According to Toby Tanser, who trained and helped coach top Kenyons, runners “who do nothing but run, have pure symmetry because they have only strengthened the muscles needed for running”. He believes that a runner’s arms are needed only for balance, and that the legs are best strengthened with running and plyometric drills.
These may be logical conclusions if you see that runners only do running training. However to fully evaluate the running, it is necessary to look closely at how much hill running is done, the angle of incline of the hills and how much speed work is done and the terrain upon which it is accomplished. These are crucial factors in regard to development of strength.
It should also be understood that strengthening the legs with only running is a very poor and ineffective way of developing strength. And it should be recognized that this occurs only when first starting to run, not when you are an accomplished runner. When you do weight training you can easily monitor volume and intensity by recording resistance and repetitions. In addition, you can make the strength training progressive in order to have continual strength gains. This cannot be done when running is the means used.
In addition, strength is gained when only running, only when you go above and beyond your typical pace and distance. These factors however, cannot be increased on a regular basis without fear of injury. But when you strength train first, you can then incorporate the strength training to allow you to run faster and longer and without fear of injury.
It should also be pointed out that the comment that plyometric drills are used to strengthen muscles is erroneous. Plyometrics is mainly effective in making the muscles more resilient and explosive — not stronger. For this you use strength training. Plyometrics should be used to literally convert strength to speed and explosive power.
Also lacking in the comments by Tanzer is that there is a major difference between common or general strength exercises and specialized — dynamic correspondence — strength exercises. These exercises duplicate the neuromuscular pathway used in execution of a joint action seen in running. In other words, they develop strength in the same manner as it is exhibited in running.
Thus, this type of strength training enhances one’s run. It can improve the runner’s ability to go further and faster more easily and with less chance of injury. Without this knowledge however, it is easy to assume that strength training is not needed. This is a major fallacy that is holding back the improvement needed for American runners to compete equally with the Kenyons.
For more information on specialized — dynamic correspondence — strength exercises see the second edition of Explosive Running.