Comparison

HRV4Training vs Nix Solo

Price, ratings, monetisation and update history for both apps, side by side — with what reviewers say about each.

Head to head
About

HRV4Training

Quantify stress, balance training and lifestyle, and improve performance with scientifically validated heart rate variability insights. This app accurately assesses HRV using your phone's camera or compatible devices, offering actionable interpretations for optimizing your training plan.

Highlights
  • HRV-based stress quantification
  • Training and lifestyle balance
  • Performance improvement insights
  • Camera-based HRV assessment
  • Integration with wearables
  • Training load analysis
  • VO2max and Lactate Threshold estimation
Features
Read full description

The first scientifically validated Heart Rate Variability (HRV) app. HRV4Training provides HRV based insights to help you quantify stress, better balance training and lifestyle, and improve performance. HRV4Training does not require a heart rate monitor, since it can accurately assess your HRV using the phone's camera. HRV4Training also works with Apple Watch, the Oura ring and all compatible Bluetooth SMART sensors. HRV4Training is also the first app that goes beyond simple measurements and provides insights on: 1) acute or day to day HRV changes in response to different stressors (alcohol, travel, sick days, training, menstruation, etc.) 2) long term multi-parameter trends to look at the big picture 3) correlations between physiological measurements and annotations 4) training load analysis, fitness, fatigue, readiness to perform and injury risk 5) VO2max estimation for runners linking the app to Strava or TrainingPeaks and using a heart rate monitor during their workouts 6) Training Polarization analysis (or 80/20) 7) Lactate Threshold Estimation for runners linking the app to Strava or TrainingPeaks and using a heart rate monitor during their workouts 8) Weekly and monthly summaries to track progress. All data is processed inside the app in order to provide you with actionable interpretations that can help optimizing your training plan and performance. Supported devices for the camera measurement: iPhones 5, 5S, 6, 6S, 6+, 6S+, SE, 7, 7+, 8, 8+, X, XS, XS Max, XR, 11, 11 Pro, 11 Pro Max, 12 mini, 12, 12 Pro, 12 Pro Max, 13 mini, 13, 13 Pro, 13 Pro Max, 14, 14 Plus, 14 Max, 14 Pro, 15, 15 Max, 15 Pro, 16, 16 Max, 16 Pro Currently unsupported devices for the camera measurement: iPhones 4S, iPads and iPods - These devices require a chest strap. Other features: - HRV-based advice helps you to adapt your trainings based on objective measures of recovery - TAGS: lets you tag your sleep, mental energy, muscle fatigue, stress and all other parameters that can help you understand what is influencing your physical condition and physiological stress - Works with Bluetooth SMART heart rate monitors (a Polar H7 is recommended) or simply your phone's Camera - Integrates with the Health app (writes heart rate, and sleep, read sleep time) - Configurable test (single or orthostatic, choose test duration between 1, 3 or 5 minutes) - Extracts and stores the following features while lying down and/or standing: heart rate, mean of beat-to-beat intervals (AVNN), standard deviation of beat-to-beat intervals (SDNN), square root of the mean squared difference of successive R-Rs (rMSSD), number of pairs of successive R-Rs that differ by more than 50 ms (pNN50), low frequency power (LF, 0.04-0.15 Hz), high frequency power (HF, 0.15-0.40 Hz) - Shows the PPG signal view to ensure signal quality during the test if the camera is used instead of a bluetooth SMART sensor - HRV4T Recovery Points to provide a single, straightforward metric to analyze your physical condition - Population summaries and people like you comparisons to put your data into perspective - Analysis of compliance and consistency - Data export via email or Dropbox - Integrates with TrainingPeaks, Oura, Strava and Coros HRV4Training requires to register an account with your email address, so that your data can be backed up safely and retrieved in case anything happens (app issue, changing phone, etc.). HRV4Training does not provide medical advice. It is intended for informational purposes only. Consult a medical professional before making health or lifestyle decisions. Please find an overview of the relevant scientific literature in our guide, here: https://marcoaltini.substack.com/p/the-ultimate-guide-to-heart-rate

About

Nix Solo

Monitor personal fluid and electrolyte losses in real-time during workouts. This app provides insights into sweat composition and hydration needs based on individual biology, using data from a companion biosensor.

Highlights
  • Real-time fluid and electrolyte loss monitoring
  • Personalized sweat composition insights
  • Correlates sweat data with environmental factors (Nix Index)
  • Post-workout Sweat Intelligence reports
  • Integration with Apple Watch and Garmin devices
  • Workout type and hydration input
Features
Read full description

To train at your peak, you can’t just train hard. You need to train in a way that’s informed by your personal biology. The Nix Solo app allows users to monitor their own personal fluid and electrolyte losses in real time during a workout using the Nix Hydration Biosensor. This companion app provides valuable insight into their unique sweat composition and individual hydration needs based on their personal biology. At Nix, we built the first biosensor to analyze sweat and provide endurance athletes with personal hydration data – scientifically validated, and delivered in real time. Once paired with the Nix Hydration Biosensor, you are on your way to understanding your hydration needs with Nix Solo, our individual monitoring app. We’re the first hydration biosensor of its kind — the only pod, patch, and free app combination to: - Evaluate the electrochemical biomarkers in your sweat - Send real-time updates to your phone, Apple Watch, Garmin Watch, or Garmin Bike Computer - Correlate your sweat data and your training environment with the Nix Index – a composite index of six environmental factors: temperature, humidity, dew point, altitude, wind speed, and solar load - Provide post workout Sweat Intelligence via insights on your Sweat Profile including fluid loss rate, electrolyte loss rate and sweat composition metrics Before your workout, tell the Nix Solo app what type of workout you’re doing and what you’ll be hydrating with. You can also consult the Nix Index to see how your environment impacts your hydration needs. Once you start sweating, our single-use patch begins measuring and analyzing your fluid and electrolyte losses minute by minute- metrics that represent your unique “sweat composition.” This data is instantly streamed to our app, and can also be shared with devices like your Apple Watch, Garmin Watch, and Garmin bike computer. The result is knowing when, what, and how much to drink in real time to enhance both safety and performance.

Screenshots

HRV4Training4 screens
Nix Solo4 screens

Verdict

The call

The clearest difference is review sentiment: HRV4Training at 61% against Nix Solo's 14%. HRV4Training also leads on chart position (#15 vs unranked) and ratings volume (288 vs 61). Nix Solo's advantage is price (Free vs $9.99) and rating (4.8 vs 3.6). On ads, in-app purchases and iOS requirement there is nothing between them.

Scored on Price · Rating · Positive reviews · Number of ratings · Update frequency · Ads · In-app purchases · Monetization · Best chart rank · Devices · Requires iOS

CostPrice · In-app purchases · Ads · Monetization

Nix Solo is free to download; HRV4Training costs $9.99 up front. Neither carries in-app purchases, so what you see is what you pay.

ReceptionRating · Positive reviews · Number of ratings

Nix Solo holds the better App Store score, 4.8 against 3.6. Both are backed by a comparable volume of ratings — 288 and 61 respectively. Written reviews complicate that: 61% of HRV4Training's written reviews are positive against 14% of Nix Solo's, so the app with the better star average is not the one people write more warmly about.

UpkeepUpdate frequency

HRV4Training ships an update every 6 weeks, Nix Solo every 10 days. The most recent releases landed on September 27, 2026 and September 26, 2026 respectively.

Scorecard8 real differences · 3 level
HRV4Training versus Nix Solo: the parameters behind the verdict, then further details
ParameterHRV4TrainingNix Solo
Price$9.99Free — better
Rating3.6 (288 ratings)4.8 (61 ratings) — better
Positive reviews61.1% of reviews — better14.3% of reviews
Number of ratings288 — better61
Update frequencyEvery 6 weeksEvery 10 days — better
AdsNoNo
In-app purchasesNoNo
MonetizationPaidFree — better
Best chart rank#15 — better—
DevicesiPhone, iPodiPhone, iPad, iPod, Apple Watch — better
Requires iOS15.615.1 — better
Further details — not scored
Size35 MB65 MB
Age rating4+4+
DeveloperA.S.M.A. B.V.Nix, Inc.

Customer experience

HRV4Training

All ratings288 ratings
Positive62.2%
Neutral5.6%
Negative32.3%
Written reviews450 reviews
Positive61.1%
Neutral7.3%
Negative31.6%

Nix Solo

All ratings61 ratings
Positive93.4%
Neutral3.3%
Negative3.3%
Written reviews21 reviews
Positive14.3%
Neutral14.3%
Negative71.4%

Bugs and requests

HRV4Training

Reported bugs

3 issues
  • App crashes frequently, especially with iOS updates
  • Syncing issues with Strava, TrainingPeaks, and Apple Health
  • Camera-based measurements often unreliable or inconsistent

Requested features

4 requests
  • Allow multiple measurements per day
  • Improve integration with Apple Watch and other wearables
  • Add more customization options and data visualization
  • Expand support for non-athletic use cases

Nix Solo

Not enough reviews yet to summarise bugs or requests.

In-app purchases

None

HRV4Training

No in-app purchases

None

Nix Solo

No in-app purchases

Questions

Is HRV4Training free?
HRV4Training costs $9.99, with no in-app purchases.
Is Nix Solo free?
Nix Solo is free to download, with no in-app purchases.
Which has better reviews, HRV4Training or Nix Solo?
Nix Solo is rated higher. HRV4Training averages 3.6 out of 5 from 288 ratings, while Nix Solo averages 4.8 from 61 ratings.
Do HRV4Training or Nix Solo have ads?
Neither HRV4Training nor Nix Solo shows ads.
Which is updated more often, HRV4Training or Nix Solo?
HRV4Training ships an update every 6 weeks, and Nix Solo every 10 days. Most recently, HRV4Training was updated on September 27, 2026 and Nix Solo on September 26, 2026.

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