Thai
Elite Member
As some of you may know, Apple Watch has plethysmograph — it looks and acts like a pulse oximeter. It can measure pulse and oxygen saturation in your blood. Most (including me) assumed that the O2 sensor part is deactivated and awaiting software to turn it on and/or FDA regulation.
But, a very smart Reddit user wrote this: (I bolded what I think is important.)
"I'm a Bio-molecular engineer and sensor design expect, so I guess I'm pretty qualified to answer this. Long ass answer incoming.
So your body has an average amount of O2 in your blood stream.
The rate at which O2 is consumed is directly related to how much ATP is produced by the electron Transport chain in the Mitochondria of your cells.
ATP is pretty much only produced at the rate at which it is needed, aside from maintaining a certain concentration in your cells.
Every time your muscles contract they are using up some ATP and have to replenish it.
So... Every muscle contraction uses ATP, and every breath you take adds more oxygen so more ATP can be made
Since ATP is an energy molecule used by your body as currency, and is directly related to exactly how much oxygen you are consuming, through some calculus we can relate the rate of O2 consumed, and the amount of O2 in your bloodstream to the exact amount of energy you are burning, giving us one of the most accurate calorie counts available.
Let's say that 2.81E22 ATP = 1 Calorie because We also know that one glucose molecule can make 30 ATP. And we know that 685 calories come from exactly 1 mole (6.02 E 23 molecules) of glucose.
We also know that the transport chain that creates ATP uses Oxygen as the final electron acceptor, and generates about 10 NADH molecules and 2 FADH2 molecules we know that PER oxygen used, 2.5 ATP are made from NADH, and 1.5 ATP are made from FADH2. So... that's 30 ATP created, PER 12 Oxygen (O2) used. Doing more math we find that 7.03E22 molecules of oxygen = 1 Calorie (calorie with a big C = food calorie or kcal)
Going off your weight, gender, and height, we can figure out the required basal metabolic rate of your body. Say, you need 1600 Calories to keep your body running if you did nothing except lay in bed and breathe all day.
So during rest, the oxygen in your blood stream will be saturated. If we know the concentration... And we know the flow rate (provided by your pulse) We know exactly how much oxygen is flowing, and thus exactly how much is burned.
If you start working out, the concentration will drop, until you breathe harder and the pulse (flow rate) will increase and we'll see that and say... Well, his oxygen is now being consumed at 7E22 molecules per 10 seconds and that means he's burning 6 Calories Per minute. Combine that with a time (say 30 seconds) and I know you burned EXACTLY 3 calories.
Tl;Dr: the amount of oxygen in your blood * your heart rate * a time interval = directly relates to how many calories are burned.
Edit: to clarify further about why it doesn't need your blood volume, the sensor detects a blood oxygen concentration, and can reasonably approximate the volume because it can be guessed from the amount of flow through the slice of your wrist that the sensor "sees" and the lag time between when you start to lose blood oxygen and the time your heart rate starts to increase. That lag gives you the circulation time from the wrist to the hypothalamus in the brain, which is approximately [some known percentage]% of the total length of circulation. So you can back calculate the total length of circulation to fill in the "missing piece" of the equation."
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So, if the above user is correct, then Apple may already be using the O2 sensor to accurately calculate your calories burned during activity.
But, a very smart Reddit user wrote this: (I bolded what I think is important.)
"I'm a Bio-molecular engineer and sensor design expect, so I guess I'm pretty qualified to answer this. Long ass answer incoming.
So your body has an average amount of O2 in your blood stream.
The rate at which O2 is consumed is directly related to how much ATP is produced by the electron Transport chain in the Mitochondria of your cells.
ATP is pretty much only produced at the rate at which it is needed, aside from maintaining a certain concentration in your cells.
Every time your muscles contract they are using up some ATP and have to replenish it.
So... Every muscle contraction uses ATP, and every breath you take adds more oxygen so more ATP can be made
Since ATP is an energy molecule used by your body as currency, and is directly related to exactly how much oxygen you are consuming, through some calculus we can relate the rate of O2 consumed, and the amount of O2 in your bloodstream to the exact amount of energy you are burning, giving us one of the most accurate calorie counts available.
Let's say that 2.81E22 ATP = 1 Calorie because We also know that one glucose molecule can make 30 ATP. And we know that 685 calories come from exactly 1 mole (6.02 E 23 molecules) of glucose.
We also know that the transport chain that creates ATP uses Oxygen as the final electron acceptor, and generates about 10 NADH molecules and 2 FADH2 molecules we know that PER oxygen used, 2.5 ATP are made from NADH, and 1.5 ATP are made from FADH2. So... that's 30 ATP created, PER 12 Oxygen (O2) used. Doing more math we find that 7.03E22 molecules of oxygen = 1 Calorie (calorie with a big C = food calorie or kcal)
Going off your weight, gender, and height, we can figure out the required basal metabolic rate of your body. Say, you need 1600 Calories to keep your body running if you did nothing except lay in bed and breathe all day.
So during rest, the oxygen in your blood stream will be saturated. If we know the concentration... And we know the flow rate (provided by your pulse) We know exactly how much oxygen is flowing, and thus exactly how much is burned.
If you start working out, the concentration will drop, until you breathe harder and the pulse (flow rate) will increase and we'll see that and say... Well, his oxygen is now being consumed at 7E22 molecules per 10 seconds and that means he's burning 6 Calories Per minute. Combine that with a time (say 30 seconds) and I know you burned EXACTLY 3 calories.
Tl;Dr: the amount of oxygen in your blood * your heart rate * a time interval = directly relates to how many calories are burned.
Edit: to clarify further about why it doesn't need your blood volume, the sensor detects a blood oxygen concentration, and can reasonably approximate the volume because it can be guessed from the amount of flow through the slice of your wrist that the sensor "sees" and the lag time between when you start to lose blood oxygen and the time your heart rate starts to increase. That lag gives you the circulation time from the wrist to the hypothalamus in the brain, which is approximately [some known percentage]% of the total length of circulation. So you can back calculate the total length of circulation to fill in the "missing piece" of the equation."
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So, if the above user is correct, then Apple may already be using the O2 sensor to accurately calculate your calories burned during activity.
