April 2004 -- Question: I have read and heard over the past several years that people who run long distances over long periods of time might be actually training to make themselves slower. I have trouble believing this. Why would strengthening one set of muscles (those used in jogging a long distance) cause problems in another set of muscles? Why wouldn't the effect of jogging be neutral with respect to sprint speed, or even positive since there must be some sort of overlap among the muscles used? What is the explanation for this?Answer:
This controversy and the confusion surrounding it have existed for years. I think we need to begin by defining terms to be sure we start on the same page. The muscles used in both jogging and sprinting are actually the same; they are just used differently. We do, however, have different muscle fibers that exist in various muscle groups and the numbers of each fiber type differ among people as well.
There are tonic muscle fibers that use oxygen and there are phasic fibers that are anaerobic and do not use oxygen. It is also important to note that there are other types of fibers between these two extremes. Both fiber types are found in basically all skeletal muscles, just in different proportions. The muscles of the body that react very rapidly are composed mainly of the phasic fibers, or fast-twitch, with only a small number of slow-twitch or tonic fibers. Conversely, the muscles that respond slowly, and usually with a prolonged contraction, are made up mainly of the tonic fibers. The large calf muscle (gastrocnemius) has a predominance of phasic fibers for jumping and pushing off, whereas the smaller calf muscle (soleus) has a higher number of tonic fibers and is used more for prolonged lower leg activity.
The tonic fibers are usually smaller with a more extensive blood capillarization, thus enabling them to use oxygen. They are used abundantly in slower and more continuous types of exercise. These slow-twitch fibers provide endurance over many minutes up to hours of exercise. The phasic fibers are much larger for greater strength of contraction and a less extensive blood supply. Since these fibers do not require oxygen for contraction, they are used more effectively in short explosive movements such as those used in interval training. These fast-twitch fibers can deliver extreme amounts of power for a few seconds to a minute or so. So, if you run long distances at a steady rate all the time, such as a five-mile jog at a continuously steady pace, you are recruiting the tonic fibers and you will actually train your body to call upon them on demand.
On the other hand, if you run in intervals, when you are using short bursts of power in windsprints, followed by intermittent recovery, you are employing the phasic fibers more effectively. Again, it is theorized that you can train your body to call upon the fibers you employ the most.
There has been a plethora of research examining this phenomenon. Studies have indicated that we may have a fixed number of the various fiber types but, through training the right way, we can enhance the firing of certain ones. It is important to note that you cannot change the relative proportions of fast-twitch and slow-twitch fibers, regardless of how hard you try. We all have a fixed number of the different fiber types, but we also have different proportions. Marathoners, for example, have been observed to have a ratio of 82 percent slow-twitch fibers to 18 percent fast-twitch fibers, while sprinters have been seen to have 63 percent fast-twitch and 37 percent slow-twitch fibers.
The key point of training is to understand that improved fitness and specificity of training to the particular activity enhance muscle development and contraction, as well as blood flow. You want to train in such a way that you enhance what you already have and facilitate the movements required by your activity's demands. You would not want to train in a counterproductive way that goes against the requirements of your sport.