If you have bad knees or a bad back, what exercises should you seek out to strengthen those weaknesses without hurting yourself more?
I recommend core conditioning and weight training to strengthen and stabilize the posterior chain (i.e., the muscles on back side of your body). These include exercises such as the plank, supine hip extension, leg curls and other movements that involve the glutes and hamstrings. Many times, when we feel and/or experience pain in our back and/or knees, it may be a simply mobility issue with the ankle or hip.
These are the reasons why I definitely recommend working with a fitness professional who is well experienced with teaching you the proper exercises to address specific weaknesses and prevent injury. In my experience, when you focus on whole body movements (such as in functional fitness), those aches and pains in the joints tend to recede or can be eliminated entirely.
What should you look for in an exercise class to make sure it’s not too advanced for you? Even when they say they will modify, it’s hard when you are not as in shape in competitive classes.
Seek a class that is fun, you enjoy, and where the instructor is interested in proper form and technique. Find an instructor that explains and demonstrates each movement properly until you can master it. When entering a new exercise class, do what you can until trust is built, don’t allow anyone to push you past what you perceive your limit to be. This is a set up for injury. Your only competition is with yourself, not other class participants and certainly not the instructor.
Once trust is built, then your instructor should motivate and inspire you and push you to your next level. Your instructor should take you to new levels of physical fitness safely, efficiently and effectively. (I intentionally said perceive cause if you subscribe to the US Navy SEALs “40% Rule”, what we think is our limit is really only the beginning of your limit; typically you have about 40% more to give.