For Those over 40: Do NOT Perform These 8 Muscle-Building Exercises

4. Leg Extensions

John Hornez, 609th Air Intelligence Squadron computer network technician and contractor, demonstrates how to perform proper leg extensions during his workout Wednesday at the fitness center. (U.S. Air Force photo/Senior Airman John Gordinier)

Using a leg extension machine requires lifting iron by strengthening your knees. As you already know, your knee joints are one of the first areas to become more delicate with age. Older people may end up dislocating their knees by attempting leg extensions. Walking briskly around your neighborhood is all the exercise you need.