Victoria Moore: State of the heart

The heart rate monitor I asked my brother to get me for Christmas would, I hoped, be a route out of sloth. The next time my internal whinger cranked up with a moany, "But I'm tired, I think I might not be very well, I'm really out of breath, I need to stop for a bit," I could check my heart rate and see how hard I really was working. No arguments. No equivocation.

The monitor consists of two parts. A chest strap detects the electronic activity of your heart, and transmits the data to a watch-like device on your wrist. This displays your current heart rate, and will also give high, low and average readings for your exercise session. The better models also allow you to programme them so that you can choose to work within a given heart rate zone, and will alert you with a beep if you stray outside it.

By Boxing Day I was surgically attached to mine. I fell asleep on the sofa wearing it and was thrilled to discover, on waking, that it was registering the lowest rate I had yet seen - 67 beats per minute. Actually, I was so surprised, I panicked that the precious monitor might already be broken, then watched in fascination as it responded to my anxiety by zipping up 20 points. For several days I monitored my activity at home with all the gravity of a scientist taking readings from a nuclear power plant: putting on T-shirt (119); walking into kitchen for tea (112); fluctuations on watching the scary Sopranos episode where Adriana gets whacked (+/-24).

"It's normal for heart rates to vary, sometimes dramatically, over the course of a day," says cardiologist Clive Weston. "There is a circadian variation, where heart rate and blood pressure follow a biological clock. They start to rise, very slightly, a couple of hours before you wake, are at their highest during the first four hours of waking, then drop away a bit in the middle of the afternoon."

Overlaid on this background is a great deal of jagged change. "A lot of worried people come to me after buying a monitor because they've just noticed their resting heart rate of 60 rises to 74 after eating, for example," says Weston. "Then there's the primeval fight-or-flight response, which might be activated by a row or just watching television. Some people have a more sensitive stress response than others, and will notice greater variations, perhaps seeing their resting heart rate go up to 100 under emotional stress. But variability of heart rate is actually a good thing - after a heart attack, the lack of it is seen as a predictor of sudden cardiac death."

In January, I finally took the monitor on the sort of outing for which it was actually designed: a slow canter around the park. One mile in, I glanced down to find that my heart had outdone itself. I'd only been jogging slowly, but the monitor seemed to think my heart rate had peaked at 228. Given that the only fact I knew about heart rates is that they shouldn't really go higher than 220 minus your age, this seemed excessive. Presuming that the monitor had blipped, I reset it, jogged home - and within 10 minutes had registered 232.

The idea of being a freak medical specimen was, I confess, seductive, but it was scotched as soon as I boasted about it on a discussion thread on the Runner's World website.

"Sounds like it was caused by an external source, like a power line, rather than being a max reading," one runner responded drily.

Personal trainer Chris Sterling was also unimpressed. "That's nothing, and most likely just a blip - if you look at the average, you'll probably see it scarcely seems to have taken that into account. Or the monitor could have got stuck. Also, some heart rate monitors cross-talk with others, picking up their signals."

Sterling favours monitors made by Polar, which he says are very reliable. Mike Kingsley, a sports scientist at Swansea University, agrees. "Some of the better commercially available monitors, such as the Polar S810, measure exercising heart rate to an accuracy similar to an ECG [electrocardiogram]."

So how best for a normal person (as opposed to serious athlete) to use one? Most broad - and they can only be broad - guidelines involve calculating your peak heart rate, then working in a "zone" at a chosen percentage of that figure. Exercising, for example, at 70-80% of your maximum heart rate is said to develop the cardio-vascular system, while slogging away in the so-called "red-line zone" at 90-100% of maximum heart rate is good for improving your speed, though should only be done in short bursts.

Sterling warns that it is important not to overdo things. "Usually, if I was working with someone new, I'd keep them at lower percentages for as much as 18 weeks. People often don't realise that it's good to work across all the zones, not just aim for the most intense that you can manage."

It takes a little while to get the hang of. I've learned through trial and error roughly how fast my heart beats at various levels of effort, and how sustainable they are, so I decide before I set out what I'm going to aim for - and stick to it.

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