Heart rate monitors are highly useful tools for competitive and recreational endurance training. They provide an objective measure of how your body is performing and recovering, thereby painting a more complete picture of health and training progress in addition to factors like pace, time and ‘feel’.
Many people are discouraged from training with a heart rate monitor due to some of the inherent problems of using a conventional chest strap. They can be uncomfortable, they sometimes slip, they suffer from accuracy issues due to conductivity problems and static, they are a faff to put on etc etc. The wrist-mounted Mio Link has generated a great deal of interest among the running and cycling community as it strives to overcome the flaws of chest straps using some innovative new technology.
Mio Link Features
How Does the Mio Link Work?
Conventional chest straps work by detecting electrical impulses produced each time the heart beats. The Mio Link is different in that the sensors detect the volume of blood flow under the skin to determine heart rate. This is similar to the technology used by various ‘activity monitors’ that are available on the market but I’m pretty sure that the Mio Link is the only product that also offers ANT+ technology for connection with sports watches and cycle computers, which is why it will be so interesting to ‘serious’ and competitive athletes.
The Link is small (just over and inch wide), stylish and the build quality is impressive. The strap allows precise adjustment to suit the size of your wrist and I’ve found that I soon forget that I’m wearing it once I put it on. My wrists are pretty skinny so I decided to go for the small strap but I came to partially regret this decision once I realised that I needed to wear it on the same wrist as my watch (see below for more on this).
Mio Link Accuracy and Performance
I’ve been waiting for a product like the Mio Link for some time and I was really excited to try it straight away so I immediately paired it with my Suunto Abit2 via Ant+ and strapped it securely to the opposite wrist to give it the ultimate test…. my weekly food shopping trip. As i zig-zagged down the aisles I was shocked to find that according to my indicated heart rate, I was experiencing a serious coronary event signified by multiple spikes, dips and flat-lines. This was clearly being caused by a problem with the unit so I resolved to return home and do a bit of research to figure out what the issue was.
After a few minutes of trawling through various forums I quickly realised that I was not the only person who had experienced these issues. You can find a great deal of information on the subject within the comments section here at DC Rainmaker’s review but they can mostly be condensed down to the following two factors:
1: Light-Loss and Boney Wrists
For the Link to work accurately it needs to be fitted very close to the skin so that no light can escape from the sides of the sensor. If like me you have bony wrists there is a good chance that there will be a natural ‘dip’ between your radius and ulnar which can allow light to escape as shown in the picture below. This can be overcome by wearing the sensor on the inside of your wrist or further up and your arm where there is a bit more ‘meat’ so the sensor can fit more closely. Once the fit has been perfected, the dips / spikes disappeared and the Link produced readings that are consistent with my HR belt.
Personally, I don’t have a big issue with this as we are all different shapes and I don’t think it’s unreasonable that some people are going to have to experiment with the sensor position to get a good reading. However I do wonder if the situation could be improved by having more of a ‘lip’ around the sensor to reduce light-loss.
2: ANT+ Transmission Problems
The second issue of flat-lining is caused by the Ant+ connection dropping out. The manufacturer claims that this happens because ANT+ cannot travel through the human body. In practice, this means it can be blocked by the torso as you swing your arms whilst running or by reaching up for a object on a high shelf at the supermarket 😉 This doesn’t completely make sense to me because I’ve conducted a couple of tests of my ANT+ chest strap where my wife held my watch and stood a couple of metres away whilst I ran on the spot with my back to her so that the signal had to pass through my body. The chest strap seemed to work fine under these circumstances and this implies to me that the problem lies with the signal strength of the Link more than an inherent characteristic of ANT+.
How to Get Accurate Readings From the Mio Link Whilst Running and Using Ant+
The best way of getting hassle-free, accurate readings from the Mio Link whilst pairing it with an Ant+ sports watch is to wear it next to the watch on the same arm. Whilst it looks a bit weird, I’ve been using the Link this way for a couple of weeks now for running, scrambling, walking, even sleeping and the solution works and produces reliable results. I’ve also found that it isn’t necessary to wear the sensor on the underside of the arm when it’s in this position because it can be fully in contact with the skin. This is fortunate as it puts the sensor right next to the watch to minimise the change of dropout. When ordering the link it is therefore important that you measure your arm about 3-4 inches proximal to the wrist to ensure that the strap is the correct length. I ordered a size-small and found that it was close to the limit of adjustability when I had it next to my watch so look out for this.
The sensor position that worked best for me
Cycling with ANT+
If you are cycling whilst using the Link and pairing it with a bar-mounted Ant+ computer, the transmission problems should not arise as the device will be in close proximity with the computer for the vast majority of the time.
If you choose to pair the Link with a smartphone app via bluetooth you are in luck as I’ve found that the data transmission is consistantly good and I’ve had no problems whatsoever.
Sample Data From Recent Runs
Below you can find a comparison between the Mio Link connected to my phone by Bluetooth and my chest strap paired to my Ambit2 on the same run to demonstrate the accuracy of the sensor (click for a larger image).
The next two graphs are both based upon the Mio Link with one showing the data with it connected to my phone and the other with it paired to my watch to demonstrate the relative difference between Ant+ and Bluetooth.
Ant+ vs Bluetooth Comparison
Both of the above data sets are examples which are representative of the typical accuracy I have experienced over the last couple of weeks since I’ve optimised the position of the sensor. As you can see, based upon my experience you can get accurate results from the Mio Link which are consistent with a chest strap once you get the setup right.
The Link has performed well since I overcame the initial issues with positioning the sensor but I’m still not crazy about having to wear it on the same wrist as my watch to get the ANT+ to work and I think that Mio need to make this clear in their marketing material and manuals so that potential customers know what to expect. I’ve read reports that Mio are planning on releasing a firmware update which will increase the signal strength and I will update the post if/when this happens.
Overall, I think that the Mio Link is a very promising product. It’s not flawless but it does overcome many of the fundamental issues of wearing a chest strap and I’ve found it to be hassle-free after initial difficulties getting the setup right the first couple of times I used it. Hopefully, other Link users will have similar success by following the tips above.
This product was purchased by the author
Where to Find One (affiliates)
The Mio Link is available from Millet Sports and Amazon (link below)
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