Story
Muscle System
Last updated
In one pass Exercise is more than burning calories.
Educational content, not medical advice — consult a clinician.
Motor unit The basic unit of contraction is the motor unit, one α-motor neuron with all the fibres it innervates, and it either fires fully or not at all.
Muscle as endocrine organ Contracting skeletal muscle secretes hundreds of signalling molecules that travel through the blood to many organs, which makes it the largest endocrine organ in the body.
Story path
Chapter 1
Each contraction sends three signals
Exercise is more than burning calories. Every time a muscle contracts, it sends mechanical, electrical, and chemical signals at the same time.
Calcium ions are released from the sarcoplasmic reticulum (the membrane sacs that store calcium inside a muscle cell), and is used and remade much faster. , the cell's switch that senses an energy shortfall, is turned on by the energy stress; , the master switch for growth, senses the mechanical tension and the amino acids; and the nervous system recalibrates its output for the next movement.
So exercise is not simply wearing the body down; it gives the body an instruction that it needs to adapt. One situation is an emergency, though: if after training your muscles are severely painful and swollen and your urine turns tea-colored, it may be rhabdomyolysis (large numbers of muscle cells breaking down), and you should get medical care immediately.
Calcium ions are released from the sarcoplasmic reticulum (the membrane sacs that store calcium inside a muscle cell), and is used and remade much faster. , the cell's switch that senses an energy shortfall, is turned on by the energy stress; , the master switch for growth, senses the mechanical tension and the amino acids; and the nervous system recalibrates its output for the next movement.
So exercise is not simply wearing the body down; it gives the body an instruction that it needs to adapt. One situation is an emergency, though: if after training your muscles are severely painful and swollen and your urine turns tea-colored, it may be rhabdomyolysis (large numbers of muscle cells breaking down), and you should get medical care immediately.
Mechanism · What the three fiber types do
Skeletal muscle is not uniform. It contains three types of fiber, each with its own metabolism and its own rules for being called on:Type I (slow-twitch, oxidative): rich in mitochondria and capillaries, reddish, resistant to fatigue, and fueled by burning fat and glycogen with oxygen. A marathon runner's calf is the classic example.Type IIa (fast-twitch, oxidative and glycolytic): the in-between type, able to do both endurance and bursts. Middle-distance running and soccer lean on it.Type IIx (fast-twitch, glycolytic): fueled almost entirely by glycolysis, which does not need oxygen. It is the main engine of sprinting and lifting, and it tires very quickly.
The order in which fibers are called on follows Henneman's size principle: the nervous system first recruits small motor units made of Type I fibers (low threshold), then IIa, and calls on IIx only near maximal effort.
So only movements close to maximal effort, such as heavy lifting, sprinting, and jumping, use IIx; no amount of everyday walking or low-intensity aerobic exercise will reach these fibers. With age, the fast Type II fibers shrink more noticeably than Type I fibers, which is one reason older adults need strength training too.
Chapter 2
How nerves command muscle
A muscle cannot decide to contract on its own; the motor neuron is the commander. One motor neuron plus all the muscle fibers (muscle cells) it controls make up one motor unit.
So strength depends not only on how big the muscle is but also on the nervous system's ability to give commands: how many motor units it can call up at once, and how well they fire together. In the first few weeks of strength training, strength climbs fast, and it is mainly the nervous-system side that is improving.
So strength depends not only on how big the muscle is but also on the nervous system's ability to give commands: how many motor units it can call up at once, and how well they fire together. In the first few weeks of strength training, strength climbs fast, and it is mainly the nervous-system side that is improving.
Mechanism · How a nerve hands off to a muscle
The neuromuscular junction (NMJ) is where a nerve meets a muscle fiber, and it is one of the most reliable chemical synapses in the body. The handoff happens in a few steps:1. The nerve impulse (action potential) reaches the end of the motor nerve, and calcium ions rush in.
2. Small vesicles in the nerve ending release acetylcholine (ACh) into the gap between nerve and muscle.
3. Acetylcholine crosses the gap, about 50 nm wide, and binds to nicotinic receptors on the surface of the muscle fiber.
4. Sodium rushes into the muscle cell and the muscle membrane fires. The electrical signal travels down the transverse tubules (T-tubules) into the depths of the cell, the sarcoplasmic reticulum releases calcium, the muscle filaments slide past one another, and the muscle contracts.
From the release of acetylcholine at the nerve ending to the firing of the muscle membrane takes only about 0.5 milliseconds. Acetylcholine is then broken down and cleared by acetylcholinesterase (AChE), so the muscle does not keep contracting.
Clinical conditions tied to this junction:
Myasthenia gravis (MG): the body makes autoantibodies that attack the acetylcholine receptor, so the muscles grow weaker the more they are used. A drooping upper eyelid is the classic sign.Curare and muscle relaxants: they block acetylcholine receptors and are essential in surgical anesthesia.Organophosphate pesticides and sarin nerve gas: they block acetylcholinesterase, acetylcholine piles up, and the muscles keep twitching and cramping. Organophosphate poisoning is an emergency; get medical help immediately.Botulinum toxin (Botox): it stops acetylcholine from being released and paralyzes the muscle. It is used for wrinkles, excessive sweating, and migraine.
The strength gained in the first 4–6 weeks of training comes mainly from neural adaptation rather than thicker muscle fibers: more motor units are recruited, they fire more in sync, and there is less inhibition. That is why beginners do not look more muscular but lift more.
Chapter 3
Burning fat or burning sugar
At low intensity, fat makes up a large share of the fuel a muscle burns. As intensity rises, the contribution from muscle glycogen and blood sugar climbs quickly.
This is not because one fuel is better; it is because the power demand is different. Fat is like a slow furnace: huge stores, but slow output. Glycogen is like a battery: limited stores, but fast output.
One point of training is to make the body better at switching fuels across intensities, which is called better metabolic flexibility.
This is not because one fuel is better; it is because the power demand is different. Fat is like a slow furnace: huge stores, but slow output. Glycogen is like a battery: limited stores, but fast output.
One point of training is to make the body better at switching fuels across intensities, which is called better metabolic flexibility.
Mechanism · What the breathing ratio says you burn
In the lab, what you are burning right now is read from the respiratory exchange ratio (RER): the carbon dioxide you breathe out divided by the oxygen you take in (VCO₂ / VO₂).RER about 0.7: almost entirely fat (at rest, or the easiest intensities such as a stroll).RER about 0.85: roughly half fat and half carbohydrate (jogging).RER about 1.0: almost entirely carbohydrate (near the threshold of high intensity).RER above 1.0: anaerobic energy plus the body buffering the extra carbon dioxide; this cannot be kept up for long.
The idea of a fat-burning zone comes from this curve. The absolute amount of fat burned peaks at moderate intensity (about 60% of maximal oxygen uptake, or ); slower is not better, and neither is faster. But over the same 60 minutes of exercise, high intensity burns more total calories, even though the share from fat is lower. So doing only low-intensity exercise to lose fat gets the arithmetic wrong.
Training brings adaptations: endurance training adds mitochondria to the muscle, raises the activity of the enzymes that break down fat, and adds more , the transporter that carries glucose into muscle cells. As a result, RER is lower at the same intensity and glycogen is spared. That is the molecular basis of metabolic flexibility.
Chapter 4
Muscle sends chemical messages
More and more researchers regard skeletal muscle as a secretory organ. Working muscle releases a class of signaling molecules called myokines, such as interleukin-6 (), IL-15, vascular endothelial growth factor (VEGF), and pathways related to brain-derived neurotrophic factor (BDNF).
Some of these signals act inside the muscle itself. Others influence how much sugar the liver releases into the blood, how fat tissue mobilizes fat, immune regulation, and the growth of new blood vessels, and they may also affect the brain.
This is one explanation for why the benefits of regular exercise are so broad: it is not just that the muscle gets stronger on its own, but that the muscle is communicating with organs throughout the body.
Some of these signals act inside the muscle itself. Others influence how much sugar the liver releases into the blood, how fat tissue mobilizes fat, immune regulation, and the growth of new blood vessels, and they may also affect the brain.
This is one explanation for why the benefits of regular exercise are so broad: it is not just that the muscle gets stronger on its own, but that the muscle is communicating with organs throughout the body.
Mechanism · Where each myokine's signal goes
These are some of the more studied myokine signals. Most of the evidence comes from animal and cell experiments, and how much each one matters in people varies:(the kind that rises sharply during exercise): it prompts the liver to release more sugar and fat tissue to break down more fat, and inside the muscle it also takes part in fat burning. It is not the same thing as the IL-6 that stays high in chronic inflammation.IL-15: supports growth of the muscle itself; in animal experiments it is also tied to NK cells (natural killer cells) of the immune system.Irisin: in mice, it makes white fat behave more like heat-producing brown fat. Whether it does this in people, and how much, is still debated.BDNF: working muscle makes it too. According to the 2012 review by Pedersen and Febbraio, inside the muscle it takes part in fat burning. Exercise also raises BDNF in the hippocampus and helps mood, but whether that is the same story as the BDNF muscle makes is not settled.VEGF: promotes the growth of new capillaries.Cathepsin B: in mice, it is linked to more new neurons in the hippocampus.SPARC: in animal experiments it is linked to holding back colon tumors. In observational studies, people who exercise regularly have a lower risk of colon cancer; that is an association and does not show that SPARC is the cause.
So the saying that exercise is like a drug that acts on many organs has something to it: working muscle sends signals to many tissues at once. For the same reason, no single supplement can do what exercise does.
Chapter 5
Muscle grows during recovery
Real training adaptation does not happen in the gym; it happens in the 24–72 hours after training. Training causes tiny damage to muscle fibers and sets off a chain of stress signals. During recovery, muscle protein synthesis (MPS) outpaces breakdown, and only then does the muscle grow on balance.
So training without enough sleep or enough food loses much of its effect: the step where muscle is actually built depends on sleep and nutrition.
So training without enough sleep or enough food loses much of its effect: the step where muscle is actually built depends on sleep and nutrition.
Evidence · How much protein after training, and when
After a session of strength training, muscle protein synthesis (MPS) stays raised for 24–48 hours. That does not mean there is an eating window you must catch.How much per meal: experiments that measure synthesis over a few hours have been pooled to find a break point (the volunteers drank protein from a single source). For younger adults it is about 0.24 g per kilogram of body weight per meal (about 14 g at 60 kg); for adults over 65, about 0.40 g (about 24 g at 60 kg). Above that amount, synthesis no longer rises clearly, and more of the extra amino acids are burned as fuel (the Morton 2018 cites this result). Older adults need more because of anabolic resistance: the same serving of protein produces a weaker synthesis response in older muscle.
Leucine is the switch for synthesis: it turns on , the master valve for building protein. The 2013 review by Volpi and colleagues estimated that older adults need about 3 g of leucine per meal to stimulate synthesis, which corresponds to 25–30 g of high-quality protein; this is still an idea waiting to be tested in randomized trials. For reference, 25 g of whey protein contains about 2.5 g of leucine.
When to eat: there is no golden window after training that you must catch within 30 minutes; that is a marketing oversimplification. A protein-containing meal before training and another after it already covers the period around the session. Morton 2018 pooled 49 with 1,863 participants: the timing of supplementation, the dose right after exercise, and the protein source had little or no effect on muscle gained over the weeks; the daily total matters more.
How to split it: the same 100 g of protein, split into 3 meals of 30 g plus a 10 g snack, drives synthesis higher in experiments that measure it over a few hours than 50 g in the morning and 50 g in the evening. The consensus statement by Phillips and van Loon (2011) likewise spreads the day's protein over 3–4 meals of similar size. But for muscle gained over weeks, getting the total right matters more than how you split it.
Mechanism · What next-day soreness is
Delayed onset muscle soreness () appears 12–24 hours after training, is worst at 48–72 hours, and fades within 5–7 days.Where it comes from (the current understanding is that several things stack up, and the details are still debated):
It is not lactate. Lactate is cleared within 1–2 hours, and that is when the soreness is only starting.It is tied mainly to eccentric contractions (a muscle working while it is being lengthened, as when walking downstairs or lowering a barbell). These cause tiny damage at the Z-discs (the anchor points of the muscle filaments), and the surrounding connective tissue remodels.The inflammatory response that follows makes the C fibers that carry pain signals more sensitive.Inflammation of the nerves themselves and disturbed calcium handling may also play a part.
Practical principles:
Being very sore does not mean the training worked; people who have trained for years tend to get less sore.When the soreness is bad, some light activity often feels better than complete rest.Cold-water baths and ice packs ease the acute inflammation but may slow long-term muscle growth. In a controlled trial (Roberts 2015), the group that took an ice bath after every session for 12 weeks gained less muscle and strength than the group that did active recovery. They are fine during competition but worth using cautiously during a training block.Nonsteroidal anti-inflammatory drugs (, such as ibuprofen) ease the symptoms, but research has found that they can blunt protein synthesis after training, so they are not recommended for routine use.
Warning: extremely severe DOMS together with tea-colored urine and a spike in (CK) means rhabdomyolysis; get emergency care, because it can cause acute kidney injury. Common triggers are going straight back to heavy weights after a long layoff, and training in the heat while dehydrated.
Chapter 6
The weekly minimum
The most reliable floor for adults is not a complicated plan but two things (the 2018 US Physical Activity Guidelines):
Build up 150 minutes of moderate-intensity aerobic exercise a week, or 75 minutes of vigorous aerobic exercise.Do muscle-strengthening exercise on 2 days a week, covering the major muscle groups.
This is not a gym standard of looks; it is the public-health minimum. If you want to be stronger and faster and recover better, that is when periodized training, protein, carbohydrate, sleep, and recovery come in.
Build up 150 minutes of moderate-intensity aerobic exercise a week, or 75 minutes of vigorous aerobic exercise.Do muscle-strengthening exercise on 2 days a week, covering the major muscle groups.
This is not a gym standard of looks; it is the public-health minimum. If you want to be stronger and faster and recover better, that is when periodized training, protein, carbohydrate, sleep, and recovery come in.
Clinical · How older adults keep their muscle
Sarcopenia (the loss of muscle mass and strength with age) happens quietly. A common rough estimate is that adults lose 0.5–1% of their muscle mass a year, faster with age, reaching 2–3% a year after 70. Strength falls faster than muscle size, because the junctions between nerve and muscle also wear down. It is one of the major reasons older adults fall, break bones, and lose the ability to live independently.Two things work best together:
1. Enough protein at each meal. Older muscle responds less to protein (anabolic resistance), so each meal needs more:
Younger adults: about 0.24 g/kg per meal (about 14 g of protein at 60 kg).Adults over 65: about 0.40 g/kg per meal (about 24 g at 60 kg). This is the break point cited in the Morton 2018 .In short-term experiments that measure synthesis, spreading protein evenly over three meals (for example 30 g each) beat a lopsided split such as 10 g at breakfast, 20 g at lunch, and 60 g at dinner.
2. Strength training. Without the training signal, protein is just building material piling up. Train at least 2 times a week, with a weight you can lift 8–12 times (8–12 RM), and increase the load gradually.
The two need each other: protein without training lacks the signal that tells muscle to build; training without enough protein has the signal but lacks the raw material.
So for older adults worried about getting weak, rather than just drinking more milk and taking more walks, the better plan is enough protein, plus picking up dumbbells.
References · 10
- Hargreaves, M., & Spriet, L. L. (2020). Skeletal muscle energy metabolism during exercise. Nature Metabolism, 2(9), 817–828. 10.1038/s42255-020-0251-4
- Morton, R. W., et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine, 52(6), 376–384. 49 RCTs, 1,863 participants, resistance training of 6 weeks or more. Protein supplementation added 2.49 kg to 1RM and 0.30 kg to fat-free mass; the effect fell with age and was larger in trained people. Break point for FFM gains at 1.62 g/kg/day (95% CI 1.03-2.20; 42 study arms, 723 participants; the biphasic model was not statistically significant, p = 0.079); given the CI, the authors say ~2.2 g/kg/day may be prudent for those maximising gains; timing, post-exercise dose and source play a minor if any role; they cite per-dose MPS break points of 0.24 (younger) and 0.40 g/kg (older). One author reports grant support from the US National Dairy Council (abstract and full text, PMC5867436). 10.1136/bjsports-2017-097608
- Volpi, E., et al. (2013). Is the optimal level of protein intake for older adults greater than the recommended dietary allowance? The Journals of Gerontology: Series A, 68(6), 677–681. Full text: the threshold dose of leucine for stimulating muscle protein synthesis in older adults appears to be approximately 3 g, corresponding to approximately 25-30 g of high-quality protein; the abstract concludes that new research is needed to establish older adults' protein needs (full text, PMC3660117). 10.1093/gerona/gls229
- Institute of Medicine. (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press. nap.nationalacademies.org/catalog/10490/dietary-reference-intakes-for-energy-carbohydrate-fiber-fat-fatty-acids-cholesterol-protein-and-amino-acids
- Pedersen, B. K., & Febbraio, M. A. (2012). Muscles, exercise and obesity: skeletal muscle as a secretory organ. Nature Reviews Endocrinology, 8(8), 457–465. 10.1038/nrendo.2012.49
- Whitham, M., & Febbraio, M. A. (2016). The ever-expanding myokinome: discovery challenges and therapeutic implications. Nature Reviews Drug Discovery, 15(10), 719–729. 10.1038/nrd.2016.153
- Phillips, S. M., & Van Loon, L. J. C. (2011). Dietary protein for athletes: from requirements to optimum adaptation. Journal of Sports Sciences, 29(sup1), S29–S38. Consensus opinion: leucine (possibly the other BCAAs) is prominent in stimulating MPS; 1.3-1.8 g/kg/day eaten as 3-4 isonitrogenous meals will maximise MPS; up to 1.8-2.0 g/kg/day, depending on the caloric deficit, may help prevent lean-mass loss during energy restriction (abstract, PMID 22150425). 10.1080/02640414.2011.619204
- Phillips, S. M. (2014). A brief review of higher dietary protein diets in weight loss: a focus on athletes. Sports Medicine, 44(Suppl 2), S149-S153. Argues that dietary protein should be the nutrient around which macronutrient changes are framed during weight loss: protein requirements to sustain lean mass rise in negative energy balance, and protein has satiety and fat-loss advantages, but raising it comes at the expense of another macronutrient. 10.1007/s40279-014-0254-y
- Roberts, L. A., Raastad, T., Markworth, J. F., Figueiredo, V. C., Egner, I. M., Shield, A., Cameron-Smith, D., Coombes, J. S., & Peake, J. M. (2015). Post-exercise cold water immersion attenuates acute anabolic signalling and long-term adaptations in muscle to strength training. The Journal of Physiology, 593(18), 4285-4301. Controlled trial (21 men, 12 weeks, 10 min cold water immersion vs 10 min low-intensity cycling after each session): quadriceps muscle mass (MRI, dominant leg) rose 103 +/- 71 g vs 309 +/- 73 g; leg press 1RM rose 133 +/- 43 kg vs 201 +/- 65 kg; knee extension 1RM 17.8 vs 33.8 kg. Abstract: cold water immersion attenuated long term gains in muscle mass and strength. 10.1113/JP270570
- U.S. Department of Health and Human Services. (2018). Physical Activity Guidelines for Americans (2nd ed.). health.gov/paguidelines/second-edition/pdf/Physical_Activity_Guidelines_2nd_edition.pdf