How do the following features on exercise machines actually work?
1)  Heart Rate Monitors
2)  Calorie Counters
3)  Variable Resistance Controls eg. on gym bikes pedalling can be made tougher
    or lighter by adjusting the resistance control.
4)  Profiles eg treadmills may have landscape to run over with "hills" where 
    the gradient of the treadmill will automatically lift up and down  to
    coincide with these "hills" and flat areas.