It does not "lubricate" joints. It causes a couple of things to happen:
1st - fluid retention in your muscles AND tendons & ligaments - with the swelling from fluid retention comes an increase in diameter - and generally speaking the bigger the tendon/ligament the stronger (caveat coming next)
2nd - an increase in collagen (proline & hydroxy-proline) synthesis. HOWEVER it takes a fairly long time for this new tissue to toughen up and have a high tensile strength. In the mean time, as your muscles get bigger and stronger at an incredibly fast rate, it becomes easier to actually have a tendon/ligament tear or rupture from RELATIVELY weak connective tissue.
Catch 22...