An extendable, load-bearing structure has a main structure and a driving element. The main structure has at least one driving arm. The driving element is placed in connection with the driving arm to drive the extendable, load-bearing structure between first and second positions. The extendable, load-bearing structure has a lock mechanism, and a supporting structure. The supporting structure has at least one supporting arm that is shorter than the driving arm of the main structure. The supporting arm has at least two supporting-arm elements which are connected to each other by joints. The supporting-arm elements of the supporting arm are substantially in line when the extendable, load-bearing structure is in its second position. The lock mechanism is arranged to releasably lock the supporting-arm elements of the supporting arm in position when these are in line.