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AlphaPWR vs other VBT devices: Is barbell velocity an incomplete metric?

Oppdatert: 7. jul. 2023

By Marie Levin Matre -Sport Scientist and Manuel Alfaro - Sport Performance Engineer

Velocity Based Training (VBT) is a training method that focuses on the velocity at which weights are lifted. The central idea is that by tracking the velocity at which you lift a weight, you can gain insight into your physical performance, including power, strength, and muscle fatigue. In strength training, your muscles contract, generating energy. This energy is then transmitted to the weights, such as a barbell, initiating their movement. Measuring this energy directly can be challenging, but we can represent it by look at the velocity at which you lift or move the weights. The harder you push, the faster the object moves. Because of this, the common practice in VBT involves using devices like linear positioning transducers, accelerometers, or video-based applications, which tracks and provide metrics based on the barbell's velocity. 

For example, in exercises such as bench press, barbell velocity can be giving a good representation of the total energy exerted. In this exercise, nearly all of the force that your muscles produce goes into moving the barbell. That's because the movement in the bench press is very simple and straightforward - you're just pushing the barbell straight up while the body is static on the bench. So, in this case, tracking how fast the barbell is moving, can provide us a good representation of how much energy your muscles are generating. However, things will be different for the back squat exercise. While our muscles work to lift the barbell, we're also lifting a significant percentage of our own body weight. In such cases, the velocity of the barbell alone cannot represent the total amount of work that our muscles exert on the entire person-barbell system. The work done by the muscles in this scenario also includes moving the lifter's own body weight, not just the barbell.

The AlphaPWR “Squat” module revolutionizes the traditional VBT practice. Standing out from the other VBT devices, AlphaPWR's advantage lies in its unique tracking point. While linear encoders and IMUs limit their tracking to the barbell's velocity, AlphaPWR takes it a step further by tracking the velocity of the COM. This metric provides more comprehensive information and insights, not only about the barbell but also about the entire system of person-barbell interaction. Unlike the barbell's velocity, the COM's velocity contains information about both the barbell and the person, a detail missing when focusing solely on the barbell's velocity. In this blog, we will develop a series of arguments to help understand the differences between COM and barbell velocity. We will also bring up for debate why relying solely on the barbell's velocity as a metric presents an incomplete picture, particularly in the context of a back squat.

What is the center of mass (COM)?

The COM represents is a single point within an object or a system where the mass is evenly distributed. It serves as both the balance and rotational point of the object. The COM location varies based on the mass distribution. For humans, the location of the COM will vary based on body types and distribution of mass. For instance, individuals with more mass concentrated in their lower body will have a lower centred COM compared to those with more mass in their upper body. Since everyone has a distinct distribution of body mass, each person has a unique placement of their COM. Figure 1 illustrates an example of what this could look like.

Figure 1. The Dependence of Anthropometrics on the Position of the COM. This is just a graphical illustration that demonstrates how the position of the center of mass can vary depending on the mass distribution across different body segments.

Barbell velocity vs COM velocity

Another factor that affects the position of the COM is the load we put on the barbell. During a back squat, the barbell itself does not move up or down regardless of how much weight is added or removed,it stays in the same position on your back. Consequently, traditional VBT devices, which track the position and velocity of the barbell, will always record the same position for the barbell, regardless of how much weight is added or removed. However, the COM of the person-barbell system can change depending on the distribution of weight. This includes variations in the person's body dimensions and proportions (referred to as anthropometric variations) as well as the weight loaded on the barbell. For example, if you add more weight to the barbell, a greater proportion of the total weight of the person-barbell system is now concentrated higher up, since the barbell is held across the upper back. This causes the COM of the system to shift upwards. Traditional VBT devices are unable to track an upward shift in the COM with increasing load, indicating that barbell velocity doesn't account for COM variation due to factors like the barbell load and individual anthropometrics. Consequently, this leaves a gap in your training information. This is the point where AlphaPWR distinguishes itself, unlike traditional VBT devices, AlphaPWR provides real-time tracking of these COM changes. This invaluable insight allows for the customization of your training regimen based on your unique variations.

Figure 2: the COM within a person-barbell system. This is an illustration of how the location of the COM will vary, depending on barbell load.

By this reason, differences between barbell and COM lifting velocity are expected. The findings of a study conducted at the Norwegian School of Sport Sciences (Matre & Alfaro, 2023) demonstrate these variations in lifting velocity between the barbell and COM. As seen in Figure 3 these differences become more pronounced at higher lifting velocities. This observation aligns with expectations, as higher lifting velocities correspond to reduced loads and bigger differences between the position of the barbell and the COM. A reduced barbell load shifts the COM lower (Figure 2), resulting in a movement pattern that diverges from that of the barbell.

Figure 3. Lifting velocity (ms-1) comparison between devices. Relation between lifting velocity tracked with the AlphaPWR, accelerometer (IMU), and linear encoder across all participants. The data from the IMU and the AlphaPWR are plotted against the linear encoder as the reference value. The dashed line represents the identity line. Sv refers to the slope value of the regression line.

However, when confronted with heavier barbell loads the COM moves higher and its movement becomes more similar to that of the barbell. While common VBT devices are a valuable tool in monitoring and adjusting training, it's essential to recognize its limitations and adjust for exercises where the barbell's velocity might not be a complete representation of the total work done by the muscles.

Digging into the details: The Science behind

This is not the first time the comparison between barbell velocities and center COM velocities in relation to performance metrics has been on focus in a research paper. A study conducted at the university of Chirchester (Lake et. Al., 2012) suggest that barbell kinematics should not be used to estimate power output applied to the person-barbell system COM during lower-body resistance exercise. In this study it was aimed to compare different methods of estimating power output applied to the barbell and body system during lower-body resistance exercise. The study highlighted that the velocity of the barbell was significantly greater than the velocities of the COM, aligning with the findings from the study conducted at The Norwegian School of Sport Sciences. This indicates that disregarding the kinematics of body segments during lower-body resistance exercise can lead to a significant overestimation of the power applied. We can also observe these patterns in the context of the back-squat an VBT.

(A) (B)

Figure 4. (A): Interactions between Work, Kinetic Energy and Potential Energy in the concentric phase of the backsquat. The grey line represents the total work done by the person-barbell system while the blue line represents the net work done by the person-barbell system. Note that the net work is equivalent to the total kinetic energy of the person barbell system. The total work is equal to the sum of this kinetic energy and the potential energy of the entire person-barbell system. (B): Kinetic energy estimations during the concentric phase in the backsquat. The orange line represents the Kinetic Energy(net work) calculated using the mass and the velocity of the barbell while the green line represents the Kinetic Energy(net work) calculated using the velocity of the barbell but the mass of the entire person- barbell system. The blue line represents the actual Kinetic energy(net work) of the entire person barbell system. The orange and the green line reflects what the barbell's velocity represents. The blue line represents what the AlphaPWR measurements represents. (All the data was collected by using the Qualisys 3D motion capture system)

According to the information provided in this blog, the velocity at which we lift an object is the representation of the amount of work generated by our body. Specifically, the velocity and mass of the object reflect its kinetic energy (Ek), which is associated with the net work performed by our muscles. When we measure the velocity of the barbell, we are essentially quantifying a portion of the muscular work and kinetic energy transferred to the barbell (Figure 4 (A)). However, this measurement fails to capture the complete work exerted on the entire person-barbell system. Consequently, the barbell velocity underestimates the total muscular work and (or Ek) carried out by the individual. On the other hand, using barbell velocity as a means to estimate the overall work (or Ek) of the person-barbell system leads to an overestimation of the work performed (Figure 5 (B)). This suggests that barbell velocity is limited in its ability to provide a comprehensive understanding of the true work exerted. Similar patterns can be observed when attempting to estimate the total potential energy of the system using barbell displacement (Figure 5 (A)). The estimation may either overestimate or underestimate the actual potential energy. In summary, both barbell velocity and barbell displacement are unable to fully capture the complete picture of the work and energy dynamics within the person-barbell system. They tend to miss essential information and may lead to either underestimation or overestimation of the true values.

(A) (B)

Figure 5. (A): Potential Energy estimations during the concentric phase in the back squat. The orange line represents the Potential Energy calculated using the mass and the displacement of the barbell while the green line represents the Potential Energy calculated using the displacement of the barbell but the mass of the entire person- barbell system. The blue line represents the actual Potential energy(net work) of the entire person barbell-system. (B): Total Work estimations during the concentric phase in the back squat. The orange line represents the work calculated using the acceleration, mass and the displacement of the barbell while the green line represents the work calculated using the displacement and acceleration of the barbell but the mass of the entire person- barbell system. The blue line represents the actual work of the entire person barbell-system. In both figures the blue line represents what the AlphaPWR measures. The green and the orange lines represents what the barbell's velocity represents. (All the data was collected by using the Qualisys 3D motion capture system)

In conclusion, barbell velocity and traditional VBT devices underestimates or overestimate the overall work done and the resultant kinetic energy produced. Significantly, the COM has emerged as a more complete metric that captures the interactions between the barbell and the lifter. It takes into account not just the barbell's motion, but also the lifter's body weight, anthropometric variations, and the shifting COM of the combined system under different loads. This multi-dimensional perspective provides a more complete view of the work done by the lifter and the energy produced during a back squat. The AlphaPWR's “Squat” module stands as an innovation in the field of VBT. It differentiates itself by tracking the COM's velocity, giving insights into the broader person-barbell system and offering a richer and more complete picture of a lifter's performance.

Thank you for choosing Alphatek, and for allowing us to be part of your journey to enhancing athletic performance!
Alphatek, Powered by Science


- Lake, J. P., Lauder, M. A., & Smith, N. A. (2012). Barbell Kinematics Should Not Be Used to Estimate Power Output Applied to the Barbell-and-Body System Center of Mass During Lower-Body Resistance Exercise. Department of Sport and Exercise Sciences, University of Chichester.

- Alfaro, M., & Matre, M. L. (2023). Velocity-based strength training: Differences in lifting velocities between barbell velocity measurement devices and center of mass velocity measures with the Alphatek Force Platform system in the back squat (Bachelor's thesis). Norwegian School of Sport Sciences, Department of Physical Performance.

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