On Mon, 17 Mar 2014 20:09:46 +0000, Chris ] wrote:

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.


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).
--
Regards, J B Good