Deep Dive Digest #18: Special Edition: Female Athlete Tech, What Your Wearable Gets Wrong and What It Gets Right
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This is a special edition of the Deep Dive Digest, built around female athlete tech. If your training group or circle includes female athletes who use a Garmin, Oura, or Whoop, it is worth forwarding directly. The data discussed here affects training decisions in ways most wearable reviews have not addressed.
Garmin’s VO2max algorithm was trained predominantly on male data. For female runners the output can mislead in both directions, and the research explains why and by how much. Oestrogen and progesterone affect heart rate, fuel use, body temperature, HRV, and recovery scores across the menstrual cycle. Most wearables measure some of these signals and interpret none of them in hormonal context. The guide covers what is actually happening at each phase, and what your device can and cannot tell you about it.
Optical heart rate accuracy varies across skin tones and wrist sizes. The research is less settled than the headlines suggest, and the controlled studies tell a more nuanced story than either the critics or the manufacturers acknowledge.
RED-S shows up in declining HRV baselines, recovery scores that will not improve, and menstrual cycles that shift before they disappear. The guide covers the physiology, identifies what Garmin, Oura and Whoop data can realistically flag, and is clear about what only a clinician can confirm.
The final piece is practical: how to cross-reference Garmin recovery metrics with menstrual phase to make smarter training decisions across a 28-day cycle. All five pieces are collected at the5krunner’s FemTech Hub.
This week’s Indie App Spotlight is IntervalCoach, the most read entry in the series. IntervalCoach is an AI training app that rewrites today’s cycling or running session based on your recovery data, available on web, iPhone, Mac and Android.
Female VO2max on Garmin: Why the Number Lies and What to Do
Garmin VO2max estimates are built on algorithms trained predominantly on male data. For female runners the number can mislead in both directions. Here is what the research shows and how to interpret your own figure.
Hormones and Endurance Training: What Your Cycle Actually Does
Oestrogen and progesterone affect heart rate, fuel use, body temperature, HRV, and recovery scores across the menstrual cycle. Here is what is actually happening at each phase, and what your wearable can and cannot tell you about it.
Wrist HR Accuracy: Dark Skin and Small Wrists Explained
Optical heart rate monitors behave differently across skin tones and wrist sizes, but the research is less settled than the headlines suggest. Here is what the controlled studies actually show, where the evidence is thin, and what it means for training decisions.
The Deep Dive Podcast
Listen to one of our recent discussions on the Deep Dive podcast – EP75 Why smartwatches fail female physiology
Browse every edition in the Deep Dive Digest archive.
Previous edition: Deep Dive Digest #17
Last Updated on 3 July 2026 by the5krunner
My favourite kit and nutrition
- Injinji – Runners protect your toes. Avoid discomfort and minor injury. Run more. Run faster. I use them.
- Garmin 90-degree charging adapter — The small adapter that keeps your charging cables tidy. Essential for race day. I use one.
- Garmin charging puck — the fastest and most reliable way to top up your Garmin before a session. I use one.
- Ravemen FR300 — Front light that mounts directly under your Garmin or Wahoo head unit. Keeps your bars clean and your beam pointed where it matters. I use one.
- Body Glide – The blue anti-chafe stick that all swimmers and many runners use. I use it.
- Maurten — The race nutrition trusted by elite athletes. Gels and drink mixes engineered to be easy on the stomach. I use them.
- Garmin Varia RTL515 — A radar rear light that alerts you to vehicles approaching from behind. Pairs with your Edge or Garmin watch. I use this model.
- Favero Assioma Pro RS2 — The power-meter pedals most serious cyclists choose. Accurate, easy to move between bikes. I use this model.
- Garmin Forerunner 970 — A serious choice for a pro-grade triathlon watch. I use this.
- Polar H10 — My daily driver for accurate, waking HRV readings.
- Wahoo ELEMNT Roam 3 — The bike computer that has the feature Garmin lacks: usability. I use mine on most rides.
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tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors. ID
