This is an excerpt from Developing Speed by NSCA -National Strength & Conditioning Association & Ian Jeffreys.
Potential for and Limits of Speed Development
Although speed can be improved, it is inaccurate to suggest that everyone has the capacity to become a sprint champion. A genetic ceiling exists for the top speed an athlete can reach, therefore limiting the ability of the vast majority of people to become an Olympic 100-meter champion. However, while this ceiling exists, it is likely that few people actually reach their ceiling. This is clearly demonstrated by the improvements that elite sprinters make throughout their careers. If top sprinters, with their training aimed specifically at speed development, do not always reach their full genetic potential, then clearly, the likelihood of athletes involved in other sports reaching their speed ceiling is much lower. Therefore, a majority of athletes have a great potential for improving speed, and speed development programs are fundamental to any total performance enhancement program. It is hoped that as speed training methods improve and are used by more and more athletes, more people will approach their ceiling and reach their full speed potential.
The principal genetic limits to performance are the type of muscle fiber, their activation, and the athlete's body type and structure. Great sprinters have a preponderance of fast-twitch muscle fibers. Fast-twitch fibers have a higher force-producing capacity and a higher speed of contraction but are less resistant to fatigue than slow-twitch fibers. Clearly, the higher the percentage of fast-twitch fibers an athlete has, the greater the capacity for speed.
This is further emphasized by the fact that there are two major types of fast-twitch fibers: Type IIa and Type IIx. Type IIx fibers demonstrate the greatest force-production capacity and contraction speed but exhibit very limited endurance. Type IIa fibers still have a high force capacity and speed of contraction, although not as high as Type IIx fibers, but have a greater endurance capacity than Type IIx. Elite sprinters have a high overall percentage of Type II fibers and also a high percentage of Type IIx fibers.
While the proportion of an athlete's muscle fiber types is predominantly set at birth, training can affect their characteristics and how they are activated. Prolonged endurance training for example can lead to Type IIx fibers taking on the characteristics of Type IIa fibers and to Type IIa fibers taking on the characteristics of Type I. Both of these effects reduce the force capacity of the muscle, especially in relation to the rate at which force can be applied. Additionally, excessive periods of resistance training, especially where slow movements are stressed, can lead to a change in fiber characteristics between Type IIx and Type IIa.
Also important is how effective an athlete is at recruiting the Type II muscle fibers (especially Type IIx). Untrained athletes typically recruit only a limited proportion of Type IIx muscle fibers, and training with high loads or high speeds or both is required to develop the capacity to recruit a high proportion of Type IIx fibers. Therefore, a speed development program needs to include resistance training that uses high loads and explosive movement.
Another important genetic factor in speed development is each athlete's body structure. Lever lengths (length of arms and legs) greatly determine the capacity to move rapidly, and the length of these levers is determined by both bone length and the point at which the muscles insert into the bone. This means that some bodies are designed ideally to move rapidly while others are not. Again this factor is genetically limited.
Although genetic limits provide a theoretical ceiling for speed capacity, the focus of a speed development program is improvement of this capacity and especially how it relates to sport performance. It is important, therefore, to look at the aspects of speed that can be improved. This requires an examination of the nature of running speed and identifying the elements that can be enhanced through training. In this way, athletes and coaches can focus on the elements they can adapt, which then become the focus of a speed improvement program.
Read more from Developing Speed by National Strength and Conditioning Association (NSCA) and Ian Jeffreys.