In the world of exercise and athletics, overspecialization generally leads to injury, often chronic injury, and even permanent disability. There are, of course, many aspects to fitness, and I’m not going to bother going over them in this post, but neglecting any of them can lead to weaknesses and limitations, which often lead to more issues, and then ultimately to injury and disability. Distance runners, and endurance athletes in general, are notorious for overspecialization. I see runners who have very limited strength (including leg strength), limited mobility and flexibility, and forget about power or technical proficiency.
CrossFit and CrossFit Endurance are a couple of favorite punching bags for running and endurance communities, but, in my experience, many runners improve dramatically after starting these programs. Why? Well because these programs are addressing elements of fitness that have been neglected for too long. Will these types of programs produce a world class distance runner? Probably not, but most runners are not world class, and often have issues that must be addressed before they can even think of reaching their potential as endurance athletes.
In summary, athletes should always be looking for their weakness, and when they find those weaknesses, they should address them. Runners in particular often have problems with strength, mobility, flexibility and crappy technique. Although runners are loathed to hear this message, often they will run better by running less, and focusing on other things. Don't just run, lift or whatever your thing is, but address your personal fitness deficits, or your activities will probably become unsustainable.
Links to other posts in this series.