For instance the treadmill can measure in METs (metabolic equivalent tasks) the energy expended at a given
level of exertion. At a given MET
level, vital signs are monitored, heart rate, respiration, etc. For a perspective 1 MET is the amount of energy to quietly sit, 2-4 METs to walk up a flight of stairs ( 7 METs usually qualifies for surgery. Ten METs is very good condition, etc.
No need to be scared.