Is HbA1C Value Of 6.0 A Cause For Concern?

I am 43 yrs 63 Kg. I have borderline diabetes which is under control and recent HbA1C is 6.0. Other profiles such as Renal, liver, cardiac etc are within normal range. Vit-D and B12 are also within normal range. I was advised to take Vit -d course for one month ( 4 sachets over 4 weeks). For the past 6 months to 1 year, I am having knee pain. After muscle training and weight training, I see substantial improvement in the knees. However, I feel that there is another condition developing in the body. My various bone joints such as shoulder, arms, spine etc have started to develop lot of friction in their movement. So I can feel that the movements are not smooth and that there is minor dis-comfort some times in the movement as well. Can you advise?