When on the stairclimber or any other machine within a couple of minutes my feet feel heavy tense like they want to cramp but the don't.what could this be..?

It isn't your feet, its your calves. If you do strengthening exercises for your calves, you won't get that feeling in your feet. Strong calves are good too. Better support for your knees, then your hips. Everything works in tandem and depending on how old you are, all the better for later years, say 55 or so.
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Also could be improper shoe support. Stregthen your calves, stretch your calves and achillies tendon, wear supportive shoes, and if you are getting pain under your foot see your dr and massage the area by rolling a tennis ball or golf ball under your foot.