The cross-bridge cycle

A small program steps a myosin head through the cross-bridge cycle. The states are resting, bound, pulled, detached and reset. In the model, each power stroke pulls one thin filament by half a unit, and the stroke uses one ATP.

The idea. With calcium present, a head in the state resting or reset binds the actin and becomes bound. A bound head pulls and becomes pulled. Then a new ATP binds, and the head is detached. Then it splits the ATP and is reset. Without calcium, a head in resting or reset stays in resting.

Your tasks.

1. Finish next_state. It gives the next state from the present state and from whether calcium is present.
2. Finish count_strokes. It runs the head for a number of steps and counts how many times the head reaches pulled.
3. Finish sarcomere_length. In the model, each stroke of one head pulls each of the two thin filaments by 0.5 u, so the sarcomere gets shorter by 1 u for each stroke.