The last repetition in the set should be the last one you can do with proper form. So if you could keep going, the weight is too light. If you can't get close to the end of the set before you are sacrificing form, the weight is too heavy. You want it to be challenging, but not so much of a challenge that you risk injury.
Do you measure your heart rate during your elliptical workout? That would be one way to know whether or not you're working hard enough. Here's an article on exercise intensity you might find helpful:
How do I tell whether I need to be using a higher or lower weight on a weight machine? I thought it was typically 2 sets of 8 or 10 and that within 3 reps of the first set I should feel like I am being challenged? I really am not sure about that but I do want to know how to tell whether I should use more or less weight on different machines. The same with the elliptical too, I can do 20 minutes on a level 3 or 4 and am fine though after that I get a little more tired. If I do a harder level, about 5 or 6 I usually last about 15 minutes. Any advice would be appreciated.
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