Another issue with controlling oxygen intake is how to measure it. Outside of the laboratory, an athlete must rely on a heart rate monitor. There is a fairly good correlation between heart rate and oxygen intake. Once oxygen intake is correlated with heart rate (through max VO2 measurement), an athlete can calculate his/her ideal heart rate, and attempt to run a marathon, or 5k at this heart rate; however, heart rate can lag power exerted by several minutes (up to 3). This can result in large oxygen debts, and in turn, can affect performance towards the end of endurance events.