Location matters with wearables, but why?

This article was developed in collaboration with our friends at Span.Health

Quick Read

  • Wearables today can provide a wide range of insights, including sleep, heart rate during exercise, illness detection, stress and recovery, and much more.
  • These wearables are used in many different contexts/use cases – at rest, during sleep, during exercise, during daily activities like typing, talking
  • Location matters – not all body locations are created equal for monitoring biometrics
  • Pick the right wearable for what matters most to you


Wearables are seemingly everywhere (on the body)

It seems like everywhere you look you see people using some form of wearable device today. It’s not too surprising considering there are hundreds of millions of devices in use around the world, with hundreds of millions more sold every year.

When most people think of wearable devices, they think of smartwatches and fitness bands, but wearable technology comes in many different form factors, including devices on wrists, ears, arms, fingers, head, and even embedded in clothing.

In general, most of these devices are doing similar things – measuring biometric signals from the body to generate assessments of exercise intensity, activity tracking, sleep quality and/or general health & wellness. A wide range of device types and use cases is great for consumer choice, but does come with trade-offs in user experience, performance, and capabilities. This article explores the benefits and limitations of different wearable form factors available today.

Location matters

Just like real estate, location, location, location is extremely important for wearables. Why? Accuracy, capabilities, and user experience. The vast majority of wearables use PPG sensors as the primary sensing modality (if you’ve seen the green blinking lights on the back of a smart watch, then you’ve seen one) for measuring heart rate, heart rate variability, respiration rate, blood pressure and other metrics.

PPG sensors work by shining light into the body and measuring how light is scattered from blood flow. They are most accurate in areas of the body that limit the amount of light that is scattered or absorbed by physiological characteristics that are not related to blood flow, like bone, muscle, tendons, and other tissues. They’re negatively impacted by parts of the body that experience more movement when the body is in motion, such as wrists and ankles, because motion increases light scatter making it harder to find the signal amongst the motion noise.

In general, the ear and head are very good places to measure biometrics with PPG sensors, because they enable high signal quality with good blood flow, minimal light scatter, and limited local motion (i.e. when the body is in motion, the ear is relatively stable). On the other end of the spectrum, the wrist and ankles are comparatively poor locations because of the physiology in those areas (bone, muscle, tendon) and local motion (think about arm and leg movements when walking, talking, running, etc.).

High biometric signal quality is important because it enables advanced metrics that are increasingly important to helping wearables provide deeper insights into an individual’s health and fitness. For example, heart rate is a standard feature on most wearables today and can be achieved with decent accuracy in nearly any body location. However, something like blood pressure monitoring is much more difficult with PPG sensors and requires optimal signal quality to generate accurate readings, which limits the sensor locations and devices that can be used.

User experience + location

Location is important but must be considered in the context of the overall user experience with the device, software, and capabilities it can deliver. The user experience and realities of technology and human behavior force trade-offs in product design. For example, a “wearable” device that provides every health and fitness insight you can possibly want but requires wearing a backpack full of equipment is not a viable solution for most people.

In wearable devices, these trade-offs tend to fall into these categories:

  • Location – In addition to the accuracy considerations mentioned above, a device’s location can determine its long-term wearability. For example, while the forehead is a very good place to measure biometric signals, it’s difficult to get people to wear a device on their forehead every day for long periods of time.
  • Size/comfort – In general most people want their wearables smaller rather than larger, but this limits the battery life and capabilities of the device.
  • Battery life – We’d all like devices we never (or rarely) have to charge, but in today’s battery technology there is a relatively linear relationship between size and power – the bigger the battery, the longer the battery life. Bigger battery means more capabilities, but a bigger device and more difficulties making it comfortable.
  • Capabilities – What does the device intend to measure – heart rate, sleep, stress, blood pressure, and/or something else? Does the device need a screen? Does it need to show notifications? Take phone calls? Make you a coffee in the morning? Each of these impacts the sensor technologies, battery life, form factor, data presentation and available device locations, among other things.

To illustrate, let’s look at some examples:

  • Apple Watch – the Apple Watch is one of the most capable devices on the market, with heart rate, afib detection, notifications, phone calls, high screen resolution, and much more. But packing that many capabilities into one device has limitations, including battery life which is why it hasn’t focused much on sleep – one day battery life means most people are charging their watch overnight.
  • Oura Ring – Oura has been laser-focused on sleep monitoring with a ring device and mobile app that is well-engineered, designed and executed for their use case. However, it has limitations as well. For example, finger is good for sleep tracking when the individual is relatively still, but not for exercise tracking where the motion noise at the finger is extremely challenging.
  • Whoop – Whoop pioneered the user experience of monitoring the balance between physical strain (primarily around exercise intensity) and recovery (primarily around sleep tracking and HRV), in a low-profile wrist band form factor. Whoop made very clear design choices that enabled them to make a small, comfortable device that maximizes battery life, but with no screen the device does not provide capabilities not related to their critical to their strain/recovery use case such as notifications, apps or telling time.

Find what’s right for you

Wearables can provide valuable insights into how your body is responding to your lifestyle. Unfortunately, you can’t get everything in one device right now, so it’s best to have a clear understanding of what’s most important to you and chose a wearable that optimizes for your criteria. If your budget allows, get multiple devices that provide different data and insights and use a service that aggregates the data to generate action plans and insights.

Here’s a quick table to get you started: