Product Reviews Sports Technology

A Look Into Running Power And Wearable Devices That Measure It

The introduction of running power metrics is an attempt at quantifying work done or load during running. With wearable devices and sensors becoming more robust and having even better computing abilities, runners can have access to such metrics without being confined to a running lab. We have a look at what running power is about and list out a handful of devices available out there that measures it.

We can only imagine what it’s like to run as fast as Usain Bolt. Whilst most can only dream of running 100 m in under 10 seconds or running 10 kms in under 30 minutes, or even completing 21 kms in under 60 minutes like our elite counterparts, there are ways to run faster. Importantly, there are methods to track performance without relying on expensive laboratories.

To run faster and improve performance, manipulation of two variables are needed: Stride Length (SL) and Stride Frequency (SF). Stated simply, to run faster and increase velocity, a runner has to increase the distance of ground covered (Stride Length), increase turnover (Stride Frequency/Cadence) or both. Whilst the ability to record and analyse running metrics like vertical oscillation (VO), cadence, heart rate and pace is not new, running power remains the new kid on the block despite being around for a number of years. 

How is power measured? 

Power is work done. Whilst some refer to work as the daily grind of a 9-5 office job, work in a physics sense equates to work completed over time. Work is done when a force is applied to an object which causes that object to move. The calculation is:

W=F*d

where W is work, F is force and d is distance.

Using familiar terminology, the object in this sense is the runner and the force is the muscular force that pulls on bones which causes rotation about their axes. The result is torque, or rotational motion. Other forces come into play, too, namely gravitational force – which is great when going downhill but a hindrance when going up. 

Some wearables measure the amount of work performed over a given time (usually measured in seconds) and offer insights into fitness levels, muscular force capability and effectiveness of training. 

Bike Power Meters

The familiar metric of cycling power meters (measured in Watts via a power meter attached to the cycle crank or pedal), tend to use an array of strain gauges to measure force applied through the crank arms, pedals or hub.

SRM Powermeter

The way gauges measure force is through a change in electrical resistance running through the gauge caused by slight fluctuations in the material as force is applied by the cyclist. When a cyclist pushes on the cranks (assuming that’s where the power meter is located), the gauge ever so slightly deforms, which results in the electrical resistance in the strain gauge changing. It’s this change in electrical resistance that’s associated with a known magnitude of force. The reason this chain of events is significant is that what’s being measured is at the point of force application, or when force is actually applied. This is particularly useful for cyclists and triathletes as it provides a real-time measure of how ‘hard’ the body is working.

For example, if a cyclist knows they’re capable of a functional threshold of 250/275 W for a 20 minute burst, it’s possible to use this metric to track effort and intensity when compared to heart rate, cadence and velocity during different conditions (i.e., hills and inclines).  However, unlike power on a bike, running power isn’t necessarily about increasing Watts. Sure, that’s good in some circumstances, but it’s not always helpful. So despite the name and linkage to power, a running power meter works differently. 

So how is running power measured?

A running power meter measures movement in three planes, namely vertical, anteroposterior and mediolateral. Most running power meters come in the form of a foot pod or similar device that clips on to some part of the runner or the running shoe (the shoelaces being the principal location as it provides a secure way to fasten the pod to the shoe). When running, the foot pod or sensor records measurements as a runner progresses through the various stages of the running gait cycle. 

How accurate are the devices?

Running is far more complicated than most realise as it takes a lot of internal coordination to run effectively and efficiently. For example, there’s internal work due to the arms and legs pumping back and forth and external work as the centre of mass (CoM) ‘bounces’ up and down along a sinusoidal path. There’s also positive work (pushing off with each stride, known as ‘toe off’) and negative work (braking during heel strike); and then there’s a significant contribution from elastic energy stored in the Achilles tendon and other tissues as they stretch, akin to an elastic band, upon landing and then spring back when pushing off the ground. According to some (though not all) guesstimates, the energy stored and released via the Achilles tendon contributes 50% of the power required for each stride. Consequently, there’s no single device that can measure all these contributions. 

What are the benefits of running with a power meter?

Calculating running power is a fairly complex task given the different variables that are specific to each individual (i.e. age, weight, height, foot strike contact, etc). Runners, like cyclists, need performance metrics to help with training plans, positive adaptations and ensuring they ‘peak’ at the right time during a season. A running power meter can assist with ​providing​ ​metrics​ ​that​ fundamentally ​track ​the​ ​metabolic​ ​intensity/loading​ ​for​ ​running in changed conditions. This is particularly useful when aiming for a constant power output when interval running or during hill sprints. Stryd released a white paper (link) that provides information on the methodology behind power calculations as well as background about the usability on running with power.

What products are available?

Garmin running power devices (source: DCRainmaker blog)

1/ Garmin has a range of options to measure running power. Their running power app works on some of their watches including the fenix and some of the Forerunner models. On top of that, we need to pair the watch with one of the following three devices: the HRM-Run,  the HRM-Tri, or the Running Dynamics Pod (RD-Pod). Both the HRM-Run and HRM-Tri are heart rate straps with in-built IMUs that measures motion during running. The RD-Pod is a sensor that can be attached to the runner’s waist band at the back. It also captures motion during running. The running power app compensates for wind (a head wind being the enemy of most runners) when providing power metrics. It does that by using location-based weather data to calculate the impact of wind on running power.

RunScribe Sensor placements (source: RunScribe)

2/ RunScribe developed a wearable gait analysis system which consists of a pair of running shoe pods – one for each foot. The shoe pods can be placed on the laces or hooked on the heel part of the shoe (as seen above). Besides delivering running power metrics, it also provides data such as shock (impact and braking forces) as well as symmetry which is comparing metrics between a runner’s left and right side. This can only be achieved by having a sensor on each foot. Another unique thing about RunScribe is they let users share and compare their data with the RunScribe community. This allows individuals to relate efficiency, foot-strikes, and power range with other users.

Stryd sensor (source: Stryd)

3/ Stryd developed a shoe pod sensor that is solely for measuring running power. It is a single shoe pod and runners can pick either foot to place the sensor. They claim that a single pod is sufficient for understanding overall intensity and pacing. With their latest hardware version, the pod sensor actually captures and measures wind resistance. They have actually ran experiments in different wind tunnels to validate the sensor measurements and their algorithms. For those who are interested, they have written a white paper about their tests.

Polar Vantage V Titan, Vantage V, Vantage M

4/ Polar came up with a running power calculation that simply relies on their GPS wrist watch (or wrist computer as they call it). No additional sensors or foot pods required. The team at Polar developed the algorithms by doing running tests in the lab with force plates; the ground reaction forces measured were used to determine mechanical power and calibrate the (sensors in the) watches. The final calculation does require the runner’s weight input so it is critical that users update their weight (on the watches) if they want an accurate running power measure. Polar also took a step further by coming up with a new parameter called Muscle Load to help runners quantify and monitor their training load.

In conclusion

Running power is a relatively new metric that can provide more context to a run. Runners can look at that value and know how hard they are running or even how efficient they are running. With the motion data that are collected from the different sensors, it provides a sense of the runner’s 3D motion and hence general running biomechanics/form. Though it is a stretch to call them biomechanical analysis tools, they can definitely help track and improve performance if used as part of a structured training plan. For runners who really want to understand their biomechanics, running labs with motion capture analysis might be a better option. Or they could check out more niche sensor products such as ViMove run, Arion run (an insole sensor solution) or Fathom Ai.

If there is a running power meter that we left out, or if you would like to share your own experience, feel free to leave a comment below. With that, thanks for reading!

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