# Naked Science Forum

## General Science => General Science => Topic started by: thedoc on 28/01/2014 19:30:01

Title: How long do I have to work out for to work off a Mars bar?
Post by: thedoc on 28/01/2014 19:30:01
Exercise physiologist Dan Gordon explains how he pushes athletes to their limits and answers the audience's questions...
Read a transcript of the interview by clicking here (http://www.thenakedscientists.com/HTML/content/interviews/interview/1000582/)
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: David Cooper on 29/01/2014 18:59:27
That's a good read, in places. I really appreciate the transcripts - they save a lot of time. How do you make them? I'm guessing that speech recognition is involved, but can you do that directly from the audio yet or do you have to get someone to listen to it in a quiet room and say it all again carefully? However you're doing it though, thanks.

It feels as if a lot of energy is still being burnt for a long time after the exercise ends, so it must be very hard to measure the actual amount of exercise needed to burn off a Mars bar. When Mark Beaumont cycled round the world he tried to eat six thousand calories a day to balance out the 100 miles or so of what was usually fairly gentle exercise - he wasn't going at race pace, but more like the speed of a Cyclists' Touring Club run. That's perhaps 4000 calories more than normal per day, and about 400 per hour. With high intensity exercise, I would guess that you could burn 400 calories in under ten minutes and might burn a good bit more throughout the rest of the hour.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: alancalverd on 30/01/2014 12:03:01
Standard Mars bar is 260 kcal = 1.1 megajoule

Maximum continuous useful output for most people = 40 watts

So to work off one Mars bar you need to exercise hard for 27,300 seconds = 7 hours 35 minutes - a working day.

A coalminer, farm labourer, arctic explorer or professional athlete might just about do it, but no ordinary mortal could eat a Mars a day without putting on a load of weight.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: David Cooper on 30/01/2014 19:30:35
Data from racing cyclists suggests otherwise. In the Tour de France they can average 250 watts or more on a stage, so the many ordinary fit people could do half that. That's just the power transmitted through the pedals though - far more energy than that is lost as heat.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: chris on 31/01/2014 18:41:30
That's a good read, in places. I really appreciate the transcripts - they save a lot of time. How do you make them?

We have a team member who listens to the programme and produces a computer type-written version, which we check and publish. Laborious, but worth the effort, in my view...
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: David Cooper on 31/01/2014 18:57:18
Well, please pass on my thanks and appreciation.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: syhprum on 12/01/2018 16:45:28
I was always taught that the thermal efficiency of a human was 25% about the same as a low tech IC engine
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: Bored chemist on 13/01/2018 13:46:25
A coalminer, farm labourer, arctic explorer or professional athlete might just about do it, but no ordinary mortal could eat a Mars a day without putting on a load of weight.
And yet, many of us (like me, for example)  do.
Indeed, my calorie consumption is probably something like 9 mars bars a day.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: alancalverd on 13/01/2018 16:43:01
Hail the next POTUS!

I guess I was a bit elliptical. A Mars a day on top of a normal diet. Standard 2400 calories is about 10 MJ per day, so adding another megajoule without burning it off is going to lead to a substantial weight increase.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: Janus on 13/01/2018 16:50:22
Standard Mars bar is 260 kcal = 1.1 megajoule

Maximum continuous useful output for most people = 40 watts

So to work off one Mars bar you need to exercise hard for 27,300 seconds = 7 hours 35 minutes - a working day.

A coalminer, farm labourer, arctic explorer or professional athlete might just about do it, but no ordinary mortal could eat a Mars a day without putting on a load of weight.
Using this calculator, it only takes ~90 min at a slow walk to burn off 270 Calories.
https://www.freedieting.com/tools/calories_burned.htm

I'm 59, in no way a professional athlete, and can burn that off in 20 min on an elliptical.  I have kept this pace up for 1 1/2 hrs, burning off the equivalent of 5 1/2 Mars bars.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: alancalverd on 13/01/2018 17:00:56
My mind goes back to the days of manpowered flight, when the Kremer Prize was won by the only athlete the competitors could find who could sustain 300W of useful output for 2 hours. That is 35 Calories per hour.

NB remember that nutrition Calories are kilocalories, or 4200 joules, and my calculations are on useful output.
Title: Re: How long do I have to work out for to work off a Mars bar?
Post by: Bored chemist on 13/01/2018 17:42:38
NB remember that nutrition Calories are kilocalories, or 4200 joules, and my calculations are on useful output.
The output may be useful; I'm less sure about your calculations.
The going rate seems to be about 10 mars bars to the marathon
https://www.theactivetimes.com/how-many-calories-does-running-marathon-burn
Plenty of people could burn off the equivalent of an extra mars bar during the day
A standing desk for 8 hours would do it.
http://paindatabase.com/standing-desk-calories/
You do not need to be "A coalminer, farm labourer, arctic explorer or professional athlete "
Just a slightly less sedentary office worker.