FSHP Academy
Exercise Physiology — all lessons

Muscle Physiology

Muscle physiology: excitation–contraction and the sliding filament modelConcentric, eccentric and isometric muscle actionsMotor units, recruitment and rate codingNeuromuscular adaptations to training

Energy Systems

Energy systems overview: how the body makes ATPThe ATP-PC (phosphagen) systemAnaerobic glycolysis and lactateThe aerobic oxidative system

Cardiovascular Physiology

Heart anatomy and the cardiac cycleStroke volume, heart rate and cardiac outputBlood components and oxygen transport

Aerobic Fitness and Intensity

Aerobic fitness concepts and adaptationsVO₂ maxLactate thresholdHeart rate zones and RPE-based intensity control

Muscle physiology: excitation–contraction and the sliding filament model

How a nerve signal becomes muscle force — from the neuromuscular junction to calcium release, cross-bridge cycling and the length–tension relationship.

Intermediate Evidence: Established consensus 5 min read Reviewed 2026-10-31 Status: draft

Learning objectives

  • Describe the structural hierarchy from whole muscle to sarcomere
  • Explain each step of excitation–contraction coupling
  • Describe the cross-bridge cycle and the role of ATP and calcium
  • Apply the length–tension relationship and the size principle to exercise

Before this lesson

Why this matters

Every rep, sprint and rehabilitation exercise depends on one microscopic event repeated billions of times: a myosin head grabbing an actin filament and pulling. Understanding that event explains why muscles fatigue, why range of motion changes strength, why early strength gains are largely neural, and why eccentric work feels different. It is the foundation for the lessons on muscle actions and energy systems.

From muscle to sarcomere

Skeletal muscle is organised in nested layers, each wrapped in connective tissue [openstax2022muscle]:

  1. Whole muscle (wrapped in epimysium), joined to bone by tendon.
  2. Fascicles — bundles of fibres (perimysium).
  3. Muscle fibres — single, long, multinucleated cells (endomysium).
  4. Myofibrils — chains of contractile units running the length of the fibre.
  5. Sarcomeres — the basic contractile unit, bounded by Z-discs.

Inside each sarcomere, thin filaments (mainly actin, with the regulatory proteins troponin and tropomyosin) anchor to the Z-discs, and thick filaments (myosin) sit in the middle, anchored at the M-line. The overlapping arrangement creates the striped (striated) appearance under a microscope.

The sliding filament model

In 1954 two independent research pairs published, in the same issue of Nature, the observation that during contraction the A-band stays constant while the I-band narrows [huxley1954] [hhuxley1954]. The interpretation — filaments do not shorten; they slide past each other — is still the accepted model.

Sarcomere structure, relaxed and contracted Two sarcomeres are drawn between Z-discs. Thin actin filaments attach to each Z-disc and point toward the centre; thick myosin filaments sit in the middle, anchored at the M-line, with cross-bridge heads. In the contracted sarcomere the Z-discs are closer together, the actin filaments slide further over the myosin, the I-band and H-zone shorten, while the A-band (length of myosin) stays the same. Relaxed (longer sarcomere) A-band = myosin length (260 units — unchanged) ½ I-band H-zone Z-disc Z-disc Contracted (shorter sarcomere) A-band = myosin length (260 units — unchanged) ½ I-band H-zone Z-disc Z-disc Actin (thin) Myosin (thick) M-line = dashed centre line. Filaments do not shorten — they slide.
Figure 1. A sarcomere relaxed and contracted. Actin and myosin keep their length; the Z-discs are pulled toward the M-line, so the I-band and H-zone shorten while the A-band does not. — Original diagram, FSHP Academy (schematic, not to scale).

Excitation–contraction coupling

“Excitation–contraction coupling” is the chain of events linking an electrical signal to mechanical force [openstax2022muscle]:

  1. Nerve signal. An action potential travels down a motor neuron to the neuromuscular junction.
  2. Transmission. The neuron releases acetylcholine (ACh), which binds receptors on the muscle membrane (sarcolemma) and depolarises it.
  3. Spread. The action potential travels along the sarcolemma and down T-tubules into the fibre.
  4. Calcium release. Voltage sensors in the T-tubule (dihydropyridine receptors) mechanically open ryanodine receptors on the sarcoplasmic reticulum (SR), releasing Ca²⁺ into the cytoplasm.
  5. Unblocking actin. Ca²⁺ binds troponin C; troponin shifts tropomyosin, uncovering myosin-binding sites on actin.
  6. Cross-bridge cycling begins (next section) and continues while Ca²⁺ and ATP are available.
  7. Relaxation. When nerve stimulation stops, ACh is broken down by acetylcholinesterase, and SERCA pumps use ATP to return Ca²⁺ to the SR. Tropomyosin re-covers actin.

The cross-bridge cycle

Each myosin head works like a ratchet:

StepWhat happensEnergy
1. AttachmentEnergised myosin head binds exposed actinStored from previous ATP hydrolysis
2. Power strokeRelease of phosphate (Pi) triggers the head to pivot, pulling actin toward the M-line; ADP is releasedChemical → mechanical
3. DetachmentA new ATP binds and myosin lets go of actinATP binding
4. Re-cockingATP is hydrolysed to ADP + Pi, returning the head to its high-energy positionATP hydrolysis

Two practical consequences follow. First, ATP is needed to release myosin, which is why muscles become rigid after death (rigor mortis). Second, ATP demand during intense contraction is enormous, so the body must resynthesise it continuously — the subject of the energy systems lesson.

The length–tension relationship

The force a fibre can produce depends on how much actin and myosin overlap. Classic experiments on single frog fibres showed peak isometric force at an intermediate sarcomere length, with force falling at longer lengths (fewer cross-bridges can reach actin) and shorter lengths (filaments collide and interfere) [gordon1966].

In a whole human joint, the strength curve you feel also depends on moment arms, muscle architecture and the number of muscles contributing, so the sarcomere curve is only part of the explanation. This is why the hardest part of a lift does not always coincide with the muscle’s weakest length — see levers, torque and moment arms.

Grading force: motor units and the size principle

A motor unit is one motor neuron plus all the fibres it supplies. The nervous system grades force in two ways:

  • Recruitment — activating more motor units. Units are recruited in order of size, smallest (low-threshold, fatigue-resistant) to largest (high-threshold, high-force) — the size principle [henneman1957].
  • Rate coding — increasing firing frequency, so twitches summate into a smooth, stronger contraction (tetanus).

Fibre type properties (slow type I versus faster type II) vary along a spectrum rather than as neat categories [schiaffino2011]; they are covered in muscle fibre types.

Practical application

Common mistakes and misconceptions

  • “The filaments shorten.” They slide; their lengths stay constant.
  • “Calcium binds myosin.” In skeletal muscle, Ca²⁺ binds troponin on the thin filament.
  • “Muscle turns into fat if you stop training.” They are different tissues; muscle can shrink (atrophy) and fat mass can increase, but one does not convert into the other.

Case study

Client: Daniel, 52, new to resistance training, asks why his leg press numbers rose 30% in four weeks while his thighs look the same.

Explanation the trainer gives: In the first weeks, the nervous system learns to recruit more motor units, fire them faster and co-ordinate the muscles involved. That improves force without much change in muscle size. Measurable growth (hypertrophy) generally takes longer to become visible. The trainer uses this to set expectations and to keep him progressing load gradually, linking to progressive overload.

Further learning: video lessons

Video links are only marked “verified” after a human has watched them. Unverified entries open a YouTube search for that channel and topic rather than a guessed link.

Introductory needs verification — opens a YouTube search

Muscles: muscle cells and the sliding filament model (overview)

Crash Course

You will understand: the big picture of how sarcomeres shorten

Why: Clear animated introduction before the detail below.
Technical needs verification — opens a YouTube search

Excitation–contraction coupling, step by step

Ninja Nerd

You will understand: every molecular step from neuromuscular junction to relaxation

Why: Long-form whiteboard lecture with full mechanistic detail.
Practical demo needs verification — opens a YouTube search

Length–tension relationship and exercise

Physiotutors

You will understand: how muscle length affects strength across a range of motion

Why: Links the physiology to exercise and rehab choices.

Knowledge check

Knowledge check · 0/6

1. During contraction, which band of the sarcomere stays the same length?

Answer C. The A-band equals the length of the myosin (thick) filaments, which do not change. I-band and H-zone shorten as actin slides over myosin.

2. Calcium starts contraction by binding to…

Answer B. Ca²⁺ binds troponin C, which shifts tropomyosin away from myosin-binding sites on actin.

3. What is the immediate role of a new ATP molecule binding to the myosin head?

Answer C. ATP binding causes detachment; its hydrolysis then re-cocks the head. Without ATP, myosin stays bound (the basis of rigor mortis).

4. According to the size principle, during a gradually increasing effort…

Answer B. Henneman’s size principle — smaller, lower-threshold motor units are recruited before larger, higher-threshold ones.

5. Why is a muscle usually weaker at extreme lengths (very stretched or very shortened)?

Answer B. The length–tension relationship — at long lengths fewer cross-bridges can form; at short lengths filaments overlap and interfere.

6. Relaxation of skeletal muscle requires…

Answer B. SERCA pumps use ATP to return calcium to the SR, allowing tropomyosin to re-cover binding sites.

Short-answer & application questions

  1. List, in order, the steps between a motor neuron action potential and the first power stroke.
  2. Use the length–tension relationship to explain why the bottom position of a deep squat often feels hardest — and what other factors contribute.
  3. A client asks why they get stronger in the first weeks of training before their muscles look bigger. Which concepts from this lesson help you answer?

Key takeaways

  • Muscle force originates in sarcomeres, where myosin heads pull actin filaments toward the centre; filaments slide, they do not shorten.
  • A motor neuron action potential triggers calcium release from the sarcoplasmic reticulum; calcium binding to troponin uncovers binding sites on actin.
  • ATP is needed to detach myosin from actin and to pump calcium back — so ATP is required for relaxation as well as contraction.
  • Force depends on filament overlap (length–tension) and on how many motor units are recruited, from smallest to largest (size principle).
  • Exercise selection, range of motion and fatigue all trace back to these mechanisms.

Connected topics

physiology

Concentric, eccentric and isometric muscle actions

physiology

Motor units, recruitment and rate coding

anatomy

Muscle architecture: pennation, length and cross-sectional area

anatomy

Muscle fibre types

physiology

Energy systems overview: how the body makes ATP

References

  1. Huxley AF, Niedergerke R (1954). Structural changes in muscle during contraction: interference microscopy of living muscle fibres. Nature, 173:971–973. doi:10.1038/173971a0 Classic study check pending
  2. Huxley H, Hanson J (1954). Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation. Nature, 173:973–976. doi:10.1038/173973a0 Classic study check pending
  3. Gordon AM, Huxley AF, Julian FJ (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibres. Journal of Physiology, 184(1):170–192. doi:10.1113/jphysiol.1966.sp007909 Classic study check pending
  4. Henneman E (1957). Relation between size of neurons and their susceptibility to discharge. Science, 126(3287):1345–1347. doi:10.1126/science.126.3287.1345 Classic study check pending
  5. Schiaffino S, Reggiani C (2011). Fiber types in mammalian skeletal muscles. Physiological Reviews, 91(4):1447–1531. doi:10.1152/physrev.00031.2010 Narrative review check pending
  6. Betts JG, Young KA, Wise JA, et al. (2022). Anatomy and Physiology 2e — Chapter 10: Muscle Tissue. OpenStax, Rice University. · Link Textbook check pending

Educational content only — not medical advice. Consult a qualified clinician for diagnosis or treatment.