The gym has machines by pulsefitness. There seems to be a mismatch in the numbers displayed.
After a bike session the summary was: Power average: 74 Watts. Time taken: 9 minutes. Energy used: 50 kcal.
The above energy figure would equate to 333 kcal per hour. But 100 Watts is only 86 kcal per hour!
The mismatch is so enormous that I feel I must have missed something. I would be grateful to any physicist who can shed light on the figures.
Presumably there is some assumed efficiency coefficient in use?
1 watt is 1 joule/second.
Google "(9*60)*74 joules in kcal" Gives 9.55 kilocalories.
Something ain't right. B-)
I think the 74 watts is power output (as developed in the machine doing the measuring) - and the 50 Calories is the amount of bio-energy burnt to develop that.
In this example, you have developed 9.5 Calories (or kcal) at the expense of 50, so about 20% efficient.
Whilst I have no idea for sure, that seems believable for a biological process.
You have missed the fact that ,mots people know how long 9 minutes is, and roughly how much power a cyclist produces.
They are also familiar with how many calories they had for breakfast.
But not the conversion between them
So to the average punter 'hey I pedalled hard enough to light a 60W lightbulb, and burn off three English breakfasts' will make them feel good and come back.
To tell them they didn't even burn off a slice of dried toast is simply bad salesmanship.
Cui Bono? Follow the money.
At an average power output of 74 watts for 9 minutes duration (the readily measurable quantities) you will have produced just short of
40Kj of electrical energy. Dividing this by the 4.2j per calorie conversion factor gives just under 10 Kcalories of energy output from the generator.Unless the wattage is an actual wattage output figure on a generator used to provide the required physical workload and the figure for the "energy used" is a derived figure for the total energy consumed by your body[1] to achieve that electrical output, the figures don't make any sense.
[1] The mechanical losses between the pedal crank and the electrical output of a generator are unlikely to exceed 15 to 20% unless it is a spectacularly inefficiently designed transmission system. Even when coupled coupled to a low efficiency generator (less than 80% efficiency) you'd still expect the gross mechanical energy input to stay below the 20Kcalorie mark.There are additional 'parasitic' sources of energy consumption above and beyond the useful pedalwork power being transmitted into the pedal crank (pumping losses, heart and lungs, and other muscular activity such as in the arms and torso) which can be derived from a lookup table of previous 'calibration runs' created by the exercise bike manufacturer using standard metabolic monitoring and measurement instrumentation on a representative sample of 'volunteers'.
I'm surmising that the average electrical power output and the duration may simply be the basis for calculating a total metabolic energy expenditure figure with a reasonable level of accuracy.
You'd have to ask the gym operators about this or get hold of information from the exercise bike manufacturer (possibly mentioned in the user guide and/ or on line if the manufacturer has a web page offering such support).
The 50kCal is an estimate of the energy you burned (input) but the power is the power you output.
A typical figure for efficiency of the body doing this type of work is about 20%, which gives a large difference between the input and output values.
The figures are fine, its just your body can't do a 100% efficient conversion of stored chemical energy into mechanical power. So the energy consumption by the body will be several times the plain mechanical output (it will waste a good amount just producing heat for example)
I've seen the 'chemical to mechanical energy' efficiency for biological systems quoted as being around the 50% mark but, of course, there are additional 'parasitic' loads on top of whatever useful muscular driven mechanical energy outputs are generated.
I'm guessing that this higher 50Kcal figure is simply derived from the total watt hours from the generator by some sort of lookup table based on actual test data collected from a representative sample of volunteers employed in earlier studies of metabolic energy consumption versus mechanical workloads. The manufacturer may simply have used already published data to 'calibrate' their exercise bike.
First, if it's a gym machine then I'd be sceptical of its accuracy anyway -- no disrespect to the gym owners, but public machines get a hell of a lot of use (and abuse).
Secondly, see if Pulsefitness have a website (they do - I got that far :)) and information about using the machine that you use. I sometimes use a Wattbike at our gym, and Wattbike's website was very useful for me in making more of the machine.
John
Apparently The Tour de France average is 400 watts.
No, I'm not that fit either!
Andy
I use a lot of different gym cycling machines when traveling, usually in hotels where they are going to get less maintenance than in a gym, and, somewhat to my surprise, they all pretty much agree on what power output I generate at specific pulse rates. The only one that's different is my one at home which under-registers power output, even compared with my brother's identical one at his home.
I have not done a comparison on their conversion of energy output to kCal burned, so I don't know if it's a simple 1:5 (or similar) ratio, or if it takes into account loading, cadence, pulse rate (for those that pick it up), etc.
I would also be interested to know the formula the Bremshey ones use to report fitness. It's heavily based on the rate of drop of your pulse rate for the 60 seconds after you finish exercising, but I don't know if it takes into account any other metrics which it records during the exercise.
An adult needs about 2000-2500 KCal a day - or very roughly 1-2 KCal per minute.
Even if you'd just sat next to the bike for 9 minutes, you'd have used (very roughly) 10-20 KCal, just staying alive and maintaining your body temperature.
IIRC vigorous exercise raises our heat output by about a factor of 2-3x compared to doing nothing.
So the overall figure of 50 KCal sounds reasonable - but remember that the *excess* calories used by exercise is somewhat less than that.
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