Structured workouts on a Karoo without a power meter

The strap is enough; the file format is the real question

Hammerhead states that following a workout needs a smart trainer or sensors that can report the rider's effort against the selected workout [Hammerhead Karoo Workouts]. A heart-rate strap is one of those sensors. The same page closes the other door: RPE-based workouts are not supported at all.

The format split is the whole story. A FIT workout's steps can target speed, heart rate, cadence, or power, with heart rate accepted as an absolute value or a percentage. The ZWO target list is power and cadence only [Hammerhead Workout Import]. The two are not interchangeable, and nothing about the device or your strap changes that — a heart-rate session simply has to arrive as a FIT workout.

That reframes the question you should be asking a platform. Not whether it works with a Karoo, but whether it exports a FIT workout carrying heart-rate targets. Plenty of tools that sync happily to the device produce ZWO by default, which is the right answer for a rider with a power meter and the wrong one for you. Worth checking before you subscribe rather than after.

One more constraint applies to both formats: zone-based targets are rejected, for heart rate as well as power [Hammerhead Workout Import]. A session written as ride zone 3 for twenty minutes will not import. It has to be expressed as beats per minute or as a percentage. And because the Karoo offers no RPE fallback, a rider with neither power meter nor strap has no route to a structured session on this device — some head units allow one, and this is a genuine narrowness rather than an oversight worth defending.

What heart rate can target, and what it cannot

Heart rate is a good target for long efforts and a poor one for short. Because interval training built from brief bursts produces a lag in heart rate and oxygen uptake, the rise during each interval is not representative of the absolute exercise intensity [Astorino et al. 2025]. A forty-second rep cannot be steered by a number that has not arrived yet.

The practical line falls somewhere around a few minutes. Efforts long enough for heart rate to settle — tempo blocks, threshold intervals, long endurance riding — are exactly where heart-rate targets earn their keep, and a plan built mostly from those is not a compromised plan. Short, sharp repetitions are a different matter: by the time the number reflects the effort, the rep is over, and chasing it during the rep makes you ride the first half too hard.

Two corrections belong on any heart-rate target you follow. The first is cardiovascular drift: heart rate rises over prolonged exercise even when work rate is held constant [Coyle & González-Alonso 2001], so a target satisfied at hour three represents less mechanical work than the same target at minute twenty. Hitting the number late in a long ride is not the same achievement as hitting it early, and reading it that way inflates what you did.

The second is that the relationship itself moves. Dehydration and ambient temperature have a profound effect on the link between heart rate and oxygen uptake, and there is measurable day-to-day variability in heart rate [Achten & Jeukendrup 2003]. On a hot day the same number means less work; on a cold morning after a good night it may mean more. Treat the target as a band and the session as the unit you judge, rather than grading yourself against a single figure.

Building a session the Karoo will actually accept

Two supported targets do the work between them. Heart rate carries the long blocks, delivered as FIT. Cadence carries the short ones — it is a first-class target in its own right in both formats [Hammerhead Workout Import], which gives a rider without power something legal to put on reps too short for heart rate.

The cadence route solves a real problem. Every step in the file has to carry a target, and Cadence or a CadenceHigh and CadenceLow pair satisfies that rule with no power anywhere in the workout [Hammerhead Workout Import]. For the short hard efforts you were always going to ride by feel, a cadence target makes the step valid, gives the device something to display, and quietly enforces the gear selection that usually decides whether those reps are any good.

The device itself is unaffected by any of this, which is worth saying because it is the encouraging half. Before you start, the Karoo shows which zones the workout targets and how long you will spend in each [Hammerhead Karoo Workouts]. During it you get the workout drawer, controls for skipping, scaling and pausing, and a new lap opened automatically at every interval boundary. Your ride file comes back with its structure already marked, whether the targets in it were watts or beats.

Check the numbers underneath before blaming a session that felt wrong. Your Karoo's heart-rate zones sync in from a platform you nominate, alongside FTP and rider weight [Hammerhead Training Metrics], so the zones the device shows you are only as current as that profile. A maximum heart rate estimated from a formula years ago, or a threshold set in a different season, will produce targets that feel wrong in a way no amount of re-syncing fixes.

Where this leaves you, and where it leaves us

A limitation in our own product belongs here, because the honest version of this page has to include it. AdaptCycling's Karoo push emits ZWO, and ZWO cannot carry heart-rate targets. Until we ship a FIT workout encoder, a rider training on heart rate is better served reaching the device another way.

The route that works today costs nothing. Export or author the session as a FIT workout with heart-rate targets and upload it to the Hammerhead Dashboard, or connect a provider that produces one — the Dashboard accepts FIT workout files directly, with no subscription in the chain [Hammerhead Workout Import]. That is the same free route that serves power-meter riders, and the file rules are the only thing standing between you and it.

What we do handle is everything upstream of the device. We read heart-rate rides, build and revise plans for riders training without power, and judge sessions on what your heart rate actually did rather than assuming a power file exists. The gap is specifically the last hop onto a Karoo, and it is a format gap rather than a modelling one — which is why it is worth naming plainly instead of leaving you to discover it after signing up.

The pattern is the one running through our broader guide to training with a Hammerhead Karoo. The device is a faithful delivery layer that decides nothing and renders whatever it is handed. Here the constraint is one level lower still, in the file format itself, and it is the sort of thing that is invisible until it costs you a session. Confirm the export before you build a training block around it.

Quick answers

Sources cited in this guide

1. 01
   Hammerhead Karoo Workouts. Karoo OS - Workouts. Hammerhead Support.
2. 02
   Hammerhead Workout Import. Dashboard - Importing ZWO & FIT based Workouts. Hammerhead Support.
3. 03
   Hammerhead Training Metrics. Dashboard - Training Metrics. Hammerhead Support.
4. 04
   Astorino et al. 2025. Heart Rate and Oxygen Uptake During Recovery from High-Intensity Interval Training: A Retrospective Analysis. International Journal of Environmental Research and Public Health.
5. 05
   Achten & Jeukendrup 2003. Heart rate monitoring: applications and limitations. Sports Medicine.
6. 06
   Coyle & González-Alonso 2001. Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews.

More inside Training with a Hammerhead Karoo

Other articles in this series

1. 01

   Karoo structured workouts without TrainingPeaks

   TrainingPeaks is not required, and Hammerhead's own docs are part of why people think it is. The free upload route and the providers that qualify.

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2. 02

   Training plan apps for the Hammerhead Karoo

   Hammerhead ships no training platform, so the app you connect is the plan. What TrainingPeaks, TrainerRoad, Intervals.icu and Xert each actually do.

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3. 03

   Intervals.icu to Karoo workout sync: how it works

   The official sync landed in January 2025 and is created from the Karoo itself. What syncs, what the importer rejects, and the format trap.

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4. 04

   Karoo workouts not syncing: how to find the real fault

   Check the Dashboard first — that one look halves the problem. The provider rules that block a push, the device half, and the failure that is not one.

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5. 05

   ZWO file not importing to a Karoo: the rejection rules

   Hammerhead rejects the whole file, not the bad step. The step types it accepts, the target rule, and the difference between rejected and ignored.

   →