The power stroke in muscle contraction is caused by the release of inorganic phosphate from the myosin head, which triggers a large rotation of the myosin lever arm that pulls the actin filament forward. This single molecular event, repeated billions of times across your muscle fibers, is what generates every movement your body makes, from blinking to sprinting.
How Calcium Sets the Stage
Before the power stroke can happen, your muscle fibers need a green light from calcium. In a resting muscle, a protein called tropomyosin lies along the actin filament like a guard rope, physically blocking the spots where myosin heads need to attach. This is the “steric blocking model”: tropomyosin sits in a position that prevents myosin from grabbing onto actin, keeping the muscle relaxed.
When a nerve signal reaches the muscle, calcium floods into the cell and binds to troponin, a small protein complex attached to tropomyosin. This binding shifts tropomyosin deeper into the groove of the actin filament, uncovering the myosin binding sites. Each tropomyosin molecule spans seven actin subunits, so one shift exposes multiple attachment points at once. Without this calcium-triggered repositioning, myosin and actin can’t interact productively, and no power stroke occurs.
Cocking the Myosin Head
The power stroke doesn’t start from scratch. It requires a “cocked” myosin head, and getting there takes energy from ATP. The cycle begins with myosin tightly locked onto actin in what’s called the rigor state (the same state responsible for rigor mortis when ATP runs out). When a fresh ATP molecule binds, myosin releases from actin and its internal machinery goes to work.
ATP is then split into ADP and inorganic phosphate, but both products stay trapped inside the myosin head. This hydrolysis triggers a conformational change: a series of structural shifts cascade along an internal helix that runs from the ATP-binding pocket to the lever arm. The result is that the lever arm swings backward, displacing the myosin head by about 5 nanometers into its cocked, pre-stroke position. Think of it like pulling back a spring. The energy from ATP is now stored as mechanical strain in the protein’s shape.
What Triggers the Stroke Itself
With the myosin head cocked and both ADP and phosphate still bound, the head weakly reattaches to actin. At first, this attachment is loose and nonspecific. Then something critical happens: the cleft in the myosin head closes, shifting the bond from weak to strong, stereospecific binding. This cleft closure destabilizes the phosphate in its binding pocket.
The release of inorganic phosphate is the pivotal event. It unlocks the conformational change that drives the lever arm forward in a roughly 60-degree rotation. This rotation is essentially a rigid-body swing of the converter domain and lever arm, amplifying tiny atomic-scale rearrangements near the ATP-binding site into a nanometer-scale pull on the actin filament. Interestingly, research using phosphate analogs has shown that the transition from weak to strong actin binding can begin before phosphate actually exits the binding site, but it’s the strong binding that ultimately drives phosphate out and commits the stroke to completion.
How Far and How Hard Myosin Pulls
A single power stroke moves the actin filament a surprisingly small distance. For myosin-10, single-molecule experiments with optical tweezers measured a total displacement of about 17 nanometers, roughly one ten-thousandth the width of a human hair. This stroke occurs in two phases: an initial 15-nanometer movement followed by a smaller 2-nanometer shift.
The force generated by one myosin head peaks at about 6 piconewtons and averages around 2 piconewtons over the full cycle. A piconewton is an almost incomprehensibly small unit of force, but muscles contain billions of myosin heads working in parallel. Skeletal muscle packs around 300 myosin heads into every half-micrometer of a single thick filament, and a muscle fiber contains thousands of these filaments arranged in series and in parallel. The collective output of all those tiny strokes adds up to the force you feel when you grip a doorknob or push off the ground.
How the Stroke Ends and Resets
After the main lever arm swing, ADP remains bound to the myosin head, and the head stays firmly attached to actin in a force-generating state. This matters because as long as ADP is present, ATP cannot bind and detach the head. The myosin essentially dwells on actin, maintaining tension. The final phase of the power stroke involves ADP release, which requires an additional 9.5-degree rotation of the lever arm. This small swing brings the lever arm to its fully extended, end-of-stroke position.
Once ADP leaves, the binding pocket is empty, and a new ATP molecule can bind almost immediately. ATP binding breaks the strong actin-myosin bond, the head detaches, and the cycle returns to the cocking phase. If calcium is still present and binding sites remain exposed, the cycle repeats. In an actively contracting muscle, each myosin head cycles through this loop roughly five times per second in slow-twitch fibers and much faster in fast-twitch fibers.
Why Fast and Slow Muscles Differ
Not all power strokes run at the same speed. Your body contains different versions of the myosin heavy chain protein, and the version present determines how quickly the cycle turns over. Slow-twitch fibers, the ones that maintain posture and resist fatigue, use a myosin isoform where ADP release is about 10 times slower than in fast-twitch fibers. Because ADP release is the rate-limiting step during sustained contraction, slow myosin heads stay attached to actin longer during each cycle. They spend a larger fraction of their time generating force (a higher “duty ratio”) but produce less power and consume ATP more slowly.
Fast-twitch fibers, responsible for explosive movements, use myosin isoforms that release ADP quickly, detach sooner, and cycle faster. This lets them shorten at higher velocities and produce more power, but at the cost of burning through ATP rapidly and fatiguing sooner. The power stroke mechanism is identical in both fiber types. The difference is purely in how long each phase of the cycle lasts.