Which Comes First, Cardio or Weights_ - Alex Hutchinson [24]
Untangling the intricate details of which cells produce and consume lactate and how your body maintains that balance remains an area of active research. But the practical implications are clear: studies have found that endurance-trained athletes produce roughly the same amount of lactate as untrained subjects, but they use it as fuel far more efficiently—that’s why lactate levels in their blood don’t rise as quickly. This means that, even though we’ve had the science backward all this time, we’ve somehow gotten the practical advice for training right. The goal of exercising at or just below your “lactate threshold” makes perfect sense—but the aim is to teach your body to consume lactate more quickly, not to avoid “poisoning” your muscles with too much lactate.
If that’s the case, what actually causes fatigue in your muscles? One recent theory advanced by researchers at Columbia University is that exhausted muscles begin to leak calcium, which reduces the force of muscle contractions. While this may be part of the puzzle, it’s unlikely that there’s one single factor that causes fatigue. And even though lactate is a valuable fuel, it’s still possible that increasing acidity in your muscle tissue can interfere with muscle contraction and cause the acute discomfort you feel. As for post-exercise muscle soreness that crops up a day or two later, there was never any reason to blame that on lactic acid—which, as it turns out, returns to normal levels in your blood less than an hour after hard exercise.
Why do I get sore a day or two after hard exercise?
Scientists have made great progress in discovering what doesn’t cause soreness after exercise. It’s not a low-grade persistent muscle spasm (see Chapter 6), and it’s not an accumulation of lactic acid. Instead, most researchers now agree with a theory first proposed over a century ago that blames post-exercise soreness on microscopic “tears” in your muscles. But that leaves an important mystery: if muscle damage is the cause, why does the pain peak 24 to 48 hours after you stop exercising?
You may know from bitter experience that the harder you work out, the more likely you are to be sore afterwards. But intensity isn’t the only factor, as numerous experiments with hill running and stair climbing have shown. For example, Swedish researchers compared three groups of volunteers who ran for 45 minutes on a treadmill, either on an uphill slope of four degrees, a downhill slope of four degrees, or a downhill slope of eight degrees. Even though the uphill group had to work the hardest to maintain pace, it was only the downhill groups that developed “delayed-onset muscle soreness,” or DOMS.
The reason is that downhill running involves “eccentric” muscle contractions, which occur when the muscle is trying to shorten but is being forced by an external load to lengthen. Typical examples include lowering the weight in a biceps curl or the braking action of your quadriceps (front upper leg) muscle as you run downhill. During eccentric contractions, your muscle filaments are stretched to their limits—and sometimes beyond. The resulting damage effectively weeds out the weakest links in your muscles, so that they will be stronger once they’re repaired.
Ironically, it’s the repair process, rather than the damage itself, that is thought to cause pain in the day or two after exercise. The body sends cells called neutrophils and macrophages to clear out the damaged tissue and mobilizes a host of other types of cells to begin the rebuilding process. The outer membranes of nearby muscle cells get damaged in the process, allowing fluid to rush in and cause the muscles to swell. Meanwhile, another substance called bradykinin is released by the damaged muscle, which, after a delay of about 12 hours, causes an increase in levels of “nerve growth factor” that lasts for about two days. Nerve growth factor, which is associated with chronic pain conditions, makes your nerve endings more sensitive—so that any movement of your inflamed