BSX Lactate Threshold Monitor (Gen 1)

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BSX is a simple, easy to use Running Sensor that estimates your Lactate Threshold (but its results may not be correct.) You just wear the BSX monitor in a custom calf compression sleeve that it comes with, along with the BSX app on your phone, while running on a treadmill. The BSX app tells you what pace to run at, which increases every 3 minutes until you can't continue, at which point it tells you your Lactate Threshold pace. Sadly, while the BSX simple and easy to use, the results are rather dubious and it's unclear to me if it does a better job than simply using an estimate of your 10K race pace. (Important: This review is for the first generation BSX. For the latest BSX, see BSX Review.)

The BSX sensor on the left with the Moxy sensor on the right. The BSX has no buttons, inputs or display, though you can reset it with a magnet.
A view of the BSX and Moxy showing the infrared emitters and detectors.
Here's my setup for testing the BSX & Moxy. I attached both sensors to my calfs, putting the Moxy in the same position as the BSX. I linked the Heart Rate strap to the BSX (or attempted to, repeatedly), to my watch, and to Live Recording software on the PC (initially SportTracks, then Golden Cheetah.) I used the BSX app on my iPhone and linked the Moxy to the live recording and my watch.

1 How to use the BSX Lactate Threshold Monitor

The BSX is remarkably simple to use, especially when compared with other ways of estimating Lactate Threshold. You'll need the BSX, its calf compression sleeve, an iOS or Android device, and a treadmill.

  • You'll need to have the BSX app on your iOS or Android device.
  • Make sure the BSX is fully charged. (Seriously, you don't want it to run flat part way through this, so leave it charging until near the time you do the test.)
  • Start the app and give it your details. It will want your conversational pace and your 10K pace, along with some other information about how long you've been running.
  • Review the screens that tell you what paces you'll have to run.
  • Link the app to the BSX, which normally requires the BSX and phone to be close together.
  • You can link the BSX to an Ant+ heart rate monitor, but they'll have to be close to pair. I found this to be tricky at best, and I only ever got it to work once. If the BSX loses connection to the heart rate monitor, then it will tell you the test is failed, but only after you've finished. Other people seem to have had similar issues, and BSX have released firmware that may help, though I didn't have much luck.
  • Once you've finished pairing the BSX you can put it into the small pocket in the calf compression sleeve that is sold with the BSX.
  • Start the test on the app and start the treadmill. The first 3 minutes is at a walking pace, and then you'll do 3 minutes at gradually increasing paces. For instance, if you tell the BSX app that your conversational pace is 8:00 and your 10K pace is 6:30, it will give you 9:22, 8:57, 8:34, 8:13, 7:53, 7:30, 7:03, 6:40, 6:22, 6:03, 5:42, etc.
  • Once you've reached voluntary exhaustion, stop the treadmill and the app
  • The BSX app will then tell you your Lactate Threshold pace.
The BSX sensor in its calf compression sleeve. (The Moxy has to be taped on.)
The dashboard view of the BSX app, showing previous results.
You have to input your conversational and 10K pace (your 10K pace is an estimate of your Lactate Threshold.)
The app shows you the paces you'll be running at.
You get a display of the pace to run at, along with the time left in this stage. You can see your Heart Rate if the BSX has managed to link to your Ant+ heart rate monitor (I only got this to work once). You'll also see your SmO2, but for me it didn't bear any resemblance to reality.
When you're done the BSX app shows you what it thinks your Lactate Threshold is, along with other training paces.
If you managed to get your Heart Rate monitor to pair with the BSX at the start, then you can get this message saying it lost contact and your test failed.

1.1 An Example Test

Here's a video of a test I did with the BSX. I sped up the 30 minute test so it lasts less than two minutes. Notice that my SmO2 says remarkably stable throughout the test.

Here is the Moxy data from the same test, showing a clear decline in SmO2, which is what you'd expect. This is in stark contrast to the BSX display.

2 Why Measure Lactate Threshold

The obvious question is why do you want to know your Lactate Threshold. There are several possible reasons.

  • Tempo Runs. The common reason is to know what pace to run for Tempo Runs, also called Threshold runs because they are performed at Lactate Threshold. This is based on the idea that Lactate Threshold pace was somehow more valuable than other paces. However, the preponderance of research indicates that rather than there being special benefits to Tempo Runs, they are probably the least effective form of training.
  • Race Pace Validation. If you have an accurate idea of your Lactate Threshold then you may be able to validate your target race pace. If your goal pace is faster than your Lactate Threshold indicates, then you should probably rethink your goals. On the other hand, if your Lactate Threshold is faster than you expect, then you should look for reasons why you're aiming too low.
  • Evaluate Training. If you can easily monitor your Lactate Threshold you can evaluate your training progress. A good training regime should result in a faster Lactate Threshold over time, though obviously there are diminishing results as you become fitter.

3 Advantages of the BSX

  • Simplicity. The BSX is vastly simpler than other methods of estimating or measuring your Lactate Threshold. There are no blood draws that accompany traditional tests, or the need to perform multiple runs on multiple days as required by the gold standards MLSS test. In addition, the BSX gives a clear and unambiguous indication of your Lactate Threshold without need for any interpretation.

4 Concerns with the BSX

  • Validity. While the BSX gives a clear and unambiguous result, it may not be correct. See "BSX Validity" below for a list of concerns.
  • Single Protocol. The BSX app supports a single protocol that uses a incremental load test with 3 minute stages. If you want to do any other test protocol, such as longer stages or something closer to the MLSS protocol, you can't.
  • Single User. The BSX is sold as a single user device, and the software does not readily support sharing the device.
  • Testing Only. The BSX does not support any type of use during training to monitor intensity.
  • Connectivity issues. I had many problems getting the BSX to link to a heart rate monitor, and I succeeded only once. The indications are that other users are having similar issues.

5 BSX Validity

I have a number of concerns with the validity of the results of the BSX.

  • No independent validation. There is no independent validation of the results of BSX, which doesn't mean that the device it doesn't work, but it is a significant concern. There are some independent reviews (see below), but these are not really validations.
  • Black Box. The BSX algorithm is proprietary and not clearly documented, which means it's hard to know if there is any scientific basis for their approach. BSX states that their algorithm is patented, but nowhere do they indicate what their patent number is. I believe it is patent US20130096403; see below for details.
  • Hidden Data. BSX does not give any access to the underlying data that supports their measurement of Lactate Threshold. What little data is available online seems a little equivocal, though it's hard to judge. BSX told me that a second generation of the software will give access to the underlying data.
  • BSX Validation. BSX has performed their own validation but the results are not published. The reviews give some insight into the methodology that BSX uses, but the details are unclear. These reviews indicate that BSX did not use the gold standard of MLSS for Lactate Threshold in their validation. Instead they used an incremental treadmill test with lactate measurement from a blood draw, and relied on the rather poor Carmichael Training Systems determination of Lactate Threshold. I believe that this indicates that BSX has performed their internal validation against a seriously flawed baseline. (See Lactate Threshold for details.)
  • 3 Minute Stages. The BSX protocol uses an incremental test with three minute stages, which does not appear to be a valid approach for measuring blood lactate directly. This is because blood lactate takes 20-30 minutes to stabilize for a given intensity. Of course, it is possible that muscle oxygenation would not be subject to this limitation as it is measuring a very different physiological parameter, but it is another concern.
  • %O2 Display. In my testing, the BSX application showed a constant oxygen saturation level regardless of exercise intensity, typically 86 to 87%. This is in contrast to Moxy that showed a decline in oxygen saturation with increasing exercise intensity, roughly in line with the published literature. BSX has suggested that future versions of their software will present a more realistic estimate of oxygen saturation, and this problem should not indicate an issue with their validity. Personally, I find this a lack of even a relative change in oxygen saturation rather troubling.
  • Invalid Results. In my personal testing, the results of the BSX did not seem valid, but seemed to be based on what I told the application was my 10K pace combined with the pace at which I terminated the test. The BSX indicated I had a Lactate Threshold pace of between 6:10 and 8:50 depending on what 10K pace I input and what pace I terminated the test. This of course doesn't mean that the BSX couldn't detect LT for other users; it may just be that if I don't have an obvious change in SmO2, the BSX resorts to guessing. However, it means that you wouldn't know if the LT pace shown is valid or a guess.
    • Tell BSX my 10K pace was 6:30 and terminating at 6:03 gives an LT pace of around 6:30
    • Tell BSX my 10K pace was 6:30 and terminating at 5:43 gives an LT pace of around 6:10
    • Tell BSX my 10K pace was 8:30 and terminating at 7:54 gives an LT pace of around 8:50
    • Tell BSX my 10K pace was 8:30 and terminating at 7:30 gives an LT pace of around 8:10
  • Exercise Ignored. A bigger concern is I got the same indicated Lactate Threshold pace if I tell the BSX I'm performing a test, but don't actually do any exercise. Just sitting still and terminating at the same point gives almost exactly the same result.
  • Sensor Location. A minor concern is the location of the sensor on the calf, where it may be more affected by changes in biomechanics than using the quads.

6 Patent US20130096403

Patent US20130096403 A1 (Apparatus and method for improving training threshold), which is a patent for determining the Lactate Threshold using an incremental stress test with an infrared sensor. I feel reasonably confident that this is the BSX patent, but I could not confirm this and BSX seem to carefully avoid any reference to the details of their patent. The patent describes an experiment using a 3 minute incremental treadmill run on 34 subjects, which looks like multiple aspects of muscle oxygenation (SmO2):

    • SmO2. The basic measure of SmO2 showed a good correlation between SmO2 and Lactate, but only in 19 of 34 subjects (56%). The example correlation uses Lactate ranges 0.6 to 2.5, suggesting that the correlation might only exist at relatively low intensities.
    • Rate of change of SmO2. This is the slope of the line drawn through the SmO during a stage. Here the correlation existed only for 11 of the 34 subjects (32%). Here the chart shoes a Lactate range of 1.2 to 6.8, which is a much higher intensity and crosses what BSX consider to be the marker for Lactate Threshold (even though there is a fairly linear increase up to the 6.8 mark.)
    • Magnitude of the reduction in SmO2. It looks like they do an exponential fit for the SmO2 and then find the rate of decay constant. This gives a correlation for 25 of the 34 subjects (73%). Here the charts are rather odd; the increment test chart shows Lactate in the range 0.6 to 2.0, but the regression chart only uses value of 0.6 to 1.3 (pretty much resting Lactate levels).
    • Rate of decay of SmO2. Like the magnitude, this uses the rate of decay from the exponential fit, but this only gives a correlation for 9 of the 34 subjects (26%). The charts for this have a Lactate range of 0.8 to 4.0, a reasonable range for a Lactate Threshold test.

Overall, 32 of the 34 had a correlation for one of the variables, but the patent does not suggest a way of determining which variable to use for a given subject. The patent proposes creating a model with a regression constant for each four variables, but the overall correlation for such a model is not shown. Given that the correlations are not shown for the full range of Lactate values seems odd to say the least.

7 Independent Reviews

BSX pointed me to three individuals who have reviewed their device; Tony Vienneau (, Matt Fisher (, and Ray Maker (

  • Tony's review on mentioned that he used the BSX to perform multiple tests (30-40). Tony noted that the muscle oxygenation value displayed on the BSX varied with fatigue, which was different to my results where the value seemed reasonably static regardless of activity level. Tony mentioned that he had a Blood Lactate test performed that corresponded with the BSX results, but gave no further details. Tony mentioned to me that he generally saw changes in displayed SmO2 that corresponded to intensity. He also found that he got an error if he ran at an easy pace with the BSX. Tony's review is available at and was published August 2015.
  • Matt completed a single test with the BSX at their facilities. Matt's review shows a plot from the BSX software that is not normally available to the end user, showing time against a relative concentration. This is intriguing, and I have similar plot from BSX for a couple of my tests. However, BSX wouldn't tell me what the substance was that they were plotting as "relative concentration". If you look at the chart on Matt's site, you'll see what is probably the raw data in red, showing a huge amount of noise, an orange line that appears to be a smoothed version, then a simple green line that is further smoothed. This is all very intriguing, but equally unclear. As an aside, Matt includes a blood lactate plot he had performed a few years back, which would not have a Lactate Threshold based on the criteria BSX uses (1+1 mmol/l rise). That's slightly ironic, but it highlights the problem with their blood testing methodology. You can read Matt's review at and was published August 2015.
  • Ray also completed a single test, and his write up includes the chart of blood lactate levels as well as some different charts from the BSX software. For the test Ray estimated his 10K pace as 6:05 min/mile pace, and the BSX indicated a Lactate Threshold pace of 6:03 min/mile, with the BSX blood test showing 5:42 min/mile. Ray's blog also includes a vaguely similar plot to the one Matt includes, showing noisy data in red. This chart is simply marked "estimated" on the Y axis and has a different scale with no units. This chart looks like a visual estimation of the breakpoint in the muscle oxygenation, something that other researchers have used. The review also includes some charts of probability, which is intriguing, but hard to interpret. You can read Ray's write up at
  • Another set of reviews that BSX did not mention were performed by Gabriel Hernando of Gabriel has written five articles about his experiences with BSX, mostly covering the difficulty in getting it to work. Gabriel found that the BSX gave a Lactate Threshold that compared well with his Functional Power Test from cycling. You can read Gabriel reviews at (this is in Spanish, but Google can translate it reasonably well.)

8 BSX Gen 2

BSX has announced Gen 2 of their product, with the following features:

  • improved ANT+ and BLE connectivity
  • real-time muscle oxygenation recording
  • improved graphs and overhauled design experience
  • automated KICKR integration (during endurance tests)
  • Full download of all workout files
  • ANT+ broadcasting to watches and head units
  • Improved light blocking in the compression sleeve

I will test the new generation when it becomes available (target October 2015). The Gen 2 product will be available at a discount to those who purchased the original device.