Whether you're hitting the gym again after a too-long holiday break, or you've hopped on the resolution bandwagon with some new ambitious goals, maybe, just maybe, you're waking up sore. Is that next-day ache a sign you're doing too much too soon, or an indicator of muscle growth you should be seeking out?

First, let's get technical. The soreness we're talking about isn't the burn you feel during an exercise, nor the Jell-O-like feeling you might get afterward. We're talking DOMS, or delayed onset muscle soreness. A textbook case starts six to eight hours after you work out, and peaks about two days later.

It's nothing to do with lactic acid; we can bust that myth right away. Lactate is produced whenever you feel a burn, say while lifting, but it's a bystander to the pain, not a cause. Lactate even provides more fuel for your muscles, putting it squarely on the good-guy side of the equation. Still suspicious? Lactate is like a good houseguest, clearing out as soon as its time is up. As you're walking out of the gym, the lactate is being flushed from your muscles. By morning, soreness may be raging, but lactate is long gone.

So what's really going on in your muscles after a hard workout? If you were to take a slice of sore muscle and put it under the microscope, you'd see that the muscle fibers, instead of being neatly stacked next to each other, may be torn and stretched out, showing damage to the cells' membranes and contractile proteins.

We know that when muscles are sore, they tend to be damaged. The problem is that it's hard to match up the amount of damage that's occurred with the amount of soreness you're feeling. You'd expect the soreness and the damage to peak at the same time, for example, but scientists who've tried to connect the two phenomena found that you just can't rely on soreness as a precise measure of damage. In broad strokes, though, it can get you in the right ballpark: somebody who is very sore probably has significant muscle damage, and somebody who isn't sore probably has little to none.

But about that damage. What doesn't kill you makes you stronger, right? There's a theory that a little damage is a good thing: in repairing damaged muscle tissue, the body builds it up stronger than it was before. A recent article by Brad Schoenfeld and Bret Contreras in the Strength and Conditioning Journal inspects that line of reasoning. If soreness means damage, and damage means bigger, stronger muscles are being built, can you use soreness to gauge how effective your workouts are at building muscle? It's an attractive theory, but there are a lot of dots to connect before we can say for sure.

First, here's some of what we know goes on in damaged muscle cells:

● White blood cells arrive on the scene and cause muscle cells to make chemicals called myokines that help to build stronger muscle. However, muscle cells can make myokines even without being prompted by the white blood cells, so damage may not be necessary for this.

● Muscles have their own stem cells called satellite cells. When muscle tissue is damaged, the satellite cells create new muscle cells and provide essential life support to damaged ones. They also make some chemicals that are used as signals to build more muscle. But, once again, scientists have also watched this happen in muscle tissue that wasn't damaged.

● Muscle cells swell, taking on water. Scientists believe this can trigger cells to produce more proteins, but that's an observation from the lab, and hasn't been seen in real life muscle tissue. So it's another maybe.

● A different type of white blood cell may signal other helper cells to aid in the rebuilding. And it seems that ROS, reactive oxygen species, may be what it uses for that signal. This is why taking antioxidants may be a bad idea, since they interfere with ROS.

A few years ago, scientists thought that antioxidants would be good for athletes: kill off the ROS, and you'll reduce muscle damage. But studies tended to show the opposite effect. Although there are no good studies on trained lifters, there's a growing body of evidence from animals and miscellaneous humans showing that antioxidant supplements hurt more than they help. They inhibit muscle growth in mice, aerobic gains in new Crossfitters, and recovery in competitive kayakers.

The truth is that muscle cells are mysterious bags of sarcoplasm and we don't fully understand what triggers them to grow bigger and stronger. The effects above may well contribute to muscle growth, but it would be wrong to say they're essential in the process. We know that hypertrophy (basically, bigger muscles) can happen without any detectable muscle damage.

A study of elderly people illustrated this beautifully: Five men and six women, all in their seventies and eighties, attempted to pedal forwards on what you might describe as an exercise bike from Hell: it pedals itself backward at the same time, strongly enough that its unfortunate riders were pushing with all their might and still going backward. This, by the way, was their entire exercise regimen for 11 weeks, starting at a gentle level and ramping up until they became backwards-pedaling beasts. The results were a stunning amount of muscle growth and an equally stunning lack of muscle damage. While your workouts are tougher than theirs, this proves an important point: damage and soreness aren't essential to the process of building muscle.

If you need to alter your training regimen to avoid losing days to soreness, here is some of what we know about preventing it. First, eccentric contractions, while great for building muscle, are also notorious for causing that can't-walk-down-the-stairs feeling the next day. A lesser known, but related, factor is when the peak contraction occurs. If it happens when the muscle is lengthened, that's going to cause more soreness than if the muscle is working hardest in a shorter position. Knowing this, you can plan your workouts to go easy on the negatives on days that serious soreness is a possibility.

But why worry about the pain? Mild or moderate soreness isn't a big deal; we've all pushed through that. But extreme damage, which is signaled by extreme soreness, is something to avoid—NOT a goal to pursue as a badge of bad-ass-ness.
The only problem is determining how much soreness is too much. There isn't a strict threshold, but I can give you some information that might help you find your own sweet spot.

First, the necessary caution about rhabdomyolysis. This is what happens when muscle damage is so extreme that proteins from the muscles end up in the bloodstream. At this point, an athlete's muscles are very swollen, very sore, very weak, and most concerning, their urine may be dark, and kidney failure may be imminent. To be honest, rhabdo is rare, and most cases in the medical literature come from people who were forced by an irresponsible trainer to exercise past the point where they felt they needed to stop.

Within the tolerable range of soreness, an important factor to be aware of is how much your strength has been reduced. Muscle damage is considered "severe" when the muscle's capacity is down to 50% of what it can usually do. In this case, recovering full strength may take three weeks, although soreness will have resolved sooner than that. This suggests that you're better off tracking recovery by how much you can lift (or how easily you can lift a given weight) rather than by how sore you feel.

Does it hurt (or rather, is it a bad idea) to work out while you're still sore? Studies suggest that no extra damage results. In one study, untrained subjects did a tough eccentric workout, and then did another session two days later, when they were still sore. Although their strength was at 75 percent when they started the workout, by the next day they were back up to 80 percent, the same as the control group that did the initial workout but had been resting ever since. More detailed tests showed they had no more muscle damage than the controls and no longer recovery time, although it's not proven that the same would be true of athletes doing more serious workouts.

But even if those extra workouts don't hurt, it's not clear that they're helping. If your muscles' capacity is reduced, then the weights you lift while recovering aren't going to be as heavy as the ones you could lift if you hadn't damaged your muscles so much in the first place.

In their review, Schoenfeld and Contreras conclude that a little soreness is fine, because it's a sign that you've achieved some muscle damage, but soreness that interferes too much with your ability to do your usual workouts (they don't define "too much;" that's up to you) is counter-productive. Probably the most sensible interpretation of damage is that it's not strictly necessary to build muscle, but it helps.