Soreness isn't a sign of an effective workout. Those who lift heavy regularly don't experience debilitating DOMS as often.
Rest days don't require total inactivity. In fact, physical activity is actually beneficial on non-lifting days.
Muscles don't need to rest 48 hours before they're trained again. And they can even be trained when they're sore.
Increasing training frequency can decrease soreness. Muscles will adapt to heavy training when you train them often.
You don't have to separate muscle groups by workout. You may get better results from full-body workouts and upper-lower body workouts.
Myth #1: Soreness means you got a good workout.
You haven't been to the gym in a while. You go and you feel like you get a really good workout. The next day you're pleased to find that you're extremely sore, barely able to even move the muscles you trained. Is this a sign you got a great workout? Not always. Soreness is not directly related to hypertrophy.
If soreness can be reduced with things like stretching, warming up, and increasing training frequency, would doing those things mean you're making your workouts less effective? Of course not.
The misconception that soreness equals gains comes from the notion that you build muscle from "tearing" muscle fibers and forcing them to repair. The soreness must be directly related to those tears, right? Not quite. The relationship isn't that simple.
Soreness is caused by pain sensitivity to micro-tears in muscular connective tissue (z-band filaments), but it's also heavily related to how sensitive your nociceptors (pain receptors) in your muscles are.
It's unlikely that you'll encounter high-level athletes that experience crippling DOMS after every workout. Why wouldn't they? Their muscles have already adapted, they recover faster, and their pain threshold has increased over time. They've become more tolerant of muscle soreness.
In fact, if you're able to get extremely sore after every workout, there's a good chance you're either not training frequently enough, you have some sort of electrolyte deficiency, your workout nutrition plan sucks, or you're not eating or sleeping well enough.
Also, don't be apprehensive to work out again if your muscles are still sore. If you don't work out frequently and have a generally low activity level, you might find a hard workout will leave you sore for several days.
That doesn't mean you need that long to recover. And your soreness will likely subside after a few warm-up sets to get the blood flowing. Obviously it's not a bad thing to be sore, but don't expect it all the time. Especially if you train full body or train certain muscle groups frequently.
Pick up the frequency, improve your nutrition, and debilitating soreness will become a thing of the past.
I've actually known people who would refuse to do standing overhead presses because they had worked legs the day prior and wanted to "give the legs time to rest." A sitting overhead press will spare you some core and leg work, but your legs won't shrink because you do standing presses the day after leg day.
Some people even refer to their "rest days" as days in which they literally avoid all physical activity in order for their muscles to recover. In actuality, some light cardio and even light lifting on rest days probably won't affect your recovery abilities, and may help speed recovery and reduce soreness by increasing circulation to the area.
Sure, you shouldn't overdo it, but light stretching and contracting of the muscles needing recovery will help keep blood flowing to those muscles and keep them loose. Doing a few sets of bodyweight squats (not to failure) the day after your leg day certainly won't hurt, and will help you burn a few extra calories.
Some believe that if you don't give a muscle group 48 hours of recovery you'll "overtrain" the muscle.
How long a muscle needs to recover depends on the volume and intensity of your training, and how well you eat and rest.
While it's true that it may take 48 hours or even longer for a muscle to recover after working it to its limit, it's probably also inadvisable to train in this manner consistently. One way to progress in workouts aside from increasing intensity or volume in individual workouts is to simply increase frequency.
Think of it as squeezing more training volume in over the course of weeks or months, instead of just focusing on how much you can do in your current workout. At a certain point, it'll become very difficult to progress in intensity by increasing weight or reps.
When you reach this point, increasing frequency (and thus volume over time) is an easier way to progressively overload and see progress. Plus, you're probably already training certain muscles unconsciously on back-to-back days.
Your core and abdominal muscles are used on virtually all big lifts. If you do a push/pull/legs split, you're inevitably going to hit your upper back when you bench or overhead press. If you squat or deadlift on leg day, you'll likely be using a good amount of forearms and upper back.
Tons of exercises use more than just the main muscles they target, and it can be hard to draw the line when deciding which exercises should be done on what days.
Myth #4: You must follow a body-part split.
Body part splits are workout routines in which you only train certain muscles/muscle groups on certain days. Chest day, back day, etc. While they can be beneficial depending on your goals and how advanced you are, they're overused by the vast majority of lifters.
A recent study found that a full-body routine 3 times a week induced a greater hypertrophic response compared to a 3-day split. The takeaway is, if you're only in the gym a few times a week, it's better to do full-body workouts.
Similarly, if you're in the gym 6 or 7 days a week, there's still no reason to split the body into more than two groups. Doing an upper body/lower body split has a distinct advantage over a push/pull/legs split – you hit each body part more often, and it's much easier to maintain optimal muscle balance. That means no shoulder problems resulting from push/pull imbalances.
Poor posture (rolled-forward shoulders) often results from doing too many exercises involving internal rotation of the shoulder and not enough stretching. What often happens is people get too excited on push-days and put too much work into exercises like the bench press. The next day, they don't feel as motivated to work as hard on pull day and end up half-assing their workout.
They don't fully train their back, especially those upper-middle back muscles that help retract your scapula and pull your shoulders into a better position. If anything, you should be putting more work into your pull days, because there's simply much more muscle in your back. But no one asks, "How much can you bent-over row?"
Another great benefit of upper/lower body and full-body workouts is that your legs won't end up lagging. In a 3-day split, your legs are really only getting one-third of your attention, while in reality they should be getting at least half. Try hitting legs two or three times a week instead of once or twice, especially if your legs are lagging behind your upper body. You'll wish you'd done it sooner.
One thing about leg work you need to know is, your quads and glutes have a tremendous ability to recover, while your hamstrings will not need as much frequency.
Myth #5: You can't gain muscle in a caloric deficit.
The amount of muscle you gain depends on a lot: how advanced you are, how much time you have to train, how much rest you get, and how you eat.
Eating right, training, and recovering properly are the most important factors in gaining muscle, and as long as you are not in a huge deficit (more than 500 calories) you'll still make gains.
But won't the person eating in a surplus make more gains?
Yes, possibly, but not by much. And for the "dirty bulker," when it comes time to cut, they often find the extreme change in diet stressful both on their mind and body. They might even start losing muscle during their cut or develop metabolic damage.
Over the span of a few years, the lifter who eats a balanced diet at or around maintenance level calories will likely make better gains than the individual who's constantly switching between bulking and cutting cycles.
I'm not saying that bulking or cutting don't have a place. They do. Some people find that bulking even helps them overcome training plateaus. But bulking and cutting should only be used temporarily, not long-term.