Training by fatigue and HRV: what it is, what the evidence says and its limits.

• HRV measures the variation in time between heartbeats; it reads better as a multi-week trend than as a single-day number.
• Subjective signals (sleep, mood, muscle soreness, perceived effort) are useful and free.
• Meta-analyses find a small or non-significant advantage for HRV-guided training, with few studies and mixed methods.
• Adjust the session's volume or intensity first, not the whole plan, after a single bad signal.
• No fatigue indicator is a medical diagnosis. If something worries you, see a professional.

A fixed plan assumes you arrive equally rested every week. In practice you do not: you sleep worse, have stress, accumulate load or come down with a mild infection. Adaptive or autoregulated training means deciding each day how hard to push based on signals of how you are, instead of blindly following the plan on paper.

Many coaches do it intuitively ("today we go lighter") and it can be systematized with objective and subjective indicators.

HRV is the variation, in milliseconds, of the time between one heartbeat and the next. A healthy heart does not beat like a metronome: that variation reflects, among other things, the balance between the sympathetic and parasympathetic nervous systems.

It is measured with a chest strap, a watch or an app, and it is usually advised to measure at the same time, at rest and in the same posture (for example on waking). What matters is not one day's value but its trend relative to your own average over several weeks.

• Sleep: hours and perceived quality of the last nights.
• Perceived effort (RPE): whether a known load feels much harder than usual.
• Muscle soreness and stiffness that does not ease within 48-72 hours.
• Mood and motivation: a persistent drop usually accompanies accumulated fatigue.
• Resting heart rate: a sustained rise relative to your usual.
• Performance: weights or times that worsen over several sessions in a row.

A short questionnaire of 4 or 5 questions, scored 1 to 5, at the start of the session already gives valid information and needs no device.

Meta-analyses have compared training with a fixed plan against adjusting load by HRV, mostly in runners and endurance athletes. Conclusions are cautious: there seems to be an effect on HRV indicators themselves, but the improvement in performance or maximal aerobic capacity is small and, in some reviews, not statistically significant.

• There are few studies, with few participants.
• Methods differ: different measurement time, different posture, different rules for when to lower load.
• Almost all research is in endurance sports; in strength and WODs there is much less.
• Pooled results do not say how you will do individually.

Practical conclusion: HRV can be one more signal, not an order. It makes sense to combine it with your own perception and the context.

One isolated bad day

Keep the session but cut volume 20-30% or drop intensity a notch. Warm up a bit longer.

Several bad days in a row

Swap the hard session for a low-intensity one, or rest. Review sleep, food and stress.

Persistent signals for weeks

Plan a deload week and, if it does not improve, see a professional.

You arrive especially fresh

You can keep the plan or add a little quality, without big jumps.

The percentages are indicative, not a prescription. For WODs, scaling is the natural tool: see the guide to scaling a WOD.

• It is not a diagnosis: a low value does not mean you are ill, nor a high one that you are healthy.
• Lots of noise: alcohol, coffee, stress, a bad night, heat or a badly placed strap alter the reading.
• Your own baseline rules: what is normal for you is not normal for someone else; compare only with yourself.
• Devices differ: wrist HRV can be less reliable than chest-strap HRV.
• If something worries you: chest pain, dizziness, palpitations or unexplained fatigue need a professional, not an app.

In a week with strength, running and a mixed WOD, the trick is to decide beforehand which session is non-negotiable (the week's key session) and which are flexible. If you arrive with bad signals, trim the flexible ones, not the key one. The hybrid training guide and the one on running and strength detail how to distribute sessions.

Vital Flow, from Zoku Athletics, adapts training to the fatigue you report: if you arrive saturated it lowers the load and if you arrive fresh it raises it. It is available on the Pro plan (see the home page for what each plan includes). Some adaptations can use an AI model, but only if you expressly turn it on in the privacy settings; it is off by default.

Vital Flow is guidance for training, not medical advice or a diagnosis, and it is no substitute for a coach or a health professional.

The app is not public on the stores yet. It is in testing and there is no launch date.

The Free plan is free, with no card and no expiry. Pro and Elite are paid; what each includes is shown in the plans section of the home page.

Who is behind the website and the app: About Zoku Athletics.

• How to scale a WOD: Rx vs scaled: What Rx and scaled mean, when to scale a WOD and how to adjust load, movements and volume without losing the stimulus or your technique.
• Hybrid training and hybrid WOD: sample week: What hybrid training and a hybrid WOD are, how to combine strength and endurance in the same week and a sample weekly routine you can adapt.
• Running and strength: how to combine: How to combine running and strength training: the interference effect, session order, three sample weeks and signs that you are overdoing it.