When I go to the gym and put my age (43) into the machine (treadmill, elliptical, stair climber), it tells me that my target heart rate (at 85%) is about 140 bpm. However, when I try to stay near that, I don't feel like I am getting a good workout...I don't even break a sweat. It doesn't feel like I am working out until the machine measures a heart rate around 155-164. However, I have heard or read somewhere that I should not be working out higher than 85% because then it's not aerobic, but anaerobic. I don't know how to handle this. Should I ignore the machines and just go with how my body feels or should I stay within that zone because I may be jeopardizing my health or weightloss efforts? Any suggestions?