Inside every muscle is a team of rowers: myosin proteins that grab actin filaments, take a stroke, and let go. The fuel for the stroke is the ATP molecule. When the rowers work in sync, the muscle contracts and the body moves.
For decades, two models existed: one explained why the boat moves slower with increasing load, the other — how the rowers move their arms. The new model merged them, tracing the path of each energy portion. It shows: part of the fuel goes into moving the boat (useful work), the rest into heat and friction, like splashes from oars. Most surprising: even when the boat is stationary, holding a load, the rowers keep working, burning ATP and releasing heat.
This approach created for the first time a standard model of muscle contraction without contradictions with energy conservation laws. The increase of entropy, inevitable energy losses, is built into the equations. The model applies not only to muscles but to any cellular motors.
The force–velocity curve for muscles is a hyperbola: with no load, speed is maximal; as load increases, it drops to zero.