Two technologies, not two brands
A chest strap is a wearable ECG: it reads the heart's electrical signal directly from the skin, the same signal a cardiograph reads, sampled at the chest where it is strongest. A watch or optical armband uses PPG, photoplethysmography: LEDs illuminate the skin and a sensor infers pulse from changes in reflected light as blood volume pulses through the capillaries. Electrical versus optical is the entire debate; everything else follows from it.
Where each one is strong, and where each one fails
Straps track fast changes best. Electrical detection is effectively instantaneous, so intervals, sprints and anything with abrupt effort swings show up on a strap with minimal lag. For the interval formats described in HIIT heart rate zones, the strap is the reference.
Wrist optics dislike wrist work. PPG's classic failure is motion artifact: gripping a bar, rowing, burpees, boxing, anything that flexes the forearm disturbs the optical signal, and some watches briefly lock onto movement cadence instead of pulse. Steady-state cycling, jogging or walking, by contrast, wrist readings are generally close.
Fit is half the measurement. A loose watch worn on the wrist bone will lose to a snug one worn correctly; a dry strap in a cold room reads poorly until sweat improves contact. Many "inaccurate device" complaints are fitting problems wearing a technology costume.
Individual factors are real. Tattoos under the sensor, very cold hands and certain skin conditions degrade optical readings for specific people; the strap does not care.
How much accuracy do you actually need?
Depends on the job. For a member accumulating time-in-zone across a 45-minute class, small optical errors mostly average out: the session's story, effort points included, survives a watch. For interval prescriptions, zone-boundary work, or the one-minute recovery test, lag and artifacts genuinely distort the result: that is strap territory. And for a member whose motivation runs on watching the screen respond the second they surge, the strap's responsiveness is the experience.
The wrong conclusion is "watches are junk": the right one is that accuracy is task-relative, and a club should let both coexist, which is also where the industry has landed, as covered in the smartwatch shift.
A practical policy for clubs
Accept whatever the member wears. The barrier to entry must stay at zero: any Bluetooth LE or ANT+ device connects, and the member's first class runs on the watch already on their wrist.
Keep house straps for precision moments. A pool of club chest straps, such as Uptivo Belt-D, serves assessments, interval-heavy formats and members who want maximum responsiveness, with Uptivo Lightband armbands as an optical alternative for comfort. Loaners also rescue the member whose watch died mid-week.
Teach fit in thirty seconds. Watch snug, one finger above the wrist bone; strap moistened, centred, firm. It is the cheapest accuracy upgrade in the building and belongs in member onboarding.
Coach the trend, not the beat. A member comparing 143 to 145 across devices is auditing noise. Zones, trends and accumulated effort are robust to small error, which is exactly why group heart rate training scores time-in-zone rather than raw beats.
The bottom line
Straps are the reference instrument; modern watches are good enough for most of what a gym measures; the club that forces a choice loses members it could have kept by accepting both. Openness plus a strap pool covers every case, at the lowest total cost. To see mixed devices running honestly on one screen, speak to an expert.