Watts & Weights — a hybrid strength/cardio app built on top of Intervals.icu

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Hi all,

I’ve been building a training app (with AI) for the hybrid-athlete niche (lift + ride/run) and would value feedback from this community. Intervals.icu sits at the heart of the integration and you’ll spot the rough edges faster than anyone else.

What it does

Watts & Weights is an app for strength training that treats Intervals.icu as its ground truth for anything cardio + wellness. The one thing I couldn’t find in any other strength app: the planner sees both sides of your week at once. When it prescribes cardio it knows what you’re lifting tomorrow; when it prescribes lifts it can see your stats from ICU. That coordination is the whole reason this exists.

Where Intervals.icu shows up

  • Reads — 21 days of wellness (HRV, RHR, sleep, CTL/ATL/TSB, cycle if set), 21 days of activities per sport with power/HR summaries, per-sport thresholds (FTP, indoor FTP, W′, LTHR, run threshold pace), and any scheduled events. All of it feeds the AI context so suggestions cite your actual numbers, not vague “based on your history” hand-waving.
  • Writes strength sessions — completed lifts upload as WeightTraining activities with icu_training_load (Foster-derived or TRIMP if HR-strap), kg_lifted, icu_rpe, feel, average_heartrate, and TCX HR stream. Load formula falls back to a √(kg_lifted) × 1.2 baseline when RPE/HR are missing, so tonnage sessions don’t upload as 0.
  • Writes structured cross-training — AI-generated Z2 / interval / mobility sessions push to the calendar as real structured workouts using ICU’s step-text description syntax (- 8m 88% warmup, 4x blocks with %FTP/%LTHR targets, cooldown). ICU parses them into a workout chart the head unit or trainer can execute, not just a calendar note.
  • Recovery import from ICU — a “pull my strength history back from ICU” flow that reparses the descriptions we uploaded and re-hydrates local workouts (HR streams included, muscle-group tagged from the local library by exercise name). Built after I switched Google accounts on my phone and lost several months of lifting history I hadn’t backed up.

What the AI actually does

Four capabilities, all grounded in your ICU numbers:

  • Training readiness — subjective self-report + HRV/RHR trend + TSB → verdict (go / modify / rest) with muscle groups to focus/avoid.
  • Long-term — 21-day rollup + strength benchmarks + goal → focus areas plus a cardio prescription (weekly hours, intensity split, key session types anchored to your FTP or LTHR).
  • Today’s workout — generates a strength session tuned to readiness + goal + equipment.
  • Cross-training — generates 1-4 weeks of non-strength sessions that avoid stacking hard cardio on heavy lift days. This is the one that surprised me most in daily use.

Honest limits (before you try it)

  • BYOK Anthropic key required for AI features. No hosted-key option yet. Non-AI workout logging works fine without one.
  • Android Chrome or desktop Chrome for HR-strap Bluetooth. iOS Safari has no Web Bluetooth — live HR features don’t work there. Everything else works on iOS; the strap has to sit on your Garmin/Wahoo instead.
  • PWA only, no App/Play Store. Add to Home Screen from the browser.
  • Alpha-ish. I use it daily but it isn’t a polished commercial product yet; expect rough edges. Issues and PRs welcome.

Link: https://fittrack2-ruby.vercel.app​

I’d especially value feedback on three things:

  1. Whether the description-text workout_doc approach for structured workouts renders correctly on your side (I’ve tested against my Elite Suito setup but ICU’s builder syntax has corners).
  2. Whether the “strength-load-around-cardio” scheduling is producing plans that feel right when you use them for a couple of weeks.
  3. Anything obvious I’m missing in the ICU API that would let me do this better — I’ve mostly stuck to /wellness, /activities, /sport-settings, /events and the standard POST /events for pushing.

Not going to pretend this replaces TrainerRoad/Zwift for pure endurance work. It’s for people who lift AND ride/run and want them coordinated — which is a smaller niche but one nobody else seems to be serving well.

Obviously, this is built with AI and would value any feedback if you find this type of app usefull.

Cheers, Peter.