Product Reviews Sports Technology

Flex (Barbell) Sensor Review – Our 1st Look

Technologies for Velocity Based Training have evolved over the years from mainly using Linear Position Transducers to cameras and IMUs. One product that has taken a rather different approach is the Flex barbell sensor which uses laser technology. We tried out the Flex and did an initial review. Read more below.


In this review, I introduce the Flex Sensor, and do a (really) brief explanation of what Velocity Based Training is about. Then I go through my experience with unboxing, setting up, using the Flex for tracking lifts and estimating 1RMs. Finally I share some of my initial thoughts about using the sensor and the App.

Key Points:

  • The Flex Sensor is targeted at athletes who do VBT or wants to do VBT
  • It tracks up to 40 barbell exercises and works best with iron barbells
  • Besides tracking lifts, the App has a feature for guided 1RM tests based on velocity measures
  • The sensor and App interface is well designed and has various customisable options to improve the user experience
  • The App is currently on iOS and an Android version will soon be released

What is Flex?

Flex is a barbell tracking device that’s developed by Kinetic Performance. It’s secured on one end of an olympic barbell and when in use, it automatically counts the number of reps of different exercises, tracks the velocity and distance of each rep and calculates the power of each rep. And because it tracks all these metrics, that makes it a suitable tool for Velocity Based Training or VBT for short.

[A brief background on Kinetic: they’re a sports technology company based in Canberra, Australia, and they first developed GymAware – a tethered velocity and performance monitoring system built for elite athletes, coaches and teams. Their technology and approach to measurement has been considered the gold standard, and therefore has been widely used by researchers and sports teams globally. Then to cater to a broader market, they developed Flex – a smartphone connected, wireless device that does almost the same thing]

Velocity Based Training

In case this is the first time you heard about Velocity Based Training, here is my quick (& simple) rundown. Typically when doing strength training, we might start off with a certain amount of weight/load (e.g. 30kg) and do a certain number of reps (e.g. 10 reps). Then as we progress, we could either increase the load or the number of reps or the number of sets. But when we can measure the velocity or how fast we do each rep, it adds another perspective of how we are lifting/pushing the load. For example, lifting 30kg through the same distance at 0.5 m/s versus 0.8m/s might look the same to the naked eye but in actual fact there is a significant difference in power output. So having (quantified) VBT can help athletes be more specific with their training plans and goals (ie whether to focus on strength, power or speed). {For more in depth reading about VBT, feel free to check out this article by science for sport (link) or this quick start guide by complementary training (link).}

Unboxing The Flex

There are two main components to the Flex sensor: One is the actual sensor that comes in a box, and two, the reflective mat that comes rolled up in a bag. The reason the reflective mat is needed is because the Flex sensor technology is based on a laser array and not IMUs (accelerometers & gyroscopes). We will go into more of that later.

The sensor has got a really solid feel and build to it. The inside of the sensor or where you would attach the barbell has 4 strong magnets that sticks really well onto most iron bars. And to secure it further, you could tighten the locking ring. Each sensor has got an identification number on it, so when it connects to an iPhone or iPad via bluetooth, you know you are connecting to the correct sensor.

Flex Sensor with serial number

The App

At the time of writing this up, Flex only supports the iOS platform but they are working on an Android version. The App is free to download and as mentioned earlier, it works on the iPhone and iPad.

Signing In

With the iOS App, signing in is a bit different. I don’t need to create a login with a password. All I had to do was put in my email, then they will send me a link via email to authorise the log in. If I am not wrong, my workout data is stored in the cloud. So if/when I sign in with another device and go through the same email authorisation process, my data will be retrieved and displayed on this other device (until I sign out of the device). Personally, I think this is a pretty good approach.

Profile section + 1RMS

Once in the App, I could set up my profile including my weight, training level, and training interests. Personally, I think this is only really relevant if I want to make my profile public and allow other Flex users to find me within the Community section. But one thing that is useful in the profile section is entering my 1RMs for bench press, squat (back) and deadlift (that is, if I already know what values they are). If I don’t already know them, there is an option in the App to do a IRM test to estimate/predict what it is for me. I will also get to the 1RM test later on.

Training + Setting Up

Currently with the Flex sensor, it is able to track up to 40 different exercises using the barbell. When starting a workout session, I can either go to WORKOUTS and follow a previously saved workout, or select TRAIN and start training with one of the 40 exercises.

40 exercises that are tracked

SENSOR SET UP: If I wanted to start off with Deadlifts (Trapbar), I would: Set up the Trapbar on the ground, place my desired weights, secure them (or not), and attach the Flex sensor on one end of the bar. Then I place the reflective mat below where the sensor is. Make sure that the sensor will still be above the reflective mat when the barbell is used during the Deadlifts. Once the physical set up is done, I turn on the sensor and pair it to the App on my iPhone/iPad.

Deadlift (Trapbar) set up

APP SET UP: On the App, after I have selected “Deadlift (Trapbar)”, I will be asked to enter the amount of weight. Once that is done, the sensor and App starts “listening” for any movement to begin tracking. Usually, lifting the bar will activate tracking immediately, so it’s advised that for starting positions that are from the floor, to set a prep timer.

Setting Prep Timer for floor starting positions

AUDIO & VISUAL FEEDBACK: Once I start lifting, the App will sound out the average speed at the end of each rep. If I prefer the audio to be a rep count, that can be changed in the settings. If I place my iPhone/iPad in front of me, I could also get a visual feedback of the average speed, number of reps, average power, distance, position plot, or even bar path. Although the last 2 items might not be that useful compared to just checking in the mirror.

Velocity – Distance – Power; values for every rep.
Distance Plots + Bar Path (Beta)

DATA: After each set, users get to review the metrics from that set for every single rep including average velocity, average power, distance, distance plot and bar path. For people who are into data and algorithms, the distance plot will be an interesting one because the highlighted parts on the graph seems to be how the app identifies each rep and the data points within the highlighted area is where the metric calculations are applied. Also the Bar Path visual (which is in Beta) looks quite useful for identifying poor technique or possibly fatigue (which leads to poor technique/posture).

The three velocity zones

INSIGHTS FROM EACH SET/SESSION: The key feature of the Flex sensor is that it can measure velocity of the barbell. So naturally the insights would revolve around velocity. The Flex user guide includes a velocity zone chart which shows the “Strength”, “Power” and “Speed” zones. When the Flex sensor detects reps that are in the various velocity zones, it puts them in the respective buckets. At the end of each set, the “Insights” page would show a breakdown of what percentage of each set was in which zone. When multiple sets and/or different exercises are completed, the Insights will reflect a breakdown of the entire exercise sets or entire session.

1RM Testing

1RM (or One rep max) for an individual is the maximum load that they can lift and complete for one rep. There are a couple of purposes for measuring or testing 1RM. One is to assess strength. The other is to use it as a gauge/guide for designing training program – for example if my Back squat 1RM is 80kg and I wanted to train at 70% 1RM, then I would use 56kg for my squats. There are a number of ways to determine 1RM for each exercise and it usually involves starting from a certain load/weight and doing a certain number of reps. Then increasing the load and doing less reps. This goes on till only 1 rep is completed for a certain load.

The 1RM Test in the Flex App works similarly. There are three exercises to test:- Bench Press, Squats (back), and Deadlifts. Based on my profile, the App recommends a starting load, the number of reps and up to 5 sets of lifts. At the start, it prompts me to lift as fast as I possibly could. Then based on the velocity measured, it determines if the first set was too light or too heavy and recommends the second set to be either lighter or heavier. If, by the end of 5 sets, the App detected that I was still lifting at a comfortable speed, it would suggest another heavier set.

1RM Screens

The time estimated for the test duration (15 minutes) is quite close but to be safe I would say give it 20 minutes or more because adding or removing weight stacks requires removing of the Flex sensor each time and that bit of time can stack up.

Some Observations

That was a quick run-through of using the Flex sensor and the App. There is probably a bunch of other features or functions or metrics stuff that I didn’t touch on but I think I went through the main basic ones. While using the sensor and App for a number of weeks, there were a few things that I noted and I thought I would share it here.

Hardware Stuff – Mats & Barbell

A couple of athlete/coachs I spoke to about the Flex sensor were pretty impressed with the sensor. But they felt the reflective mat felt like an extra thing to have to bring around. So I tested the sensor without the reflective mat and it was just above a typical rubber gym mat flooring – the reading was horrible (see video below of tracking squats with and without the mat). So you really can’t do without it! The support page on Flex suggests if you forgot to bring the mat, to use white coreflute or white laminated timber.

The Flex sensor works best on iron barbells, and you will know it is an iron barbell when the magnets inside the sensor stick on strongly. There are, however, certain barbell designs that are aluminium or have a non-ferrous (or not iron) coating and that causes the magnets to not stick onto the bar. This causes the sensor to think that it is not attached to a barbell and won’t work normally. To rectify that, there is a “Disable Bar Sensor” option in settings when the Flex sensor is connected. But having the Bar Sensor turned on will provide a better user experience, including automated tracking of when you finished a set.

Tracking + Reps Identification

The tracking and reps identification was pretty spot on, which shouldn’t come as a surprise considering that the team that developed Flex had prior experience with building and supporting GymAware (which has been doing VBT for over 15 years). The main difference between GymAware and Flex is their measurement technology (Linear Position Transducer vs Lasers). So though the tracking algorithms or processing of data would be largely similar, the steps to using the devices are quite different and the user interaction and user experience with the devices would be different.

GymAware unit

User Experience / Guidance

There are a few useful interfaces in the App that guides the user on different things. For example, the starting page reminds the user how to place the reflective mat and to use the prep timer if the barbell starting position is on the ground. The 1RM test is also very well guided. There are also lots of useful features within that can help manage a training session – including setting a custom rest timer, changing the audio feedback, reminding me of my personal record etc.

Audio feedback – average velocity per rep

One thing I thought would be useful is providing guidance on the use of “Workouts” on the App. Things like how to best start a “Workout” or create a “Workout” or use “Workouts” again. I know the support page provides explanation on the various usage but I think having a guided interface on the App would be helpful.

In Closing

As mentioned earlier, having used the Flex sensor for only a short time (less than 10 sessions), there are quite a few things I haven’t properly explored like the Community page/feed or using Workouts to start a session.

My experience with it so far has been pretty good. I can tell that a lot of thought has been put into the design of the user interface and experience – there are automations that can be turned on/off or customised like the Prep Timer or the Auto Record Timeout. They may seem like little things but they all come together to make tracking the training session easier. That said, I reckon it will take someone new at least a few sessions of using/meddling with it before they will really get the hang of it.

Who is this for? I would say this suits experienced and serious athletes who want to be or are on a Velocity Based Training program. Better still if they are under the oversight of a coach. For athletes who are part of a team and already train with the GymAware system, having the Flex makes sense especially if they don’t always train with the team. Flex developed an integration platform called Bridge to connect Flex sensor data to the GymAware cloud (so it only works for existing GymAware cloud users).

Lastly, in the time that I was testing the sensor, the Flex team released a couple of updates including a new feature called Buddies. That shows they are constantly improving and working on usability and considering more use cases. So kudos to them!

That’s all I have for this post. I might do another post next time comparing the Flex with other sensors including the GymAware. If there was anything you would like to find out about the Flex sensor that wasn’t mentioned, feel free to leave a comment or reach out to us here (link). Till next time!

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