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Human Physiology I · Muscle Physiology

Sliding Filament Theory and the Cross-Bridge Cycle

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

The states that muscle shortens because thin (actin) filaments slide past thick (myosin) filaments toward the M line, shortening each sarcomere without the filaments themselves shortening. This sliding is powered by the cross-bridge cycle: an energized ("cocked") binds actin, pivots in the (releasing phosphate and ADP), and then detaches when a fresh ATP binds. ATP is required both to detach the cross-bridge and to re-cock the myosin head; without ATP (as in ) bridges cannot release. occurs when calcium is pumped back into the sarcoplasmic reticulum by the , allowing tropomyosin to re-cover actin.

Why this matters

The cross-bridge cycle is where fatigue and several muscle disorders converge. Sustained contraction and rigor reflect the ATP dependence of , and laboratory muscle preparations use ATP and calcium manipulations to isolate the steps of the cycle. Measurements of muscle force in response to controlled calcium and ATP (permeabilized-fiber experiments) are a standard research and diagnostic tool for evaluating contractile function. Note that diagnostic criteria, protocols, laboratory ranges, and scope of practice vary by institution and jurisdiction; these notes support education but do not replace clinical instruction or supervision. Concerning symptoms require evaluation by qualified clinicians or local emergency services.

The college version

1. The sliding filament theory and sarcomere shortening

The sliding filament theory holds that during contraction the thin filaments slide inward over the thick filaments toward the M line, so the Z discs are pulled closer together and the sarcomere shortens. Filament lengths do not change; instead the degree of overlap increases. This produces the classic : the I band and H zone shorten (and the H zone may disappear), while the A band, set by the thick filament's length, stays constant.

2. The cross-bridge cycle: bind, stroke, release

A is a myosin head attached to actin. The myosin head cycles through four states: (1) to the head causes it to release actin (detachment); (2) splits ATP into ADP and phosphate, and the released energy swings the head into a high-energy, "" position; (3) actin binding — the cocked head attaches to actin, and phosphate release triggers the power stroke; (4) the power stroke pivots the head, pulling the thin filament toward the M line, and ADP release completes the stroke. The head then stays bound until a new ATP binds and causes detachment, restarting the cycle.

3. ATP requirement, rigor, and relaxation

Every cycle consumes one ATP, so contraction has an absolute ATP requirement. If ATP is absent, cross-bridges cannot detach, and the muscle becomes rigid — the mechanism of rigor mortis. Relaxation is the reverse of activation: calcium is actively pumped back into the sarcoplasmic reticulum by the SERCA pump (sarcoplasmic/endoplasmic reticulum calcium ATPase), cytoplasmic calcium falls, troponin releases calcium, and tropomyosin re-covers actin so new cross-bridges cannot form.

How it works

  1. Calcium exposes actin's myosin-binding sites.
  2. A cocked myosin head (ADP plus phosphate) binds actin.
  3. Phosphate release triggers the power stroke; the head pulls actin toward the M line.
  4. ADP is released, and the head stays attached.
  5. ATP binds, causing detachment; ATP hydrolysis re-cocks the head.
  6. With calcium still high, the cycle repeats and the sarcomere shortens.
  7. When calcium falls (SERCA pump), tropomyosin re-covers actin and the muscle relaxes.

Common confusions

Do not confuseWithDifference
ATP bindingATP hydrolysisBinding detaches the bridge; hydrolysis re-cocks the head
Phosphate releaseADP releasePhosphate release starts the power stroke; ADP release ends it
Rigor mortisNormal sustained contractionIrreversible (no ATP) stiffness vs. ATP-fueled cross-bridge cycling
RelaxationPassive lengtheningRelaxation is active calcium removal, not just "letting go"
CalsequestrinSERCA pumpCalcium-buffering protein in the lumen vs. the pump that moves calcium in

Memory aids

"Bind, Stroke, Detach, Re-cock — ATP turns the lock." The head binds actin, strokes, detaches when ATP binds, and re-cocks when ATP is hydrolyzed. No ATP = no detach = rigor.

Quick review

Topic Recap

Muscle shortens by the sliding filament theory: thin filaments slide over thick filaments, shortening the sarcomere while filament lengths stay constant (I band and H zone shrink, A band constant). The cross-bridge cycle drives this — a cocked myosin head binds actin, phosphate release triggers the power stroke, ADP is released, and ATP binding detaches the head so ATP hydrolysis can re-cock it. Without ATP, detachment fails (rigor mortis). Relaxation returns calcium to the SR via the SERCA pump, with calsequestrin buffering the store.

Knowledge Check

  1. According to the sliding filament theory, what actually shortens during contraction?
  2. What two things does ATP do in the cross-bridge cycle?
  3. Which release event triggers the power stroke, and which ends it?
  4. Why does rigor mortis occur after death?
  5. How does the muscle relax once stimulation stops?

Answers and Rationales

  1. The sarcomere shortens as thin filaments slide past thick filaments toward the M line; the filaments themselves do not shorten. Rationale: the band changes (I band and H zone shrink, A band constant) prove sliding.
  2. ATP binding detaches the cross-bridge from actin, and ATP hydrolysis re-cocks the myosin head. Rationale: these two roles explain why ATP is needed both to continue and to stop contraction.
  3. Phosphate release triggers the power stroke; ADP release completes it. Rationale: the energy stored in the cocked head is released as phosphate leaves, and ADP follows.
  4. Without ATP, cross-bridges cannot detach from actin, so muscles lock in a stiff state. Rationale: detachment requires ATP binding, which ceases after death.
  5. The SERCA pump pumps calcium back into the sarcoplasmic reticulum, cytoplasmic calcium falls, tropomyosin re-covers actin's binding sites, and cross-bridge cycling stops. Rationale: relaxation is an active, calcium-removing process.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Picture a team rowing a boat. Each rower's oar dips into the water, pulls, and then lifts out to reach forward again. The myosin heads are the oars, actin is the water, and ATP is the rower's energy: it "cocks" the oar for the next stroke and lets the oar release from the water afterward. Hundreds of heads stroke, release, and re-grab in sequence, and the boat (the thin filament) slides along.

Where it stops being exact: in rowing the oar and water are separate, but in muscle the myosin head chemically binds actin, and each individual stroke moves the filament only a few nanometers. Also, the "release" step is not voluntary — it absolutely requires ATP, so when ATP runs out (as in rigor mortis) the oars stay stuck in the water and cannot let go.

Simple Example

Lifting a heavy book, holding it still, and slowly lowering it all use the same sliding filaments. During the lift, myosin heads stroke rapidly; if the ATP supply is interrupted, the muscle stiffens with cross-bridges locked onto actin — the same molecular event behind the stiffness of rigor mortis after death.

Worked example

  1. Calcium binds troponin, tropomyosin shifts, and myosin-binding sites on actin are exposed.
  2. A myosin head with ATP already hydrolyzed (holding ADP plus phosphate) is in the cocked, high-energy state; it binds actin, forming a cross bridge.
  3. Phosphate release triggers the power stroke: the head pivots toward the M line and drags the thin filament with it; ADP is then released. The direction of the force is toward the center of the sarcomere, which is why the sarcomere shortens.
  4. A new ATP molecule binds the head; this binding, not the energy of hydrolysis, is what causes detachment of the cross bridge from actin.
  5. The bound ATP is hydrolyzed to ADP plus phosphate, re-cocking the head so it is ready to bind again — as long as calcium and ATP are both present, the cycle repeats and the sarcomere keeps shortening.
  6. When stimulation stops, the SERCA pump returns calcium to the SR lumen (where calsequestrin buffers and concentrates it), tropomyosin re-covers actin, and the fiber relaxes.

Key takeaways

  • High yield: ATP has two jobs in the cycle — binding detaches the cross-bridge, and hydrolysis re-cocks the head. Rigor mortis occurs because without ATP there is no detachment.
  • High yield: The power stroke is triggered by phosphate release and completed by ADP release; it pulls the thin filament toward the M line.
  • The A band stays constant while the I band and H zone shrink — the signature of sliding.
  • Relaxation is active: the SERCA pump must use ATP to resequester calcium.
  • Calsequestrin concentrates calcium inside the SR, maintaining the gradient the pump works against.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • State the sliding filament theory and describe how sarcomere shortening and the band changes occur without the filaments themselves shortening.
  • List the steps of the cross-bridge cycle and the role of ATP binding and ATP hydrolysis in each.
  • Explain the events of the power stroke, including ADP release and phosphate release.
  • Relate detachment, the ATP requirement, rigor mortis, and the roles of the SERCA pump and calsequestrin to relaxation.

Key vocabulary

Sliding filament theory
Contraction occurs by filaments sliding, not shortening
Cross bridge
A myosin head bound to actin
Myosin head
The motor domain of myosin that binds actin and ATP
ATP binding
New ATP attaching to the head
ATP hydrolysis
ATP split into ADP plus phosphate
Cocked myosin head
High-energy head holding ADP plus phosphate
Actin binding
Cocked head attaching to actin
Power stroke
Head pivot that pulls actin toward the M line
ADP release
ADP leaving after the power stroke
Phosphate release
Phosphate leaving to trigger the power stroke
Detachment
Cross-bridge release triggered by ATP binding
ATP requirement
One ATP consumed per cross-bridge cycle
Rigor mortis
Stiffness from cross-bridges that cannot detach
Relaxation
Return to rest as calcium is removed
SERCA pump
SR calcium ATPase that resequesters calcium
Calsequestrin
Calcium-buffering protein in the SR lumen
Sarcomere shortening
Z discs pulled together as filaments slide
Band changes
I band and H zone shrink; A band constant

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