Let your larger, stronger joints and muscles work for you. Rest a flat of plants on your palms and forearms rather than gripping with your fingers. Lift by bending at the knees, not the back. Use a stool that is padded on top and bottom when getting down to ground level. It's a stool for resting or, turned over, it's a padded kneeling bench with legs that make it easier to get up again.
If bending or kneeling is difficult, bring the garden to you. Window boxes, hanging baskets, containers on wheels, trellises or raised beds are for all gardeners but especially for those with back or knee problems. Vegetables as well as flowers can be grown in containers and raised beds.