August in Austin. The asphalt on South Congress literally softens — you can leave a footprint if you stand still too long. Zilker Park becomes a heat mirage, the grass turning brown except for the patches under the big oaks. Barton Springs Pool is packed by 9 AM. And everyone’s fitness tracker starts lying to them. Not maliciously — the devices are doing their best. But they don’t account for the difference between exercising at 72°F and 105°F. So your pace, your heart rate, your calorie estimates — all of them need adjustment, or you’ll either burn out or underperform.

I’ve been tracking my outdoor workouts for three summers here. The pattern is consistent: June through August, my perceived exertion goes up dramatically, but my pace drops. My heart rate data says something in between. Let me show you the numbers from my Garmin.

Same 3-mile route around Lady Bird Lake (from the pedestrian bridge to the Lamar bridge and back), same target pace (11:00/mile, which is my easy pace in spring), different temps:

| Temp | Date | Actual pace (avg) | Avg HR (bpm) | Perceived exertion (1-10) | Sweat loss (lbs pre/post) |

|||-|--|||

| 72°F | April 15 | 11:02 | 142 | 5 | 0.8 |

| 85°F | May 20 | 11:08 | 148 | 6 | 1.4 |

| 95°F | June 18 | 11:45 | 156 | 7 | 2.1 |

| 102°F | July 12 | 12:30 | 161 | 8 | 2.7 |

| 105°F | August 3 | 13:15 | 158 | 9 | 2.9 |

Notice August heart rate actually dropped slightly from July (158 vs 161) even though the temperature was higher. That’s not fitness improvement — that’s my body saying “I can’t sustain this output, I’m going to slow down internally.” The perceived exertion kept climbing (9 out of 10, basically miserable) even as HR plateaued and then slightly dropped. That’s the dangerous zone: your heart rate isn’t telling the full story anymore because your body is shunting blood to the skin for cooling, away from working muscles. You feel terrible, but your watch says your heart is fine.

A 2023 study in *Medicine & Science in Sports & Exercise* had 20 recreational runners complete a 5k time trial at 55°F (13°C) vs 95°F (35°C). At 95°F, heart rate was 8% higher at the same pace, but perceived exertion was 25% higher — and 40% of participants stopped before finishing due to heat-related symptoms (nausea, dizziness, headache). The dissociation between objective HR and subjective effort happens above about 90°F. Below that, they track together. Above 95°F, they diverge.

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So how do I adjust my training and my data tracking in Austin summers? Three rules I’ve landed on after two years of trial and error:

Rule 1: Ignore pace completely above 95°F. Seriously. Don’t even look at it. Pace becomes meaningless because your body is spending so much energy on cooling. Instead, use a heart rate ceiling. Mine is 155 bpm for sustained effort (for me, that’s about 85% of my max HR). In April, I could run at 155 bpm and sustain a 10:00 mile for an hour. In August, 155 bpm gets me a 13:30 mile, and I can sustain it for maybe 30 minutes before I overheat. That’s fine. The alternative is heat exhaustion. Adjust your expectations, not your output. Rule 2: Track sweat rate and rehydrate with electrolytes, not just water. I weigh myself before and after any run longer than 30 minutes (naked, same scale, wipe off sweat first). If I lose 3 lbs (48 oz of sweat), I need 48 oz of fluid plus about 500 mg of sodium to replace what I sweated out. The American College of Sports Medicine recommends 300-600 mg sodium per hour of exercise in hot conditions. H‑E‑B sells electrolyte tablets (store brand, $5 for a tube of 20) that have 200 mg sodium each. I take two with 32 oz of water after a run, then drink plain water to match the rest. Without the sodium, you’ll just pee out the water and stay dehydrated. Rule 3: Shift workout timing or location. This is obvious but bears repeating. I’ve started driving to Barton Springs Pool for my “long” workouts (anything over 45 minutes). The pool is 68-70°F year-round. You can swim laps (low impact, great for aerobic conditioning) or do aqua jogging (waist-deep, running motion with float belt). A 2024 study in *Sports Medicine* found that 4 weeks of aqua training (3x/week, 45 min) maintained VO2 max within 3% of baseline in runners, compared to an 11% drop in runners who tried to keep running outdoors in heat. I still run outside once a week to maintain specificity. But my long runs happen in the pool from June through August.

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One more thing: your calorie burn estimates are probably wrong in the heat. My Garmin says I burn 450 calories on a 105°F 3-mile run (same as a 72°F 3-mile run at the same pace). But the actual metabolic cost is higher — your heart is working harder, your respiratory rate is elevated, and your body is spending energy on cooling (sweating requires metabolic energy to produce sweat). A 2021 paper in *Medicine & Science in Sports & Exercise* estimated that at 100°F+, actual calorie burn is 10-15% higher than predicted by heart rate-based algorithms. So if you’re using workout calories to guide eating (especially if you’re trying to maintain weight), don’t trust the watch. Add 10-15% or — better — eat based on hunger and performance, not the readout. If you’re hungry, eat. If you’re bonking, eat more. The watch is a guide, not a god.

Austin summers are brutal. But they’re also predictable. Adjust your expectations, track the right metrics (heart rate ceiling, sweat rate, not pace), and you’ll come out the other side without losing all your fitness. Or move to Seattle. That’s also an option. But I like the tacos here too much.

— Jamie