Literature Review

Generating force during short time intervals is the name of the game for athletes across the sporting world. An athlete should apply forces at a greater rate to achieve faster acceleration (8, 11). With the forceful extension of the hip, knee, and ankle, overcoming of inertia, this creates a ground reaction force, a horizontal and vertical force. Acceleration refers to the rate at which an object’s velocity changes over time (8) and the mass of an object changes its velocity.

Within the sporting context, at the takeoff of a sprint, acceleration begins with keeping a low recovery angle of the lower leg. This is imperative for the rate of force production, fast and forceful steps to accelerate as fast as possible. The ability to produce force rapidly is arguably a more desirable trait than maximal force production (8, 14, 15). Rate of force development is the maximal force in minimal time and can be quantified by explosive strength and can determine anathlete’s explosive ability (1, 4, 8). Impulse is imperative for generating force but has a tradeoff with trying to generate maximal force, more time spent on the ground (8). The angles you will see for the recovery leg during acceleration will be below 50 degrees in almost all athletes trying to perform at a high level and producing maximal force with triple extension of the driving leg. Upper leg position is below 180 degrees on takeoff and full-extension, with full flexion above 230 degrees (10). Two phases describe leg motion, in the air and on the ground. In the air, there is high speed and low resistance so an open-kinetic chain vs. lower speed, high resistance which is on the ground and a closed kinetic chain. Torque comes into play to create the force, the hip flexors and extensors to accelerate using rotary motion, creating a horizontal ground reaction force (GRF) that initiates displacement of the athlete’s body. Creating horizontal GRF are the hamstring muscles just before ground contact and had the greatest capacity to produce eccentric hamstring torque. The bicep femoris along with the knee flexors have the highest EMG activity at the end-of-swing phase over the entire sprint acceleration (11). The ‘pawing’ action with high hamstring muscular activity during the end-of-swing phase, is to produce high backward force once the foot is in contact with the ground, this decreases the associated braking GRF. This can describe why hamstring injuries occur so often during sprinting along with some hip flexor injuries, due to their EMG activity in the phase of the sprint in which requires a lot of eccentric torque production from these muscles This study was using a treadmill to test subjects, so this could help with the backward motion of force production, but can still apply to most athletes, standing or crouching in sport (11). This can explain doing training that involves fast and intense eccentric actions of the hamstrings such as B-skips. There is a treadmill that correlates better with ground speeds and is not motorized called the Shredmill, coined and created by Tony Villani at XPE Sports. This type of treadmill has incline and resistance settings, with a study to show that running on this non-motorized treadmill correlates with sprints at 10 yards and 30 yards, depending on the incline and resistance used. This can be a tool of use for athletes and coaches that can lead future research in acceleration and top speed mechanics in training for sport (13).

During the plant phase, the length of ground contact time of either agility or a change of direction exceeds the typical ground contact time of both the acceleration phase of sprinting and the max velocity phase. Deceleration requires a braking impulse, this is the amount of impulse needed to change momentum. Effective braking is an important component concerning high velocity and high force-eccentric contractions (8). Lowering the center of mass and pushing off the opposite leg is the best way to get into a ready position to accelerate, decelerate or change direction, in reaction to a stimulus. During sport competition or practice, there is not enough time during either scenario to produce maximal force outputs (5, 8). Rate of force production may be more imperative than most other factors like impulse and braking impulse (8). Impulse is an underlying factor, producing forces in a short amount of time along with the rate of force production, make up some implications of sprinting and accelerating. The elbow joint is supposed to be flexed close to 90 degrees throughout running and opposite of the flexed knee drive, but if an athlete is in a sport that requires the use of the hand with a ball, this could be different. Powerful arm actions to facilitate leg drive (8).

Neurophysiological factors have their place in acceleration, dealing with neural drive, force production, motor unit recruitment, impulse generation to name a few. Training the SSC (stretch-shortening cycle) and SMM (spring-mass model) is imperative when describing how force can be created. The SMM model is used to describe the relationship with the SSC, muscle stiffness and sprinting when stride frequency increases (7, 8). Using plyometrics to increase the vertical component of GRF can help overcome inertia and gravity when training for acceleration or sprinting (9, 10). Data shows that doing both plyometric and sprint training can help acceleration and sprint velocities but affect different components of acceleration. Sprint training should get you faster and increasing your skill of acceleration, if form and coaching is done correctly (8, 9, 10, 13).

Acceleration in sport might be the most important quality of an athlete because this can create a matchup and situational problem for their opponent. Other athletes might be fast at their top speed but beating your opponent to their top speed can make or break an athlete during their sport. We must look at the athlete’s needs first and see how an athlete accelerates. For resistance training and field work, see where the athlete is the weakest, and work from the bottom up and depending on the time of year for their sport season. Focusing on concentric contractions with plyometric training, unilateral and bilateral (13, 14, 15) and allowing enough time for rest, to be able to accelerate with the most force possible. For change of direction and agility, trainers should give exercises that increase neural drive while overloading the hip and knee regions involved in the SSC (8). Change of direction requires longer SSC drills or activities to recreate this situation for the athlete to see improvement. This enhances the athlete by responding to the demands of perception along with tactical situations, setting yourself up for success in sport. Training from different starts can improve acceleration and expose athlete’s to different styles of acceleration, responding to a stimulus (object, verbally or visually) (2) or voluntary sprinting. The hamstrings need to be worked since this is the most injured muscle group in sprinting, which is why performing Nordic hamstring curls have started to gain traction in practice and literature (6, 8). Ongoing athlete monitoring programs can assist coaches in determining the agenda for training the athlete(s) (8).