Place · Level 3
Muscle System
肌肉不是只会收缩 · 它是代谢炉、内分泌器官、神经训练场
Story path
Chapter 1
Contraction is signal
Contraction is signal
Exercise isn't 'burning calories' — that's the shallow read. Every muscle contraction sends mechanical, electrical, and chemical signals simultaneously.
Calcium ions release from the sarcoplasmic reticulum, adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it. turnover spikes, AMP-activated protein kinase: The cell's 'low fuel' sensor — switches on when energy is low to make energy and pause building. senses energy stress, mechanistic target of rapamycin: The cell's master 'grow / build' switch — turned on by enough protein and resistance training. senses mechanical tension and amino acids, and the nervous system recalibrates motor output.
So exercise isn't depleting the body — it's giving the body a 'need to adapt' command.
Calcium ions release from the sarcoplasmic reticulum, adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it. turnover spikes, AMP-activated protein kinase: The cell's 'low fuel' sensor — switches on when energy is low to make energy and pause building. senses energy stress, mechanistic target of rapamycin: The cell's master 'grow / build' switch — turned on by enough protein and resistance training. senses mechanical tension and amino acids, and the nervous system recalibrates motor output.
So exercise isn't depleting the body — it's giving the body a 'need to adapt' command.
Fiber types
Skeletal muscle isn't uniform — there are three fiber types, each with different metabolic and recruitment rules:Type I (slow oxidative, slow-twitch): rich in mitochondria and capillaries, red muscle, fatigue-resistant, uses fat and glycogen oxidation. The marathon runner's calf is the archetype.Type IIa (fast oxidative-glycolytic): intermediate, capable of both endurance and burst. Mid-distance running, soccer.Type IIx (fast glycolytic): nearly all anaerobic glycolysis, the explosive workhorse for sprints and lifting, very easily fatigued.
Recruitment follows Henneman's size principle: the nervous system fires Type I small motor units first (low threshold), then IIa, and only at near-maximal effort recruits IIx.
So 'without lifting weights you can't engage IIx' — daily walking and low-intensity aerobic work, no matter how much, won't activate this fiber type. In the elderly, IIx atrophies 2–3× faster than Type I, which is why strength training matters more than aerobic for fall prevention.
Chapter 2
Neuromuscular junction
Neuromuscular junction
Muscle can't decide to contract on its own — the motor neuron is the commander. One motor neuron + all the muscle fibers it innervates = one motor unit.
So 'strength' is half muscle and half the nervous system's ability to send commands.
So 'strength' is half muscle and half the nervous system's ability to send commands.
The 0.5 ms handshake
The neuromuscular junction (NMJ) is the body's fastest and most reliable chemical synapse:1. Action potential reaches the motor nerve terminal → Ca²⁺ rushes in
2. Synaptic vesicles release acetylcholine (ACh) into the synaptic cleft
3. ACh crosses the ~50 nm cleft and binds nicotinic receptors on the muscle fiber
4. Na⁺ rushes into the muscle cell → end-plate potential → muscle membrane AP → T-tubule → SR releases Ca²⁺ → myofilament sliding
The whole process is < 0.5 ms. ACh is then cleaved and cleared by acetylcholinesterase (AChE) to prevent sustained contraction.
Clinical NMJ stories:
Myasthenia gravis (MG) — autoantibodies against the ACh receptor → the more you use it the weaker it gets; classic ptosis (drooping eyelid)Curare / muscle relaxants — block ACh receptors → essential in surgical anesthesiaOrganophosphate pesticides / sarin gas — inhibit AChE → ACh accumulates → sustained convulsionsBotulinum toxin (Botox) — prevents ACh release → muscle paralysis (wrinkle treatment / hyperhidrosis / migraine)
Early strength-training gains (first 4–6 weeks) are nearly all neural adaptation, not muscle fiber growth: more motor units recruited, more synchronous firing, less inhibition. This is why beginners 'don't look bigger but lift more'.
Chapter 3
Fuel switching
Fuel switching
At low intensity, fat oxidation accounts for a larger share; as intensity rises, muscle glycogen and blood glucose contributions climb sharply.
It isn't that one fuel is 'better' — it's that power demand differs. Fat is a slow furnace: large stores but slow output; glycogen is a battery: limited stores but fast output.
One meaning of training is making the body better at switching fuels across intensities — better metabolic flexibility.
It isn't that one fuel is 'better' — it's that power demand differs. Fat is a slow furnace: large stores but slow output; glycogen is a battery: limited stores but fast output.
One meaning of training is making the body better at switching fuels across intensities — better metabolic flexibility.
RER
In the lab, 'what are you burning right now' is measured by RER (respiratory exchange ratio) = VCO₂ / VO₂:RER ≈ 0.7 = almost pure fat (rest / walking / Zone 1)RER ≈ 0.85 = 50/50 fat and carb (jogging)RER ≈ 1.0 = almost pure carb (near high-intensity threshold)RER > 1.0 = anaerobic + CO₂ buffering, not sustainable
The 'fat-burning zone' myth comes from this curve — absolute fat oxidation peaks at moderate intensity (~60% VO₂max), not 'slower is better' or 'faster is better'. But for the same 60-minute workout, high intensity burns more total calories even if fat percentage is lower — so 'only doing low intensity to lose fat' miscounts the books.
Training adaptation: endurance training increases mitochondria + β-oxidation enzyme activity + GLUT4 upregulation, so at the same intensity RER drops (sparing glycogen) — the molecular basis of metabolic flexibility.
Chapter 4
Myokines
Myokines
Skeletal muscle is now considered an endocrine organ. Contracting muscle releases myokines: interleukin-6: A pro-inflammatory signal molecule (cytokine) released by immune cells during inflammation., IL-15, VEGF, BDNF-related pathways, and more.
These signals influence hepatic glucose output, fat tissue mobilization, immune regulation, angiogenesis, and brain plasticity.
This explains why the benefits of regular exercise are so broad — not because muscle alone gets stronger, but because muscle communicates with organs across the body.
These signals influence hepatic glucose output, fat tissue mobilization, immune regulation, angiogenesis, and brain plasticity.
This explains why the benefits of regular exercise are so broad — not because muscle alone gets stronger, but because muscle communicates with organs across the body.
Target map
Molecularly validated myokine → target organ signals:interleukin-6: A pro-inflammatory signal molecule (cytokine) released by immune cells during inflammation. (exercise-type, acute) → liver raises glucose output + fat tissue promotes lipolysis; entirely different context-effect from chronic inflammatory IL-6IL-15 → supports muscle itself for hypertrophy + NK cells for anti-tumor surveillanceIrisin → white fat browning, increasing thermogenesis; effect magnitude in humans still debated but supported by structural biologyBDNF (exercise-induced) → hippocampus neural plasticity + antidepressant effect; the molecular root of 'running fights depression', with strong RCT evidenceVEGF → capillary new growthCathepsin B → hippocampal new neuron proliferationSPARC → colon anti-tumor surveillance; epidemiologically, regular exercisers see ~25% lower colon cancer risk
So 'exercise is a multi-organ drug' isn't rhetoric — the molecular mechanism is dozens of myokines simultaneously sending commands to a dozen tissues. It's also why no single supplement can do what exercise does.
Chapter 5
Recovery: where training happens
Recovery: where training happens
Real training adaptation doesn't happen in the gym — it happens in the 24–72 hours after. Training creates muscle fiber micro-damage + stress signals; during recovery, when protein synthesis (MPS) exceeds breakdown, muscle net grows.
This is why 'only training without sleeping / only training without eating' equals training in vain.
This is why 'only training without sleeping / only training without eating' equals training in vain.
MPS window: time × dose
Muscle protein synthesis (MPS) opens after training and stays elevated 24–48 hours (Phillips 2014). Protein supplementation during this window:Per-meal dose threshold:
Young adults: ~0.25–0.3 g/kg/meal (60 kg → ~18 g) is enough to maximize MPSElderly (> 65): threshold rises to ~0.4 g/kg/meal (60 kg → ~24 g) — the anabolic resistance phenomenonA single meal beyond ~40 g shows diminishing returns (extra protein is oxidized for energy and doesn't enter muscle)
Leucine (Leu) is the trigger: ~2–3 g Leu per meal activates mTOR complex 1: The main working form of mTOR — the switch that directly drives protein synthesis. → S6K1 → the translation machinery. Whey protein contains ~2.5 g Leu per 25 g — a naturally appropriate dose.
Timing: any time within 3 hours post-training works — the '30-minute golden window' is marketing oversimplification. But 24-hour cumulative total protein matters more than single-dose timing.
Distribution > total: the same 100 g of protein split as 50 g morning + 50 g evening is worse than 3 meals × 30 g + a 10 g snack, because MPS can be re-activated every ~3–4 hours and overcrowded doses miss the intermediate pulses.
DOMS
Delayed Onset Muscle Soreness (DOMS) appears 12–24 h after training, peaks at 48–72 h, and resolves within 5–7 days.Mechanism (modern understanding):
Not lactate — lactate clears within 1–2 hours, and DOMS only starts thenPrimarily Z-disk micro-damage from eccentric contractions + extracellular matrix (ECM) remodelingSecondary inflammatory response activates C-fiber nociceptorsAlso involves neurogenic inflammation + abnormal calcium-ion regulation
Practical principles:
DOMS severity doesn't equal training effectiveness — experienced athletes get less DOMSWhen DOMS is severe, active recovery (mild activity) > complete rest — increases blood flow and speeds clearanceCold water immersion / ice packs reduce acute inflammation, but may suppress long-term hypertrophy (Roberts 2015) — usable in competition; caution during trainingNSAIDs (ibuprofen etc.) reduce symptoms but suppress protein synthesis — not recommended for routine use
Warning: extremely severe DOMS + brown urine + creatine kinase (CK) spiking = rhabdomyolysis — emergency care; can cause acute kidney injury. Common triggers: returning to heavy weights after long inactivity, training in heat with dehydration.
Chapter 6
Minimum effective dose
Minimum effective dose
The most reliable adult floor isn't a complex plan, just two things:
Accumulate 150 minutes of moderate-intensity aerobic per week, or 75 minutes of vigorous2 days per week of strength training covering major muscle groups
This isn't a gym aesthetic standard — it's the public-health minimum. Want to be stronger, faster, recover better? Then talk periodization, protein, carbs, sleep, and recovery.
Accumulate 150 minutes of moderate-intensity aerobic per week, or 75 minutes of vigorous2 days per week of strength training covering major muscle groups
This isn't a gym aesthetic standard — it's the public-health minimum. Want to be stronger, faster, recover better? Then talk periodization, protein, carbs, sleep, and recovery.
Sarcopenia
Sarcopenia — after age 50, muscle mass loss runs 0.5–1% per year, with strength loss faster (~2–3%/year, because neuro-muscular unit degradation precedes fiber loss). After age 70, falls, fractures, and loss of independent living trace back primarily to this.Two interventions work only together:
1. Protein threshold — elderly have a raised single-meal Leu concentration threshold (anabolic resistance):
Young adults: ~0.25 g/kg/meal (60 kg = 15 g protein)Elderly (> 65): ~0.4 g/kg/meal (60 kg = 24 g) — ESPEN / PROT-AGE consensus3 meals evenly distributed beats 10 g morning / 20 g noon / 60 g evening
2. Resistance training — without the training signal, protein is just material piling up. ≥ 2 times per week, 8–12 RM intensity, progressive overload.
These two don't work in isolation: supplementing protein without training = feeding muscle that doesn't exist; training without enough protein = signal without raw material.
The prescription for 'elderly people afraid of weakness' is 'eat enough protein + pick up dumbbells', not 'drink more milk + take more walks'.