Weights, general strength circuits, plyometrics, and hip/core strength exercises all have their place in the training of a distance runner. Today, I'd like to focus on general strength circuits specifically.
Each word in that phrase has a particular meaning. General means not specific. i.e. not exercises that are very similar to running or that involve running. An example of a more specific strength exercise might be uphill sprinting or bounding. Strength means more or less what you'd expect it to be—resistance exercises for muscular strength. Finally, circuits denotes that we're talking about a high-intensity strength routine with many different exercises and short recovery.
Why should a distance runner do this type of strength work? There are three reasons, and each of them illustrates one of the three domains from which a good coach will draw wisdom.
Why do strength circuits?
1. Anecdotal observations on strength
The first domain is anecdotal observation. Subjectively, I (and a lot of other coaches) have noticed that fast, injury-resistant runners tend to be stronger and more athletic than their slower, injury-prone counterparts. Of course, there are exceptions—a skinny, uncoordinated kid who wins the state meet, for example—but if you spend enough time around budding distance runners, you'll find the general trend is undeniable. This alone is reason enough to do some type of work for improving strength and general athleticism, since it's also evident to any experienced coach that "just" running won't make you strong and athletic.
2. A physiological argument for strength circuits
The second is drawing from physiology and training theory. In distance training, we know that it is advisable to build a base of general, less-specific running before moving to race-specific workouts. We can apply the same principle to both strength work in general and to high-intensity circuits in particular. Before we start doing any heavy weight lifting, high-intensity plyometrics, or hill sprinting or bounding, it makes sense to improve our general strength and athleticism so we're better-prepared for higher-intensity, more running-specific stimuli further down the road.
Additionally, there's a general-to-specific argument to be made for high intensity circuits with regards to development of a finishing kick. Think about what happens when you kick at the end of a race: you call upon your fast-twitch fibers to work at a high intensity, even though they're already awash in acidosis. We can train this in a specific way by doing certain workouts (or just by racing), but how could we train, in a general, non-specific way, the ability to recruit fast-twitch fibers in a fatigued state? A general strength circuit is the perfect solution.
3. Scientific research on strength circuits and hormone levels
If you keep up with pro running news, you'll know that there has been a lot of buzz recently about the possibility of illegal doping being much more widespread than was previously thought. There are three go-to pharmacological aids for drug cheats: EPO, which boosts your red blood cell production, testosterone, which boosts muscle growth and recovery, and human growth hormone, which also aids in muscle growth and recovery. Wouldn't it be great to be able to boost levels of these hormones naturally?
With EPO, we're out of luck unless we live at altitude, but with regards to hGH and testosterone, the situation is a little more interesting. Scientific research shows that a general strength circuit designed with a couple of guidelines in mind will boost levels of human growth hormone and testosterone in the blood for several hours post-exercise.1
By now, it should be clear that strength circuits should be a part of any competitive 800m to 10,000m runner, and should be a serious consideration for long-distance runners from a recovery and injury-resilience perspective too. The next question is how to actually design a general strength routine. To do so, we'll look to the scientific literature for guidance.