Tight muscles of the feet, foot surgery, etc. are generally the causes of stiff toes and toes not bending.
Soak the feet and toes in hot water for at least 20 minutes a day. Then rub the feet with vapor rub to keep the muscles relaxed. Start walking barefoot on the living room rug, then march in place, then start jogging, and then start running. Walking, jogging and running all require bending of the toes. However, the muscles of the feet must have elasticity to allow the toes to be able to bend.