Why are the skeleton and muscles referred to a single organ apparatus?

The skeleton and muscles belong to the organs of the musculoskeletal system, the main importance in the body of which is protection, support and movement. Moreover, these functions require the simultaneous participation of both bones and muscles. Here are some specific examples:
– in order to move, you need the muscles to contract, but for this they need a fulcrum – the bones serve as it;
– the bones in the joints are connected in a special way, without which, even with muscle contraction, movement would not be possible;
– protect the arteries of the extremities, closing them on both sides.