The mechanics of muscle damage
Our muscles are pretty good at adapting to movements we use often, but when we start doing something new (different movement, heavier load, more repititions of the movement, tension - holding the movement) that is a new stressor. These mechanical stressors will over time cause adaptation, but they produce small acute injury to the muscle tissue.
"Three main phases have been identified in the process of muscle regeneration; a destruction phase with the initial inflammatory response, a regeneration phase with activation and proliferation of satellite cells and a remodeling phase with maturation of the regenerated myofibers." (3)
The [eccentric - lengthening] contraction of the muscle tissue with the added stressor (weight, usually) damages the myofibrils, which are the long filaments that run side by side and make up muscle fibres (1). Textbook imagery makes them look like little tubes... which form to fill a bigger tube called a myofibre. A bunch of these together make a muscle. The trick to making this lengthening movement work effectively to positively damage the muscle is to complete the full range of movement to the fibres lengthen past optimum length (don't over-extend though).
The damage to the myofibrils and the muscle cell membranes (usually refered to as microtears because it is literally tiny tears from over stretching the sarcomeres within the myofibre) allows unregulated calcium through the tears which tightens the sarcomeres within the myofibrils; and activates proteases and hydrolases that contribute to muscle damage (1). Calcium is used in your muscle every single time one of your sarcomeres contracts. There's a lot of finer science details which even I don't enjoy reading, so I won't subject you to that. But, basically, muscle tissue lengthens under tension/pressure and tiny tears are created. Sometimes some of the myofibres even die (2). The microtears trigger inflammation as the cells release creatine kinase and myoglobulin (4). Macrophages come to the site to collect debris and pump out Th1 cyotkines (small signalling proteins). Water is brought into the muscles which accounts for the swelling you feel. This peaks around 24 hours post exercise. More (slightly different) macrophages (M2) are called to signal for tissue repair (myoblasts) and regulate the inflammation (1). At this point you might notice decreased perceived strength and range of motion. Your muscle is now full of satellite cells (that will turn into muscle cells), neutrophils, macrophages, T lymphocytes, mast cells, endothelial cells and fibroblasts (4). These will all play a role in repair of the muscle. Some of these cells produce mitogens (to stimulate mitosis) and motogens (to stimulates cell motility) - how fun!
The mechanics of muscle repair
The repair of muscle tissue begins with the injury that triggers the inflammatory cascade. Remember all those cells that are in your damaged muscle now you've reached peak inflammation. M2 macrophages, CD8 and T-regulatory lymphocytes in your muscle come in between 24 and 4 days after damage and secrete anti-inflammatory cytokines (different signalling proteins to before). The cytokines signal other cells to make more myoblasts (to build myofibres) and satellite cells (4). These satellite cells eventually become myoblasts. Type I/III collagens, laminin and fibronectin form bonds to stabilise the tissue, the same happens after a wound has clotted, but here the bonds provide scaffolding for the myoblasts. Myoblasts stick to the bonds to form new myotubes or fill holes in damaged myofibres (3). The fresh myoblasts that make myotubes mature into myofibres. If you've had a significant trauma injury, it's as this point where your blood vessels are re-established in the new repaired tissue and nerves are complete (3).
As a result of the repair process and the increased apparent requirement to do more or lift more, the body grows new mitochondria to make more energy for next time, and then builds more myofibres to house more mitochondria during each repair. This process is completed in approximately 7 days (4).
How to assist muscle recovery?
1) Get adequate sleep. Sleep is the most significant period of repairs, hands down.
2) Rest. You must find the balance between training and recovery to prevent muscle breakdown exceeding the muscle repair (5). Change the body part your using each time so that section has time to recover and you can still get the endorphins from working another body part. (e.g. upper body exercises day 1, lower body exercises day 2, a walk, repeat).
3) Massage the muscle - foam roll, roll a ball over the length of the muscle, an actual (gentle) massage (4,5) the next day or day after.
4) Cold water immersion improves recovery if you're brave enough (4).
5) Curcumin to act as an anti-inflammatory agents, reduced inflammatory markers and improved muscle function compared to placebo (6). So did [American, Red) ginseng (ginsenosides, rather) and Tart cherry juice.
6) Creatine showed similar effects to curcumin (and far less cost, but must be consistently ingested) (6).
7) Vitamin D3 and Polyunsaturated Fatty Acids (PUFAs) equated to less soreness (6).
3 and 4 have controversial research, 'they' haven't quite determined if it is perceived recovery or if the therapy does speed recovery. I, personally, feel that foam rolling helps reduce the tightness and therefore makes me feel less impacted during the day.
What makes this process slower?
1) The number one factors that leaves muscles chronically under-recovered and fatigued it 100% over-use. You cannot go to the gym and smash full body 7 days a week, just as you can't run long distances 7 days a week. You will feel like a train hit you at some point, and if you don't, then I assume your training is not moderate or intense and therefore not damaging the muscle fibres significantly. Which is fine - low impact, light exercise 7 days a week is completely manageable. You know why? Because the muscle damage isn't as significant from light exercise and has sufficient time to repair from that lighter stimulus. Walking, for example, is considered light exercise.
2) Other factors that slow muscle repair could be; lack of protein and other tissue connecting minerals and vitamins (C, zinc, collagen, etc). This would be purely based on resources available for muscle repair, however the body is quite resouceful.
3) An inflammatory diet may contribute to increased use of resources and potentially impact the effect of the anti-inflammatory actions taken when trying to repair muscle (theoretically).
4) genetic propensity of immune system/ regulatory cells/ muscle growth etc.
When is it more than muscle fatigue?
If you find that you've had a few days off moderate or intense exercise and you are still experiencing doms/soreness/swelling - especially if this type of exercise is not new to you, it might be time to consider if you are really taking good care of yourself. Did you sleep enough? Are you eating the right amount of the right foods? Have you foam rolled/ stretched/ done some light movements?
If that isn't effective and this has been the case for months, to the point where you have given up exercising because it's too fatiguing, there may be more to it. Chronic fatigue, lack of motivation, lack of energy to complete daily tasks can be indicators of a few things and is best to see your health practitioner (GP, Naturopath) for futher investigation.
(1) PMID 23733696
(2) PMID 11731568
(3) PMID 27447481
(4) PMID 28035017
(5) PMID 29755363
(6) PMID 30617517