This is a video about the importance of weight training even for runners. In older people squats, as shown, can be modified to strengthen, not hurt the knees. The strength of the muscles around the knees are very important to keep strong, for safety and stability. One doesn’t need to do heavy weights, but it is optimal to lift until the muscle fails. This can be tiring if you’re doing many different muscle groups at one session. I do Bar Method(barmethod.com), and one typically uses not only arms (biceps, triceps) to failure, but glutes, abs and thighs to failure as well. So if you push yourself in class, you can feel pretty wiped out.
This video (click on the photo) makes the case for using lighter weights and the case for squats since they use so many muscle groups. I did a previous blog post on the importance of squats to increase Growth Hormone, which gives an anti-aging benefit to squats, too.