You must not base the amount of Rest Days you take just on muscle soreness alone, but on Central Nervous System Recovery as well. Muscle pain such as DOMs (Delayed onset muscle soreness) really becomes a problem only if it impairs the proper movement and contraction of the muscle - or if it's simply too painful to move throughout its full range. However, the Nervous System is more silent then say, muscle soreness. With muscle soreness you know that the muscle has been worked so much that micro tears of the muscle fibre have occurred and they are in need of repair - something that your Nervous System Fatigue is not directly linked too. Your Nervous system, the system which tells your muscles to turn off + on can still be fatigued even if you do not create enough micro tears to solicit muscle soreness after your workout. Generally, if it's too fatigued it will only manifest itself subtly, with slight decreases in strength and moderate decreases in focus and concentration. These things are much harder to identify and link to poor Recovery.
NOTE: The information given here refers to a split bodybuilding routine.
1. Individualise Your Training Split
If you are a beginner, great! You must adhere to a strict structure of training so you learn and adapt quickly to the exercises and routines, and it might not be time to individualise things just yet. However, if you've been training for 1 or more years already then you can be more methodical about your training split. Just because everyone trains Chest on Monday doesn't mean it has any relevance to how your body recovers. If you aren't ready to train Chest then Training it will not help you, and with prolonged Nervous System Fatigue you might end up going backwards with your training. Conversely, if you're ready to train earlier then there is no need to wait until Monday. Shape your training split to your individual needs.
2. Do Not Be Afraid of Resting
Resting is when your muscles grow and adapt. Training only solicits adaptive potential, it does not manifest it. So ask yourself, do you want to spend more time Training or more time Resting? Everyone who is comfortable with their ability to solicit adaptions from their training wants to take more and more Rest days - why? Because they know that all their hard work is paying off the better their Recovery is! We do not hibernate for 6-8 hours every night for fun do we, as we would still need sleep regardless of training - SO with the added stimulation of micro tears and Central Nervous System Fatigue our Rest becomes even more necessary and welcome.
Continuing on from Point 1, becoming instinctive about your training split can be a little hard to manage at first, so to help you gauge what kind of rest and split is right for you then you must experiment with a few different approaches. Genetics play a big role in Recovery speed, one size does not fit all. Try taking a Rest day in between each Training day and gauge your progress and general feelings. If you think you can handle more, take a Rest day only after two Training days - if that becomes too much to handle then you know your ideal amount of Rest days is somewhere in between those two structures, something like this; 2 Training Days - Rest Day - 1 Training Day - Rest Day - 1 Training Day - Rest Day. Be open to it changing as you progress.
4. Rest Day Rule(s)
This is especially relevant if you are a beginner; Taking two weeks off before training a muscle group again is what is described as 'insufficient frequency for a perceived change in environment'. In short it means you have to train a muscle more than that for it to change and adapt. Depending on how you've structured your Training split to fulfil your Recovery requirements, try to adhere to the following:
(For the Natural Lifter)
1. Take no more than 3 Rest days in a row.
2. Train no more than 3 days in a row.
3. Train each muscle group at least once every 7-8 days.
4. Do not Train a muscle group more than twice every 7-8 days.
5. Do not Train very sore muscles.
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