Being overweight as I am, I attended the gym yesterday and amongst other endurances, I spent 5 minutes on a rowing machine. Now rowing is, of necessity, a repetitive activity and gave me plenty of time to reflect on – well, this, and that – but amongst my reflections was the realisation that the machine was measuring the efficiency with which I turned food energy into mechanical work. How so?
The rowing machine calculated the rate at which I produced useful mechanical energy. Over 5 minutes I averaged exactly 150 watts of output power. Now output power is relatively easy to measure, and so I trust this estimate. Producing 1 watt of power corresponds to producing 1 joule of energy per second. So producing 150 watts for 300 seconds corresponds to producing 150 x 5 x 60 = 45,000 joules of useful mechanical energy.
The rowing machine also calculated the rate at which I used up food calories. It told me that the work I had done was equivalent to 62 calories of food. I don’t know how the machine could have estimated this so I don’t really trust this estimate. Now one food calorie is an old unit and is misleading in two ways. Firstly, it actually corresponds to 1 kilo-calorie of energy i.e. 1000 calories of energy and converting to modern units, 1 calorie of energy corresponds to 4.2 joules of energy. So my 62 food calories amounts to 62 kilo-calories which amounts to 4.2 x 62,000 = 260,400 joules.
So in order to produce 45,000 joules of work, I need to eat 260,400 joules of food energy. In other words my body was just 17.3% efficient. I don’t really know what to make of this figure, but it’s one that one doesn’t see often, and I thought I would just write it down before I forgot.
Now I will return to letting my body ache 🙂