Biomechanics of Speed

The following is an exclusive excerpt from the book Developing Speed, published by Human Kinetics. All text and images provided by Human Kinetics.

We have discussed how horizontal propulsion is generated during the stance phase. Now we will look at the rules governing motion and force production. Biomechanics is the study of forces and their effects on living systems (McGinnis 2005), and because forces determine motion, a fundamental understanding of the biomechanical principles that affect speed can assist coaches and athletes in developing running speed.

Speed clearly involves motion, and so to maximize its effectiveness, speed training should be guided by the scientific principles of motion. In 1687, renowned British scientist Sir Isaac Newton published his famous book, commonly referred to as Principia. Published in Latin, the language of science at that time, the book contained his three laws of motion, the fundamentals of which still hold true today and that contribute greatly to understanding the training concepts for speed development. Newton’s three laws of motion—the law of inertia, the law of acceleration, and the law of action and reaction—translated into English read as follows:

• Law 1—Law of inertia. Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.
• Law 2—Law of acceleration. The change of motion of an object is proportional to the force impressed and is made in the direction of the straight line in which the force is impressed.
• Law 3—Law of action and reaction. To every action there is always an opposite and equal reaction or the mutual actions of two bodies upon each other are always equal and directed to contrary parts.

At first, these may appear to be overly scientific and have little importance to speed training. However, when examined more closely and worded more simply, they play a vital role in planning effective speed training. An understanding of these rules, together with the application of their impact will help coaches and athletes make informed decisions in many elements of speed development.

Law 1 states that any time motion needs to be started or changed, a force must be applied. In terms of running speed, this force comes from within the body in the form of a muscular action, and so, every time an athlete wants to start moving or change the motion (for example increase speed, decrease speed, or change direction), the athlete needs to apply a force. Without the application of force, motion cannot be initiated or changed. A change in the direction or quantity of motion is termed acceleration and so any acceleration requires the application of a force.

This then leads to the second law, where the rate of change of motion (acceleration) is proportional to the amount of force applied. This is a cause-and-effect relationship, where force directly causes acceleration. This is indeed one of the most important messages an athlete or coach needs to take onboard when designing a speed training program: The rate of acceleration depends on the force applied. The second law of motion is summarized by the following equation:

Force = Mass × Acceleration

In terms of running, mass can be assumed to be constant; therefore, acceleration is directly dependent on, and proportional to, the force applied. The ability to produce force effectively and rapidly is essential. The amount of force required, though, depends on the athletic task or movement to be performed. Therefore, training must be specific to each task to develop a range of movement patterns.

This also brings up another important element for long-term speed development: the role of mass, or the athlete’s weight. Increased mass requires greater force to achieve the acceleration. Therefore, when athletes use resistance training they must make sure an increase in body weight coincides with an increase in strength. Increasing muscle size without also increasing the ability to produce force does nothing to improve the ability to accelerate. Strength training for speed development should focus on increasing force-producing capacity not on increasing muscle size.

The key message from the third law—for every action there is an equal and opposite reaction—is illustrated when running forces are applied into the ground, which then pushes the athlete upward and forward with an equal and opposite force. This brings into focus the importance of the direction of the application of force as well as its quantity. Force needs to be applied in a direction opposite to the intended direction of motion. Speed, therefore, is maximized when both the quantity and direction of the force are optimal.

Taking all of these laws together, it can be seen that ground forces largely determine acceleration and running speed. Thus, improving the application of ground forces needs to be a major focus of any speed training program.

With Developing Speed, the National Strength and Conditioning Association (NSCA) has created the definitive resource for developing speed training programs that optimize athletic performance. Including assessments and the application of speed training to eight specific sports, this authoritative guide provides all the tools needed for maximizing speed. The book is available in bookstores everywhere, as well as online at the NSCA Store.