Biomechanics - Change of direction , agility , and spring model

While an athlete is in locomotion, they are consistently applying force into the ground and achieving high movement velocities. In order to change their direction, they must quickly apply force in order to decelerate, and then quickly accelerate in another direction. Agility overlaps with change-of-direction (COD), as the athlete has to quickly react by changing direction as a result from an external stimulus, such as being pushed or tackled from behind. Therefore, the athlete must have the perceptual cognitive ability to stop and change direction quickly and efficiently. Here, enhancing the stretch-shortening-cycle (SSC) can greatly improve change-of-direction and agility performance. The SSC is the body's ability to quickly eccentrically contract (lengthen) and then concentrically contract (shorten). The amortization phase is the pause between the eccentric and concentric contractions. The ability to shorten this phase as much as possible is the first step to enhancing COD and agility abilities. Now, let's further breakdown the SSC to better understand the biomechanics of how it works. This brings us into the spring-mass-model/system. The spring-mass-model explains how the muscles and tendons act as springs to produce a great force through compression and muscle stiffness. When looking at a sprinter, as soon as the athlete strikes their foot on the ground, compression of spring begins, which results in braking forces. This very short deceleration of the foot, allows for the swing leg to come forward for the next step, which places the athletes center of mass in a midstance. At this point, the athlete is at the lowest center of mass, as the spring is compressed to the lowest point. The resultant energy from the ground reaction force propels the sprinter forward. This same principle can be applied to change-in-direction and agility (Haff & Triplett, 2021). 

All movement requires the application of force. In sporting activities there is limited time to apply this force. Therefore, rate of force development (RFD) and impulse are two factors that will help sports and conditioning professionals plan programs properly to enhance COD and agility performance. RFD is the maximal amount of force in a minimal amount of time, while impulse is the amount of generated force and the time required for its performance. Furthermore, a change in impulse can change momentum, which gives the athlete the ability to accelerate or decelerate, depending on their upcoming movement. With that said, impulse is greatly dependent on the angle and velocity in the COD (Haff & Triplett, 2021).. Dos’Santos et al. (2018), studied the correlation between angle and velocity on COD and agility. They looked at the kinematic and biomechanical breakdown of multiple angles in COD from 45 degrees to 180 degrees. In regard to ground reaction force (GRF) and whole body center-of-mass velocity, when they compared multiple studies, they found greater propulsive and braking forces in sharper COD angles (i.e. 90 degrees versus 45 degrees). They also found that GRF percentages were significantly greater with higher angle changes in direction. More specifically, the results showed that these sharper cuts require greater posterior and lateral directed force (Dos’Santos et al., 2018). In order to apply this information to our athletes, it's required to understand the kinematic breakdown of what's happening internally. Dos’Santos et al. (2018) cites Harder et al. illustrating the muscular activity and energy expenditure from the different angles in COD. During sharper cuts (higher degrees), greater muscle activity from the biceps femoris and vastus lateralis was found. Also, the hamstrings aid in stabilizing the knee, preventing any knee valgus or irritation of the ACL. Therefore, a co-contraction of the knee flexors and extensors is required when changing direction, and strength and conditioning professionals should focus on eccentric knee flexor and extensor strength. Additionally, Haff and Triplett (2021) bring up the importance of maintaining strong trunk positioning during the deceleration and reacceleration. Now, when looking at agility versus COD, we must consider the perceptual cognitive abilities that must be applied. High velocity and high force eccentric contractions are specific to an athlete's agility performance, due to the fact that eccentric training adaptations show specific correlation to the velocity of eccentric loading (Haff & Triplett, 2021). These factors demonstrate that the success of an athlete is dependent on eccentric training, enhancing the stretch-shortening cycle, and progressing the athlete by adding in different techniques to improve the perceptual-cognitive ability during sporting activities. Below are three different drills that help to enhance these factors. I will include progressions for each of them based on the information summarized here. 

5-10-5 drill

During the 5-10-5 drill, athletes run forward 5 meters, change direction, run 10 meters, change direction again and run 5 meters back to the starting line. The ability to decelerate and then reaccelerate quickly is essential for the success of this drill. In addition, lateral change in direction and mobility in hips, knees and ankles, can aid in creating more explosiveness as the athlete changes direction. To break it down visually, as the athlete drops the hand down to the first cone, their body weight is leaning in the opposite direction, as this is the direction they must accelerate in. Therefore, mobility in the hips, knees and ankles is essential, in order to properly apply force into the ground while leaning in the opposite direction. In order to progress this drill, we can focus on technical aspects such as, limiting the ground contact time, decreasing the stride frequency (less steps), putting a larger focus on maximal and eccentric strength, and obviously decreasing the total time it takes to finish the drill. However, we can also add in movements or other tools that require further perceptual cognitive ability. Therefore, adding in additional stimulus can be beneficial. For example, catching a medicine ball and throwing it back at the 5 meter point will add a stability component, as well as challenge the athlete to still properly decelerate and reaccelerate with external stimulus. Another progression could be shortening the meters, challenging the athletes ability to accelerate and decelerate in a shorter amount of time. These progressions would be added in after the athlete displays complete postural control and mobility, while performing the 5-10-5 drill. They also should display explosive eccentric strength capabilities. These progressions are more sport specific. Therefore, perfecting the 5-10-5 drill during the first half of preseason, and then introducing the progressions during the last 2-3 weeks, and continuing these progressions in season, would be ideal for the athlete. 

Lateral depth jumps

A lateral depth jump helps the athlete learn how to absorb the force from their body weight, using the SSC to create an explosive concentric contraction. You can do this by jumping laterally off of a platform on the ground and then immediately jumping vertically as high as you can into the air. A progression for this could be adding in an additional box to laterally jump onto. Therefore, the athlete would jump off of the first box, and in one hop jump on to the second box, jump off the second box, and then perform the vertical jump as high as they can from the ground. The athlete is now being challenged to produce enough force to effectively jump laterally twice more, and still gain vertical velocity on the second jump. This progression can be done when the athlete has shown adequate hip, knee and ankle stability, as well as enough vertical height to be able to progress to two platforms. Landing mechanics are also essential to avoid any potential injury. If the athlete is displaying any knee valgus or foot pronation, mobility and flexibility exercises should be included to improve the performance. 

Hurdle hops

Hurdle hops are typically done by hurdling or jumping over small cones or hurdles on the ground, on a single leg. The objective is to have minimal ground contact time, and explosive force from the hip and knee flexors and extensors, to successfully jump over each hurdle/cone. Common errors in this drill are not gaining enough height/force and knocking over one of the cones, displaying longer ground contact times between jumps, or showing an excessive pelvic tilt proving poor postural control and core strength. In order to progress this movement, the athlete's ground contact time must meet the standards for the specified sport. They also must display proper knee and hip flexion without any knee valgus, and show minimal ankle pronation/supination, as well as proper core stability and strength while hopping. Finally, the athlete must demonstrate the ability to produce enough force to hop over each cone successfully without tripping, falling or knocking over a cone. Therefore, a progression for this drill would be adding higher hurdles, placing the hurdles further apart, and/or additional hurdles which builds muscular endurance. 

References

Dos'Santos, T., Thomas, C., Comfort, P., & Jones, P. A. (2018). The effect of angle and velocity on change of direction biomechanics: An angle-velocity trade-off. Sports medicine (Auckland, N.Z.), 48(10), 2235–2253. https://doi.org/10.1007/s40279-018-0968-3

Haff, G., & Triplett, N. T. (2021). Essentials of strength training and conditioning. Human Kinetics.