Hi Kristie, I guess it depends on your goals as to how much cardio you do. If you want to build muscle mass then normally you won't be doing cardio every day. If you want to increase cardio fitness and are only lifting to stay toned then you would do more cardio and probably use lighter weights when you do lift so you could combine cardio and strength on the same day. I tend to do cardio and lifting on separate days with the occasional total body metabolic workout that combines both cardio and lifting. I don't measure heart rate but work off the 10 level perceived exertion scale and I find I can get up to level 6 and sometimes even 7 when I lift, especially if I am doing leg related lifting exercises like squats, lunges, high step, etc. On cardio days I'm working from level 6 up to 9 if I'm doing HIIT cardio. Even though your heart rate isn't going up are you breathing harder and sweating when you lift?