The Core Sensor on a sports bra. All photos: iRunFar/Reese Ruland
The Core Sensor ($290) is a real-time core body temperature sensor that can help you monitor your body’s temperature throughout your daily activities, including runs. As an athlete striving for peak performance, understanding your body’s core temperature and how it reacts to different conditions is paramount, especially if running and racing in the heat. This device, made by the brand Core and worn on the outside of your body, uses a thermal energy transfer sensor to accurately measure and calculate your core body temperature, not just your skin temperature.
There are many reasons to track core body temperature, from preventing gastrointestinal distress to understanding hormone fluctuations during the menstrual cycle. This data can help you make informed decisions both while planning workouts and while out on a run. This review first discusses the benefits of tracking core body temperature and how it can be used to optimize training, then offers a review of the device’s set-up, functionality, and data analysis.
Why Use a Core Sensor for Body Temperature Measurement?
You might wonder why you’d want a Core Sensor to monitor your body temperature, and maybe you think you don’t need to care because you’re not a professional athlete or don’t race in hot conditions. But you’d be mistaken. First and foremost, your core temperature serves as an early warning system against heat-related illnesses such as heat exhaustion and heatstroke. By detecting increases in core temperature before symptoms manifest, athletes can take proactive measures to prevent serious health consequences.
Moreover, maintaining optimal core temperature is directly linked to athletic performance. When core temperature rises beyond the ideal range, the body’s physiological systems strain to dissipate heat, diverting resources away from muscular performance and cognitive function. As a result, athletes experience decreased endurance, diminished strength, impaired decision-making, and increased fatigue — all of which can hamper their ability to perform at their best. Deviations from a narrow range of optimal core body temperature, even slight ones, can profoundly affect performance. Additionally, it’s been observed that core temperatures above 39.5 degrees Celcius (103.1 degrees Fahrenheit) can result in gastrointestinal distress.
By continuously monitoring core temperature, athletes can gather data to help fine-tune training, optimize recovery, and prevent heat-related issues. Athletes can adjust their intensity, hydration, and cooling strategies on the fly using real-time data to maintain optimal core temperature throughout training and competition.
Using the Core Sensor for Heat Training
Aside from knowing when to cool yourself down, the Core Sensor can be your ally in heat training when preparing for a hot event.
With proper heat training, the body can perform better in hot conditions. When athletes get hot, their power output drops as the body diverts blood away from the muscles to the skin to increase cooling. Heat training creates a physiological trigger that increases plasma and blood volume. As a result, the body becomes more efficient at cooling and can produce more power in the heat.
Core notes that many experienced coaches have identified approximately 38.5 degrees Celsius (101.3 degrees Fahrenheit) as the ideal core body temperature for heat training. Training at too low temperatures doesn’t trigger increased plasma production, and training at excessively high temperatures can be counterproductive by causing excess fatigue, slowing recovery, and potentially being dangerous. Core offers Heat Training Zones, which define a safe and effective core body temperature range tailored to each individual that can be calculated using a heat ramp test.
A rear view of the Core Sensor.
Core Sensor for Menstrual Cycle Temperature Tracking
The Core Sensor is a great aid for navigating the intricacies of athletic performance throughout the menstrual cycle. Core body temperature is affected by fluctuations in hormones like estrogen and progesterone, and monitoring it can help you understand hormone impact on fluid regulation, thermoregulation, and metabolic responses. This can help an athlete optimize training and performance.
Different parts of the menstrual cycle introduce myriad changes that can affect an athlete’s physical and mental state. From fluctuations in strength and endurance to variations in pain tolerance and motivation, there are unique challenges and opportunities for female athletes to navigate and take advantage of. While scientific research on this topic remains limited, anecdotal evidence suggests that many women experience fluctuations in performance throughout their cycle.
If worn throughout the day, the Core Sensor can provide monitoring of daily basal body temperature, giving athletes insights into patterns throughout the menstrual cycle. Armed with this information, athletes can communicate effectively with coaches to optimize training programs to align with their hormonal fluctuations.
You can learn more by reading The Effects of Menstrual Cycle Phase on Elite Athlete Performance: A Critical and Systematic Review.
How the Core Sensor Works
The Core Sensor, which you wear on an armband or heart rate monitor strap, uses a new type of thermal energy transfer sensor to provide accurate and non-invasive tracking of core temperature — an achievement that has long been elusive in the realm of sports science. In the past, you’d likely have to swallow an e-pill to get your internal body temperature.
Thermal energy transfer, although fundamental to our perception of temperature, has been difficult to measure with a compact unit. Unlike conventional temperature sensors that merely detect the ambient temperature or skin temperature, thermal energy transfer sensors assess the actual flow of heat in and out of the body, offering a more comprehensive understanding of thermoregulation dynamics.
It’s important to note that core temperature rises mostly due to the mechanical work done by the body itself rather than the environment. While outside temperature has an effect, it isn’t nearly as significant as the rise generated by moving. When contracting muscles, the body is only about 20% efficient, and 80% of the energy produced is in the form of heat. For instance, if a cyclist using a power meter sees that they are producing 200 watts, their body is actually creating about 1,000 watts of energy, but 800 of those are in the form of heat. The body has to dissipate that energy to maintain a steady core temperature.
Placement of the Core Sensor on the torso.
Understanding thermal energy transfer illuminates how the body exchanges heat with its environment during physical exertion, shedding light on the mechanisms underlying performance fluctuations and heat-related stress. By monitoring thermal energy transfer in real-time, athletes gain insights into their physiological responses to environmental conditions, enabling them to adjust their training and performance strategies accordingly.
Review of the Core Sensor
I was pretty excited to use the Core Sensor. I’ve been an Oura Ring user since 2016, and I love all the data that little device provides about recovery, core temperature, activity, and more. Needless to say, I’m a fan of actionable data. Additionally, with the Javelina 100 Mile on my race calendar this year, I wanted to do some heat training to prepare.
To test the device, I wore it while running and riding my bike in both warm and cold temperatures. I also tested it across my entire menstrual cycle to see if there were significant changes in my core temperature or performance throughout various stages.
I did not wear the Core Sensor throughout the day and night. Instead, I use my Oura Ring to help track hormone and body temperature changes continuously. This review is primarily focused on the Core Sensor’s capabilities during activity.
Core Sensor Set-Up
Downloading the app and setting up the Core Sensor was incredibly simple and straightforward. The device has no buttons, and you just shake it a few times to turn it on. A green light blinks to confirm it’s actually on. The app shows a real-time reading of your core temperature as soon as you start wearing it, even if you just place it next to your bicep. The app creates a graph of your temperature as a function of time.
The Core Sensor pairs with most running watches and cycling computers, which is good since it’s not practical to open a phone to see core temperature throughout a run. I did encounter a bit of a problem connecting the device with Garmin Connect, which I believe was an issue with the Garmin app, which honestly doesn’t surprise me. Eventually, I connected the sensor to my Garmin fēnix 7s and my Garmin Edge 830. I wasn’t able to connect the device to the Garmin Forerunner 165 Music, which I was also testing at the same time. I suspect that the entry-level watch doesn’t support the Core app. If you plan on buying the Core Sensor, ensure your watch is compatible.
Close-up view of the Core Sensor.
Core Sensor Pre-Run Checks
Using the Core Sensor is pretty simple. Shake it to turn it on, and attach it to your heart rate monitor, armband, or sports bra. For optimal accuracy, it should be worn on your chest or torso. The specific location, whether on the front or back, is based on your personal preference. However, it’s advisable to maintain a consistent position each time you wear it. I wore mine on a heart rate strap or my sports bra. The only time I had issues with the device coming off the strap was while cycling, and that was because I forgot to use the clips that Core provides to keep the device from slipping. The device also comes with medical-grade adhesive patches, so you can literally stick the device to your side.
For those with a heart rate monitor, I recommend pairing it with the Core app for a more comprehensive analysis of your data. The app overlays various metrics, including heart rate, helping you identify what factors affect your core temperature.
After the device is turned on and placed on your body, it’s a good idea to check and see if it is connected. This is done easily by opening the app and ensuring it says “device connected” on the status bar and that readings are starting to appear on the data screen.
Using Core Sensor Data While Running
When connected to the Core Sensor, my watch and bike computer didn’t display the same pretty graph as the phone app did, but it did show my core temperature at that moment. I could see my core temperature rise as I went from sitting in my car at the trailhead to running up a hill. And then I could watch it fall when I started descending or going easier.
While using the device during activity, I found it fascinating to observe how my temperature fluctuated based on the intensity of my effort. During a training run in the mountains, my cooling strategies were limited — I could slow down, stop, or rest in the shade, but I couldn’t carry ice or enough water to splash on myself, nor did I want to sit in a stream to cool down — so I couldn’t test how these cooling strategies would affect my core temperature.
Once I was done with my run and could open my phone, I could study the graph of my core temperature and the overlay of my heart rate. In the screenshot below, my temperature stayed mostly steady during the activity — around 100 degrees Fahrenheit. I could see my core temperature response, which was delayed, when it rose to 101 degrees Fahrenheit after a big climb that caused a rise in my heart rate. My temperature and heart rate both declined after that peak because I was headed downhill, doing less mechanical work and creating less heat. As I began to return back to a sustained effort, my temperature returned to its steady-ish 100 degrees Fahrenheit.
An overlay of core temperature, heart rate, and skin temperature.
Using the Core Sensor for Heat Training
While I couldn’t do much during normal training runs and rides to mitigate a high core temperature, the data is valuable and actionable, namely when specifically heat training rather than just running normally. For instance, if I were specifically training in the heat for a race like the Western States 100 or Javelina 100 Mile, I could monitor when my core temperature exceeds its optimal range and identify what efforts or distances cause this. This data could help me determine when cold relief is needed, and I could experiment with wearing a cooling vest, using ice in my arm sleeves, or an ice pack to see which strategy are most effective.
If you want to heat train, you should do so with caution and under the supervision of someone who understands heat exhaustion and can help you safely establish your heat zones. Training too hot is always a bad idea and can lead to some pretty serious medical conditions. If you have concerns, consult a doctor.
Post-Review Update: Recently, Core has launched a Heat Adaptation Score to help athletes easily assess just how heat adapted they are. While untested on our end, this feature has promise for those training specifically to prepare for hot-weather events.
Core Sensor Overall Impressions
Overall, I’m really impressed with the Core Sensor and plan to use it closer to races like the Javelina 100 to help with heat training beforehand and body temperature regulation during the race. The only thing missing for me with this device is the ability to overlay an activity profile captured by my GPS device, but this is minor.
This device isn’t for everyone — you’ll need the motivation to use the data to take action, whether it’s cooling yourself down, knowing when to stop exercising or slow down, determining if your menstrual cycle is affecting your training, or identifying if a high core temperature is causing stomach distress. But if you’re training for a hot race or looking to optimize your overall performance, using the Core Sensor to keep track of your core body temperature can provide invaluable data.
Call for Comments
- Have you struggled with overheating during a run or a race?
- Would you find a real-time reading of your core body temperature useful during hot runs?
