Measuring and Training with Power Data

Power Measurement

Now that you’ve got the basics of power and how to use a bike power meter covered, hopefully you're hungry for a little more.

We'll take you step by step through some definitions and explanations that will give you the edge in making your power meter work for you. Let's dive in.

Power Defined

Power is simply the amount of work or energy you expend in a given time frame and is measured as a watt. Normally, work or energy is represented as a joule, while time is represented in seconds.

So 1 watt is equal to 1 joule of energy per second, while 100 watts is equal to 100 joules per second. As a point of reference, 1 horsepower is equal to 746 watts or 746 joules of energy per second. In contrast, a professional cyclist can hold just over 400 watts for 30 minutes.

Measuring Power on a Bike

On a bicycle, there are three places to measure power: the hub, pedals or crank.

Power is measured as the amount of force or torque generated at these points of measurement (pedals, crank or hub) multiplied by the speed or angular velocity of the pedals, crank or hub.

So power output on a bicycle is simply a product of how hard you push on the pedals and how fast you are pedaling. To produce more power, you can either push harder or pedal faster.

Hubs

PowerTap hub-based power meters work by measuring the amount of force, or torque, at the rear hub using strain gauges embedded within the hub.

The PowerTap hub also measures the speed of the hub by using a speed sensor within the hub and a magnet attached to the axle.

Chainrings

The PowerTap C1 Chainring power meter measures the amount of torque applied to the chain rings using strain gauges embedded into each arm of the spider attachment.

Angular velocity measured with an accelerometer on the chain ring is combined with torque to produce a true measure of power.

Pedals

The PowerTap P1 Pedal power meter uses the amount of force applied to each pedal to measure power.

The direction and magnitude of the applied force is then analyzed in real-time to determine which portion of that force is directed to drive the bike forward

Bicycle power is also a product of your speed and all of the forces that resist forward motion. Forces like aerodynamic resistance or wind, gravitational resistance or the grade of a given hill, rolling resistance or the quality and pressure in your tires, and the resistance in moving parts like your chain or bearings all impact your power

Practically speaking, the higher your power output, the faster you go.

P = $\frac{Work}{Time}$ = $\frac{Joule}{Sec}$

It’s important to note that two individuals may produce very different power outputs to achieve the same speed. This is due to differences in body and bicycle weight and in aerodynamics.

For example, a person weighing 200 lbs will need more power or energy in a given time frame to go 15 mph up a hill compared to a rider who weighs 150 lbs . Likewise, riding in a more aerodynamic position, like the drops, will require less power at a given speed on a level road compared to riding in a poor aerodynamic position.

Power and Energy

There are a number of different ways to represent energy. A kilojoule (Kj or Kjoule) is a mechanical representation of energy. In our everyday world we typically represent energy, thermally, as the amount of heat released when burning a quantity of food. Thus, we normally think of energy in terms of the amount of food we can eat in kilocalories.

Since power is really just a measure of energy over a given timeframe, if you know your average power output and the duration of a given ride, you can calculate the amount of energy you use on that ride. The PowerTap system does this continuously throughout a ride and represents that energy in Kjoules, where 1 Kjoule is equal to 1000 Joules.

To get an idea of how many Kcals you burn for a given number of Kjoules of energy transferred to the bicycle, you need to know that 1 Kcal is equal to roughly 4 Kjoules (4.184). So if you do 1000 Kjoules on the bicycle, you’ve really transferred about 250 Kcals of energy to the rear hub. But that doesn’t mean that you’ve burned 250 Kcals worth of food. This is because while riding a bicycle, the average person is just under a quarter or 25% efficient.

That means if you burn 1000 Kcals of food while riding a bicycle, only about 250 gets transferred to the hub to make the bicycle move. The rest just gets wasted as excess heat.

4.184 kJ x 25% body efficiency = 1 kcal

1 Kjoule is also equal to one Calorie with a capital C, which is how kcals are listed on US food labels.

So by a quirk of nature, 1000 Kjoules measured by a power meter is equal to just over 1000 Kcals burned by your body.

During a typical five hour stage in the Tour De France the average rider can do close to 4000 Kjoules of work and burn about 4000 Kcals worth of energy. In contrast, the Surgeon General currently recommends that the average American accumulate about 1500 Kcals, or150 minutes, worth of exercise each week to maintain a healthy lifestyle.

The take home? If you want to ride in the Tour, try and do 4000 Kjoules in 5 hours. If you want to stay healthy, just try and do 1500 Kjoules over the course of a week. Our power meters will help you measure your efforts in either case.

The Significance of Measuring Power

Stimulus vs. Response

A basic tenet in training is that there is a distinct relationship between an individual’s training load, or the training stimulus, and that person’s adaptive response or performance.

First conceived by Hans Selye, a German physiologist, this “Stimulus-Response" relationship is generalized as an inverted U relationship, where too little and too much stimulus or training load results in sub-optimal performances.

Simply put, if you don’t train, you won’t perform, but if you train too much you run the risk of breaking down and hurting your performance.

Stimulus vs Resonse by Hans Selye

Too little or too much stress can hurt performance. Determining the optimal or perfect amount of training for a given individual is often the biggest single problem faced by athletes and coaches.

PowerTap power meters supply the tools and technology to easily measure the detailed training stimulus, where variations in wind, terrain, and drafting, make speed and distance an inconsistent measure of training load.

Because power output is an absolute and objective measure of the training stimulus or intensity, PowerTap technology makes it possible to accurately quantify an individual’s training load during training and competition.

In the same way that strength athletes can measure the actual mass that they lift in the gym, cyclists measure the actual power they produce using a PowerTap power meter, rather than relying on responses that may or may not represent the actual load.

Power (Stimulus) vs. Heart Rate (Response)

In the laboratory, there is a strong and linear relationship between power output and an individual’s heart rate response. In a controlled laboratory environment, as power output increases heart rate increases in a predictable fashion.

Because of this strong relationship between power and heart rate in testing environments, heart rate became a common way to measure exercise intensity outside of the laboratory in real world conditions.

But just because it was a common practice, doesn’t mean it was correct - the relationship between heart rate and power output in the laboratory is not the same as it is in the real world.

A number of factors can change the relationship between power output and heart rate outside of the laboratory. The brain is always sending signals to the heart to adjust the rate of blood flow.

Influential Factors

Other than pedaling the bike, here are just a few factors that may influence that rate:

• Anticipatory Response: Heart rate can rise by simply thinking about effort, which is referred to as anticipatory response. Your body reacts to emotions by going into a state of readiness, called Fight or Flight. The body prepares for activity by increasing heart rate and releasing glucose from the liver for energy.

• Calm and Relaxed: Tense muscles and death gripping the bars or using muscles not involved in cycling, will signal the brain to send blood to these areas as well. Therefore, relaxation and efficient use of muscles will assist in managing the rate of blood flow and ensure the blood flows to the muscles truly needed to move the bike.

• Medications: Can affect heart rate. For example, antihistamines and anti-depressant drugs typically increase blood flow, while beta-blockers and blood pressure medications tend to slow down blood flow.

• Temperature: In hot temperatures, heart rate increases to increase the ability to release the heat and cool the body. However, in cooler temperatures, heart rate is typically lower as the brain is diverting the blood throughout the entire body to keep it warm.

• Gender: When looking at the same workload, women typically have higher heart rates than men, due to the fact women have lower stroke volume, in general.

• Altitude generally demonstrates higher heart rate, as the heart needs to work overtime to pump blood for re-oxygenated and boost the removal of lactate from the blood and muscles.

• Fitness elicits a decrease in heart rate due to its ability to work harder with less effort.

• Fatigue, cramping and dehydration can cause an increase in heart rate, due to the body attempting to transport additional nutrients and remove more toxins.

• Not Enough Recovery: Being in a state of needing more recovery may lower heart rate during exercise, as the brain signals blood flow for recovery, in lieu of sending blood flow to dynamic muscles.

As a result, heart rate is not a good predictor of the training stimulus or power output in the field, especially in competitive events where power output can vary tremendously. This doesn’t mean that the heart rate response is un-important. It just means that the heart rate response should not be confused with the actual stimulus. Where power is the actual exercise stimulus, heart rate is one of many physiological responses.

In the past, when heart rate was used as the primary measure of the exercise stimulus, the basic training tenet of “Stimulus-Response" was actually a “Response-Response" relationship. By monitoring both power and the resulting heart rate response, you can track the actual exercise stimulus as well as an important physiological response. This allows for better tracking of short and long term response to training.

For example, on one day you might notice that your heart rate is really high at 100 watts and that you feel terrible. A week later, you might find that your heart rate at 100 watts is significantly lower and that you feel better, indicating an improvement in your fitness.

Heart rate and power represent two distinct things. The value of each is greatly enhanced by knowing one relative to the other.

Summary of Power vs Heart Rate
• Power is the actual exercise stimulus and heart rate is a physiological response.
• As power increases, heart rate increases.
• Heart rate has a limit (max), while Power is subject to change.
• Humans possess more power than we demonstrate, and can therefore be trained.
• Heart rate takes several seconds to “catch up" or “drop", whereas power is instantaneous.
• Power is a direct measure of mechanical work and heart rate measures the response.

Power Output Provides Objective Feedback

Monitoring and evaluating power output during training and competition provides the most objective and immediate feedback about one’s performance. Ultimately, accurate and reliable feedback is a critical aspect of any training program whether you’re an Olympic athlete or just trying to stay healthy.

In the same way that thermal energy or heat can be measured with a thermometer, mechanical energy or power can be measured with a power meter. Using this analogy, if you wanted to bake a pie and you didn’t know the temperature, you may use trial and error to eventually get a sense for how hot the oven needs to be to bake a perfect pie.

However, if you had a simple tool, like a thermometer, to give you direct and immediate feedback, the learning curve for baking that pie would be greatly accelerated. Similarly, understanding your own response to training relative to an objective measure like power output takes the guesswork out of training because of direct, consistent, and immediate feedback.

Training with Power

The basic principles have not changed – the ability to apply them has.

PowerTap does not change the basic nature of training, it simply allows you to quantify it and thus better implement training principles.

You still have to work hard if you want to improve, but you can also work smarter by applying basic training principles that were difficult to apply without power.

Principal of Training

The principle of training is based on progressively rebuilding cells. The cells are stressed or “overtrained" to establish molecular breakdown. This is known as the overload phase. With proper rest and nutrition, the cell is repaired and restored to a changed level. This is known as the recovery phase.

Complete fitness development is achieved when training includes a gradual progression of stressing all muscle fiber and taxing all energy systems all while providing appropriate recovery to repair the cells. Without this breakdown or a proper rebuilding period, the cells stop adapting in a positive manner.

Your genetics and how you develop metabolic fitness plays a key role in how quickly the rebuilding process takes place. Realize that there are added stresses on the body that need to be included in cellular breakdown, such as lifestyle, additional activities, intensity, lack of sickness, injury and poor nutrition. Accordingly, the principles of training are defined to individuality, specificity, overload/recovery and progression.

Individuality

Everyone has different genetic characteristics, possessing their own abilities, capacities and responses to training. It is unlikely that two people will show the same responses and adaptations to training compared to another. A training program that works well on one individual may not work as well on another. The principle of individuality reflects the uniqueness of each individual’s strengths and weaknesses and one’s ability to respond to various programs.

It is important for an individual to develop techniques and strategies to realize characteristics that are strengths that need only be maintained and that are weaknesses that need to be developed. The more of these factors you are aware of, the more specific training can be.

PowerTap technology provides data specific to an individual’s muscle fiber type, energy systems, recovery rate, and more. Since Power Tap powermeters provide scientifically accurate data that is specific to the individual user, you or a coach can figure out a specific training plan for your goals rather than generalizing from a program designed for a completely different rider.

Specificity

A fundamental principle of training is that training should be as specific to the demands of competition as possible. In other words, to optimize your training for a given event, your training should either partially or entirely replicate the competition.

Using PowerTap technology you can measure different aspects of a given race, such as the average power output, total energy required, and time spent in different intensity ranges. Individuals and coaches can then use that information to develop better training strategies and to monitor whether the training is specific to the competitive goals.

In the same way that power helps athletes accomplish specific training goals that help them in competition, monitoring power output can help regular individuals accomplish specific health or fitness goals.

For example, there are 3500 kcals in one pound of fat. If you want to lose weight, most doctors recommend that you not try to lose more than one pound per week. In order to do that, you would need decrease your food intake by 500 kcals, which is the same as 500 Calories on US food labels, each day or burn an extra 500 kcals through exercise each day.

Because dieting alone can lead to a decrease in muscle mass, and in turn slow your metabolism down and hurt your fitness, most doctors and physiologists recommend that you lose those extra calories through exercise.

By using a PowerTap power meter, you can create a very specific goal of riding at least 500 Kjoules each day. The exact amount of time it takes to do 500 Kjoules depends on both weight and fitness. Regardless, you can match a specific training regime to the demands required for a specific goal.

The overload principle states that in order for the cardio-respiratory and muscular system to develop endurance, strength and power, the systems must be gradually stressed with a stimulus greater than it is used to (over-trained).

With proper rest, the body adapts to the stimulus, programming the body to become stronger and develop stamina. By also overloading the body based on how it specifically needs to perform, known as specificity, the significant muscles adapt purposely to realistic applications of activity.

To build strength, tax the body with resistance greater than it is used to. To build endurance, work for longer durations than it is used to. To develop a fast spin, hold a faster cadence than you are used to. Once the overload ends, restoration is required to ensure a positive adaptation.

The overload breaks the body down on a cellular level, ending with fatigue and a drop in performance. This breakdown accumulates unless time is allowed for adequate recovery. The period of recovery rebuilds the body to be subjected to a new training load. The body is restored and adapts to the stimulus, increasing fitness and/or performance. It is then time to begin the overload-recovery process again. This will ensure progression to an optimal level.

If there is no regular overload, then there is also too much recovery (over-resting). In either case, there is no need for the body to adapt, resulting in a decrease in fitness -known as reversibility. While this turnaround is considered detraining and a negative adaptation, reversibility can also be used to allow complete physical and mental recovery from extended periods of training, allowing one to recharge and get motivated.

It is important to allow 2-4 weeks of full recovery after major events or a long training season. When you return to training, you will quickly regain your previous level of fitness and performance.

Periodization

Another basic training principle is the idea of periodization. Since different responses take place during different intensities and techniques, you must add variety to your training to improve your fitness level.

If you do similar training rides (the same load) every day, year round, the body will plateau and your fitness will stay at the same level. In other words, if you do the same thing, you get the same results. If you are looking for different results, it is necessary to include variety.

These diverse cycles of training incorporate individuality, specificity, overload and rest. Periodization simply refers to the balance between hard days of training and easy days of training or recovery.

Essentially, the training stimulus needs to be greater than recently experienced - an idea referred to as overload. At the same time, any period of overload needs to be followed by a period of rest or recovery to allow the body to heal and grow stronger.

Coaches and athletes can create proper periodization schedules using PowerTap power meters to evaluate one’s training load. If done correctly, an individual’s training load looks very similar to a stock chart for a successful company. Despite periodic highs and lows, the general training load that person is able to handle continues to grow.

Power and Aerodynamics

Over a flat road on a windless day, 90% of the resistance impeding forward motion on a bicycle is due to aerodynamic drag. As a result, a cyclist’s speed is largely a function of aerodynamic resistance and power output.

Aerodynamic resistance changes exponentially with speed, therefore a small change in aerodynamic resistance created by a subtle change in body position or equipment can cause dramatic changes in power output at a given speed. For example, going from the hoods to the drops can save 30 to 50 watts at 25 mph, creating a time saving of 3 to 5 minutes in a 40 km time trial.

Due to the fact that a decrease aerodynamic drag can dramatically improve performance, many professional cyclists pay large sums of money to optimize their body position and aerodynamics in a wind tunnel. Essentially, these athletes are attempting to decrease the power required for a given speed.

The same improvements normally limited to a wind tunnel can be made by simply analyzing the relationship between power output and speed with a PowerTap power meter. For example, if a change in body position decreases the power required for a given speed, then that body position is more aerodynamic.

By experimenting with different body positions and measuring power, significant improvements can be made in aerodynamics - and ultimately speed.

Next: Setting Goals Using a Power Meter

A power meter can be used for setting goals at all levels of training. Get started in using your new tool to set goals for interval sessions and for your overall training load.