The way in which K2 is good for your joints is it keeps calcium and other minerals in the bones. Usually western and bodybuilding diets are high in calcium, which is a good thing as calcium is a necessary nutrient/mineral, but over time certain deficiencies and age can cause the calcium we intake through our diets and supplements to be stored in the blood/arteries instead of the bones/ligaments. This can cause hardening of the arteries or "atherosclerosis". Andgrens and possibly sarms like ostarine can increase this. So yes supplement with Vit K. Will it help with joints...??? With consistent and long term supplementation it will force the calcium you intake to be absorbed by the bones which will reduce your likely hood for fractures and or arthritis in the long run.
Note: The 4 micro nutrients over looked by athletes especially those enhanced are;
Vit k,
Vit D,
Calcium,
magnesium...
They all play integral roles in long term health and the proper utilization of each greatly plays off presence of the others. I.e you cant absorb calcium without vitamin D, but you also wont store it in the right places, i.e bones, without vit k.
You'll notice when buying vit K2 there are two forms...menaquinine 4 and menaquinine 7. Menaquinine 4 is the synthetic form of k2 while menaquinine 7 is naturally produce form of vit k. Both are superior to regular vit k1 (Phytonadione), but menaquinine 4 is the only form of vit k shown to reduce the occurrence or fractures and arthritis in a 2 year clinical study. Therefore I recomend vit k2 as menaquinine4.