You have had the experience: You've gone out for an extra-long run, worked out on some steep hills for the very first time, or completed an unusual number of work intervals on the track - and then paid the grisly price. For a few days after your effort, your legs felt stiff, your muscles and tendons were tender and sore, and your usual leg strength was missing in action.

What did you actually do to your legs to create so much discomfort and weakness? Did you set back your training, or are such occasional bouts of pain and feebleness a normal part of the training process? Was there anything about your soreness induction which would actually be good for you during subsequent training? Before we respond to these key questions, let's take note of a fact which will help us with our answers: You have probably also had an interesting, follow-up experience with soreness. That is, it's likely that you performed - at a later date - a workout similar to the one which produced so much leg distress initially, after a few weeks of other sorts of training, for example. Somewhat surprisingly, this second session produced no ill effects at all - not even a whisper of protest from the sinews and cables in your lower appendages. Why did the first effort lead to misfortune, while the second failed to perturb your legs at all?

This scenario, in which a specific workout produces pain after its initial completion and then rubs milk-and-honey balm on your legs after its second and subsequent fulfillments, has been noticed by exercise scientists and is often called the "repeated-bout effects" (1 & 2). Amazingly enough, the "protection" from soreness and enfeeblement which occurs after the first training session can last for several weeks - and possibly for as long as six months in some cases (3).

Why should we care about this? If we can understand the underlying mechanism which produces protection from significant soreness, it might be possible to train in ways which invoke this mechanism (without producing significant tissue damage) and thus protect ourselves from muscle strains and training-related tendon damage. There might, in fact, be a general routine, a combination of strength training and running, which, when carried out during an initial phase of training, could provide many protective benefits over the course of a training year.

To date, investigations which have attempted to unravel the mystery of the repeated-bout mechanism have produced some extremely interesting results. Some research has suggested, for example, that a muscle group does not have to be exercised in the same manner in the initial and subsequent bouts of exertion in order for a protective effect to occur (a clear violation of our hallowed specificity-of-training principle).For example, one study found that 100 maximal, eccentric contractions of the quadriceps muscles furnished protection against quadriceps damage following a subsequent bout of downhill running (4).

That word "eccentric" will appear repeatedly as we talk about post-workout soreness, so let's deal with it for a moment. Recall that eccentric muscle contractions are notorious for producing soreness and that an eccentric muscular contraction is one in which a muscle is exerting force and attempting to shorten - and yet ends up being elongated by other forces acting on the muscle. A good example of this is what happens to your quads as you run. The poor fellows' contract when your foot hits the ground, but the forces of impact make your knee flex anyway, and the quads get temporarily stretched and lengthened - as they are trying to shorten and keep the knee joint under control. Put yourself on a significant hill and run in a downward direction - and things get much worse for the quads. Since your foot is falling farther with each step, the leg is accelerating downward to greater extent than usual, and thus the forces on the quads are considerably augmented. The eccentric-strain damage to the quads is more extensive, and post workout quadriceps pain is likely to appear - if you have not done much prior downhill running.It is clear that eccentric strains produce a significant amount of leg discomfort which is part of running training. However, there is also something about eccentric straining/training which ultimately provides a considerable amount of protection for muscles and tendons (5). In short, eccentric strains damage muscles - but lead to adaptations which are highly protective.