New to intervals.icu?
You have probably not heard of it. Serious athletes use it anyway.
intervals.icu is a free training platform built by a single developer and used by hundreds of thousands of endurance athletes worldwide. It is what IcuSync is built on, and it is worth understanding before you connect the two.
A free analytics and planning platform for endurance athletes
If you currently use Strava to log your runs or TrainingPeaks to follow a plan, intervals.icu sits in similar territory, but it goes significantly further, and it is free.
Built and maintained by David Tinker, intervals.icu gives you the kind of training analytics that used to require an expensive coaching platform or a sports science background to interpret. Fitness and fatigue curves, training load over time, zone distribution analysis, structured workout scheduling, and automatic sync to Garmin, Coros, Wahoo, and other devices, all available without paying a subscription fee.
It is used by self-coached athletes who want to take their training seriously, by coaches managing athlete calendars, and by anyone who has outgrown Strava and wants more than a social feed and a weekly summary.
IcuSync is built on top of intervals.icu because it has an open API, a genuinely powerful data model, and a developer who cares deeply about the platform. When Claude pushes a workout through IcuSync, it lands on your intervals.icu calendar and syncs to your watch automatically. That chain only works because of how intervals.icu is built.
intervals.icu is not a coaching app and it is not a social platform. It is a serious training tool for athletes who want to understand and control their own data. IcuSync adds Claude as the intelligence on top of it.
How intervals.icu compares to what you are probably using now
This is an honest comparison. TrainingPeaks is a mature, well-built platform with things intervals.icu does not have, including a coaching marketplace, a large library of training plans, and a polished mobile experience. If you are working with a coach who uses TrainingPeaks, it may still be the right tool for that relationship.
What intervals.icu offers is the analytics depth and open API access that makes IcuSync possible, at no cost.
Cost
Fitness and fatigue tracking (CTL/ATL/TSB)
Structured workout builder
Planned workout sync to Garmin
Planned workout sync to Coros
Zone analysis and distribution
Claude integration via IcuSync
Open API access
Coaching marketplace
Social feed and segments
Free to use
You do not need to leave Strava or TrainingPeaks to use intervals.icu. Many athletes connect all three, Strava for social logging, TrainingPeaks for coaching, and intervals.icu for detailed analytics and IcuSync. intervals.icu connects to both platforms as data sources.
The three numbers worth understanding
intervals.icu surfaces a lot of data. Most of it is optional. Three metrics are worth understanding from day one because Claude references them when assessing your training.
Fitness (CTL)
Chronic Training Load. A rolling average of your training stress over roughly the last six weeks. Think of it as a measure of how much consistent work your body has absorbed and adapted to. A rising CTL over time generally means your fitness is building. A falling CTL means you are doing less than your body is used to.
Fatigue (ATL)
Acute Training Load. A shorter rolling average covering roughly the last week. It rises quickly when you train hard and falls quickly when you rest. High ATL relative to your CTL means you are carrying accumulated fatigue from recent training.
Form (TSB)
Training Stress Balance. The difference between your fitness and your fatigue. A negative TSB means you are in a fatigued state, common during heavy training blocks and not necessarily a problem. A positive TSB means you are fresher than your fitness baseline suggests, typically where you want to be on race day.
You do not need to manage these numbers manually. Claude reads them and translates them into plain language when you ask about your training. Understanding what they represent helps you follow Claude's reasoning when it makes a recommendation.
Do you need to give up Strava or TrainingPeaks?
No. intervals.icu works alongside both platforms.
If you use Strava, you can connect it to intervals.icu as a data source. Your activities will flow from Strava into intervals.icu automatically, giving you the analytics layer on top of the logging you already do.
If you use TrainingPeaks, the situation is similar, and many athletes run both in parallel, using TrainingPeaks for a coaching relationship and intervals.icu for deeper personal analysis and IcuSync access.
The only thing IcuSync requires is an intervals.icu account. What else you use alongside it is entirely up to you.
Getting set up takes about five minutes
Create a free account
Go to intervals.icu and sign up. No credit card required.
Connect your device
Link your Garmin, Coros, Wahoo, or other device under Settings → Connections. Tick the Upload planned workouts checkbox so sessions Claude builds sync to your watch automatically.
Set your threshold values
Go to Settings → Sport Settings and enter your threshold pace for running, or FTP and Critical Power for cycling and power-based running. Claude uses these to set workout intensity targets correctly.
Get your API key and Athlete ID
Go to Settings → Developer to find your Athlete ID and API key. You will need both when connecting IcuSync.
Once you have your API key and Athlete ID, you are ready to connect.
Connect IcuSync, $20.00 USD per yearintervals.icu is free. IcuSync is $20.00 USD per year. Claude does the thinking.
Set up intervals.icu, connect IcuSync, and start talking to your training data.
Get Started, $20.00 USD per yearRequires a Claude account. Paid Claude plan recommended for full access across web, mobile, and desktop.