Muscle Contraction: Sliding Filament Theory

The sarcomere: muscle's repeating engine

Skeletal muscle is built from myofibrils, and each myofibril is a chain of identical repeating units called sarcomeres — the actual contractile machine. A sarcomere is bounded by two Z-discs, anchoring thin actin filaments that point inward from each end, interdigitated with thick myosin filaments sitting in the middle. Contraction is not the filaments themselves shrinking — they stay a fixed length — it is the thin and thick filaments sliding past each other, pulling the Z-discs closer together. This is the sliding filament theory, established by Huxley and Hanson and independently by Huxley and Niedergerke in 1954, and it is the model this simulation renders directly: watch the Z-discs pull inward as filaments slide, not shrink.

The cross-bridge cycle: one ATP, one power stroke

Sliding is driven by myosin heads — small protein motors projecting from the thick filament — repeatedly grabbing the actin filament, pulling it a few nanometres, letting go, and resetting, hundreds of times per second across billions of heads. The cycle has four chemically distinct steps:

1. attached, rigor state:  myosin head bound tightly to actin, no ATP
2. ATP binds myosin:       head releases actin (this is the "detach" step -
                            without ATP the head stays locked on, which is
                            why rigor mortis is caused by an ATP shortage)
3. ATP hydrolyzed -> ADP+Pi: head cocks into a high-energy conformation
                            and re-attaches weakly, further along actin
4. power stroke:           Pi then ADP are released, the head snaps to
                            its low-energy shape, dragging actin ~10 nm -
                            this is the force-generating step

Each cycle consumes exactly one ATP molecule and produces one discrete power stroke; a sarcomere's macroscopic shortening is the statistical sum of enormous numbers of asynchronous heads independently cycling, which is why muscle force output is smooth rather than jerky even though the underlying mechanism is a series of discrete ratchet steps.

Why rigor mortis happens, mechanically

Step 2 of the cycle — ATP binding — is what releases a myosin head from actin. After death, cellular ATP production stops and existing ATP is rapidly consumed and not replenished, so myosin heads that are in the attached, rigor-like state have nothing to bind and cannot detach. Every head is stuck cross-bridged to actin simultaneously, across every sarcomere, and the muscle locks rigid — rigor mortis is a direct, mechanical consequence of the cross-bridge cycle stalling at exactly the step that requires fresh ATP, not a separate biological process.

The force-length relationship

Because force comes from the number of myosin heads that can physically reach and bind actin, the force a sarcomere can generate depends on how much the thick and thin filaments already overlap — and that overlap is a direct function of sarcomere length. Stretched too far, actin and myosin barely overlap and few cross-bridges can form: force is low. At an intermediate, optimal length (roughly the sarcomere's resting length in vivo), overlap is maximal and force peaks. Compressed too far, the thin filaments from opposite ends start to overlap each other and myosin filaments jam against the Z-discs, again reducing effective cross-bridge formation and force. The resulting force-length curve is a characteristic ascending-plateau-descending shape, and it is directly why a muscle's strength depends measurably on the joint angle it is contracting from — the sarcomeres inside it sit at different points on this same curve at different joint angles.

Calcium: the switch, not the motor

Cross-bridge cycling does not happen continuously by default — it is gated by calcium. At rest, a regulatory protein complex (troponin-tropomyosin) physically blocks myosin's binding sites on actin. A nerve impulse triggers calcium release from the sarcoplasmic reticulum; calcium binds troponin, which shifts tropomyosin out of the way and exposes the binding sites, letting cross-bridge cycling begin. When the nerve signal stops, calcium is pumped back into storage, tropomyosin re-blocks the sites, and the muscle relaxes. Calcium therefore acts purely as an on/off switch for whether cycling can occur — the ATP-driven cross-bridge cycle described above is what actually generates force once the switch is open.

Frequently asked questions

Try it live

Everything above runs in your browser — open Muscle Contraction and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.