Spark 3 Cardio & Heart rate

FarreG Registered Users Posts: 1
New Traveler
edited January 24 in TomTom Sports
Something bizarre is going on for a while now and i have done the comparison with my second sports watch.

When I practice fitness training, I often find that the heart rate on my TomTom does not match my actual heart rate. Often if I have an effective higher wrist (eg 145), the tomtom only gives 78.

Anyone who knows how or why this is happening?

For clarification, I posted a picture with the noticeable difference between the two sportwatches during an exercise this week ...


  • tfarabaugh
    tfarabaugh Posts: 16,943
    It will depend on what sports you are doing, as optical HR does not work well with certain types of exercise, like weight lifting, cycling (in certain conditions), rowing and others. Anything that involves forearm tension and flexing will throw off the watch (so weight lifting, rowing, bearing down your weight on the bars in cycling, pull ups, etc.) will be an issue, as you are squeezing the blood vessels the watch is reading, so it sees this as a reduced pulse. It is not that the watch is having a problem reading your pulse, it is that you pulse has actually dropped because you are temporarily cutting off blood flow to the vessels it is reading. I have experienced this with every optical HR I have used, including a Mio and a Scosche unit. i wear a chests strap linked to the watch as an external HR for these sorts of activities.

    On the other hand, spikes in HR are generally from poor blood flow producing weak pulse strength, so the watch reads cadence instead. This is particularly apparent early in a workout or during a non-intense workout when you are not warmed up. You have to think of the optical heart rate as an algorithm that is attempting to track a signal in a set frequency range (30-230 or whatever it uses). If the pulse signal is weak it latches onto the next strongest rhythmic signal, which is your cadence in running and the vibrations of the bike in cycling. For most people who experience this while running it spikes to around 180-200 bpm which is also the average cadence people run at. Additionally, each person has a different HR signal ‘strength’, depending on a range of factors, so some are prone to get it more than others. But usually their signal strength is lower for the first 5-10 minutes until they warm up properly. So in that time, it is prone to latching onto cadence, which is a common fault with all optical HRs, unfortunately.

    I hope this helped answer your question. If so, please mark it as a solution so others can look for it if they have the same question.