Improve Your Burst

“Burst” is a key performance indicator for many team sports. When wanting to make a tackle, evade a tackle, cut an offensive player off, score a goal, etc., it is important to get from point A to point B faster than your opponent. This requires a rapid change of speed, which is known as acceleration. 

Physics of Acceleration

Let’s break down acceleration using Newton's laws of motion. Physics tells us acceleration speed depends on the athlete to apply forces to the ground that are high in magnitude relative to their bodyweight, and in the proper direction. 

Law 1: An object will remain still or keep moving at a constant speed until acted upon by an unbalanced outside force. 

How does this apply to acceleration speed? Let’s use a wide receiver as an example. Pre-play, the receiver(object) is still. To begin movement, he must apply a force to the ground that is greater than his bodyweight. This is where relative strength comes into play. The extent of the receiver’s change in speed depends on the magnitude of excess force applied to the ground, relative to his bodyweight. We will get into the direction of force later in this article, but the relative strength plays a large role in accelerating from his stationary position. 

Law 2: Force= mass x acceleration (f=mxa)

This law describes the relationship between force, mass, and acceleration. Based on this equation, we know acceleration=force/mass. So, this tells us that the higher the mass of the object, the greater force required to accelerate it and vice versa. This further points to the importance of relative strength and body composition. In terms of sprinting, mass refers to body weight. So, acceleration speed depends on the ability to apply a force in excess of body weight. Get strong and don’t be fat. 

Law 3: Every action has an equal and opposite reaction. 

Let’s visualize this with a bouncy ball. If I slam a bouncy ball straight down as hard as I can, it will bounce back up with a lot of height. If I slam it straight down with less force, it will bounce back up to a lesser height. If I throw the ball straight forwards at a wall, it will bounce straight backwards towards me. The path of the ball after it bounces off the ground depends on the magnitude and direction of how I throw it. When accelerating, force needs to be applied horizontally to propel forward. 

Strength

As described above, acceleration performance begins with relative strength, producing high mass specific force. An athlete must produce force in excess of their bodyweight. This is improved through general strength training. Exercises that apply high load to large groups of muscle mass, through a full range of motion are most effective. Bilateral squats, unilateral squats, and hinges are the bread and butter here.

 Bilateral squatting trains a strong neural drive in a full body extension pattern. It is the most effective way to load the lower body, as the powerful leg extensor muscles (quads, glutes, hamstrings) are highly loaded through a full range of motion. They allow for long term progress and health. 

Unilateral squat variations are beneficial due to the fact sprinting is done on one leg. The first few steps of acceleration require the bodyweight to be supported on one leg in deep knee and hip flexion angles. Unilateral squats also develop smaller muscles more effectively than bilateral variations.

Hinges (RDLs) target hip extension to a greater degree, applying a high load to the posterior chain. The hamstrings are being loaded at a long muscle length, while simultaneously stressing the spinal erectors, which are responsible for maintaining postural integrity in the early phases of a sprint.

Rep ranges should be in the 1-5 range, with no more than 3 reps left in the tank. When using %s, the majority of work should be done over 75% of your 1RM. This is to ensure recruitment of high threshold motor units (HTMU). These are the motor units responsible for high force, high speed activities. Accelerating is not only about force production, but rate of force production. Time to peak force has shown to be a strong predictor of sprint success, as faster sprinters produce more force in less time than slower sprinters. When in the weight room, we must train the nervous system to recruit HTMU.

Technique

High relative force production is only one part of the equation. The force must be applied in the proper direction. Direction of force application has shown to have a stronger correlation to sprint times than amount of force.

https://pubmed.ncbi.nlm.nih.gov/21364480/

This study measured mean horizontal, total vertical, and total ground reaction forces (GRF) during a 100m sprint in physically active male subjects. They found that net horizontal force production was significantly correlated to 100m performance, but total and vertical GRF were not.

Horizontal force application is a product of shapes and rhythm of the sprinter. The athlete must achieve timing and body position that allows for optimal direction of force. This is where technique comes into play. Two main elements to focus on with acceleration technique is projection of the center of mass, and fast/efficient switching of the legs. 

Projection of the Center of Mass

Projection refers to full extension. As you can see in the image above, Bolt is maximizing his horizontal force production by fully extending at the hip, displaying a straight line from head to toe. Referring back to Newton’s 3rd law, every action has an equal and opposite reaction. Full extension provides the foot with a large angle of attack to the ground to produce a horizontal impulse by striking the ground directly underneath or slightly behind the hip. If his hips raise vertically out of the blocks, the foot will be attacking at a vertical angle, producing braking forces and slowing down horizontal acceleration. Below are examples of projection drills:

Acceleration specific broad jump Cues: shift body weight forward, shoot forward

The slingshot start forces the athlete the to fully project from the hip, then attack backward with a low heel recovery. Cues: Punch knee forward

Skipping for distance teaches the athlete how to produce horizontal force in a rhythmic manner Cues: Cover said distance in as few reps as possible

The half kneeling start can force the athlete into proper angles, projecting forward with a low center of mass. Cues: push with front hip, throw shoulders forward

Others include: prowler marches/bounds, med ball starts

Drilling the athletes into projection allows them to feel the body angles required to produce horizontal force. 

Switching of the Limbs

The only way to move forward is to push off the ground. Therefore, projection needs to be followed by a rapid switching off the limbs to get the foot back on the ground. This refers to thigh angular velocity. Elite sprinters show higher thigh angular velocities than their sub-elite counterparts. Velocity is calculated by dividing displacement by time. Therefore, elite sprinters take their thighs through a greater range of motion (displacement) in less time. The switch should be initiated from the hips. Hip flexion and extension should drive movement of the legs, as it allows for the greatest attack angle for the foot, maximizing ground reaction force. Below are examples of drills to emphasize proper switching:

Cues: Heel under knee, drive elbow back, get air time

The lunge switch drills limb speed, while focusing on initiating movement from the hip. Cues: keep head still

Wall switches emphasize speed of the switch, while allowing the athlete to feel the proper inclined trunk angle.

Conclusions

Accelerating is dependent on applying high force into the ground at a high rate in the proper direction. Force application ability is dependent on relative strength, this is the lowest hanging fruit. However, if the goal is to move forward, force must be applied down and backwards due to Newton's 2nd law of motion. Applying force in the proper direction is dependent on technique. Drilling athletes into projection of the center of mass, followed by fast switching of the limbs allows their brain to process the optimal acceleration positions.