this is mainly just to help put some numbers with what i am seeing in training, but with a SRM what does the Kj come out to in cal burned?
if i am remebering correctly isnt it like 4 cal to the Kj, but with aerobic activity it drops to 1/4th of that, so a Kj = 1 cal?
i may be completly off here.
David,
There are 4.1868 Joules in 1 calorie, so 4.1868 kJ in a kcal (a food calorie, Cal, is a kcal).
So take the kJ from your SRM and DIVIDE by 4.1868 to find the Calorie equivalent.
If you google “kilojoule calorie conversion” there are online calculators that will convert for you…
Hope that helps!
John
From the Cyclops site: http://www.saris.com/CalorieCalculator.aspx
If I do 1000 Kjoules on a ride, can I calculate how many Kcals my body has burned?
One Kcal is equal to 4.186 Kjoules. Based on this conversion, if a rider does 1000 Kjoules on a ride the Kcal equivalent would be 239 Kcals (1000 Kjoules / 4.186). Remember, however, that the energy measured by the Power Tap in Kjoules only represents the mechanical energy delivered to the rear hub by the body, not the total energy expended by the body. If our bodies were 100% efficient then all of the metabolic energy produced by the body would be converted to mechanical work and a 1000 Kjoule ride would cost the body 239 Kcals. Unfortunately, we are not 100% efficient. On average a pedaling cyclist is only about 18 to 24% mechanically efficient. For every 100 Kcals our body burns, only about 18 to 24 Kcals actually goes into moving the bicycle. The other 76 to 82 Kcals gets wasted as heat or is used for the maintenance of other bodily functions. If we were to assume that a person was about 22% efficient, then to do a 1000 Kjoule ride, he would actually have to burn 1086 Kcals (239 / 22% or 0.22). Because 1 Kcal is about 4 Kjoules, and because only 1 Kcal out of every 4 Kcals burned by our body actually goes into moving the bicycle, 1 Kjoule of work performed on the bicycle is about equal to 1 Kcal burned by the body.
In the Applied Exercise Science Laboratory we actually measure mechanically efficiency and calculate personal conversion factors that allow athletes to convert mechanical Kjoules to metabolic Kcals. This conversion ranges from about 1.05 Kcals per Kjoule for the most efficient athletes to about 1.15 for our least efficient athletes. It’s important to note, that a number of factors like training status, temperature, and biomechanics may change a person’s efficiency. When it comes to calculating energy expenditure, however, the potential errors introduced by changes in an athlete’s efficiency are small compared to the potential errors that may occur when heart rate is used to calculate energy expenditure. While there is no perfect way to calculate the total Kcals used during a ride, measuring power output is currently our best estimate in real world conditions.
It’s important to note that food labels generally refer to energy as a Calorie and that a Calorie with a capitol C is the same as a Kcal. So a 250 Calorie Power Bar is actually a 250 Kcal Power Bar.
1 Kcal = 4.186 Kjoules
For every Kcal burned by the body only about 22% is used to move the bicycle.
1 Kjoule on Power Tap ~ 1.1 Kcals burned by the body Can I use my power meter to estimate how much food I should or should not be eating during a training ride or race?
Although our bodies can use carbohydrate, fat, and protein as a fuel source during exercise, once carbohydrate or glycogen stores are depleted an athlete’s ability to maintain a given pace becomes compromised. To maintain a high intensity for a prolonged period of time the ingestion of carbohydrate becomes critical. In order to determine how much carbohydrate should be ingested a rider needs to determine the following; 1) The Kcals burned, 2) The percentage of energy coming from carbohydrate, and 3) The total amount of carbohydrate stored in the body.
To estimate the number of Kcals being burned simply multiply the Kjoules of work done by 1.1 or just add 10 to every 100 Kjoules of work done. For example, if a person rides 2.5 hours at an average power output of 222 Watts, they would do about 2000 Kjoule of work and burn about 2200 Kcals.
If the pace is easy for a person you can assume that about 50% of the Kcals burned will come from carbohydrate. If the pace is somewhat hard about 80% of the Kcals might come from carbohydrate. And finally, for a hard to very hard pace carbohydrate would supply 100% of the Kcals. If we assume that the rider in the example above felt that 222 Watts was a somewhat hard pace, then about 1760 Kcals out of 2200 Kcals would come from carbohydrate (80% of 2200 = 1760). Because carbohydrate can be stored in the body as liver and muscle glycogen, the actual amount of carbohydrate that would need to be eaten would be equal to the difference between what was needed (1760 Kcal of carbohydrate) and what might be stored as glycogen.
The amount of glycogen available for energy during exercise is dependent upon a number of factors including an athlete’s training state, the type of activity, and most importantly a person’s diet. On a normal diet (50 to 60% of calories from carbohydrate), the amount of carbohydrate stored in the body is approximately 18 Kcals per kilogram of body weight. On a high carbohydrate diet (70 to 80% of calories from carbohydrate) the total is approximately 30 Kcals per kg. For a 70 kg (154 lbs) rider, the total energy from stored carbohydrate might range from 1260 Kcals (70 Kg x 18 Kcals per Kg) to 2100 Kcals (70 Kg x 30 Kcals per Kg), while the range for a 60 kg (132 lbs) rider would be 1080 to 1800 Kcals (Table 2).
Based on these values a 70 kg cyclist on a normal diet and riding at what he felt was a somewhat hard pace (222 W) for 2.5 hours (2000 Kjoule total work) would have to consume approximately 500 Kcals (1760 –1260 = 500) of carbohydrate to maintain his pace. This would be equal to about 4 bottles of Gatorade or 2 Power Bars. If the rider wanted to maintain body weight, they would eventually have to replace the additional 1700 Kcals (2200 Kcals – 500 Kcals = 1700) through their normal diet in addition to the Kcals needed to maintain their basic energy needs.
In contrast, if a recreational cyclist trying to lose weight does a 1hour ride at an easy pace of 100 watts he would only do about 360 Kjoules of work and only burn about 400 Kcals. Since this ride was at an easy pace only about 200 Kcals would come from carbohydrate. At 70 Kg and a normal diet, they would have about 1260 Kcals of stored carbohydrate. Theoretically, this person could do the whole ride without eating.
Estimating Carbohydrate Need
1) Figure out how many Kjoules of work you expect to do:
Kjoules = Average Wattage x Hours x 3600 / 1000
E.g., For a 200 Watt average for 4.0 hours the total Kjoules would equal:
200 watts x 3 hours x 3600  1000 = 2,880 Kjoules
Figure out how many Kcals you expect to burn:
High efficiency = Kcals = 1.05 x Kjoules
Normal efficiency = Kcals = 1.1 x Kjoules
Low efficiency = Kcals = 1.15 x Kjoules
E.g. For a person of low efficiency 2,880 Kjoules would be equal to:
1.15 x 2880 = 3312 Kcals
3) Figure out how much of the Kcals will come from carbohydrate:
Very Easy Ride = 30% of total Kcals from carbohydrate
Easy Ride = 50% of total Kcals from carbohydrate
Somewhat Hard = 80% of total Kcals from Carbohydrate
Hard to Max = 100% of total Kcals from Carbohydrate
E.g., For a somewhat hard ride, the total Kcals from carbohydrate would be equal to:
3312 Kcals x 80% = 2650 Kcals
Figure out how much is on board:
Normal Diet = 18 Kcals per Kg x Weight (Kg)
High Carbohydrate Diet = 30 Kcals per Kg x Weight (Kg)
1 Kg = 2.2 lbs
E.g., 70 kg male on a high carbohydrate diet 165 lb rider / 2.2 = 75 Kg 75 kg x 30 Kcal per kg = 2250 Kcals
Take the difference between the Kcals from carbohydrate and the carbohydrate on board:
E.g., 2650 Kcals – 2250 Kcals = 400 Kcals
400 Kcals = 2 Power Bars or 3 water bottles of Gatorade
Use the calculator at the upper left of this page to find your own calorie needs.
thank you!