EXERCISE PHYSIOLOGY - ENERGY REQUIREMENTS OF BICYCLING
- the weight of the cyclist and equipment
- the distance
- the terrain (flat versus hilly)
- the speed of the ride
- headwinds or tailwinds
ENERGY - POWER, CALORIES & WATTS
Before we go any further, let's review the terms energy, force, power, Calories, and watts which are often used interchangeably. Energy is the ability to perform work. The presence of energy is revealed only when change takes place. Potential energy is stored energy (the energy which will let you roll down the hill on your bike, starting from a dead stop, without ever pedaling). Kinetic energy is the energy of motion (the energy contained in you - and your bike - when already rolling down that hill and evident if you run into someone while in motion). The measurement units for energy (either potential or released) are calories or Calories.
Force is the ability of that energy to make a change - to change the state of rest or motion in matter. When force is actually applied, work (force applied over some distance) is done. The same amount of work is done if the task is accomplished in 5 seconds or 5 minutes. The rate at which the work is done is power - the more work per minute or second, the more powerful the force applied to do that work. And watts are the units used to measure power. The more force applied to accomplish the task in a shorter period of time, the more work done and the more power required to do it.
Energy output can be expressed in absolute terms (time interval independent) or in as energy released over a specified or defined time interval (time interval dependent). The most common time independent energy unit used in the cycling literature is the Calorie. In the physical sciences (physics, chemistry), a calorie (small "c") is the quantity of energy required to raise the temperature of 1 gram of water 1 degree centigrade. As this unit is too small to easily express the energy needs of biologic systems, the Calorie (large "C"), which is equivalent to 1000 calories (small c again) or 1 kcal is often used. Unfortunately most nutritionists forget to capitalize the "C" when they are writing about "calories" (they really mean Calories), so don't get confused. If the energy released is measured over a set period of time, it is expressed in watts, and is a reflection of power.
Approximately 60% of the Caloric energy from the food we eat is lost as heat during the fabrication of ATP (adenosine triphosphate), the high energy, intermediary molecule actually used by the muscle cell to power muscle contraction. Additional energy, again reflected as heat production, is lost when ATP is metabolized in the actual mechanical work of muscle fiber contraction. The net result - only 25% of the Caloric energy in the food we eat is actually used to power the mechanical work of the muscle cells. The initial heat loss associated with the conversion of Calories in food into ATP occurs slowly over several hours and is easily compensated for by our body's temperature control mechanisms, but the heat produced with the metabolism of ATP to power muscle contraction is concentrated over a shorter period of time and is why our body temperature rises (and we sweat to compensate) when we are exercising.
Our bicycle, on the other hand, is very efficient in terms of energy loss. Over 95% of the muscle energy we use at the pedals is translated into forward motion and less than 5% is lost (again as heat) from the rolling resistance of the tires, bearing friction, etc. Some of the things we can do to increase the efficiency (decrease resistance losses) are:
- keep bearings and chain well lubricated
- use light oil in bearings and bottom bracket for time trials
- use light greases - paraffin gives more resistance than grease
- use tires with a small "footprint"
- keep tires maximally inflated to decrease rolling resistance
- use thinner, more flexible tires (less energy taken up in sidewall deformation)
Curt Austin has put together a nice calculator to estimate power output (in Watts - you enter your own parameters) on his website. As energy used in Watts is directly proportional to Calories, this calculator will let you play with the numbers for weight, position on the bicycle, road grade, and air resistance/wind which we will discuss below.
Here is an interesting question re the ability to "train" to increase power - with a f/u. It is annecdotal, without proof, but is worth considering.
Q. I have added a ballast of 5.5 kg's to my hydration pack (so my buds don't see) to see if I can train with it and then shed it on race day. I also always ride with my Sigma light and battery firmly secured in my water bottle cage(another kg at least) telling my mates its just too much trouble to take it off and put it back on again. I also have my race wheels that are 600g lighter than my traing wheels.
I have read a lot of hill training tips and routines but the underlying goal is to increase your power to weight ratio. So I figured that if I weigh 70KG's and upped that to 77 for training, then shed it on race days, I would be scoring an an increase in my P 2 W ratio which would help me get to the top of the hills in touch with the real climbers. What I have found is, that I don't notice the extra weight once I have the pack on and I'm riding. I just find that when we dice for the crest of the hills my legs are on fire and I may come second, but I am not even thinking about the extra weight. Slowly I have managed to get back to where I was in the ranks of my chain gang with carrying the extra weight. I don't have any power measuring equipment only HR and my HR on the climbs is +-8 BPM higher than before, depending on how steep the climb. I do manage to stay with my mates though. Do you think my plan has merit?
A. If one believes that training (cardio and strength) is the body responding to stress, then adding extra weight for training and shedding for the race should work. I'll be interested to hear about your results. It is the saem concept as doing intervals to increase your cruising speeds. Don't forget to let me know.
F/U Hi Dick, I had a great race today rode 2Hrs 29min 15secs for 100k's. There was no real wind to speak of. I did have some niggly feelings in my legs at about 95k's but no full blown cramps. If I can repeat this performance in November(19th), when I go up to ride the 94.7 in JHB, I'll be really chaffed. It is at an altitude 1500m higher than Cape Town and has a "sort of" climb in the middle of about 7k's 3.2% gradient. The rest of it is rolling hills. My goal there is a sub 2H30.
WEIGHT
The combined weight of the cyclist and equipment impact the energy requirements of a ride. This relationship is directly proportional i.e. a doubling of the weight on the bike doubles the number of Calories expended. And 2 pounds on a cyclist is just as much a problem as 2 pounds of equipment on the bike frame itself. Austin did a nice analysis on the effect of weight on performance. Here's his conclusion: I thought it would be interesting to see how weight would influence these curves. If I lost 10 lbs (about 5%), I would be able to go about 5% faster on the steepest hills, 0.4% faster on the level, and about 2% slower on the downhills. Over a simulated 20-mile closed-circuit ride with a variety of grades, a 10-lb difference produced a 33 second difference. This may or may not seem significant in the context of a time trial. On the other hand, there are two hills on this simulated route where the heavier rider falls back 14 seconds. That is, about 200 feet back and well-dropped. A two-lb difference that you can buy at a bike shop for $500 amounts to only 7 seconds on this circuit, but again, this could mean cresting a hill 50 feet behind your better-sponsored buddies.
HORIZONTAL DISTANCE
Horizontal distance. We all know that it takes more energy the further we carry any object. The same is true in cycling. On level terrain, the number of Calories expended is directly proportional to the distance and doubling the distance (weight remaining the same) will double the number of Calories required.
VERTICAL DISTANCE (hills)
Vertical distance, i.e. climbing a grade or hills requires additional energy as you overcoming gravity (essentially lifting the cycle/rider to a higher elevation). A common question is how speed on the flats compares to speed on an uphill slope. Using Austin's calculator, I first calculated the power output for a 170 pound cyclist & 22 pound bike on the flats at 20 mph. It was 210 watts. Keeping energy output steady (at 210 watts), I then calculated the speed on a 1% (17.25 mph), 2% (14.6), 3% (12.3) and 5% (9.0) grade.
What about descents and hilly terrain? How does weight factor into these riding conditions? You may have noticed that a heavier rider descends a hill faster (energy expenditures being applied to the pedals being equal) than a lighter one. This seems to fly in the face of a fact you learned in physics class about all objects falling at the same speed independent of their weight. But when going biking down a hill, the slope factor needs to be taken into account. The final speed down a long hill is the balance between the propulsive forces - total rider/bike weight x the sine {that's a trigonometric function} of the angle of the hill - and the resistive forces - wind resistance is the big one. And the heavier rider comes out ahead. If one does the exact calculations with twin brothers weighing 175 pounds, descending a medium slope hill, riding similar bikes, and in exactly the same aerodynamic positions, with one carrying 25 pounds of lead shot, the heavier one would go 26.73 mph while the lighter one would be slightly slower at 25 mph.
And what about rolling terrain?? With climbing, the lighter rider has a definite advantage over the heavier one. And in rolling terrain with repeated ups and downs, the lighter rider comes out ahead.
INERTIAL WEIGHT
Finally, weight is a factor in sprints where inertia (the resistance to setting an object into motion - why it is harder to get up to speed on a bike than to maintain that speed) comes into play. It definitely takes more energy to accelerate a heavier rider/bike combination in a sprint. And extra weight in some bike components (rims for example) may require twice as much energy to accelerate as an equal weight in the frame. This is a result of the fact that with rotational speed you are accelerating these components much more quickly. (Note: this means you should upgrade
The bottom line - the heavier you are, the greater the total energy requirements for your ride. And except for the special case of inertia, all weight is equal. So don't forget that the extra water bottle, the larger heavier tool set, and even that extra pancake you ate in the morning all require additional energy on the ride. And saving a few ounces by eating one less pancake will have as much impact on your performance as that expensive titanium item you've been saving to buy.
AIR RESISTANCE, WIND, AND DRAFTING
Along with the Calories needed to
- counter the effects of gravity
- over come the friction and rolling resistance in the bicycle
Air resistance increases with your air speed (the velocity of our travel through that mass of air). Even with the best riding technique, a head wind will increase your energy expenditure per mile for any specific ground speed (the speed indicated on your bike computer). With the head wind, your air speed (and air resistance) is now GREATER than your computer indicates, the air resistance is higher than at a similar ground speed in calm conditions, and your energy needs are greater. Likewise a tailwind will decrease our air speed relative to your ground speed and make it easier to maintain any specific ground speed. And worst of all, this relationship is an "exponential" one which means that doubling our air speed MORE THAN doubles the Calories expended per mile traveled.(This graph visually demonstrates the fact.)
A headwind on an out and back course always results in a slower total ride time than for the same course ridden in calm conditions as the time gained on the return trip with a tail wind doesn't make up for the loss from grinding into the wind on the way out. For a 12 mph wind, total time will rise by about 7%.
Remember that the "speed" that determines your energy needs to overcome air resistance is your AIR speed, not the GROUND speed which is read from your computer. When you are calculating energy needs for a ride, it is the air speed that is used. A head wind should be added to your average ground speed to determine your air speed (and thus air resistance) while a tail wind should be subtracted from your ground speed. If you think about it, this makes sense - it is always easier to ride with a tail wind, ground speed staying the same.
At cycling speeds greater than 15 mph (24.1 km/h), the energy needed to overcome AIR RESISTANCE greatly exceed those of the rolling and mechanical resistance in your bike. For example, in going from 7.5 mph to 20 mph:
- mechanical resistance increases by 225%
- rolling resistance by 363%
- air resistance by 1800%.
In order to benefit from drafting, you've got to be in the drafting bubble behind the cyclist immediately in front of you. And in a crosswind the bubble will NOT be directly behind the rider in front but will be some angle away from them. The effectiveness of this bubble decreases with the distance, being the greatest if you draft closely and falling off until there is minimal benefit at 5 or 6 feet. The important fact is that you will get some benefit 3, or even 4 feet, back - and it's a lot safer than being directly on the rear wheel of the rider in front of you.
The rider being drafted also gains a slight advantage. This is explained by the fact that the low pressure behind the lead rider is increased in a pace line, giving the leader a slight "nudge" due to the pressure differential between the high pressure ahead and the low pressure behind. This is why a NASCAR racing car will go 1-2 mph faster when being drafted.
Since wind resistance plays such a great role in the overall resistance we get when riding, it makes excellent sense to draft. Better if closer, but that comes with practice and skill as well as trust in the front-rider's smoothness and consistency.
Your frontal surface area affects your air resistance. Wind tunnel results show that eliminating the drag created by projecting 4.5 inches of a pencil into the airstream will provide a 158 foot finish line advantage to a cyclist in a 25 mile time trial. That baggy jersey or upright position may be costing you minutes.
Let's review the factors in air resistance again:
Air resistance =.5*(rho/g)*Area*Cd*V^2
- rho=air density
- g=gravity
- area= frontal area of the rider and bike (scrunch down, less area, faster ride)
- Cd=coefficient of friction (smoother rider and helmet, and less protrusions from the bike, the lower the Cd. This also refers to the shape of the frame, wheels, etc. A tube, spoke, fork shaped like a wing has a lower Cd than round spokes, tubes,or forks.)
- V=air speed - which is squared (ie going from V=7mph to 21mph is a 3x increase in speed which is then squared and the force required is now 9x)
Shocks, both front and rear, will affect your riding over uneven terrain on a mountain bike. Front shocks decrease vibration transmitted to the shoulders and allow more concentration on the course (no energy issues here). The older rear suspended bikes without a rigid rear triangle could absorb some pedal/rear wheel energy, but this is less of an issue with the newer rear suspensions. One study did compare rigid frame (RIG), front shock (FS), and fully suspended (FSR) mountain bikes using the same riders and course. The front suspended bikes finished 80 seconds ahead of the RIG and FSR bikes over a 31 minute course!
THE BOTTOM LINE - HOW MANY CALORIES DO YOU "BURN" WHILE CYCLING?
To calculate the Caloric requirements of cycling, you need to total the Calories needed to maintain your basic life processes (your basal metabolic rate or BMR) which are needed even if you were not exercising and the Calories used for the physical activity itself. A third component called the "thermic effect of food" refers to the energy expended in digesting, absorbing, and transporting food energy to the cells in the body. Thus your total Caloric needs can be expressed as:
CALORIC NEED = CAL(bmr) + CAL(physical efforts) + CAL(thermic effect)
As a rule, the average American, pursuing the average recreational activities and chores of daily living (mowing the lawn, etc.), uses:
- 23% of their Calories for physical activity
- 10% of their daily Calories for the thermic effect
- 67% of their Calories for the BMR
This is a straight 10% of all the Calories you actually eat, so you can easily calculate it. (You add up CAL(bmr) and CAL(physical effort) that need to be replaced and add another 10% to cover the energy needs of digestion and absorption.)
ENERGY REQUIREMENTS IN A COLD ENVIRONMENT
It was mentioned that a cold environment does NOT increase the BMR but requires the expenditure of additional Calories to produce heat energy and thus maintain a constant body temperature. Generally this is from muscle activity and is most noticeable with shivering to generate extra heat energy when your core temperature is falling. While riding there will be some "waste" energy (from the inefficiency of converting eaten of stored Calories into power at the pedal) but then again, the wind chill effect from riding will accentuate heat loss and tend to counterbalance this effect. How many additional Calories are neededin the cold? At rest, roughly 16 Calories per day for every degree F below 98.6. Although one can argue about exact BMR and find different formulae to calculate basal Caloric requirements, the following gives an estimate of the approximate extra energy needs (again, per day): Additional Calories/day for a cold environment = (98.6 - ambient temperature in degrees F) x 16 which would then be added to the BMR calculation and Calories used for exercise.
Does exercising in the cold markedly increase Caloric needs? Probably not by a factor that is of any significance to a cyclist, but it once again demonstrates the multitude of variables that need to be considered as one estimates the Caloric needs of exercise and cycling.
QUESTIONS
Question:I have a heart rate monitor that calculates Calorie burn based on my activity level and I was wondering if I should feed just that number or add that number to my daily requirements. - WTD
Answer: I wouldn't calculate your Caloric needs from a HR monitor. For example, does a 200 pound muscular guy with a HR of 180 burn as many Calories as an out of shape 200 pounder at the same heart rate?? Watts expended relate to work done. Heart rate doesn't. If your basal is 1700 and you really burn 1000 with exercise, you need to eat 2700 between the 3 meals and supplements during that 24 hours.
sursa: http://www.cptips.com
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