There could be multiple reasons your joints hurt when flexing. You should see your health care provider to rule out health issues.
exstend
your joints help your fingers bend and flex
a component of physical fitness that refers to your ability to extend and flex your joints and major joints is called flexibility.
Your joints will hurt if the synovial fluid hardens and joints become stiff.
Worn U joints, flex disks, or CV joints.
because your joints r hurt
Osteo Bi Flex caplets are used to help treat osteoarthritis. It works by repairing damage to your joints while reducing swelling in the joints, which relieves pain.
Anemia
I don't think it should. Since meat has plenty of protein it should do anything BUT make joints or muscles hurt.
Arthritis
Joints. The joints connect the bones into place, so it is your bones that hurt when you get bad joints in old age.
Watch me flex my biceps. My biceps hurt.