The basic definition of speed states that it is the shortest time that is required to move along a set distance, which is basically the same as velocity without any indication of direction (2). With the human body, speed is not constant over the entire distance and is divided into stages (8) once the body is in motion. These stages include acceleration and maximum speed. Acceleration can be defined as the rate of change in speed up to the point where maximum speed is reached. Acceleration will be covered more extensively in another article in this issue. In the maximum speed stage, the top speed is maintained as long as possible for the remainder of the distance. With shorter distances, this is not a problem; however, deceleration can occur if the distance is long enough, usually with distances greater than 60 m. The actual length of these stages can vary depending on total distance covered and the athlete's level of ability and training. For example, the acceleration distance for a 40 m sprint is approximately 10 m and the acceleration distance for a 100 m sprint is approximately 30–40 m. However, a novice runner will only accelerate for 20–30 m, whereas an elite runner may accelerate for 40–50 m in the 100 m sprint (8), which would typically result in a greater maximum speed and potentially less deceleration.
All sports, even endurance sports, rely on some type of speed for the performance of selected movements or in specific situations. The most common movement in sport where speed is important is running. The body reaches top running speed through the combination of force and power production in the lower body, which translates into stride length and stride rate. Although these components of the stride can be influenced by an individual’s body dimensions, they can also be trained to be optimized for a particular individual. Faster runners typically have higher stride rates and stride lengths (14); so, training to improve running speed often involves trying to improve one or both of these components. However, simply increasing stride length and/or rate may not be the most effective method for improving running speed. It is possible to overstride or to have a stride rate that does not result in a faster movement velocity because stride length may be shortened as a result. Therefore, the best approach to maximizing speed is to determine the optimal stride length and stride rate based on an individual’s physical dimensions and force/power capabilities. Because force capabilities are related to maximum strength, strength is a major factor that should also be considered when attempting to improve maximum speed (14).
FORCE, POWER, AND IMPULSE
Strength is a physical quantity that is defined in physics terms as force (F), and in the case of athletic performance, peak force. Power is the amount of force exerted through a certain distance (or displacement) per unit of time (t), which is really the rate at which one can perform work. Power is also often described using the rate of force development (RFD) (5,6). In addition, these variables can be expressed as a function of impulse (F × t), or the average force exerted over a given unit of time, which is equal to the change in momentum (mv2 − mv1) (Figure 1). Momentum is calculated as the product of the mass of an object and its velocity, so changes in momentum are most often the result of changes in velocity because the mass of an object (i.e., a runner) is not changing. These concepts in physics are the basic tenants that determine athletic performance, including activities such as running. If the body is treated as an object with some mass, then the force applied to it determines its acceleration rate and then this body reacts as a projectile with a trajectory defined by Newton’s laws of motion. The impulse and change in momentum relationship is simply a reorganized equation based on Newton’s second law (F = ma). Athletic performance must fall under these relationships, which do not change because the object happens to be a human body. Peak force, power, and impulse, in particular, thus define the essence of sprinting capabilities in that the function of these abilities is to move the body as fast as possible in the horizontal direction (5,15,17).
Unlike walking, running involves putting the body into flight where it loses contact with the ground, which falls under the definition of a projectile. Projectile motion involves an object with some trajectory or flight path based on its velocity and angle of projection when it leaves the ground. According to Newton’s laws, acceleration as a result of gravity is constant at 9.81 m/s2 for vertical motion and the horizontal acceleration rate (for a slow moving object) is 0 m/s2. Using the equations for trajectory, the flight path in terms of vertical (dV) and horizontal (dH) displacement can be defined (4).
Impulse, which is thought to be of great importance when determining sprinting performance, is related to trajectory because impulse defines the velocity change of an object (via momentum). It has been observed that elite sprinters generate an impulse of approximately 276 N-s in comparison with well-trained sprinters with an impulse of 215 N-s (12). Figure 2 depicts how average force applied to the ground at a given angle (7°) determines the flight path of the runner’s center of mass. This is important because the largest percentage of a runner’s stride length/step length (DSL) is determined by force application to the ground; DSL is calculated as shown in Figure 2a. If the force applied is increased, then a corresponding increase in dF and thus stride length/step length would be observed. Salo et al. (10) clearly showed that average stride length/step length quite effectively determines 100 m time (Figure 2b). In practical terms, if strength is increased, there would be an increase in force for a given time period, and running velocity would show a corresponding increase. Studies consistently show that force, RFD, power, and impulse determine sprinting capabilities (Table 1) (1,3,6,7,13,15,16).
To optimize stride length and rate, it is essential that proper running technique is used. Proper running mechanics can be described as a sequence of body positions for optimal force and power output. These are often split into the support and recovery phases (9). If a stride begins with the footstrike, there is rapid but slight flexion of the knee and hip as the landing is cushioned, then extension of both joints and propulsion of the body up and forward as the foot leaves the ground surface (support phase). In the recovery phase, the knee flexes first, followed by the hip. The toe is also dorsiflexed throughout in preparation for another footstrike.
At the beginning of the support phase, the body is nearly vertical and only the forefoot is touching the ground. This contact should be brief; elite sprinters can generate peak ground reaction forces in approximately 0.4 seconds (11). The end of the support phase is where the body moves forward, and the hip, knee, and ankle extend and propel the body off the ground (triple extension). This push-off angle is typically in the 50–55° range (11). The recovery phase begins with the knee flexing fully and the hip extending fully. Although these movements are not active (i.e., they are in reality eccentric knee extension and eccentric hip flexion), the end result is the heel nearly touching the buttocks. This shortens the lever so that rotation of the leg back via hip flexion to the position required for the support phase can be accomplished more quickly. The hip is rapidly flexed and then extended, and the knee extended and foot positioned to touch the ground in a “pawing” motion. This contact should ideally occur directly under the center of gravity (8).
Although most sprinting descriptions are focused on the lower body, the position of the upper body is also vital for optimizing running speed (9). The shoulders should be firmly positioned, so that the elbows can remain flexed at a 90° angle and kept close to the sides during the running motion. The arms should be moving explosively, and arm drive ranges from a position where the hand is close to the ear with the upper arm perpendicular to the body to where the hand is close to the hip and the upper arm is about 45° behind the body. The neck and facial muscles should remain as relaxed as possible throughout.
TRAINING TO IMPROVE RUNNING SPEED
Aside from a general repetitive sprint regimen that serves to condition the body, there are technique drills and strengthening and power exercises that are also necessary to improving speed. The drills focus on improving and maintaining running mechanics and include the following: A-march, A-skip, B-march, B-skip, fast leg-left every third step, fast leg-right every third step, fast leg-left every step, fast leg-right every step, fast leg-each foot into a sprint, claw/paw-right, claw/paw-left. Proper form for these drills is depicted in the videos labeled Supplemental Digital Content. The A-march and A-skip drills can be seen in Supplemental Digital Content 1 (https://links.lww.com/SCJ/A72); B-march and B-skip drills can be seen in Supplemental Digital Content 2 (https://links.lww.com/SCJ/A73); the fast leg every third step and fast leg every step drills can be seen in Supplemental Digital Content 3 (https://links.lww.com/SCJ/A74); the fast leg into a sprint and claw/paw drills can be seen in Supplemental Digital Content 4 (https://links.lww.com/SCJ/A75). These drills are typically performed in the workout after the dynamic warm-up, but before conditioning or resistance training, and the rest periods are similar to those in the dynamic warm-up. Because the main purpose of the drills is for running technique, these drills do not have to be periodized like the conditioning or resistance training and are typically performed daily for track athletes and 1–3 times per week for other sports.
The other main component of sprint speed is force output that contributes to both power and impulse. Improving force output during running is done via increasing peak strength, especially in the lower body. Multiple joint exercises that can be loaded more heavily are best for increasing lower-body strength, such as the squat and leg press. Power output should also be addressed directly through the use of explosive exercises across a spectrum of loads. These exercises include jump squats and power cleans or power snatches. Power training is important because it more specifically addresses the triple extension of the running technique and the velocity components of power and impulse. All these exercises should be periodized based on sport training and competitive seasons, and serve as an adjunct to the primary sport training. A sample program design for general speed improvements is provided in Table 2.
Speed is an important component of most sports and running speed is vital in both team and individual sports. Maximal running speed is dependent on running technique, stride length and rate, force capabilities, and production of power and impulse. Training programs can be designed to address all these characteristics.
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