Wednesday, May 11, 2022

2.2kg Fat Loss in 44 hours: Sled Push Challenge

Marathon Sled Push 

In December 2020, I decided to participate in my first-ever marathon.  Although I've completed various Ultra Marathons of 100 miles+ before, I've never covered just the 26 miles (42.2km).  

SO for my first ever marathon I decided to add a little twist, I wanted to push and pull a sled (prowler), weighing my body weight (90kg), back and forth on a 20m astro turf track, 2110 times.  As far as I know, no one has ever done this before...and now I know why.

To give you a kind of idea of the things I do on a weekend, here's some of my accolades:
  • Ran in the Worlds Toughest Mudder (a 24hour obstacle course in the Nevada dessert)
  • Raced 230km across the Artic circle (5 days)
  • Ran 100 miles through the jungles and mountain of Borneo (38 hours)
  • Spent 12 hours on a Watt bike
  • Spent 24 hours on a Concept 2 rower
  • Completed 100m on a concept 2 Ski Erg (17 hours)
  • Ran across 66km across Singapore in 10 kg weighted vest and 7kg ruck sack (9 hours)
  • Completed a 300 mile Concept 2 Triathlon on the Bike, Rower and Ski Erg (44 hours)
  • Pushed and Pulled a 90kg Sled 42.2km (60 hours and 30 seconds)

Well done you, so what?

After all these challenges, other than feeling absolutely fucked, I never noticed any kind of results that I un-expectedly did pushing and pulling the prowler.

After 60 hours of physical discomfort and minimal sleep (maybe 2 hours of 5 minute naps here and there), my body competed transformed.  In three days my body fat was shredded.  I was able to increase the weight on my deadlift.  My Achilles pain had gone (I'd been suffering with that for over a year).  My mobility in my hips increased (as noticed by my sports massage therapist) and, I took 4 minutes off my 8km run time.  

No one could understand what had happened and quite frankly, it was written off as a bit of a fluke.  But get this, my body went down from about 9-10% to 3.2% in 60 hours.  Colleagues, put it down to dehydration and glycogen depletion...thing is, I was able to keep the body fat down below 4.5% for a couple of months, before I went on a booze cruise. 

For me, this wasn't a fluke and I really felt that there was some-'thing' in this and was I convinced I had to do it again to prove it.

Because 42.2km wasn't enough 

I decided I wanted to give it go in again 2022.  This time, I thought I'd combine the prowler which I'd done before and the distance across Singapore (50km as the crow flys), because, why not?

Now, I gotta say, I didn't really train for this.  I generally decide I'm going to do these things and then just crack on with it.  I don't like wasting time to stressing myself out about the complexities.

This time, it was a bit of an error as it turns out, because I only completed 35km.  To be honest I got bored. But there was more to it psychologically, which I'll come back to later...


Despite not making to the full 50km, I still got some unbelievable fat loss results-like ridiculous! 

Friday morning to Monday Morning

You can clearly see visible results.  According to the Inbody (body composition measuring equipment), I'd dropped 2.2Kg of FAT-yes, fat!  I would be happy if a client managed that in four weeks, training x3 per week and eating a balanced-portion conscous diet.

The Science Bit

Now this is where is gets really interesting and for the sciencey bit, I'm going to hand you over to my lovely assistant, Stu Bauld...

Before the challenge started, we decided to measure Phil’s resting metabolic rate (RMR). This informs us of Phil’s resting energy expenditure, or how many calories Phil burns at rest. 

RMR is the highest contribution to daily energy expenditure and It makes up around 60-75% of your total daily energy. It represents the energy needed to sustain basic life process such as breathing, heartbeat, renal function and blood circulation.  This is thought to account for about 65% to 80% of total energy expenditure.

Due to the continued weight loss experienced in the days following Phil’s last sled push attempt of 42km, we predicted that his RMR must have be elevated above the normal level for a period of time once he had stopped pushing the sled. This increase in RMR could have contributed toward the rapid weight lost that Phil experienced.

To measure Phil’s RMR we use a method called indirect calorimetry. This is performed by using a gas analysis device strapped to Phil’s face which measures how much oxygen is inhaled as well as the amount of carbon dioxide exhaled. These numbers can then be entered into an equation (Weir equation) which will then inform 3 things: 

  1. Total energy consumption in kcal per minute 
  2. The ratio of oxygen consumed to carbon dioxide produced (RER value). This RER value can then inform exactly how much energy is coming from fat oxidation and glucose (carbohydrate) oxidation in grams per minute. 
  3. How slow/fast the metabolic rate is compared to someone of a similar gender, height, age and weight

Total energy consumption in calories per minute

The ratio of oxygen consumed to carbon dioxide produced (RER value). This RER value can then inform exactly how much energy is coming from fat oxidation and glucose (carbohydrate) oxidation in grams per minute.

How slow/fast the metabolic rate is compared to someone of a similar gender, height, age and weight.
Once we had measured Phil’s RMR we attempted to measure the energy expenditure of actually pushing the sled. To do this we used the gas analysis system and attached it to Phil as he pushed the sled up and down the 40m track. We took a 30 minute measurement that could help us extrapolate approximately how many calories Phil would expend per hour, and provide a gauge as to the intensity at which Phil was working at.

What Did We See?

After measuring Phil’s expenditure during the 30 minute trial we were able to establish some helpful numbers. The volume of oxygen required for Phil to perform each interval was on average 33.9ml/min/kg. The peak amount of oxygen that he used per interval was approximately 42.7ml/min/kg. 

We know that as exercise intensity increases, there is a concurrent increase in oxygen uptake in order to meet the demands of the exercise. We also know that Phil has a VO2max (maximum amount of oxygen you can take in and use during heavy exercise) of 67.4ml/min/kg, as we have tested this previously. Therefore, this means that each interval Phil was having to work at an intensity of between 50-63% of his VO2max.

When we looked at heart rate, this was on average 107bpm with a peak no higher than 128bpm which equated to approximated 58% of his maximum heart rate (Phil’s maximum heart rate was 183bpm observed from previous testing).

This all indicated that from a cardiorespiratory perspective the level of intensity that Phil was working at was relatively low and this was also shown when we looked at the energy substrate utilisation, with a high reliance of fat over carbohydrate to power each interval (as exercise intensity increases, there is an increase in carbohydrate use). The energetic demands can been seen in the tables below.

The energetic demands show that there would be an estimated 875kcal expended per hour with approximately 51g coming from carbohydrates and 74g coming from fat.

Although the intensity may appear to be relatively low, it is important to recognise that there would be significant neuromuscular stress on Phil’s body as a result of pushing and pulling his bodyweight on the sled. This combined with high amount of inflammation due to tissue stress would also increase the energy expenditure of the challenge.

Post Challenge Measurements

After the completion of the sled challenge, we continued to measure Phil’s RMR to see whether any significant differences existed in his resting energy expenditure. 
We re-tested on day 2 following the challenge and observed a 19.2% increase in the RMR. This equates to a 370kcal increase in daily resting energy expenditure after 48 hours! 

We measured again of day 4 and day 8 post sled push. By this time RMR had appeared to return to normal pre challenge levels.

Physical activity may have 2 distinct effects on RMR. One effect of physical training is an increase in muscle mass. Muscle mass is a highly metabolic tissue, therefore by increasing the amount of muscle mass, you also increase the amount of calories required on a daily basis. A second effect of physical activity on RMR may result from the physiological process that are involved in the recovery period following exercise.

As this challenge was too short in duration to result in any alteration in muscle mass, it is likely that the increase is resting energy expenditure is due to these physiological processes of Phil’s recovery. The changes in RMR following physical activity have been termed the excess post-exercise oxygen consumption (EPOC), which describes the increase in oxygen consumption after exercise has ended.

There have been 2 components of EPOC identified. Firstly, a fast component may occur immediately after exercise and last up to approximately 2 hours. This is believed to be required in order for body temperature/blood oxygen levels to return to normal, ATP re-synthesis and blood lactate removal (amongst other things).

The second and long lasting component can elevate RMR up to 48 hours post exercise; by 72 hours there has been no elevation detected. The long term EPOC has been shown to increase RMR by 5-10% so in Phil’s case this challenge really has had a significant impact on RMR. The mechanisms behind this are not clear, however, it may include a switch from carbohydrate to fat metabolism.


Ultimately, the driving force of fat loss is a calorie deficit. The energy cost of pushing and pulling the sled for that duration of time as well as the elevated RMR in the following days has clearly been the key mechanisms behind Phil’s body composition change. Measuring markers of inflammation may have been a more informative approach as Phil’s body is likely to have to come under extreme levels of stress; particularly in the liver and kidneys.

There is relatively little research on the health related effects of ultra-endurance challenges. However, there are some studies showing that repeated prolonged strenuous exercise may be hazardous to health manifested as cardiovascular complications, excessive muscle/cartilage damage and systematic inflammatory reaction.

The increase in Phil’s RMR corresponds with the research into the long lasting EPOC, displaying a significant increase in the 48 hours following the challenge. The actual increase of 19.2% is huge and an indication of just how difficult and taxing the actual challenge was.  

Stu Bauld of Functional Performance.  


As always with these challenges, I'm most fascinated by the psychology.  I had this challenge in mind for a while, but I kept putting it off and procrastinating about it for weeks.  I didn't prepare my body physically, ie didn't train specifically for it.  

For me, training for these challenges means overthinking and boredom.  But actually, on a deeper level, is actually procrastination that we all experience.  I did't want to commit, because that makes it real and there might be failure; and it might make me look a failure and so naturally want to avoid that. 

To get over that, I had to tell everyone, and it out there, what I intended to do, then there was no backing out. Lesson #1 of accomplishing a challenge.

I sort of had a plan going in, and that was to break the challenge down into bit sizes.  The strategy I used was 1km per hour.  To stick to this, I used EMOM (Every Minute On the Minute).  On the start of a minute, I'd push the prowler and on the start of the next minute I would pull the prowler.  If it took me 30 seconds to complete 20m, then I'd get 30 seconds rest.  Based on timings and distance, that would make 50 minutes equate to 1km, giving me 10 minutes rest, before I'd start the next kilometre on the next hour.

I used a very basic tally on a white board.  It was a great visual aid, as I could see the progress and just chip away at the total distance.

Accomplish a challenge Lesson #2

Have a visual of your progress and break that down into smaller chunks.  I tallied 5 laps as a group.  x5 groups made a kilometre. Then in my head, that was 'one kilometre down, only X more to go...'

One of the main goals for me as mentioned earlier, was the fat loss.  I wanted to prove it could be done again.  On the second day you could actually see the difference is body my composition, obviously that was very motivational.  But, THAT got into my head.

The fat loss was the main goal for me, not the distance and because of that, once I started getting tired and things started to hurt, then the more I felt more tired and more hurt I felt.  Funny init.  

As a result, I start getting more fed up and bored.  Then I started justifying finishing earlier and cutting down the distance, because I didn't 'need to' anymore, as I'd already hit my primary goal.  

First it went down to 42.2km, because I'd already done that.  Then it went down to 40km and by the early evening on the third day, I was like 'fuck it, just go to 35km'.

Am I disappointed with myself? Nah, couldn't give a monkeys, as I got a great result and we got some great findings, that seem to be completely new in the science of fat loss-as in, never been found before!  Like very exciting findings, that need further testing.

Lesson #3

The 'real' goal and the 'real' reason for completing it must be inline.

So why wasn't my goals and 'why' inline?

Well, I had some personal stuff going on the first time and this was certainly taking my mind off of it in one way or another.  But also, the original challenge was for a cause/for charity.   There was a lot more riding on the original challenge.  It wasn't about me, there was more going on, it was for a 'greater good'.  Much more motivation. 

Lesson #4 Find your 'true' motivation.

What's next?

I do believe there's further testing on the fat loss to be done.  Bear in mind I did less distance and got more sleep, compared to the first challenge and I still saw fat loss results.   So the question now is, what's the minimal amount of time/distance i can do, to still get credible results?  

What I'm thinking next is a 10x3, ie 10 hours on, get a good nights rest for three days on the trot.  It's still a long time, but it's certainly a lot more appealing to say an actor, that needs to drop 2kg quickly for a role.  A personal trainer/model/body builder that needs to drop body fat fast.  2kg in a weekend, is a lot more inviting than 2 kg in weeks!

But I'll keep you posted whatever I do,  but always up for suggestions.

Keep it real folks.

If you have physical challenge to overcome and you'd like some assistance, reach out and get in touch.

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