Negative Split: Free Local Strava Running Analytics

If you run with Strava, 2026 has been a year of doors closing. Year in Sport went behind the subscription last December. Fitness & Freshness, custom zones and Best Efforts live there too. Then the developer program changes arrived in June, API access began requiring a paid Strava subscription, and long-loved third-party tools began losing their sync: the best-known analytics extension has been frozen on the Chrome Web Store since late 2024, and its desktop companion lost Strava sync entirely this summer. If your training analysis depends on someone else’s developer link to Strava, you have learned this year that it can be switched off.

Negative Split is a free Chrome extension, built by a runner, that takes a different route: it computes everything locally, in your browser, from the Strava pages you are already looking at. There is no API, no account and no server. There is nothing for Strava to revoke.

The problem: the analysis moved behind two paywalls at once

A runner on free Strava in 2026 sees distance, pace and a map. The interesting questions sit elsewhere. How hard was that run really, I mean, once the hills are accounted for? Is my training load building, or am I digging a hole? What would I run a marathon in today? Strava answers those questions behind its subscription; the same company’s API pricing is squeezing and pushing away the independent tools that used to answer them for free. The result is an odd gap: your own running data is on the screen in front of you, and the arithmetic on top of it is the thing being sold back to you.

What is a Negative Split

Negative Split is a Manifest V3 Chrome extension for runners (now you know!). Install it, open Strava as usual, and your own pages grow new sections: a pacing and training-load card on every activity, a sortable personal-best list and race predictions on your profile, Fitness / Fatigue / Form on your dashboard, and a free Year-in-Sport-style recap with a shareable, holographic “Year Card”. It is free, it works with a free Strava account, and the developer’s stated plan is to keep the core free.

How it works

TL;DR: Negative Split reads the Strava pages you already view and computes every metric locally in your browser; no server, no login, no API token that can expire or be priced out.

Technically, it is a set of content scripts. When you open one of your activities, the extension reads the same streams the page itself loads, runs the sports-science arithmetic on your machine, and renders the results next to Strava’s own content. The math is deliberately boring: Minetti’s energy-cost model for grade-adjusted pace, an exponentially weighted impulse-response model for Fitness/Fatigue/Form, Riegel-style scaling with a personal fade exponent for race predictions. No machine learning, no cloud inference, no “AI insights” (!!!). Every number on screen has a formula behind it, and most of them show that formula in a tooltip. If you distrust a value, you can check it with a calculator.

Because nothing depends on the Strava API, this year’s developer program changes do not apply to it. There is no sync to break and no token to re-authorise. The honest trade-off is that if Strava redesigns a page, a section can temporarily disappear until the extension is updated. It is built to fail silently, so Strava itself is never touched, and fixes ship in days rather than months.

A worked example, with real numbers

The developer’s own 28-km-long run makes the arithmetic concrete. Threshold pace is the one number you enter by hand once in the popup: here, 4:00/km, roughly a one-hour race pace. The run itself averaged about 5:15/km moving. Grade-adjusting each second of the run with the Minetti model and normalising gives an Intensity Factor of 0.77; in other words, the run sat at 77 per cent of threshold effort once the hills were priced in. Duration times intensity squared gives a training load of rTSS 147 for the session. Those two numbers, plus a decoupling figure that shows whether pace-per-heartbeat drifted in the second half, tell you more about the run than anything on the free activity page.

Two more examples of the same transparency. A 6×6-minute interval session is detected automatically from the velocity stream: the extension found exactly six work repetitions, at paces stepping from 4:12/km down to 4:00/km, and suggested “6×6′ @4:12→4:00” as the workout title, which matched what the runner had typed by hand. And on profiles, where a threshold has not been entered, the extension estimates one from visible personal bests: a 39:52 10K PB produced an estimated threshold of 3:59/km, one second off the manually configured value.

The free Year in Sport, plus a holographic Year Card

The recap is the feature with the obvious backstory: Strava paywalled Year in Sport in December 2025. Negative Split builds a private one from your own history on demand, any day of the year: totals, sport splits, a daily heatmap, monthly volume, season bests, and previous-period comparisons. On top of that sit honest fun stats (cheeseburgers burned, Everests climbed, the walk to Mordor, each with its formula one hover away) and a runner archetype (“The Dawn Metronome”, “The Weekend Volcano”) rendered on a tilting, foil-shimmering trading card you can download as a share image. It is a toy, in the best sense, and every number on it is computed in the same deterministic way as the training load.

Compatibility, and what it does not do

It works with Chrome on desktop, as well as Chromium-based browsers installed from the Chrome Web Store: Edge, Brave, Opera, and Arc. Any watch or platform that syncs with Strava is supported, whether Garmin, COROS, Polar, Suunto, Apple Watch, or phone apps, because the extension reads Strava’s processed data rather than communicating with devices. Both free and subscribed Strava accounts work.

The extension does not cover the Strava mobile apps, since it is a browser extension that runs on strava.com in a desktop browser. Cycling gets basic support, but the analytics are runner-first and lack a power-meter suite. There is no synced full-history database; the recap and trends are built from what Strava’s own pages provide. Negative Split is an independent project, not affiliated with Strava.

Privacy, by architecture

The extension requests one named host permission (strava.com) and local storage, and that is the whole story. The Chrome Web Store data disclosure reads “no data collected”, and the architecture is why: there is no server to send anything to. Threshold pace and body weight (used only for the fun-stat calorie estimates, with the estimate formula shown) live in local browser storage. Privacy is enforced by architecture rather than by policy, which is the strongest form.

Summary

Negative Split is what independent Strava tooling looks like after 2026: local-first, deterministic, and structurally immune to API repricing. Free users get back a version of the analysis Strava sells (training load, form, predictions, a year recap), and data-minded runners get arithmetic they can audit. The catch with any DOM-based tool is that Strava redesigns can cause short outages. For a free install with nothing to sign up for, it is a very easy tool to try and a hard one to argue with.

Get it: Negative Split for Strava on the Chrome Web Store. More at ownward.hu/negative-split.

Quick answers

Does it work with my Garmin, COROS or Apple Watch?
Yes, indirectly. If your runs land on Strava, Negative Split can analyse them. It reads Strava’s web pages, so the recording device never matters.

Do I need a Strava subscription?
No. It works on free accounts, and most of what it adds (training load, Fitness/Fatigue/Form, race predictions, the year recap) overlaps with what the subscription sells. Subscribers can run both side by side.

What happens when Strava changes its website?
Affected sections disappear rather than break the page, and updates restore them, typically within days. There is no login or token involved, so recovery never needs anything from you beyond the automatic extension update.

Related reading on the5krunner

• Klimat Strava Weather App: 80 Million Activities Later: another independent Strava tool that adds detailed weather context to activities, and has proved that the free-to-users model can survive at scale.
• My Running Dashboard: Analytics Built From One Runner’s DNF: a free running dashboard with CP-anchored fitness, race predictions and a Time Machine that rewinds your training history.
• Pace It: Running Analytics That Show Whether Your Training Is Actually Working: pace-at-effort trends and personalised training paces for Apple Watch and iPhone, sitting on Strava or HealthKit.

Author: Lajos Gere, developer of Negative Split, edited by the5krunner.

Last Updated on 13 July 2026 by the5krunner