You've probably heard the term DOMs before, and likely even experienced it yourself. DOMs (Delayed-onset Muscle Soreness) is a form of muscle soreness that occurs a day or two after resistance training in the muscles that you worked. Some people even enjoy the feeling, thinking it is a sure sign that they've trained hard enough. But is this really the case? What exactly causes DOMs in the first place? Here's what the research says about DOMs and what it does or doesn't say about your workouts.
What causes muscle soreness?
Muscle soreness can occur for many reasons. Currently, we don't know the direct mechanism(s) that cause muscle soreness. What we do know, however, is what it's related to. This can provide us with clues as to what DOMs can and can't predict.
We know that muscle soreness has a relationship with muscle damage which occurs when we resistance train . This soreness seems to be more closely related to the repair process of this damage but not the damage itself , so it is not predictive of how much muscle damage has occurred from your training. In fact, we know that a loss in force production (temporary reduction in strength) IS predictive of the extent of muscle damage, but a study by Kazunori Nosaka et al. did not find any correlation between a loss in force production and the degree of muscle soreness . This means that while muscle soreness may indicate that muscle damage has occurred, it can't predict the degree of damage.
A better predictor of how much muscle damage has occurred would be a temporary reduction in performance. If your muscles feel weak and unable to produce as much force as they would normally, it's likely your muscles are still damaged and not yet repaired. This may or may not overlap with muscle soreness; you can still be sore while your muscles are no longer damaged.
What causes muscle damage?
Muscle damage typically occurs when we resistance train . The same training that is required to grow muscle and get stronger will inevitably cause some degree of muscle damage. It seems that the harder we train, the more muscle damage is likely to occur.
There are also other ways that muscle damage can occur. Any time you place a novel stimulus on your muscles, more damage will occur. You've probably experienced this yourself. When starting a new workout routine filled with exercises you aren't used to doing, or changing up your rep ranges drastically, you end up becoming excessively sore in the following days after the workout.
Low-frequency training has also been shown to produce higher levels of muscle damage. This is likely because the body never fully acclimates to the training and each session is seen as novel, causing excessive amounts of muscle damage. If you train your muscles only once a week, it's likely you'll experience more soreness than someone who trains more often.
Does muscle damage predict hypertrophy or strength stimulus?
Since we now know that DOMs has a relationship with muscle damage and training in general, you may be wondering if muscle damage is predictive of hypertrophy or strength stimulus. Does more muscle damage mean more muscle and strength? According to the research, there is little to no evidence to support a correlation between muscle damage and hypertrophy or strength stimulus. A study by Kyle L Flann et al. has shown that when comparing a group with extensive muscle damage after training with a group that had very little, hypertrophy was the same across both groups , indicating no correlation. Many other studies have found similar results  .
This means that muscle soreness isnot a good predictor of hypertrophy or strength stimulus. Being very sore after a workout does not indicate how productive that workout was in terms of stimulating strength or hypertrophy, but is instead an indicator of whether or not muscle damage occurred recently.
Excessive amounts of muscle damage may actually have a negative impact on hypertrophy and strength.
Before the body can allocate resources to building muscle, it must first repair any muscle damage that has occurred, making muscle damage a "barrier to hypertrophy" . This means that if you're causing a lot of muscle damage, it can have a detrimental effect on building muscle due to the body needing to allocate resources to repairing muscle damage before building muscle. For example, lower frequency training has been shown to cause more muscle damage than higher frequencies due to the muscles never acclimating properly to training. This is one of the reasons it is believed that low-frequency training produces less hypertrophy compared to higher-frequency training. When training at least twice a week, the muscle can acclimate much easier, resulting in less repeated bouts of muscle damage, freeing up recovery resources to synthesize new muscle rather than repairing damage. Therefore, not getting excessively sore can be a good sign.
It's perfectly okay to get slightly sore after a workout, or even very sore on occasion. But if you find yourself getting very sore after almost every workout, you may want to evaluate your training and consider finding ways to minimize muscle damage so it doesn't act as a barrier to building muscle -- you don't want your body constantly needing to repair muscle damage before it begins the muscle building process. This can easily be done by avoiding very low-frequency training (1x a week), not training to failure too often, and avoiding switching exercises too often to allow your muscles to acclimate to your training, resulting in less muscle damage.
Does being sore mean you shouldn't train?
Many people think that if a muscle is sore, it shouldn't be trained. This would only be true if muscle soreness had a direct causal relationship with muscle damage, however, it does not. As mentioned before, the best indicator of muscle damage is a loss in force production, which in simple terms just means a temporary loss in strength. Because soreness and this loss in force production often do not overlap, we know that a muscle can sometimes be completely ready to train again even when it is still sore. If the muscle is still sore, but its force production is back at peak levels, then it is ready to be trained again.
Force production, not muscle soreness should be used to determine whether a muscle is ready to be trained again or not. If your muscle is feeling weak and not ready to produce normal amounts of force, you would probably be better allowing the muscle to rest, regardless if it's sore or not.
So what can muscle soreness tell us?
Muscle soreness can predict that damage to a muscle recently occurred. This can help indicate whether or not you're properly stimulating the muscles you intended to stimulate. For example, if you're not sure if you're using your back properly during pulling movements, but your back muscles become sore, this is a good sign that you're stimulating your back adequately.
Since soreness is an indication of damage, you can also use soreness as a way to monitor whether your muscles are being damaged too often. If you're in a constant state of extreme soreness, this may indicate that your training is off in one way or another and needs to be reevaluated to make sure muscle damage is not acting as a barrier to hypertrophy.
Since muscle damage is likely to occur when trying to optimize hypertrophy and strength stimulus, not ever getting sore can be a sign that your training volume and/or intensity should be increased. If you're never sore, you're likely not training as hard as you could be. Remember, it's only excessive amounts of muscle damage that is detrimental to hypertrophy, not muscle damage in general.
Soreness is correlated with muscle damage.
Muscle soreness is correlated to muscle damage, but can't predict the extent of muscle damage. If you are sore, it is likely that you have incurred muscle damage recently. Being sore, however, does not indicate if your muscles are still damaged, as muscle damage and repair do not directly correlate with the timing of soreness -- you may still be sore when the muscle is done repairing. It's thought that muscle soreness results as a biproduct of one of the mechanisms involved in muscle repair.
But muscle damage is more closely correlated with a loss in performance.
A better predictor of muscle damage is a temporary loss of performance. The more weak a muscle feels and the larger the performance drop, the higher the amount of muscle damage it has incurred. When trying to predict muscle damage, don't use soreness, but muscle weakness / loss in performance instead.
The amount of muscle damage you incur is not predictive of progress.
Muscle damage will likely always occur to some extent when trying to build muscle and strength, because the training required to do so produces some muscle damage as well. However, it does not appear to have a causal relationship with muscle and strength adaptation. More muscle damage will not produce more hypertrophy or strength stimulus. Thus, muscle damage, and therefore muscle soreness, cannot be used to predict rate of progress.
To the contrary, too much muscle damage can actually negatively impact muscle growth.
To the contrary of what was thought initially, excessive amounts of muscle damage may actually have a negative impact on hypertrophy, because the body must first allocate resources to repairing muscle damage before it can begin to synthesize new muscle tissue. This has been shown to be the case in many studies; groups with higher amounts of muscle damage often have worse hypertrophic outcomes. This is because the body can only allocate a finite amount of resources towards repairing damage and synthesizing new muscle tissue, so the more muscle damage you incur, the more of those finite resources must be used to repair that damage instead of muscle growth.