To complete the other comment: the MuscleMemory is updated through learning, so a more complete example would be:

    function muscleAction(Vec3d target, Vec3d environment, MuscleMemory memory) -> {actions: MuscleActivation[], result: Vec3d}

After executing the muscleAction function, through "practice", the MuscleMemory will be updated.

    function updateMuscleMemory(Vec3d target, Vec3d environment, MuscleMemory memory, MuscleActivation[] actions, Vec3d result) {
        memory.update(target, environment, actions, result);
    }

Sort-of like backpropagation.