On Tue, 18 Mar 2014 03:23:50 +0000, John Rumm
wrote:

On 17/03/2014 20:09, 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.

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.
--
Regards, J B Good