Our Muscular System

Your muscular system is what gives you the ability to move, the strength to do work, and a substantial fraction of your metabolic capacity. It's the largest organ system in your body by mass, roughly 35-40% of your total body weight in lean adults, and it's the system that responds most dramatically to how you train, eat, and age.

Muscle is one of the biggest determinants of healthspan. Muscle mass and strength independently predict mortality, metabolic resilience, and how well you age into your 60s, 70s, and beyond. It's metabolically active tissue that regulates glucose disposal, and contracting muscle releases signalling molecules that affect the brain, immune system, and fat tissue.

The three types of muscle

Your body has three distinct types, each built differently and each doing a different job:

Skeletal muscle is the muscle attached to your bones via tendons, and it's the only type you control consciously. Every voluntary movement, lifting a weight, walking, typing, blinking, comes from skeletal muscle contracting and pulling on a bone. It makes up roughly 95% of the muscle mass in your body, and it's the type the rest of this page focuses on.

Smooth muscle lines the walls of your blood vessels, gut, airways, bladder, and other hollow organs. You don't consciously control it. It contracts and relaxes automatically to regulate blood vessel diameter, move food through the digestive tract, and handle a range of other involuntary functions.

Cardiac muscle is found only in the heart. Like smooth muscle, it operates automatically. But it has some properties unique to itself: it's striated like skeletal muscle (built for strong, coordinated contractions). I contracts roughly 100,000 times per day for your entire life without rest.

The rest of this page is about skeletal muscle, the type you can train and modify directly.

How muscle contracts

Two protein filaments physically slide past each other, powered by ATP and triggered by calcium, and that sliding produces every movement you've ever made.

Sarcomeres

Sarcomeres are linked end-to-end inside each muscle fibre, billions of them in a row. When all the sarcomeres in a fibre shorten at the same time, the whole fibre shortens. Multiply that across all the fibres in a muscle, and the muscle visibly contracts.

Inside each sarcomere, there are two types of protein filament arranged in a specific pattern:

Actin (thin filaments), anchored at each end of the sarcomere

Myosin (thick filaments), sitting in the middle, with hundreds of small heads extending toward the actin filaments on either side

The whole arrangement looks like two combs facing each other, with the myosin heads acting as the teeth reaching between them. When the muscle gets the signal to contract, those myosin heads physically grab the actin filaments and pull them inward, which shortens the sarcomere. That's the entire contraction mechanism, scaled up across billions of sarcomeres.

The contraction

The contraction is called the sliding filament model. The filaments themselves don't change length, they just slide past each other. The muscle shortens because the overlap between the filaments increases.

A single myosin head can perform this pull-release cycle roughly five times per second under load, and there are thousands of them in each sarcomere. That's how rapid, coordinated force generation happens at the molecular scale.

Calcium

Muscle doesn't contract until it gets a calcium signal.

Muscle is built to stay relaxed by default. The myosin heads can't grab onto the actin filaments because there's a physical blocker sitting on top of the actin (a protein called tropomyosin) covering all the spots where myosin would attach.

Calcium is what removes the blocker. When your brain tells a muscle to contract, the signal travels down a nerve to the muscle fibre and triggers a flood of calcium ions inside the muscle cell. The calcium binds to the blocker and physically pulls it out of the way, exposing the attachment spots on the actin. The myosin heads can now grab on, and contraction begins.

When the brain stops sending the signal, the muscle cell rapidly pumps the calcium back into storage. The blocker slides back over the actin. Myosin can no longer grab on. The muscle relaxes.

The whole system is built around removing a brake. This is why severe calcium imbalances cause muscle cramps or weakness.

ATP

Each myosin head needs ATP to perform one pull-release cycle. Specifically:

ATP binds to the myosin head, which releases its grip on actin.

The ATP is broken down, which cocks the myosin head back into a "loaded" position.

The cocked head attaches to a new spot on actin.

The head pulls (the "power stroke"), and the cycle starts again.

Without ATP, the cycle stops. The myosin heads stay attached to the actin without being able to release. This is what causes freezing the muscles in place upon death.

During exercise, the muscle uses ATP faster than it can be replaced by normal aerobic metabolism. The body has three backup systems to keep up:

Creatine can rapidly regenerate ATP from ADP. This is the fastest source, but the stores last only about 10 seconds of all-out effort. It's the system powering short, explosive movements like a heavy lift or a sprint start. Supplementing creatine increases the size of this reserve

Anaerobic glycolysis breaks down glucose without oxygen, producing ATP fast but only in small amounts per glucose molecule. This is the dominant system during high-intensity efforts lasting 30 seconds to 2 minutes, and it generates lactate as a byproduct

Aerobic metabolism in the mitochondria produces the most ATP per fuel molecule (about 30 per glucose) but is slower to ramp up. It dominates during sustained efforts lasting more than a few minutes, and it can use either glucose or fatty acids as fuel

Fiber types

Not all muscle fibres behave the same way. Skeletal muscle contains two main types, optimised for very different jobs.

Slow-twitch (Type I)

Slow-twitch fibres are built for endurance. They contract more slowly than fast-twitch fibres, generate less peak force, but can keep working for hours without fatiguing. The reason is metabolic: slow-twitch fibres are packed with mitochondria and run almost entirely on aerobic metabolism, burning glucose and fatty acids with oxygen to produce a steady supply of ATP. They also contain high levels of myoglobin, the oxygen-storing protein that gives these fibres their characteristic red colour.

These are the fibres doing the work during long-distance running, cycling, swimming, or any activity that requires sustained output over time. The muscles in your back and core that hold your posture all day are also predominantly slow-twitch, they need to keep firing for hours.

Fast-twitch (Type II)

Fast-twitch fibres are built for power. They contract faster than slow-twitch fibres, generate more peak force, but fatigue quickly. They contain fewer mitochondria and rely heavily on anaerobic metabolism, breaking down glucose without oxygen for fast ATP production. This is why hard sets and sprints leave you breathless and burning, your fast-twitch fibres are producing lactate as a byproduct of their fuel system.

Fast-twitch fibres are actually a spectrum. The further along the spectrum you go, the faster and more powerful the fibres get, but the less fatigue-resistant they become:

Type IIa (intermediate) sits between slow-twitch and pure fast-twitch. Reasonably fast, reasonably powerful, and more fatigue-resistant than pure fast-twitch. Endurance training shifts fibres in this direction

Type IIx (pure fast-twitch) is the most powerful and fastest-contracting, but fatigues fastest. Powerlifting, sprinting, and explosive sports favour these fibres

Fast-twitch fibres are responsible for jumping, sprinting, lifting heavy, and any movement requiring rapid force production. They're also the fibres that respond most dramatically to resistance training, hypertrophy.

Why fibre type matters

The fibre composition of your muscles is mostly determined by genetics. Most people sit somewhere around 50/50 slow to fast-twitch, but the variation between individuals is enormous. Elite distance runners often have 70-80% slow-twitch fibres in their leg muscles. Elite sprinters often have 70-80% fast-twitch. This isn't because their training shifted them that far, it's because they were born close to those ratios and self-selected into the sports where they could excel.

You can shift your fibre composition somewhat through training, but the shifts are modest and happen mostly along the fast-twitch spectrum (Type IIx becoming Type IIa with endurance training). You can't meaningfully convert slow-twitch fibres into fast-twitch ones.

What you can do is make whatever fibres you have stronger, larger, and more metabolically efficient. A slow-twitch-dominant lifter can still build significant muscle and strength, they just take longer to recover from heavy training and may never be able to sprint as fast as someone with more fast-twitch fibres.

A mix of strength training, hypertrophy work, and conditioning develops both types regardless of your starting composition. The genetics determine your ceiling in extreme specialisation.

Muscle as an endocrine organ

For most of the 20th century, muscle was considered a purely mechanical tissue: something that contracts when you tell it to, gets bigger when you train it, and otherwise doesn't do much. That view is now known to be wrong. Working muscle releases dozens of signalling molecules that travel through the bloodstream and act on the brain, immune system, fat tissue, and other organs. Muscle is one of the largest endocrine organs in your body, alongside the thyroid, adrenals, and pancreas.

These signalling molecules released by contracting muscle are called myokines, and they're a reason why exercise produces effects far beyond muscle itself.

What myokines actually do

Contracting muscle releases over 600 distinct myokines, with new ones still being identified. Most of them are released only during or shortly after muscular contraction, which means a sedentary body simply doesn't produce them:

IL-6 from muscle. This is the same molecule that shows up as a chronic inflammation marker in bloodwork, but the role is completely different depending on where it comes from. Chronic IL-6 from inflamed visceral fat or low-grade systemic inflammation is harmful. Acute IL-6 released by contracting muscle is anti-inflammatory and metabolically beneficial. Muscle-derived IL-6 spikes during exercise and drops back to baseline within hours, and during that spike it improves insulin sensitivity, mobilises fat for energy, and signals the immune system to reduce inflammation elsewhere in the body.

BDNF (brain-derived neurotrophic factor).Exercise causes muscle to release BDNF into the bloodstream, and the brain takes it up. BDNF is the master signal for neuroplasticity, the formation of new neurons and new connections between existing ones. This is one of the main mechanisms behind exercise improving learning, memory, and protecting against age-related cognitive decline. Both endurance and resistance training reliably raise BDNF, with higher intensity sessions producing larger spikes.

Irisin. Released by working muscle, irisin travels to fat tissue and triggers the conversion of energy-storing white fat into energy-burning brown fat. Brown fat is metabolically active, generating heat and burning calories continuously. This is one mechanism behind why consistent training increases resting metabolic rate even outside the gym.

Cathepsin B. Crosses the blood-brain barrier during exercise and contributes to hippocampal neurogenesis (the birth of new memory-related neurons). Another mechanism linking exercise to cognitive function.

Myostatin. This one runs in the opposite direction. Myostatin is released by muscle and tells muscle to stop growing, it's a negative feedback signal that limits muscle size. Resistance training transiently reduces myostatin release, which is part of how muscle grows.

This is also why muscle mass is such a strong predictor of mortality and healthspan, more muscle that's actively used means more myokine production and more downstream signalling across every other system.

Sarcopenia

Sarcopenia is the progressive loss of muscle mass and strength with age. It's one of the strongest predictors of mortality, disability, and loss of independence in older adults.

Starting around age 30, you lose roughly 1% of muscle mass per year if you do nothing. After 60, the rate accelerates to 1.5-2% per year. Strength declines even faster than mass, the muscle that remains gets weaker per unit of size. By the time someone reaches their 70s without intervention, they often have less than half the muscle and a third of the strength they had at 25.

Resistance training and adequate protein can flatten the curve substantially, and in many cases reverse it.

Loss of muscle is one of the most consequential variables in how someone ages:

Falls and fractures. Weak muscles can't catch you when you stumble. Hip fractures in older adults often start a downward spiral.

Independence. Standing up from a chair, climbing stairs, carrying groceries, all of these require a baseline of strength.

Metabolic health. Muscle is the largest disposal site for blood glucose. As muscle mass shrinks, insulin resistance worsens, blood sugar control deteriorates, and the risk of type 2 diabetes climbs.

All-cause mortality. Grip strength and leg strength in middle age and beyond are among the strongest predictors of how long someone will live.

Myokine signalling. Less muscle means less myokine release, which means weaker cognitive, metabolic, and anti-inflammatory signalling across the body.

Several mechanisms work together to drive sarcopenia, none of them are inevitable, but most of them get worse if you do nothing:

Anabolic resistance. Older muscle responds less to dietary protein and training than younger muscle does.

Hormonal decline. Testosterone, growth hormone, IGF-1, and oestrogen all decline with age, and all of them support muscle maintenance.

Mitochondrial decline. Aging muscle has fewer, less efficient mitochondria, which limits both training capacity and metabolic function.

Satellite cell exhaustion. The stem cells that repair and grow muscle become less responsive with age, and the pool shrinks.

Chronic inflammation. Sustained low-grade inflammation suppresses muscle protein synthesis and promotes muscle breakdown directly through TNF-α and IL-6

All of these matter, but the largest factor by far is simply not using your muscles. Older adults move less, so their muscles atrophy. Most "age-related" muscle loss is actually inactivity-related muscle loss happening to people who got older.

Sarcopenia is one of the most reversible aging mechanisms:

Resistance training. Single largest intervention by a wide margin. Lifting heavy at least twice per week reverses sarcopenia. Studies on 70 and 80 year-olds who start lifting consistently show measurable gains in muscle mass and strength. Muscle that gets loaded regularly maintains itself. Muscle that doesn't, atrophies.

Protein intake. Anabolic resistance means older adults need more protein per meal than younger adults to trigger the same muscle protein synthesis response. The typical recommendation rises from roughly 0.8g/kg/day (the bare minimum to avoid deficiency) to 1.6-2.2g/kg/day for active older adults.

Hormonal status. Testosterone, growth hormone, IGF-1, and thyroid hormones all support muscle maintenance, and all of them tend to drift downward with age. Maintaining adequate levels through whatever means available significantly affects the rate of muscle loss.

The other levers (sleep, addressing inflammation, adequate vitamin D, creatine supplementation, omega-3 intake) all contribute meaningfully but are secondary to the first three. None of them work without training and adequate protein in place.