Home  Cyclingnews TV   News  Tech   Features   Road   MTB   BMX   Cyclo-cross   Track    Photos    Fitness    Letters   Search   Forum  

Recently on Cyclingnews.com

Giro finale
Photo ©: Bettini

Tech Review - December 1, 2004

Polar S725 cycling heart rate monitor

Polar's S725 heart rate monitor is intended specifically for cyclists, combining a multitude of HR measuring tools with a number of useful on the bike functions and some pretty handy analysis software. Cyclingnews' Chief Online Editor Jeff Jones explores the capacity of the Polar S725.

Polar's S725
Photo ©: Polar
Click for larger image

Finnish company Polar Electro was founded in 1977 and can be considered largely responsible for the boom in the use of Heart Rate Monitors for measuring athletes' performance. The company now operates in 50 countries and employs 1400 people, with net sales of €144 million (2002).

Despite the rise of affordable power measuring devices in recent years (Polar makes one of these), just measuring your heart rate both during and post exercise can still be very informative. As our fitness panel will tell you, there are many things that can affect your heart rate, such as weather conditions, training duration, nerves, illness and food, but armed with this knowledge and a bit of experience of how your body works, you can certainly use a heart rate monitor to your advantage when training.

Case in point: I had a nasty virus at the beginning of the year which kept me off the bike for 10 days. Doctors also recommended that I don't train "hard" for an additional four weeks until the virus had completely gone, otherwise there was a risk of dropping dead from a heart attack. I have to say that I wasn't particularly enthused by that happening. To me, not training hard meant not going anywhere near my red zone, and fortunately I had just received a beta model of Polar's new S725 cycling heart rate monitor. So I followed the advice and was able to ride at a low to medium intensity for a month.

Now, many months down the track, I'm writing this review, and while I might feel like death warmed up on some occasions, at least that's just a feeling and not the reality. Did Polar Electro save my life? Well, it didn't stop me from stupidly riding in the snow...

The test unit

The components of the Polar S725:
Photo ©: CN
Click for larger image

I tested a beta model of the Polar S725, which back in February, 2004 was not yet available to the general public. The wrist unit is a fairly solid piece of equipment: the large LCD display is encased in polycarbonate and glass fibre with a slightly gimmicky carbon fibre weave finish. There are four function buttons (two on each side of the unit) and one large red button which is the start/stop button. The wrist strap is made of polyurethane with a stainless steel buckle. It is 30m water resistant (which in wristwatch water resistance terms means you can get it wet, but not go swimming in it) and takes a CR2354 battery, which Polar claims will last two years if used for two hours a day, every day.

The chest transmitter is made of polyamide, takes a CR2025 battery which will last an average of one year if used every day for two hours, according to Polar. The transmitter is also 30m water resistant. The chest strap is made of a combination of polyurethane, polyester, polyamide, nylon and natural rubber, and is designed to be comfortable without creating any problems for data transmission to the wrist unit.

The Polar S725 also came with a cordless speed sensor, constructed of polyamide and mounted to the front fork via two plastic cable ties. There was a handlebar mount for the wrist unit that also attached by means of a couple of cable ties. The Polar S725 can be used with Polar's chainstay mounted power sensor, but that wasn't supplied with this unit.

The IR Interface
Photo ©: CN
Click for larger image

For communication with a personal computer/laptop, this beta unit came with Polar's infra-red (IR) interface for a USB port. However, the Polar S-series Toolkit software hadn't been tweaked to interface correctly with this device. Fortunately, the wrist unit's IR sensor could communicate with the IR port on the back of my laptop, so I could download all the data and analyse it via the S-series Toolkit software. Another option is to download the data to a Nokia 5140 mobile phone (Finland rules, OK?). You are then given a summary of the session and can send it to a PC, Polar's Personal Trainer web service or another mobile phone.


The Polar S725 is much more than a basic HRM. It's a cycling specific device that includes a complete cordless cycle computer. Program in the wheel size and voila, you're off and riding. There is no shortage of functions: current speed, average/maximum speed, trip distance, odometer, stopwatch, time of day, altitude and temperature are all included. The latter two functions - which I find to be quite useful - are based on the comparison of air pressure to standard average altitude according to ISO 2533. The average altitude in Gent, Belgium, where I live for much of the year is approximately five metres above sea level, give or take five metres for the occasional bridge. But I do venture out into some hillier territory now and again, and have been known to climb to the oxygen-starved, dizzying height of about 150 metres above sea level somewhere between Kluisbergen and Ronse.

I'm not going to attempt to list all the functions of the heart rate monitor itself in detail: Polar's instruction booklet is over a hundred pages thick! Suffice to say, this is the most function-laden piece of cycling equipment that I have ever used. Apart from the cycle computer functions listed above, there are six different "usage levels" for the HRM, ranging from Basic Use (measures current and average heart rate plus cycling data) to E1-E5 (where you can exercise according to five different permutations of pre-set heart rate zones). The E1-E5 sets are designed for interval workouts, which, being the seriously dedicated cyclist that I am, I avoid doing whenever possible.

Nevertheless, it's often interesting to see how much time I've spent above certain limits when doing post-exercise analysis. There's also a "recovery" function, which measures how quickly your heart rate drops after doing an interval - another way of gauging your fitness.

Another useful feature is the Calories burned during exercise. I was a bit curious how this worked, and it prompted a little bit of discussion among the Cyclingnews fitness panel. The "food calories" required to ride for a certain length of time is related to power output on the bike and the rider's efficiency level. The body typically turns food into work with about 22-23 percent efficiency, so the S725 calculates how much work you have been doing, based on your heart rate and other measurements, and converts that into food Calories burned. Calories burned is not really dependent on heart rate: Fitter people can burn more Calories per hour than less fit people because they can produce more power for longer, and this is not greatly offset by the rider's own efficiency value.


A feature unique to Polar is the "OwnIndex" value - essentially an estimate of your maximal oxygen uptake (VO2max), which is one way to measure aerobic fitness. VO2max values are expressed in either mL.min-1 or mL.min-1.kg-1. VO2max can vary between 25 (unfit, sedentary) and 95 mL.min-1.kg-1 (very gifted endurance athlete). VO2max is highest in aerobic sports that utilise the biggest muscle groups, such as cross country skiing and cycling.

A proper VO2max test requires you to go into a lab and exercise flat out while someone measures your oxygen uptake. This is painful and can run into a bit of money if you do it regularly, but it's the only way to get a true value for your VO2max. The Polar OwnIndex is a bit simpler to measure - program in your weight and maximum heart rate (if known, otherwise an estimate will be given) to the HRM and then take the Polar Fitness Test. I was pleasantly surprised to learn that this involved lying in bed for about five minutes while my heart rate was recorded. At the end, I was given an OwnIndex number that is supposed to correspond to my VO2max in mL.min-1.kg-1. I measured mine several times and always got 70 or 71. Unfortunately I couldn't compare this to a real VO2max test because I've never done one.

Given my current level of training and fitness, I'm also unlikely to see a change in my OwnIndex over time, whereas for an unfit person who has just started exercise, it can change by 12-15% in 10-12 weeks of regular exercise. Importantly, Polar notes that, "The purpose of Polar Fitness Test is the same as that of all fitness tests; to monitor this progress. It is not so much the exact OwnIndex values, but rather the trend, that is the most important thing to follow."

According to Polar, the fitness test measures the variability in your heartbeat while at rest. Fitter people have more regular heartbeats, and there have been several scientific studies done comparing heartbeat variation, height, weight and age with measured VO2max values. Polar claims that its fitness test will give you a VO2max that is accurate to within eight percent of your "real" value, and is one of the best submaximal VO2max estimations available.

Recording and viewing data

The S-Series Toolkit software
Photo ©: CN
Click for larger image

There are three different types of recording options, and you can measure your workout at 5, 15 and 60 second intervals for later analysis. The recording interval combined with the optional functions that you have turned on (Altitude, Speed, Cadence, Power) determine the total length of recording time. This can vary from just under 5 hours to just under 100 hours in one file. For example, the standard setting that I used was 5 seconds with Altitude and Speed "on", giving an ample recording lifetime of 11 hours 10 minutes.

After hammering yourself stupid for x number of hours, it's then a matter of downloading the recorded data and viewing it with the S-series Toolkit software. The software is possibly even more functional than the heart rate monitor itself, and you can view your hard-earned data in a variety of ways. The standard is a plot of time, heart rate, speed, altitude and temperature (and more, if you have the extra bits of equipment). The graph is zoomable for a detailed analysis of a climb or a specific interval - quite useful. With the altitude data, it's possible to calculate your climbing rate in metres/min or m/h, which is handy for comparison across different climbs of similar gradients.

There are several histogram options which show time spent in various heart rate zones; a diary section where you record your weight, ride description, general feeling and so on; and lots of ways of looking at your accumulated data, e.g. by distance, hours, average speed or average heart rate.

I found the software is relatively easy to use, and there was more than enough in it to tell me what I wanted (or didn't want) to know.


Amazingly functional as it is, the Polar S725 is not perfect. One irritating issue is the sensitivity of the barometric altimeter to changes in atmospheric pressure. I found that one day my baseline altitude reading would be 5m above sea level, and the next it would be 60m. The relative altitude differences seemed to be OK, but one time when I was doing laps of Centennial Park, I noticed a difference of about 10 m at the same point on different laps, depending on whether it was raining or not. This is somewhat unavoidable with altimeters that depend on barometric pressure - if they're sensitive enough to be useful for the relatively small changes in altitude you experience on a bike, then they will also be sensitive to the weather. Polar could deal with the issue of the reading changing from day to day by ignoring any change that happens while the bike is at rest, but then you'd have to reset the altitude if, say, you drove up into the hills for the day. Nevertheless, the altitude measurement was accurate enough to obtain useful training data. I wouldn't be using it for surveying though.

The other problem I experienced from time to time was the main red button sticking down, usually related to how wet or muddy it had been on the previous training ride. Sometimes when the button was pressed, it stayed down and had to be prised up again with a sharp fingernail. Polar was aware of this problem too, and told us that the buttons had been made smaller to fix this. My beta unit had a larger button, but Polar said it was a relatively trivial matter to replace it with a smaller one.


The Polar S725 takes heart rate monitor functionality to a new level, with its myriad of features in a simple, lightweight unit. As the S725 is cycling specific, it fairly well eliminates the need for a separate cycle computer, reducing the number of gadgets on your handlebar dashboard. Combined with a Polar power measurement attachment, this is a very powerful training tool. Features such as mobile connectivity, IR transfer both to and from your laptop and detailed analysis software all add to its usefulness.

The only drawback I could find was the variability in altitude measurement, given that the large button problem was restricted to the beta-unit. The S725 should thus do all you want it to, and more besides.

Recommended retail price: US$350
Total weight: 170g
Wrist unit: Polycarbonate/glass
Transmitter and chest strap: Polyamide, polyurethane, polyester, polyamide, nylon and natural rubber
Speed sensor: Polyamide
Colour: Black/carbon weave
Pro: Incredibly functional
Con: Variability in altitude measurement
More information: www.polar.fi
Cyclingnews Rating: Click for key to ratings