I was wondering why everyone in past talked that we should do high reps and now I don't see that anymore. I researched that and I saw this:Build muscle mass while preventing joint damage
A major misconception is that heavy lifting and joint pain are cause and effect. Many bodybuilders and powerlifters just deal with the accompanying joint pain. They reach for glucosamine, chondroitin and fish oil to get relief.
The previously mentioned research team also established that training with very light weight (a weight that allows the completion of 15-20 reps) prevented cartilage deconstruction and even enhanced its reconstruction.
The scientists found that exercising with light weights reverses the processes that occur when joints are stressed by heavy weights. Light weights seem to inhibit the activation of genes in cartilage cells that cause inflammation and an ensuing attack on cartilage. This prevents the breakdown of cartilage and leads to it being rebuilt.