They have already become a part of the professional circus and are continuing their triumphant progress in amateur sports. We are talking about powermeters. Powermeters are designed to measure performance when cycling. More and more cyclists use a powermeter to control their training. Many trainers advise ambitious hobby cyclists to use a powermeter, because this way the training can be evaluated and controlled more precisely. In competitions with constant cycling sections such as time trials or cycling marathons, a powermeter is ideal for pacing. The power is measured in the unit Watt.
What does a power meter do for training?
The goal of any training plan is to set training stimuli to which the body reacts with adaptation and to give the body the time it needs to adapt between the stresses. It is a balance between stress and regeneration. The perceived load, however, can differ from the real load. There is a danger of overstraining the body or, on the contrary, not enough stimulation. Therefore a neutral guideline is needed. On the basis of this guideline, firstly, the stimuli set in the past are classified, secondly the stimuli occurring during training are controlled and thirdly the stimuli of the training are evaluated afterwards. Before powermeters existed, training was analyzed with the help of pulse values, among other things. However, the pulse is not only influenced by the training load, but also by factors such as heat, fatigue, time of day, hormone levels or diseases. This can falsify the training evaluations. In order to know how high the stress during a training session is, people now rely on performance measurement.
What do I do with the performance values?
In competition, the absolute wattage values have a more or less great significance depending on the discipline. A sprinter on the track or a time trialist can already make an estimation of the result in the competition based on his possible peak values or his continuous performance. In a mountain stage or road race, other factors such as weight or tactics also play a decisive role. In training, it is the individual, relative values that count most for the training design and to recognize progress. The starting point for the classification of training intensity is the determination of the so-called threshold performance. It is the power at the transition from a main energy supply through fats to carbohydrates. Thus, at rest or during low to medium stress, the body uses mainly fat to provide energy. With increasing sporting intensity and rising energy requirements, the body makes more and more use of carbohydrates. Above the threshold performance level, the energy in the muscle comes almost exclusively from carbohydrates. While below the threshold power can be maintained for a very long time, above the threshold fatigue occurs rapidly. Threshold power is an endurance power that can be maintained for about 40 to 90 minutes before the performance drops. Thus, the threshold power is one of the most important parameters for classifying the performance of an athlete. The physiological processes and thus the threshold performance are attempted to be recorded in the laboratory with the help of performance diagnostics by measuring and analyzing lactate or spiroergometry. This procedure is naturally complex and cost-intensive. Therefore, in practice, people often start to determine the so-called functional threshold power FTP (functional threshold power) and base their training on this. The FTP functional threshold power is defined as the maximum possible continuous power over 60 minutes. Since it would be a great strain for an athlete to push for one hour, some shorter test protocols were developed to determine the FTP. The athlete only needs his bike, a power meter and a device to record the performance. The FTP is determined by a subsequent analysis of the performance data.
The agony of choice
Initially, only a few manufacturers took up the challenge of “performance measurement in cycling”. The first power meters were used in professional sports a good 30 years ago. Meanwhile there is a big market for powermeters and the boom continues. Numerous manufacturers are fighting for market shares. The athlete is now spoilt for choice when buying a power meter. I will not give a complete list of all products on the market, but an overview of the most common measuring systems.
The measuring principles are similar in most cases. However, there are major differences in the specific design and thus also at which interface in the drivetrain the power is determined. The power produced on the wheel is made up of two measured variables: the applied force and the angular velocity. The angular velocity is easy to measure and is reflected, for example, in the cadence. The crux of power measurement is to measure the force as accurately as possible. Most power meters use so-called strain gauges. If a force is applied when pedalling a bicycle, the strain gauges deform. This deformation is evaluated by the powermeter. Behind this is the physical fact that the electrical conductivity of an electrical conductor (strain gauge) depends on its expansion (geometry).
The powermeters available on the market differ in many aspects, especially in their acquisition costs. A major difference, which is partly reflected in the price, is the scope of measurement. Cheap powermeters usually consist of a sensor that measures the power of one leg. On the other hand there are power meters which measure the total power of both legs together with one sensor: Depending on whether these sensors are permanently integrated in a crank, connected to an exchangeable part of the crank or mounted on a crank, the prices vary considerably. A precise analysis using two sensors, one on each side, tends to be one of the more expensive, but not necessarily better measuring systems. Depending on the manufacturer, ANT+ and/or Bluetooth are available for data transfer. The number of measurements and the transmission per revolution differ depending on the powermeter and of course also influence the accuracy of the measured values. Most of the power meters are battery operated. Some models have a rechargeable battery.
Powermeter pedals measure the power at the pedal. The sensor can be located either on the pedal axle or in the pedal body. This type of force measurement should measure the force exactly where it is applied by one or both pedals. Here there is the possibility of one-sided or both-sided power measurement. With the one-sided measurement the sensor is located on the left side – with all current manufacturers of watt pedals. To estimate the total power during one crank revolution, the measured value of the left side is doubled. So the assumption is that the same power is produced on the left and right side. By using another sensor on the right side, separate measurements can be taken and the total power is calculated for this type of measurement. The advantage of pedal measurement is the high flexibility when using the pedals on different wheels. However, some manufacturers require a sometimes more complex calibration after assembly. Watts pedals should generally be calibrated regularly. Some manufacturers even recommend a calibration before every ride. It is important to enter the crank length in a corresponding app and/or on the bike computer, because this information is necessary for the factor “angular velocity” (see above). A clear disadvantage of watt pedals is that you are bound to a special shoe type (Roadbike/MTB) and the cleats on the shoes worn so far have to be changed from time to time. Manufacturers of power meter pedals include e.g. Assioma or Garmin.
Widely used are power meters, which are installed on or in the crank arm. With some manufacturers you buy the powermeter together with the corresponding crank. There is also the possibility to retrofit an existing crank arm with a power meter. Often the existing crank arm is replaced by a crank arm with a built-in power meter (usually of the same manufacturer). Mostly one side is measured on the left side. With some manufacturers, the measurement is made in the crank axle. Manufacturers of power meter pedals include e.g. Stages, Rotor or 4iiii.
The one-sided measurement is a relatively cheap form of power measurement. However, it has its limits. Hardly any athlete has an equal distribution of power from the left and right leg. The distribution is often not constant, but depends, among other things, on cadence, intensity, fatigue and terrain. A one-sided measurement quickly results in incorrect watt values. Let’s assume a rider determines his FTP while cycling in the plane, with 50 percent of the power considered coming from one leg each. With an FTP of 300 watts, this would be 150 watts for each leg. On climbs, however, the rider pushes more with his right leg, which reaches 53 percent of the total power. For the left leg, where the one-sided power measurement takes place, 47 percent remains. If the rider were now trying to climb the mountain at 300 watts, he would be riding well above his FTP and thus lose grains unnecessarily in a race or even collapse unexpectedly. The 300 watts on the mountain would result from the 150 watts on the left leg. In this case, however, the 150 watts only correspond to 47 percent of the total power. The left leg power meter would double the power, although the value is less than half of the total power. The total power from both legs would be 319 watts, or 106 percent of its FTP. The differences between the two legs can be even greater in practice.
Most manufacturers of one-sided watt measurement, whether pedal or crank arm, offer a second sensor for the other side, i.e. a double-sided measurement. The price increases naturally and the argument “cheap” is then sometimes no longer given. But with the use of two sensors the probability of errors increases as well. Dropout or connection interruptions lead then more often to wrong values, which are more difficult to detect. Depending on the manufacturer, the two separate sensors first communicate with each other and the main sensor then sends the measured values, including calculated total power, to a bicycle computer. Since each sensor has a certain inaccuracy, the potential measurement error is doubled when using two sensors. Depending on the manufacturer, there is a dependence of the measured value on the ambient temperature. Thus, changes in temperature can lead to distortion of the performance. This is partly compensated electronically. Here, however, the complexity of the system increases again. At the very least, inertial effects occur, because an externally mounted electronic system takes on the ambient temperature faster than, for example, an entire crank arm can be evenly tempered.
When measuring separately, possible negative power values are not always considered. These negative power values can be derived from the analysis values of the individual sensors from some manufacturers. Thus, when using two separate powermeters, one has the possibility to access additional information through two independent sensors. The Torque Effectiveness can be used to output the positive and negative torque separately for each leg. This means that the force of the active leg and the negative force of the passive leg can be displayed. The balance shows the left-right distribution of the power in percent. Pedal Smoothness shows how evenly the pedalling movement is performed by relating the average power during a crank revolution to the maximum power during the revolution.
Many cyclists rely on powermeters to measure their power output. They measure the total power directly at the transmission between the crank axle and the chainring or crank spider (used for example in the World Tour by Team EF, Education First). These powermeters measure the total power of both legs with one sensor. The accuracy of the spiderpowermeters is usually higher and due to the position in the crank the sensors are better protected (falls etc.) and more durable, which is why they are used offroad (e.g. MTB Team Centurion Vaude). The calibration is sometimes done by the sensor itself and temperature dependencies can be better compensated. Depending on the manufacturer, the installation is more or less complex. Depending on the crank, the spiderpowermeters can partly be exchanged between different wheels. In this category there is the biggest price range from cheap to high priced depending on the manufacturer. Manufacturers are among others power2max or SRM.
For whom is which powermeter suitable? – My conclusion and experience
Beginners avoid high acquisition costs at the first purchase. If the new Powermeter is to be used on an existing wheel, there is the possibility of one-sided measurement or the installation of a crankspider (check compatibility) as a favorable solution. For beginners I would personally recommend the measurement of the total power, because the one-sided measurement can be accompanied by the inaccuracies mentioned above and especially beginners in power measurement have little experience with the classification of the values. In my opinion, a power meter that shows frequent dropouts or disconnections to bike computers is not usable. A noticeable temperature dependence can also disturb the training, especially in the cold season. Trainers usually recommend measuring the overall performance for training control. If you are planning to buy a new bike, depending on the type of crank, a built-in power meter can be purchased at the same time. For my indoor training I use a noticeably temperature-dependent, left-sided power meter, outside the power meter would be much too inaccurate. For this use the device fulfills its purpose, because the temperature and the terrain can be considered as nearly constant. If you only ride a road bike, you have the option to use watt pedals, which are more or less reliable depending on the manufacturer. For the MTB sector with SPD system, the first power meter pedals are now available on the market, but I haven’t read really good reviews yet. For those who are not limited to one type of bike, the Powermeter version crank arm or crank spider is recommended to be free in the choice of the pedal system. The most robust are the crankspider powermeters and are suitable for extreme conditions, like Christoph Strasser had during his RAAM victories with heat, snow, continuous rain and flooded roads. Measuring overall performance is the gold standard for ambitious training and competition.
Any reliable performance measurement via powermeter is better than no performance measurement. Performance measurement should not be seen as a dogmatic guideline, but as a chance for more control in training and competition.
Further links (contains advertising):
Dual-sided power measurement and its advantages-power2max
Validity of the Stryd Power Meter in Measuring Running Parameters at Submaximal Speeds
Validity and Reliability of the PowerTap P1 Pedals Power Meter