., 2018; Havens and Sigward, 2015b), so the ‘plant’ step acts to facilitate propulsion into the intended direction. In 90° side-step cutting, braking occurs in the foot contacts prior to the execution. In contrast, during a 180° turn due to the need to bring the horizontal velocity to zero before turning and reaccelerating back the other way, more substantial braking takes place takes place in the execution (Jones et al., 2016). Specifically, due to the dual-support nature of sharper angle COD, eccentric strength of the knee extensors and flexors, and hip extensors will aid in an athlete’s stability from touch down to maximum knee flexion, allowing a safer execution and firm base to push from (propulsion). Indeed, due to longer ground contact times ≥0.4 s (increased braking) the propulsion (maximum knee flexion to take-off) sub-phase of the execution phase maybe facilitated by use of the slow stretch-shorten cycle (concentric strength of the lower-limb extensors) as the athlete extends at the hip, knee, and ankle into the first re-acceleration step. Thus, alongside the hip and knee extensors, the gastrocnemius and soleus help to contribute to displace the center of mass forward by storing and releasing elastic energy to aid the body’s forward projection out of the COD. It is likely that a greater transfer of resistance training can be achieved if the conditioning programme emphasizes a similar fast SSC contraction type to the performance movement. The trunk contains approximately half of the body’s mass, and during COD the entire body’s mass must be balanced and supported on one leg (i.e., side-step cutting) or both legs (i.e., turning), thus trunk control and positioning is a critical factor influencing knee-joint loading. During the execution phase, control of the trunk suggests isometric trunk strength may be essential for controlling dynamic stability during this phase as well as alleviating high knee joints loads of the ‘plant’ step (Sasaki et al., 2011).
Follow-Through (‘Re-Acceleration’)
Like the approach, change in velocity during the re-acceleration phase is reliant on horizontal impulse and facilitated through the interaction of stride length and stride frequency. The force generated in the desired direction during each ground contact (Newton’s 3rd Law) during re-acceleration could be enhanced with greater extension range of motion at the hip, knee, and ankle joints (range of motion principle). Application of force through a greater range combined with a longer ground contact time supports the increase of horizontal impulse, resulting in an increased rate of change in horizontal velocity (impulse-momentum relationship / work-energy principle). From a technical standpoint, the optimal angle of forward lean is determined by the extension capacity of the limbs of the hip, knee, and ankle; thus, higher available forces during ground contact allow forward inclination of the force vector since the required vertical impulse is accumulated more rapidly. It can be suggested that more explosive athletes should present more aggressive angles of forward lean through an acceleration phase associated with their greater acceleration ability, whereas athletes with lower force production capabilities are not able to achieve the same angle of forward lean. Yet, it could be argued that for team-sport athletes a more upright head and body position allows maintenance of visual scanning, which is key for both the approach and re-acceleration phases. Furthermore, If the environmental constraints are tight, then a shorter stride length and increased stride frequency may be advantageous to allow more rapid changes in motion (i.e., a secondary COD). Thus, concentric strength of the knee and hip extensors (quadriceps and glutes) will be strength qualities important for technical execution of the re-acceleration phase.
Summary
- Muscle roles shift within the distinct COD phases, suggesting that a better understanding of “what role is happening at what phase” is very important to design specific training methods and therefore enhance muscle function during COD.
- The focus of training should be on 1) the eccentric strength and rate of force development of the hip and knee extensors as the main force dissipators, 2) the trunk and ankle/foot complex as the transmitting muscles during braking, and 3) the hip and knee extensors as the main force generators during propulsion and re-acceleration.
- This framework approach allows the COD to be broken down into its sub-phases and link key movement principles to the key body positions in each phase. Therein, the underpinning strength qualities (and their role(s)) are identified to diagnose and implement targeted training interventions to transfer to COD.
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