The following is an exclusive excerpt from the new book, Developing Speed, which is now available in bookstores everywhere, as well as online at Amazon. It is the latest release in the Sport Performance Series from the National Strength and Conditioning Association (NSCA) and Human Kinetics.
Acceleration is the rate of change in velocity, or the change of velocity in a given time. The first phase of acceleration involves overcoming the body’s inertia in order to get moving, as in Newton’s first law of motion. Inertia refers to the propensity of a body to resist changes in motion and is at its highest when a body is stationary. Therefore, initiating motion requires great force and depends on maximal strength.
The importance of maximal strength and power during the start and acceleration phase of a sprint can be understood more clearly by remembering that sprinters develop force to overcome inertia when their feet are on the ground. This requires large movements through the hip, knee, and ankle to extend the leg on the ground. Stronger athletes, who can create more force, are better able to use greater forward lean during acceleration. This enables them to assume an effective line of force and minimize the tendency of the body to rotate from side to side, which helps them apply the forces necessary to complete the pushing action in the acute forward lean.
The magnitude of body lean is greatest during initial acceleration. The lean involves the whole body from the ground and not just from the waist (figure 2.2). The athlete is not bent over. The greater the acceleration, the greater the forward lean. Therefore, after the body overcomes inertia and starts moving, the rate of acceleration decreases along with the body lean, and with each step, the body becomes more upright as speed increases.
Figure 2.2 Stride sequence from a stationary start, exhibiting forward lean characteristic of acceleration phase.
This gradual transition to a more upright posture and eventually erect running posture coincides with a change in the leg and arm actions from initial acceleration to maximum speed. The hips rise as the torso rises, knee lift is higher relative to the hip, and the foot of the recovering leg (the leg moving forward) gains height with each stride. This transition is a progression to maximum speed technique and doesn’t happen all at once. Coaches should not encourage the athlete to stay low. A normal transition to an upright posture takes place and should not be restricted. This is especially pertinent for field athletes who also have to consider the technical requirements of the game and whether staying low will hinder their ability to carry these out.
The first step of a sprint is rapid but relatively short, with a comparatively long ground contact time. Each subsequent step involves a shorter ground contact time and a longer stride length until, ideally, stride length is optimized at maximal speed. During the initial driving strides, the sprinter pushes backward and downward, beginning with a bent knee and flexed hip on contact of the support leg (figure 2.3). The athlete drives the leg back behind the body, fully extending the hip, knee, and ankle to maximize the drive of the legs (figure 2.4). A straight line can be drawn from head to toe running through the hip, knee, and ankle joints.
Figure 2.3 Early acceleration stride sequence, exhibiting foot strike with flexed hip and knee.
Figure 2.4 Sprinter pushing and extending through hip, knee, and ankle.
Fundamental to this technique is having an acute, positive shin angle, as illustrated in figure 2.5a, when the driving leg contacts the ground. As the athlete accelerates, this angle becomes less acute with each stride to nearly perpendicular at maximum speed. A negative shin angle in initial acceleration, as shown in figure 2.5b, indicates that the foot is too far in front of the body to create a pushing action and creates excessive braking forces during foot strike. This means that after the initial foot strike, the athlete has to carry the body over the foot using a relatively weak pulling action before attaining proper posture to apply force backward and downward. The emphasis of the leg action during acceleration is on back-side mechanics and the pushing action. To achieve this, the arms must propel and lift the athlete during an effective forward lean. Because the force applied to the ground is through the foot, the arm action is sometimes underemphasized. However, the arm drive is fundamental to applying force through the legs, particularly during acceleration, and poor arm action results in inefficient sprint technique.
Figure 2.5 Sprinter strides with (a) a positive shin angle that provides the best drive and (b) a negative shin angle that hinders acceleration.
In front of the body, the arms provide lift when driving forward and upward from the shoulder in synchronicity with the forward drive of the opposite knee (refer to figure 2.4). The arms should remain nearly in line with the shoulders. The rearward drive of the arm downward and backward applies force against the ground, and at this point the athlete opens the elbow angle.