Why Your Zone 2 Heart Rate Is Wrong on Clen, T3 and Tren

A student once joked to an exercise physiologist that he had found the shortcut to zone 2: drink enough caffeine to get your heart rate into the zone and sit on the couch. It's a good joke because everyone can see why it fails. Your heart rate went up, but nothing made it go up except the drug.

Now look at a typical cut. Clen in the morning, a bit of T3, tren in the background, a stim pre-workout, and you're 2 kg down on water by Thursday. You get on the treadmill, walk at a pace that would have been a stroll in the off-season, and your watch says 135 bpm, zone 2, job done. That is the caffeine joke with extra steps. Below: why it happens, how to find your real zone 2 on cycle, and when a high resting heart rate stops being a training problem and becomes a medical one.

Heart rate is the passenger, not the driver

Exercise physiologists use one equation for this, the Fick principle:

Oxygen used (VO2) = heart rate x stroke volume x oxygen extracted by the muscles (a-vO2 difference)

Heart rate and stroke volume together are cardiac output, the delivery side. The a-vO2 difference is the demand side: how much oxygen your working muscles actually pull out of the blood. Training raises maximal oxygen uptake mainly by increasing how much blood the heart can push per minute, which is stroke volume, not a faster heart rate (Bassett and Howley, 2000; Levine, 2008).

The muscles set the demand. When working muscle needs more ATP, it burns more oxygen, and heart rate rises to deliver it. Push heart rate up with a drug while the muscles are idle and you've changed one term of the equation without creating the demand behind it.

The cleanest proof runs the other way: lower heart rate with a drug and see if training still works. In trained men, propranolol cut heart rate at 60% of VO2max from about 135 to 107 bpm, yet oxygen uptake at that workload didn't change, because stroke volume rose to make up the difference (Joyner et al., 1986). In an 8-week cycling study, men training on propranolol gained the same VO2max as men on placebo, and muscle capillary density rose 17 to 19% in both groups (Svedenhag et al., 1984). Their heart rates during training were much lower and they still adapted. Not perfectly: the gains in mitochondrial enzymes were smaller on propranolol. But the engine grew without the heart rate.

Heart rate is a convenient readout because, in a sober and hydrated person, it tracks workload closely. On cycle, that link breaks.

What about caffeine itself?

Caffeine itself is the weakest example. At normal doses caffeine barely moves heart rate: 5 mg/kg didn't change resting heart rate in one controlled trial (Glaister et al., 2025), and 250 mg left resting heart rate, maximal heart rate and VO2max unchanged in another (Björkman et al., 2026). What caffeine does reliably is make the same work feel easier, by about 0.8 points on the effort scale (Glaister and Gissane, 2018). That matters later: stims distort how hard you think you're working too.

Does lifting weights count as cardio if your heart rate is high?

Mostly no. A hard set sends heart rate up for reasons that have little to do with oxygen demand. In a leg press to failure, heart rate climbed from 82 to 143 bpm while blood pressure peaked at 270/183 and stroke volume tended to fall, from 94 to 77 mL, though not significantly (Lentini et al., 1993). That's a pressure load from straining, not the sustained, high-volume blood flow that builds an aerobic engine. During a bench set to failure, most of the energy came from anaerobic sources and recovery, not aerobic metabolism (Scott et al., 2011).

Resistance training does nudge VO2max up in untrained and older people, about 1.8 mL/kg/min in adults over 60 (Kelley et al., 2024). Aerobic training beats it by about the same margin head to head (An et al., 2024). For a trained lifter, a 140 bpm session of supersets is good lifting. It isn't your zone 2.

What pushes your heart rate up on cycle

Each of these raises heart rate at a fixed pace, so each one shifts your zones.

Two of these need more detail.

Potassium. Beta-2 agonists like clen pull potassium into cells, so blood levels drop. In a series of clenbuterol poisonings the median low point was 2.5 mmol/L (Hieger et al., 2016), and one bodybuilder on clen plus thyroid hormone presented with potassium of 1.9 and muscle weakness, with no diuretic involved (Bonnar et al., 2021). Low potassium and magnesium make the heart more irritable, which is the bridge from "my heart rate is high" to "my heart rhythm is wrong". Our clenbuterol and potassium page has the detail.

Haematocrit. Thicker blood raises the load on the heart and blood pressure, and many users assume it raises heart rate as well. We couldn't find solid human data putting a bpm number on it, so treat high haematocrit as a reason to check blood pressure and heart rate together, not as a known heart rate driver. Our haematocrit guide covers the thresholds.

What pulls it down: nebivolol and other beta blockers

Beta blockers break heart rate zones the other way. Nebivolol 5 mg lowered heart rate at rest, at submaximal work and at maximal work in physically active people with high blood pressure, while maximal work capacity didn't change (Predel et al., 2001). It drops heart rate less than atenolol and doesn't reduce exercise tolerance the way older beta blockers can (Van Bortel and van Baak, 1992). Forum users commonly report around 10 bpm off resting heart rate at 5 mg.

So on nebivolol, a heart rate of 120 might be real zone 2 work, and chasing 135 pushes you above it. Cardiac rehab research treats percentage-of-peak heart rate as unreliable on beta blockers and leans on heart rate reserve, workload or effort ratings instead (Zanettini et al., 2012; Tsai et al., 2015).

The bigger risk is stacking. Someone on clen plus nebivolol has one drug pushing heart rate up and another pulling it down, and the watch number reflects neither the workload nor the cardiac stress. Our blood pressure on cycle article covers where nebivolol fits and why it shouldn't be used to mask a clen or T3 heart rate.

Worked example: 135 bpm on and off clen

This is illustrative, built from the effect sizes above, not a measured case.

Off cycle, a 95 kg lifter walks at 6.0 km/h on a 10% incline. Heart rate settles at 128 bpm, he can talk in full sentences, and that's his zone 2.

Three weeks into a cut with clen and T3, his morning resting heart rate is up 12 bpm. The same 6.0 km/h now reads about 140. His watch says he's above zone 2, so he slows to 5.0 km/h to get back to 128. He's now walking at a lower workload than his off-season zone 2, the muscles are doing less, and the adaptations he wants (more capillaries, more mitochondria, a bigger stroke volume) get a smaller stimulus. He logs 45 minutes of "zone 2" and gets a lighter session than he thinks.

Don't ignore heart rate. Just stop letting it set the pace while drugs are moving it.

How to find your real zone 2 on cycle

Zone 2 is the intensity just below your first threshold, the point where breathing and lactate first start to climb. Most guides give it as a percentage of maximum heart rate, but fixed percentage-of-max prescriptions "have little merit" for individuals (Jamnick et al., 2020). In 50 cyclists, the different markers used to define zone 2 varied between people by 6 to 29% (Meixner et al., 2025). That's before any drugs are involved.

Ranked by how well each method holds up on gear:

1. Talk test, then lock it to a speed or wattage. If you can recite a short passage comfortably, you're below the first threshold. When it starts to feel uncomfortable, you're at it. The talk test is consistent across sessions and tracks the ventilatory and lactate thresholds well (Persinger et al., 2004; Foster et al., 2008; Quinn and Coons, 2011). Your speaking is driven by breathing, and breathing is driven by what the muscles are producing, so a stimulant-driven heart rate doesn't fool it much. In runners, speed-based anchors estimated the first lactate threshold more accurately than heart rate or effort ratings (Nuuttila et al., 2025).
2. A lactate meter. Training below the first threshold usually means blood lactate at or under about 2 mmol/L (Seiler and Kjerland, 2006). Finger-prick meters are cheap now and they measure the muscle side of the equation directly.
3. Effort rating (RPE). Zone 2 feels like 3 out of 10, or 11 to 12 on the 6 to 20 scale. It's useful but less reliable on stimulants, because caffeine and similar drugs make the same work feel easier (Glaister and Gissane, 2018), and people on beta blockers drift to higher workloads when they set intensity by feel (Zanettini et al., 2012).
4. Nose breathing. It's a rough cue at best. In one study, nasal-only breathing didn't stop people training harder than intended (Rappelt et al., 2023).

Some watches now estimate your threshold from heart rate variability (DFA alpha1). It lines up with the first threshold in sober athletes, though individual errors run from about 20 bpm low to 10 bpm high (Rogers et al., 2021; Schaffarczyk et al., 2023). It's still built on your heartbeats, and ectopic beats distort it (Tarkiainen et al., 2007). Nobody has tested it on clen or tren, so don't trust it there.

A 20-minute treadmill test you can do this week

Do it off stims, hydrated, ideally at the start of a cycle or in a cruise:

1. Warm up 5 minutes at an easy walk.
2. Set the incline to 8 to 10% and start at 4.5 km/h. Every 3 minutes, add 0.5 km/h.
3. In the last 30 seconds of each stage, say a fixed sentence out loud (the same 30 words each time) and rate it: comfortable, a bit hard, or can't.
4. The last stage that was comfortable is your zone 2 speed. Write down the speed, incline and heart rate.
5. On a bike, do the same with 20 W steps.

From then on, your zone 2 is that speed and incline, not that heart rate. Repeat the test every 6 to 8 weeks or when your stack changes, and log what you were taking each time. If your heart rate at that speed is 10 or more bpm higher than on your clean test, that number is now telling you about the drugs, not your fitness.

Decision tree: my watch says zone 2, is it?

• On clen, T3, a stim pre-workout, or more than 2% down on bodyweight from water? Your heart rate reads high. Use your locked speed or wattage and the talk test. Expect a higher number on the watch at the same pace, and don't slow down to chase it.
• On nebivolol or another beta blocker? Your heart rate reads low. Use your locked speed and the talk test. Don't speed up to reach your old heart rate target.
• On both? Ignore heart rate for intensity completely. Use speed plus the talk test.
• Heart rate at your locked speed has risen 15 or more bpm compared with your clean test, or keeps climbing during the session? Shorten the session, drink, check your electrolytes, and read the next section.
• Can't hold a conversation at a speed that used to be easy, or you're breathless on stairs? That isn't a zones problem. Stop and get checked. Forum reports of tren breathlessness are common, but "it's just tren" is not a diagnosis.

Resting heart rate as a cycle marker

Your training heart rate is noisy. Your morning resting heart rate is one of the best cheap signals you have, as long as you compare it with yourself.

How to measure it: seated or lying, first thing in the morning, after 5 minutes still, before coffee or anything else. Use a 7-day average, not a single day. Overnight resting heart rate from a decent wearable is accurate: Oura matched ECG closely, WHOOP moderately (Dial et al., 2025). Optical wearables underestimate heart rate more as intensity rises, so for lifting and hard intervals a chest strap is the safer reading (Boudreaux et al., 2018).

Why it matters: across large cohorts, every 10 bpm higher resting heart rate carries about 17% higher all-cause mortality and 15% higher cardiovascular disease risk (Aune et al., 2017). In the Copenhagen Male Study the link held even after accounting for fitness, and men above 90 bpm had about three times the mortality of men at 50 or below (Jensen et al., 2013). That's population data on sober people, not proof that a few weeks of clen shortens your life. It is a reason not to run a resting heart rate of 95 for months.

What our marker page uses: the VitalMetrics resting heart rate page uses 60 to 100 bpm as the general reference and 45 to 90 for enhanced athletes. The more useful rule is relative: a rise of 10 to 15 bpm above your own off-cycle baseline is meaningful even if the number is still "normal".

What the forums report, paraphrased from Evo, EliteFitness, UGL Talk and ASF threads:

• Tren acetate at 250 mg a week for 7 weeks: resting heart rate up about 10 bpm on a wearable, back to normal about 10 days after stopping.
• Stage prep on test propionate and tren acetate at 210 mg each: average heart rate 88 bpm three weeks out.
• A long-term clen user posted a year of wearable data with resting heart rate "through the roof" even after he stopped feeling the drug. Tolerance to the effects isn't tolerance to the heart rate.
• The folk ceiling on clen is "keep resting heart rate under 85". One experienced poster pointed out that the often-quoted target of 70 comes from heart failure management and doesn't apply to healthy users.
• The worst case: a 37-year-old on test, masteron, 200 mg of tren and growth hormone sat at a resting 105 to 110 with blood pressure up to 190/70. His ECG was flagged and a cardiologist suspected cardiomyopathy.

Red flags that end the cycle conversation

• Resting heart rate persistently above 100, especially with a 24-hour average above 90 (the same cut-offs doctors use to define inappropriate sinus tachycardia, Ali et al., 2021)
• Palpitations, skipped beats you can feel, or an irregular pulse
• Chest pain or pressure, fainting or near-fainting, or breathlessness at rest
• Cramping, weakness and palpitations together on clen (think potassium)
• Resting heart rate rising week on week with rising blood pressure and rising haematocrit

Clenbuterol misuse produces tachycardia, low potassium, troponin rises and ECG changes in poison centre data (Spiller et al., 2013), and a tenfold dosing error has caused heart rates of 254 with atrial fibrillation (Daubert et al., 2007). Excess thyroid hormone roughly doubles the risk of atrial fibrillation in subclinical hyperthyroidism (Singh et al., 2024). Those cases are why the red flags above matter.

What to test

Bloodwork can't see your heart rhythm, but it explains a lot of high heart rates.

Beyond bloods:

• A 12-lead ECG is cheap and the right first step for palpitations, a resting heart rate above 100, or a long history of heavy cycles. In one study, long-term AAS users showed autonomic changes and longer QT intervals than sedentary controls (Barbosa Neto et al., 2018).
• A Holter monitor (24 to 48 hours) if palpitations come and go.
• An echocardiogram after years of cycling. Long-term AAS users had a lower pumping fraction than non-users, 52% vs 63%, and more coronary plaque (Baggish et al., 2017). A 2026 meta-analysis found septal thickening detectable before 6 months of use; users overall had a pumping fraction 2.8 points lower than non-users, and 7.4 points lower after more than 6 years of exposure. Those are cross-sectional comparisons, not proof of decline in one person (Fernandes et al., 2026).

Practical recommendations

• Do one clean zone 2 test (off stims, hydrated) and lock zone 2 to a speed and incline or a wattage.
• On clen, T3, tren or a stim pre-workout, train cardio by pace and the talk test. Expect a higher watch number and don't slow down to chase it.
• On nebivolol, same rule in reverse: don't speed up to hit an old heart rate.
• Track morning resting heart rate as a 7-day average against your off-cycle baseline. Treat +10 to 15 bpm as a signal to review the stack, not something to push through.
• Check potassium, magnesium and thyroid markers if you run clen or T3, and get an ECG if resting heart rate stays above 100 or you feel your heart skipping.
• Don't use a beta blocker to make a clen or T3 heart rate look normal. Lower the dose of the thing raising it.

Key takeaways

• Heart rate is a response to work. A drug that raises it without the muscles doing more gives you a high number and a smaller training stimulus.
• Clen, T3, heavy AAS cycles, retatrutide and dehydration all push heart rate up at the same pace, so your watch's zone 2 becomes too easy. Beta blockers push it the other way.
• People training on a beta blocker still largely adapt at much lower heart rates, which shows heart rate isn't the main stimulus.
• Lifting at 140 bpm is good lifting, not zone 2.
• Set zone 2 by the talk test and lock it to a speed or wattage. Use lactate if you have a meter. Treat effort ratings and heart rate as secondary on stims.
• Morning resting heart rate against your own baseline is a cheap cycle marker. Above 100 at rest, palpitations or chest symptoms need a doctor, not a beta blocker.

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