A Deep Dive on the Different VBT Technology: An Evaluation of Research and Practical Applications

To effectively incorporate VBT (Velocity-Based Training) into your workouts, minimizing measurement errors is crucial!

Coaches and athletes looking to enhance their training with VBT often wonder which tool best suits their needs. In this article, I’ll explore this question from a Sports Science researcher’s and a Strength and Conditioning coach’s perspective.

When I first encountered a Motion Capture lab equipped with 16 optoelectronic cameras, two things struck me: first, the system’s impressive ability to gather data and produce a detailed visualization of human movement; and second, the cost. It was clear to me that my department would never allocate €150,000 for scientific research, let alone to track the speed of a barbell in my athletes’ workouts.

The marker-based motion capture system is considered the gold standard for measuring human movement in biomechanics. This system uses reflective markers placed on key anatomical points to precisely track three-dimensional movement. Although it’s a labor-intensive process and can be affected by tester reliability issues, it remains the primary reference for validating other methods or tools for movement measurement.

Figure 1: Schematic example of a motion capture setup from Weeks, B.K. et al (2012)²

Experience is the Collection of Mistakes You No Longer Make: From Linear Encoders to Inertial Measurement Units (IMUs)

My journey in tracking load displacement speed began over 15 years ago, when I used my first linear encoder during my doctoral research. At that time, these sensors required integrated systems for signal acquisition and processing. A system still in use today, the Muscle Lab by Prof. Carmelo Bosco—a VBT pioneer—was an early example (Bosco C. 1995)³.

Figure 3: Linear encoder of the Muscle Lab system (year 2000)

What is a Linear Encoder?

A linear encoder consists of a wire, a transducer (usually a potentiometer or magnetic sensor), and an analog or digital interface. The wire attaches to an object to be monitored, such as a barbell. When the object moves, the transducer slides along the wire, generating an electrical signal that reflects the wire’s position. This signal is processed by the control system (DAQ) to calculate movement.

More on this topic:

Difference Between Rotary and Linear Encoders

How Does It Measure Speed?

The linear encoder captures the object’s position at regular intervals, typically hundredths of a second (100Hz). It calculates speed by determining the difference in position between successive readings and dividing this by the time interval. This approach is simple yet precise.

Linear encoders remain among the most reliable tools for measuring barbell speed in linear movements. However, in the 2000s, systems like the Muscle Lab, which integrated multiple sensors, were bulky, costly, and not very portable.

The Emergence of Accelerometers

An alternative approach emerged with the first accelerometers, designed to assess vertical jump (height and contact time) and monitor load displacement speed. My first accelerometer, the Myotest Pro, offered a balance of affordability and measurement validity.

Initially, these devices used mono- or triaxial accelerometers. They quickly evolved to include gyroscopes and magnetometers, improving accuracy. Today, they’re known as Inertial Measurement Units (IMUs). Thanks to their compact size, IMUs can be integrated into wearable devices, such as smartwatches, bracelets, and elastic bands, to track body segment movements.

How Does an IMU Calculate Speed?

IMUs measure acceleration forces along different axes (X, Y, and Z). Because raw acceleration data often contains noise, filters are applied to remove high-frequency interference. Speed is then calculated by integrating the acceleration data over time, allowing speed estimation based on acceleration and time intervals.

Accuracy Limitations of IMUs

The main drawback of IMUs is their tendency to accumulate errors over time, known as “drift.” Since the device continuously measures changes relative to itself without relying on an external reference, it rounds off small fractions during calculations. This rounding can lead to significant errors, especially since many sensors sample data at high frequencies (up to 1000Hz). In one second, errors can multiply significantly if not corrected, making IMUs unsuitable for applications that demand high precision without complex and frequent calibrations ⁴.

Smartphone Apps with Video-Based Motion Tracking

Several smartphone apps now use the device’s camera to track movement speed. After noticing data variability with the Myotest, I looked for more precise, portable, and affordable solutions. In recent years, mobile apps have gained popularity in sports performance monitoring, offering innovative and cost-effective methods for collecting kinematic data ⁵.

Markerless video tracking systems require high-definition video and software to track pixels as virtual markers, enabling analysis of movements like squats, bench presses, and deadlifts. Generally, a frame rate of at least 120 fps is recommended for good movement analysis on smartphones, with higher rates (e.g., 240 fps or 480 fps) preferable for detailed analyses of rapid movements. Although promising, this method still faces challenges, such as calibration and data handling ⁶.

Main Issues with Smartphone Applications

While this method is valid, as recent studies confirm, practicality often suffers due to several limitations:

• Calibration of joint segments is usually needed.

• Filming requires a tripod set up at least 1.5 meters from the subject, often demanding videography skills.

• Gyms are rarely empty, complicating attempts to avoid interference from other people.

• For VBT, a tool must provide real-time results. Unfortunately, most apps involve multiple steps before delivering results, which limits their practicality, especially when training multiple athletes simultaneously.

Which Tool is Currently the Most Accurate and Practical?

Over years of testing and using various tools for VBT in my athletes’ training, I have closely followed the technological evolution of these methods. IMUs, for instance, have achieved impressive precision and practicality at relatively low costs. Yet, I consistently found myself returning to linear encoders for their unmatched data accuracy—something I’m unwilling to compromise.

Recently, several studies have evaluated the reliability and validity of different tools and methods for monitoring load displacement. A key study by Alejandro Pérez-Castilla et al. (2019) ⁷ compared the reliability and validity of seven commercially available devices for measuring movement speed during the bench press exercise.

Figure 3: From Pérez-Castilla, A. et al 2019. Distribution of the measurement devices during the testing protocol: (1) Trio-OptiTrack, (2) T-Force, (3) Chronojump, (4) Vitruve, (5) Velowin, (6) PowerLift, (7) PUSH band, and (8) Beast sensor.

The results of this study suggest that linear encoders, camera-based optoelectronic systems, and smartphone applications can be used to obtain accurate speed measurements for linear movements, while inertial measurement units were found to be less reliable and valid.

Key Points from the Study

• Devices were ranked by reliability as follows: (I) VITRUVE (II) Velowin™, PowerLift™, T-Force™, Chronojump™ (III) PUSH™ band, and (IV) Beast™ sensor.
• All devices, except for the Beast™ sensor, showed high concurrent validity* with the Trio-OptiTrack™ system.
• VITRUVE was the only device that did not show variance errors** within the sample, indicating consistent and precise measurements.

* Concurrent validity occurs when the results of a new measurement tool are similar to those of an already validated tool, thus confirming the effectiveness and accuracy of the new tool. 

** The absence of variance errors suggests that the tool’s results are consistent and homogeneous, without significant differences between measurements in the sample, indicating that the tool is reliable and produces stable and precise measurements.

Reliability and Reproducibility Alone Are Not Enough!

While reliability and reproducibility are fundamental requirements for these devices, they’re not sufficient for choosing the best option. Practicality and cost must also be considered.

Criteria for Practicality

To be practical, a device must be compact, easy to use, wireless, equipped with streamlined software, and accessible via smartphone or tablet. It should handle data in gym settings with multiple athletes performing various exercises, be easy to update, and allow error correction for load or athlete entry mistakes. Finally, it must provide immediate and realistic feedback.

To get the best device: Optimal price and superior quality!

Below is a table summarizing a comprehensive evaluation of scientifically validated velocity-based devices. This table includes technology type, sampling frequency, device placement, cost, and my personal experience with these devices. The information draws from various scientific studies ^8-10, supplemented by my personal assessment.

Table 1

Encoders Reimagined: Back to the Future

It feels as though we are moving backward and forward simultaneously. Encoders, with their precision and reliability, have returned as the top-performing solution. Thanks to technological advancements, they’re now miniaturized, equipped with AI for automatic calibrations, and can wirelessly transmit real-time data.

From the early days of linear encoders and motion capture challenges, through the emergence of accelerometers and IMUs, to the use of smartphone apps, each method has enriched our understanding and measurement of movement. Despite these advances, accuracy, practicality, and cost-effectiveness remain essential. Today’s encoders embody these qualities, blending cutting-edge technology with proven reliability.

It may seem futuristic, but it’s already within reach. The choice is yours!

References

1 Das, K., de Paula Oliveira, T. & Newell, J. Comparison of markerless and marker-based motion capture systems using 95% functional limits of agreement in a linear mixed-effects modelling framework. Sci Rep 13, 22880 (2023). https://doi.org/10.1038/s41598-023-49360-2

2 Weeks, B.K., Carty, C.P. & Horan, S.A. Kinematic predictors of single-leg squat performance: a comparison of experienced physiotherapists and student physiotherapists. BMC Musculoskelet Disord 13, 207 (2012).

3 Bosco C, Belli A, Astrua M, Tihanyi J, Pozzo R, Kellis S, Tsarpela O, Foti C, Manno R, Tranquilli C. A dynamometer for evaluation of dynamic muscle work. Eur J Appl Physiol Occup Physiol. 1995;70(5):379-86. doi: 10.1007/BF00618487. PMID: 7671871 

4 Elodie Piche, Marine Guilbot, Frédéric Chorin, Olivier Guerin, Raphaël Zory, Pauline Gerus, Validity and repeatability of a new inertial measurement unit system for gait analysis on kinematic parameters: Comparison with an optoelectronic system, Measurement, Volume 198, 2022, 111442, ISSN 0263-2241,https://doi.org/10.1016/j.measurement.2022.111442.

5 Onat Cetin, Ozkan Isik, Validity and   reliability   of   the My Lift   app   in determining   1RM   for   deadlift   and   back   squatexercises. European Journal of Human Movement Vol. 46 (2021): First Semester. EISSN: 2386-4095 DOI: https://doi.org/10.21134/eurjhm.2021.46.599

6 Peart DJ, Balsalobre-Fernández C, Shaw MP. Use of Mobile Applications to Collect Data in Sport, Health, and Exercise Science: A Narrative Review. J Strength Cond Res. 2019 Apr;33(4):1167-1177. doi: 10.1519/JSC.0000000000002344. PMID: 29176384.

7 Pérez-Castilla A, Piepoli A, Delgado-García G, Garrido-Blanca G, García-Ramos A. Reliability and Concurrent Validity of Seven Commercially Available Devices for the Assessment of Movement Velocity at Different Intensities During the Bench Press. J Strength Cond Res. 2019 May;33(5):1258-1265. doi: 10.1519/JSC.0000000000003118. PMID: 31034462.

8 Fritschi, R.; Seiler, J.; Gross, M. Validity and Effects of Placement of Velocity-Based Training Devices. Sports 2021, 9, 123. https://doi.org/10.3390/sports9090123

9 Weakley J, Morrison M, García-Ramos A, Johnston R, James L, Cole MH. The Validity and Reliability of Commercially Available Resistance Training Monitoring Devices: A Systematic Review. Sports Med. 2021 Mar;51(3):443-502. doi: 10.1007/s40279-020-01382-w. Epub 2021 Jan 21. PMID: 33475985; PMCID: PMC7900050.10 Weakley J, Chalkley D, Johnston R, García-Ramos A, Townshend A, Dorrell H, Pearson M, Morrison M, Cole M. Criterion Validity, and Interunit and Between-Day Reliability of the FLEX for Measuring Barbell Velocity During Commonly Used Resistance Training Exercises. J Strength Cond Res. 2020 Jun;34(6):1519-1524. doi: 10.1519/JSC.0000000000003592. PMID: 32459410.