But our bodies can adapt to the heat<\/a>. That first hot summer day is a doozy, but by August I\u2019m usually surviving pretty well, if not thriving. Athletes who need to perform well in the heat will often follow heat training protocols to be sure they\u2019re ready in advance for hot-weather races.\u00a0<\/p>\n And this is where a gadget called the Core 2 Thermal Sensor<\/a> comes in. It\u2019s a little smart slab of plastic that you attach to your heart rate chest strap, and it measures your body temperature while you train. From that data, you can learn how well you\u2019re adapting to the heat, and you can use it to guide you through a heat-training protocol so you can adapt faster and more efficiently. You can also use it when you\u2019re running a race to make sure you don\u2019t overheat\u2014train hot, race cool, as the saying goes.<\/p>\n The Core company sent me a sensor to train with, and I\u2019ve been using it for a few weeks. Spring weather is beginning to turn to summer, so I\u2019m feeling the effects of hotter weather, but I also need to seek heat out if I want to encourage my body to adapt to the hotter conditions that are coming.<\/p>\n I\u2019ll give my initial thoughts on this device based on these past few weeks of use, and then I\u2019m going to continue working with the sensor over the summer. Whenever I write about heat adaptation or hot weather running, I\u2019ll pull in data from my experience with the Core 2 sensor\u2014and then I\u2019ll return at the end of the season for some final thoughts on whether I thought training with the sensor was worthwhile.\u00a0 This gadget, cool as it is, costs $294.95 (plus, at the moment, an extra $31 for shipping and tariffs), so it needs to be pretty darn useful to be worth that cost.<\/p>\n The metrics and what they mean<\/p>\n Credit: Beth Skwarecki\n <\/p>\n The Core 2 sensor gives you three numbers whenever you wear it:\u00a0<\/p>\n Your skin temperature<\/p>\n<\/li>\n Your (estimated) core temperature<\/p>\n<\/li>\n A \u201cheat strain index,\u201d or HSI, broken into one of four zones.<\/p>\n<\/li>\n<\/ul>\n Your core temperature is the temperature inside your body, specifically inside your torso (your core). Some studies test this with a rectal thermometer, or with a probe that can be swallowed. When you take your kid\u2019s temperature at their mouth, ear, or armpit, you\u2019re trying to get a measurement that is close to this number. A normal core temperature for a healthy human at rest is 98.6 degrees, give or take a degree or two<\/a>.\u00a0<\/p>\n Skin temperature is, of course, the temperature on the outside of your body, wherever it\u2019s being measured. The Oura ring and the Apple Watch, to name a few, have temperature sensors that let them monitor skin temperature.\u00a0<\/p>\n I was perplexed at first: how can one device know the temperature in two different parts of your body? I asked Brian Maiorano, the coach liaison at Core. He explained that the device contains two sensors. Skin temp comes from a regular temperature sensor, similar to what\u2019s in other wearables. And then there is a heat flux sensor, which for the moment I am content to not entirely understand. (It measures the flow of heat energy<\/a> rather than temperature, which is different somehow.)\u00a0<\/p>\n Core has an algorithm that can estimate core temperature from heat flux sensor data, but it\u2019s only valid during exercise if the device knows the athlete\u2019s heart rate at that moment. That\u2019s why you need to pair the Core 2 sensor to a heart rate chest strap. Importantly, the sensor is for use during exercise, but not during passive heat sitting in a sauna\u2014you have to log sauna sessions manually.\u00a0<\/p>\n From the skin temperature and core temperature, Core can generate a heat strain index, or HSI. This gets categorized into one of four heat zones<\/a>, so if you\u2019re used to heart rate zones<\/a> or cycling power zones<\/a>, you\u2019ll be right at home. With a glance at your watch during a workout, you can tell whether you\u2019re in:\u00a0<\/p>\n Heat zone 1<\/strong>: No heat strain. Either your core temp isn\u2019t elevated, or it is but your skin temperature is low enough that you must be cooling yourself off quite well. This is the ideal zone for running a race, or for taking a break from heat training.\u00a0<\/p>\n<\/li>\n Heat zone 2<\/strong>: Moderate heat strain. You\u2019re warm, and maybe your performance is slightly affected.\u00a0<\/p>\n<\/li>\n Heat zone 3<\/strong>: High heat strain. Your skin and core temperatures are both high, you\u2019re probably sweaty and exhausted, and you\u2019re definitely not running as fast or performing as well as you would in a lower zone. This is the zone that helps you the most in heat training, though!<\/p>\n<\/li>\n Heat zone 4<\/strong>: Extremely high heat strain. This is the danger zone, and you\u2019ll want to get out of it as quickly as possible\u2014stop exercising, cool down, etc. You should also be on the lookout for signs of heat illness<\/a>, like dizziness, cramping, and nausea.\u00a0<\/p>\n<\/li>\n<\/ul>\n Safety is important when you\u2019re exercising in the heat, and I wouldn\u2019t trust a device (no matter how good its algorithm claims to be) to tell you when you\u2019re safe to keep going.\u00a0<\/p>\n Everything I\u2019ll discuss about heat training should be taken in the context of stopping if you don\u2019t feel good, and ideally being around others who could help you if something goes wrong. At a core temperature of about 104 degrees, your body can no longer cool itself effectively, leading to potentially deadly consequences for your brain and heart. This is known as heat stroke. At that stage, you may not be coherent enough to call 911 for yourself; somebody may have to do it for you.\u00a0<\/p>\n Once you dive into the Core app, you\u2019ll see a few more numbers, including a Heat Adaptation Score and a Heat Training Load. The app also totals up the amount of time you\u2019ve spent in each heat zone. I\u2019ll show you more about those metrics in the heat training section below.\u00a0<\/p>\n Setup<\/p>\n Credit: Beth Skwarecki\n <\/p>\n To use the Core 2 temperature sensor, you\u2019ll need a heart rate chest strap and a compatible watch, like a Garmin. I\u2019ve got both\u2014my trusty Coospo<\/a> (25 bucks and it\u2019s lasted me years) and a selection of watches. I mostly used it with my Garmin Forerunner 265s<\/a> and the Forerunner 570<\/a> that I\u2019m testing for a review.\u00a0<\/p>\n Currently, pairing is available for watches and bike computers from Garmin, Coros, Wahoo, Suunto, and Hammerhead. The Apple Watch has been supported in the past, but a note<\/a> on the Core website says there\u2019s an issue with it at the moment and it\u2019s been removed\u2014hopefully temporarily.<\/p>\n![\"Home](\"https:\/\/www.europesays.com\/uk\/wp-content\/uploads\/2025\/05\/images-5.fill.size_2000x1125.v1748636654.jpg\")
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