My Fitness Watch Said I Was 'Overreaching' During Austin's Hottest Week. It Was Wrong.
James "Jamie" Whitfield
I was halfway through my Tuesday morning run on the Lady Bird Lake trail when my watch buzzed with a notification that made me stop mid-stride. "Overreaching detected," it said. "Consider reducing training intensity. Your HRV is 23% below baseline." I looked at the screen, then I looked at the sky, which was already a pale white from the heat haze at 7:45 AM. Then I looked at the temperature display on my watch: 91°F. Ninety-one degrees. Before 8 AM. And this piece of consumer electronics was telling me that my body was failing, not that the environment was absurd.
I've been a personal trainer in Austin for twelve years. I've worked with marathoners, powerlifters, weekend warriors, and people who just want to climb the stairs at their office without getting winded. I know what overreaching looks like. It's chronic fatigue, declining performance, irritability, disrupted sleep. It's not a single run on a day when the heat index is 108°F. What my watch was detecting wasn't overreaching. It was thermoregulatory stress. My body was diverting blood to my skin to cool down, which meant less blood for my muscles, which meant higher heart rate at lower intensities, which meant lower HRV, which the watch's algorithm interpreted as "you're training too hard." The algorithm was not built for Texas in July. It was built for Silicon Valley in April.
Here's the thing about fitness wearables in extreme heat: they become liars. Not malicious liars, just incompetent ones. The sensors are fine. The heart rate monitors, the accelerometers, the pulse oximeters—they work. But the algorithms that interpret that data were trained on datasets collected in temperate climates from people exercising in controlled conditions. When you take those same algorithms and expose them to a 105°F heat index with 65% humidity, they break. They misinterpret heat stress as cardiovascular stress. They misinterpret dehydration as poor recovery. They misinterpret the body's entirely appropriate response to a hostile environment as a sign that you're doing something wrong.
I tested this systematically because I'm a nerd and I have access to a physiology lab at UT Austin through a colleague in the kinesiology department. For two weeks during the worst of the heat wave, I wore three different fitness watches simultaneously—a Garmin, an Apple Watch, and a Whoop strap—while also wearing a chest-strap ECG monitor and having my core temperature measured via ingestible sensor. Yes, I swallowed a thermometer pill. Twice. Science is weird. The results were consistent across all three consumer devices: they overestimated my training stress by 30-40% on days when the heat index was above 100°F. They told me I needed more recovery when what I actually needed was air conditioning and a cold shower.
The BMI calculation was the most ridiculous part. One of my clients, a 34-year-old software developer who lives in East Austin and runs with me three mornings a week, got a "fitness age" reading from his watch that jumped from 28 to 41 in a single week. He panicked. He thought he was having some kind of cardiovascular event. I looked at his data. His resting heart rate had gone from 52 to 61. His VO2 max estimate had dropped from 48 to 39. His sleep score had crashed. All of this happened during the same week that Austin set a record for consecutive days over 105°F. "Your watch thinks you're getting older because it's hot outside," I told him. He didn't believe me until I showed him the correlation between the daily temperature and his "fitness age." The correlation was 0.87. His body wasn't aging. The algorithm was confused by heat.
The AI coaching features are even worse. One of my watches has an AI coach that generates daily training recommendations based on my recovery metrics. During the heat wave, it recommended "light active recovery" for twelve consecutive days. I ignored it and kept training, because I knew the data was garbage. But imagine someone who doesn't have a background in exercise physiology. Imagine someone who trusts their watch implicitly. They'd spend two weeks doing nothing but gentle yoga because their AI coach thinks they're overtrained, when in reality they're just hot. That's not coaching. That's misinformation with a premium subscription fee.
I started telling my clients to ignore their wearables during heat waves. This is hard for them. These devices are expensive. They represent a significant investment of money and attention. People don't want to hear that their $500 watch is giving them bad advice. But it is. The HRV data is meaningless when your body is fighting to maintain core temperature. The sleep data is meaningless when your bedroom is 85°F. The recovery metrics are meaningless when you're chronically dehydrated from sweating out two liters a day. The only metric that matters in a heat wave is "did you stay cool enough to not get heat illness?" And no consumer wearable measures that accurately.
The body composition estimates are particularly egregious. Most modern wearables use bioimpedance to estimate body fat percentage. They send a tiny electrical current through your wrist and measure the resistance. The assumption is that fat has different conductivity than muscle. Which is true, in a lab, at 72°F, with proper hydration. In a heat wave, your skin temperature is elevated, your sweat changes skin conductivity, and your total body water is fluctuating wildly depending on how much you've hydrated. I measured my own body fat percentage via DEXA scan— the gold standard— and compared it to my watch's estimate during the heat wave. The watch was off by 4.2 percentage points. That's the difference between "healthy body composition" and "needs improvement" in most fitness apps. Four percentage points because it was hot outside.
I've started doing something that would make the wearable companies furious: I manually override my data. When I finish a run in 100°F heat, I don't let the watch log it as a "high stress" workout. I annotate it: "heat stress, not training stress." I keep a separate spreadsheet where I track actual training load, perceived exertion adjusted for temperature, and how I actually feel versus what the watch says. It's extra work. It's annoying. But it's the only way to get useful data from a device that doesn't understand my environment. My clients think I'm crazy. "You paid for the watch to track this stuff," one of them said. "I paid for the watch to track my fitness," I told him. "It can't do that if it thinks heat is fitness."
The broader problem is that the fitness industry has sold us a narrative of personal optimization that ignores the environment. Your watch tells you to sleep better, eat cleaner, train smarter. It doesn't tell you that the reason you feel terrible is that your city is experiencing a once-in-a-century heat wave. It doesn't tell you that your "poor recovery" is actually your body doing exactly what it should be doing in extreme heat. It individualizes a collective problem. It makes you think you're broken when the world is broken. And that's dangerous, because it diverts attention from the real issues—climate adaptation, urban heat islands, infrastructure that can't handle the new normal—and focuses it on your bedtime routine.
I'm not saying fitness wearables are useless. In normal conditions, they're great. They motivate people to move more, sleep better, pay attention to their bodies. But normal conditions are becoming abnormal. Austin has had more 100-degree days this year than any year on record. The heat isn't a temporary inconvenience. It's the baseline. And the wearable industry needs to catch up. They need algorithms trained on hot-climate data. They need temperature-adjusted metrics. They need to stop telling people they're overreaching when they're actually overheating. Until then, we're all going to be getting bad advice from expensive bracelets, and some of us are going to make training decisions based on data that has no relationship to reality.
So here's what I tell my clients now. Wear your watch. Collect the data. But don't trust the interpretations during a heat wave. Your HRV is low because it's hot, not because you're overtrained. Your sleep score is bad because your bedroom is 86°F, not because you need magnesium. Your BMI hasn't changed, even if your watch's bioimpedance sensor says it has. And if your AI coach tells you to take it easy for the third week in a row, tell it to check the weather forecast. Because the problem isn't your body. The problem is that Austin in July is not a place where consumer electronics should be making health decisions.
Anyone else getting garbage data from their watch this summer? I'm starting to think I should just go back to a stopwatch and a notebook.
The thing that really gets under my skin is the way these devices have become authority figures. People trust them more than their own bodies. I had a client last month, a woman in her forties who started training with me because she wanted to run her first 5K. She was doing great. Her times were improving. She felt good. Then her watch gave her a "readiness score" of 32 and told her to skip her run. She skipped it. Then she skipped the next one. By the end of the week, she'd missed three training sessions because her watch said she wasn't ready. I asked her how she felt. "Fine," she said. "A little tired from the heat, but fine." Her watch disagreed. Her watch, which had never been to Austin in July, which had never felt what 105°F feels like when you're trying to maintain a 9-minute mile, had decided she wasn't ready. And she listened to it. Because it has graphs. Because it has numbers. Because we've been conditioned to believe that data is more trustworthy than embodied experience.
I started running without my watch on the hottest days. Just me, a stopwatch, and a bottle of water with electrolytes. It was liberating. I didn't know my heart rate. I didn't know my pace per mile. I didn't know my "training load" or my "recovery score." I just knew that I was running, that it was hard, that the heat was making it harder, and that I would stop if I felt dizzy or nauseous. That's how humans ran for two million years. We didn't need wrist-mounted computers to tell us when we were tired. We felt it. And somehow, in the rush to quantify everything, we've forgotten that our bodies come with built-in sensors that are actually pretty good at detecting heat stress. Dizziness, nausea, excessive thirst, chills—these are real signals. They're not as precise as a heart rate monitor, but they're more relevant to the actual question of "should I keep running?"
The calorie burn estimates during heat are another joke. My watch told me I burned 847 calories on a 45-minute run in 102°F heat. That same run at 75°F would have registered around 620 calories. The difference—227 calories—is supposedly because my heart rate was higher. But my heart rate was higher because my body was pumping blood to my skin, not because I was working harder. The actual mechanical work of running was identical. The thermoregulatory work was extra. But the watch can't distinguish between "working hard at running" and "working hard at not dying of heat stroke." It just sees elevated heart rate and assumes you're burning more calories. You're not. If anything, you're burning fewer calories because your performance degrades in heat. You run slower. You cover less distance. The watch sees the time and the heart rate and invents a calorie number that has no basis in physiology.
I tested this with a metabolic cart at the UT lab. We measured my actual oxygen consumption during a treadmill run at 75°F and again at 95°F. The oxygen consumption—which is the direct measure of calorie burn—was 8% lower at 95°F. I was working less hard, not more. My heart rate was 12 beats per minute higher, but my oxygen consumption was lower. The watch would have reported a higher calorie burn in the hot condition. The reality was the opposite. This is not a small error. This is a fundamental misunderstanding of exercise physiology that affects millions of people's dietary decisions. People eat back their "extra" calories based on data that is literally backwards.
The wearable companies know about this. They have to. They have exercise physiologists on staff. They have data scientists who can see the same correlations I'm seeing. But fixing it would require admitting that their devices don't work well in heat, which would require admitting that climate change affects their product category, which is not a conversation that tech companies want to have. So they don't. They keep selling us devices that give us bad data in extreme weather, and they keep marketing them as "personal health coaches" that work "anytime, anywhere." Anytime, anywhere except Austin in July. Or Phoenix in August. Or Houston in September. Or anywhere, increasingly, during the new normal of extreme heat.
I've started thinking about this as a justice issue, which might sound dramatic, but hear me out. The people who are most affected by heat-related wearable errors are the people who live in the hottest places. Texas, Arizona, Florida, the Deep South. These are also places with lower average incomes, less access to climate-controlled training facilities, and higher rates of outdoor labor. A construction worker in Houston who relies on his watch to tell him when he's overexerting himself is getting bad data that could put him in the hospital. A runner in Phoenix who trusts her "recovery score" to tell her when it's safe to train again is getting advice that ignores the 110°F reality outside her door. The wearable industry is selling a product that works for people in temperate climates and pretending it works for everyone. It doesn't. And the people who are paying the price for that pretense are the ones who can least afford it.
So what do we do? We adapt. We learn to read our own bodies again. We use wearables as data collection tools, not decision-making tools. We cross-reference their advice with the actual weather conditions. We remember that a high heart rate in 100°F heat is not the same as a high heart rate in 70°F heat. We drink more water than the apps tell us to. We slow down when our bodies tell us to, not when our watches tell us to. And we demand better from the companies that are taking our money. We demand algorithms that account for heat and humidity. We demand transparency about the conditions under which their devices were tested. We demand that they stop pretending that climate change doesn't affect their products, because it does, and ignoring it is not a business strategy. It's a public health failure.
My watch is sitting in a drawer right now. I'll put it back on in October, when the temperatures drop and the data becomes useful again. Until then, I'm running by feel. I'm training by intuition. I'm listening to my body instead of my algorithms. And you know what? I'm sleeping better. My "recovery score" might be terrible, but I don't care. I know how I feel. And in a heat wave, that's the only metric that matters.
