Applications of Power Measurement in Running Training

*Original article from Sportraining

Contextualizing Power Measurement

To understand power as an indicator of an athlete's external load, it must first be placed in context. This helps explain how a concept initially applied to cycling evolved through a series of assumptions until it reached running, where it offers both advantages and drawbacks.

For this reason, and in line with the consensus on monitoring athletes' training load, load in cyclic endurance sports can be assessed through internal- and external-load variables. Power is the main variable used to assess external load in cycling (Allen and Coggan, 2006). In fact, the use of a power meter allows us to assess athletes' fatigue and recovery, individualize training load, and determine training loads.

Power is calculated by multiplying the force exerted by velocity (Marroyo and López, 2015). In other words, mechanical power is the variable that indicates the rate at which work is performed, and it is calculated by dividing the work performed by the time required to do it.

Force, however, is a complex quantity to measure because no devices measure it directly, so strategies are used to measure it indirectly through direct proportionality with other variables. Some of these strategies exploit the electrical characteristics of certain materials, whose electrical properties change when force is applied, producing a measurable linear relationship. In practice, this means using force platforms, devices that measure ground-reaction forces and are based on two technologies. An example of the first, piezoelectric technology, is a household scale, based on quartz's physical property of changing its surface electrical charge when the mechanical load it supports changes. The second is strain-gauge technology, which changes electrical resistance when deformed by mechanical stress; in other words, a pressure platform. This second technology is used in SRM cycling power meters^® or Powertap^®, which record the force generated between the chainring and crank and at the rear-wheel axle.

What Some Authors Say   

Once power and the different ways of measuring it are known and understood, it is difficult to understand why pioneers of running power such as Vance (2016) state in the book “Run with Power” that power has revolutionized running. With the arrival of running power meters, they argue, there is now a tool that measures performance directly, objectively and repeatedly. They state that the power produced can be measured not only during training days or weeks, but also in competition, and that running power is the most suitable tool for analyzing an athlete's fitness and tolerance to stress. They find it surprising that this tool was not used earlier, because it makes it possible to analyze data, metrics and measurements that were previously unknown, opening the door to new levels of elite sports performance. This author gives numerous reasons why using running power can positively affect training and fitness, including specificity, technical improvements, objective performance measurements, load quantification, better recovery, precision in tapering, warm-up effectiveness, watts per kilogram, speed per watt, and objective feedback within periodization. Or… what we could describe as an intensification of the key training principle: individualization of load.

The Reality
But if this were true, it would be difficult to understand why many coaches and athletes do not answer with an emphatic yes to the question: have running watts replaced pace (min/km) or heart rate (bpm)? In that case, we would have to assume that watts have done for running what they once did for cycling, giving other
sports such as trail running a new, more objective approach rather than relying on distance, elevation gain, pace and beats per minute to estimate how hard an event, training session or competition is. Extending that emphatic yes to the scientific field would mean discussing a power meter based
on accelerometry that measures performance directly, objectively and repeatedly—in other words, a scientifically valid and reliable power meter. It would measure power rather than estimate it, and it would do so every time with the same accuracy and no variation between measurements. If we compared the Stryd running power meter with a Mercedes engine, we could say that this engine is the most reliable on the Formula 1 starting grid because it always runs at the same revolutions per minute with the least possible fluctuation. If that were so, and Stryd had been scientifically validated, there would not be studies in which power is more sensitive to changes in cadence and other biomechanical factors than to running economy or caloric cost (Austin, Hokanson, McGinnis and Patrick, 2018). This is logical because power is based on accelerometry.
We should add that running-power data are not obtained through a power meter or watt meter as in cycling. Instead, they are estimated through a complex calculation and assumptions that the accelerometer processes using different formulas.

The Stryd Power Meter
First of all, we are discussing Stryd, one of the best-known brands on the market, solely because it predominates in the scientific literature and is the device most widely used by elite athletes and triathletes, with no commercial interest involved.

The second version of the Stryd power meter—unlike the pioneering model, which was a chest strap—is a pod resembling a pedometer, with a sensor that contains three accelerometers. It measures foot acceleration in three directions during running: horizontal, vertical and lateral. Horizontal acceleration receives particular attention because it is the primary objective of running: the aim is always to maximize acceleration in the horizontal plane while minimizing, as far as possible, acceleration in the vertical plane (vertical oscillation) and lateral plane (rotation of the shoulders, hips, hands, elbows, knees and feet). The importance of recording lateral and vertical movement to optimize running economy (RE) should not be overlooked. Power (P) is therefore calculated using body mass (m) in kilograms (kg), acceleration (a) measured in meters per second (m/s²) and velocity (v) in meters per second (m/s). One of the main advantages provided by Stryd and its software is a real-time calculation of running power based on three-dimensional acceleration data. In other words, it gives the athlete immediate feedback  . As for further drawbacks, although the basic formula described is simple, it requires a complex algorithm to integrate acceleration and direction correctly and consistently.

What Formulas Are Used?

Some of these formulas and algorithms are explained in “The Secret of Running” by Van Dijk and Van Megen (2017). This book provides the scientific basis for power-based training, using data collected with Stryd itself, although with the pioneering model (a chest-strap accelerometer). Together with “Run with Power” by Vance (2016), it completes the scientific literature on running power. However, as with the cycling book “Training and Racing with a Power Meter” by Allen and Coggan (2016), bibliographical references are scarce.

Van Dijk and Van Megen (2017) propose a comprehensive model of running physics that determines how much power is required to run under different conditions. Running into the wind obviously requires additional power, so the amount needed to overcome air resistance (Pa) must be known. Running uphill also requires additional power, so the amount required to overcome climbing resistance (Pc) must also be known. And, of course, running on flat ground without either of those resistances still requires power; this required power is called running resistance (Pr). In conclusion, under equilibrium conditions, the power available from the human body must equal the sum of the power required to overcome these three forms of resistance: running resistance, air resistance and climbing resistance.
This model is expressed and developed using the following formula:

P= Pr + Pa + Pc

Running Resistance

Pr = cmv

In theory, the amount of power (“Pr”, in watts) required to overcome resistance can be calculated from the runner's specific running cost (“c”, in kJ/kg/km), body weight (“m”, in kg) and velocity (“v”, in m/s), as shown in the formula. This running-resistance formula shows that power requirements are directly proportional to body weight and velocity. Running resistance can be explained generally, showing the relationship between power in watts (W) and speed in kilometers per hour (km/h), or specifically, showing the relationship between specific power expressed in W/kg and speed in km/h.

The book under discussion explains that a value of 0.98 kJ/kg/km can be used for specific energy cost (“c”, in kJ/kg/km). Several textbooks and scientific articles set the specific cost of running at a standard value of one kcal per kilogram of body weight per kilometer. Since one kcal equals 4.184 joules and metabolic efficiency can be set at 25%, this would be equivalent to 4.184 x 0.25 = 1.05 kJ/kg/km, making this value slightly higher than 0,98.

Air Resistance

Pa = 0.5pCdA (v + vw) 2v

The power (“Pa”, in watts) required to overcome air resistance depends on air density (“p”, in kg/m³), the air-resistance factor (“CdA”, in m²), velocity (“v”, in m/s), and wind velocity or “velocity wind” (“vw”, in m/s).

Climbing Resistance

Pc = (i / 100) mgv Climbing resistance depends on gradient (“i”, in %), body weight (“m”, in kg), and velocity (“v”, in m/s).

Once this is understood, we can deduce that running “rFTPw” (“Running Functional Threshold Power”), or “Functional Threshold Power”, expressed in watts (W), refers to estimated watts associated with a given intensity. This can be calculated on the device's own website in the section called “Power Center” when using Stryd, or theoretically through a complex calculation model. Data from different tests can be uploaded to calculate FTP and thereby estimate a specific time for a 5k or 10k. These tests are explained both in Stryd's theoretical manual and in Vance's book (2016), and are summarized in the following table (Table 1).

Conclusions

In conclusion, and considering the concepts of cycling and running power within a cyclic endurance sport such as triathlon, all coaches who plan the season and quantify training load in this sport through TrainingPeaks^®, the well-known virtual software based on the methodology of Allen and Coggan (2006), should reflect on the following. When triathlon is analyzed by discipline, watts can be measured validly and reliably with a power meter—directly, objectively and repeatedly—only in the second discipline, cycling. If critical speed is used in the other two disciplines, swimming and running, we are using the external variable of “pace” on a platform whose main variable or “gold standard” is not pace, but power. The same issue arises when quantifying load.

For all these reasons, running power may, for now, be useful as a complement to training by helping us understand improvements in running technique. Aubry, Power and Burr (2018) demonstrated this when they assessed running power as a training metric for elite and recreational runners. They concluded that
among recreational runners, improvements in the mechanical components of running could be associated with reductions in ground-contact time, vertical oscillation and cadence. In the elite population, however, Stryd is not accurate enough because, although metabolic demand differed at different speeds, it did not differ across different surfaces. It can also measure parameters such as stiffness quickly and easily, as greater stiffness has been positively associated with flight time and foot-strike angle, and negatively associated with ground-contact time (García-Pinillos, Latorre-Román, Ramírez-Campillo, Párraga-Montilla and Roche-Seruendo, 2018). We must not forget, however, that the power value obtained is based on accelerometry, so it may be more sensitive to changes in cadence and other biomechanical factors than to running economy or caloric cost (Austin, Hokanson, McGinnis and Patrick, 2018). At present, the external variable “pace” (minutes/kilometer) and the internal variable “heart rate” (beats per minute) remain the primary variables, both in day-to-day work on an athletics track and when planning and quantifying running training. This would explain why planning and load-quantification platforms
such as TrainingPeaks® have not yet replaced sport-specific load-quantification models such as Objective Load Equivalents (ECOs) (Cejuela
and Esteve-Lanao, 2011) in triathlon.

AUTHORS

Javier Olaya Cuartero. Doctoral candidate at the University of Alicante. Master's degree in Sports Performance and Health. Graduate in Physical Activity and Sport Sciences (CCAFyD). National Triathlon Coach.
Roberto Cejuela Anta. PhD in Physical Activity and Sport Sciences. Senior coach in triathlon, athletics, cycling and swimming. University of Alicante.

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BIBLIOGRAPHY

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