Analysing a single rep - VBT metrics use case


Repetitions are the atoms of a training session. The fundamental building block of everything we do in the gym.

Just like an atom is made up of even smaller components, all repetitions are made up of smaller components, and elements with a range of different metrics that can be used to describe it and quantify the rep.

Describing displacement in a rep

Displacement is the most basic metric when it comes to performing strength and power exercises. In VBT, displacement looks at the change in position of the bar during a repetition. An isometric exercise such as a plank has no displacement, but more typical isotonic (eccentric-concentric) exercises cover a certain distance as the load is moved.

Range of motion (ROM)

Displacement in lifting is more commonly called range of motion (ROM). This is a description of how far the implement moves from the bottom of the rep through to the very top position. ROM is most commonly measured in centimetres (CM).

A good range of motion will depend on your goals. If we take powerlifting as an example, with deadlift, the goal is to minimise the range of motion as a way to increase lift efficiency and maximise the load lifted - a goal that has seen the rising popularity of sumo deadlifting with its wide stance. In athletic development the goal of a trap bar jump for example might be to maximise the range of motion of each rep as this would indicate a higher jump and therefore more power.

Bar path

Displacement or ROM can also be valuable as a tool to improve technique, this is best done with the visual aid of a bar path trace. Bar path describes how an implement moves in 2D or 3D space, for many common exercises this is basically a straight line from top to bottom, while on more technically complex movements like the Olympic lifts it can be more curved as the lift follows the different portions of a rep. The shape of these curves on an exercise can be used as a coaching tool to create more optimal performance.

Bar path for a front squat (red dots fro eccentric, green for concentric).

Eccentric, isometric and concentric portions

For most standard strength and power exercises we control the weight through a range of motion working the entire time.

Some movements are eccentric first, others start with the concentric, in some movements you can avoid working through the eccentric phase by dropping the weight instead of lowing it with control and in others you never perform the concentric portion because you failed during the eccentric (Nordics). These eccentric only exercises are known as supramaximal eccentrics.

Lifting Tempo

While data around the eccentric and concentric lift phases is not currently common, there is huge potential for velocity tracking technology to help athletes and coaches gain insights.

One key possibility is to accurately measure the tempo of a repetition.

Tempo counting is a common approach to training in both strength development and hypertrophy work, teaching athletes to control every last centimetre of the rep to develop eccentric control, concentric force development by avoid the elastic effects of bouncing the weight.

Athletes and coaches will often count the tempo or use an interval timing application to keep a consistent tempo, however cheating and inaccurate mental counting (especially late in a set) is all too common. Velocity tracking technology like MetricVBT can precisely measure and deliver tempo data for a given set to deliver objective feedback and analysis on repetition tempo keeping lifters honest and helping you improve your tempo regulation.

Describing velocity and power in a rep

This is a velocity time chart for a single repetition of the back squat, this chart isn’t used very often in day-to-day training, but it is still valuable to understand how a rep is analysed. With the velocity of the barbell across the repetition being measured in metres per second (m/s)

A velocity-time chart taken from the MetricVBT app
The velocity-time chart for a back squat

The graph shows the moment-by-moment velocity over time during a repetition of the back squat, when the bar is stationary velocity is 0.0m/s. The steepest sections of the velocity trace are showing the weight being accelerated (moving away from the central 0m/s line), or the bar being decelerated as the trace returns to the 0m/s.

When training with velocity, most of the time we are focused on concentric velocity, how fast you moved the weight up. That's not to say that eccentric velocity isn't important (it is - but that's a topic for another time), it's just that the concentric phase is the primary focus in the vast majority of training whether you track velocity or not.

For the rest of this article (and the series) we will refer to concentric velocity as simply velocity.

There are four different ways we can calculate and label the velocity of a rep. Peak, mean, propulsive and working phase velocity. Each with their strengths, weaknesses and specific applications.

Peak velocity

Peak velocity is the single moment in a rep when velocity reaches its maximum. Most velocity tracking technology records peak velocity for 1/100th of a second, an incredibly small and sensitive sample. Peak velocity occurs at the end of the accelerative phase of a movement towards the top of a repetitions range of motion (or the point before the float phase for jumping/Olympic Lifting), this occurs once you have built momentum through the first portion of the rep and are moving out of the sticking point and into a position of biomechanical advantage.

The Peak velocity metric - velocity based training

Mean velocity

This is the average velocity across the entire concentric portion of a rep. From the moment you start pushing against the bar until the moment the bar reaches its top position. Mean velocity is traditionally the most common form of velocity metric.

The mean velocity metric - velocity based training

Propulsive velocity

Propulsive velocity is a reimagined form of mean velocity designed to eliminate the active deceleration lifters often apply to submaximal sets on strength exercises. Propulsive velocity factors in the effects of gravity on the bar, recording velocity from the start of the concentric part of the lift and stopping the recording once the bar begins being actively deccelerated. This point occurs shortly after peak velocity is reached.

The propulsive velocity metric - velocity based training

Working phase velocity

Working phase velocity is a novel metric that I have invented aiming to take the concept of propulsive velocity one step further, creating a universal velocity standard that can be applied to all exercises.

Working phase velocity corrects for the problems of mean velocity and peak velocity respectively, capturing only the portion of the movement where effort is applied. Working phase velocity is recorded from the start of the concentric phase and ends when the athlete is no longer pushing it upwards, a float phase begins or the top of the repetition is reached.

I have written in detail previously about the Working Phase Velocity concept, but of all the benefits, eliminating the need to be swapping velocity variations is a huge step in simplifying the VBT process and learning curve without sacrificing validity or precision.

Picking the best metric

To put a bow on the velocity variations, I recommend the working phase metric on all exercises, having a universal metric like this means no swapping and changes in the app settings for different exercises and one less thing to remember during training. For those already training with velocity, it actually won’t be much of a change from the current mean/propulsive velocity that you are using:

  • On heavy strength reps (above ~75% 1RM), the entire ROM is effectively propulsive so it gives basically identical readings to mean velocity.
  • On strength reps below ~75% of 1RM working phase velocity will ignore the deceleration phase just like a propulsive metric, giving very similar results, with very little difference between the passive and active deceleration being applied to the bar.
  • On power exercises, work phase metrics eliminates the float or hang-time phase much like peak metrics do, but with the added improvement of less sensitivity as it measures the entire working portion of the rep.

Working phase velocity is still a concept in development and is only available on the MetricVBT application, so you might not be able to use it with your technology of choice, if that is the case, use these tips as a guide:

Mean velocity or propulsive velocity are both great for strength exercises of all loads, use whichever of these your technology offers.

Peak velocity solves the float phase issue by focusing only on the single instant of greatest velocity, which is usually the point of greatest effort. Just be wary that peak velocity is susceptible to noise, and can be overly sensitive leading to high volatility and low reliability, so take your peak velocity data with a grain of salt!

Training with power

All of these concepts and information above relates to velocity metrics but is also applicable when training with power metrics. A common point of confusion is knowing when to switch between velocity metrics and power metrics for velocity based training. I find the vast majority of my time is spent working with velocity, saving power metrics to be used when I am training specifically near maximum power on an olympic lift or loaded jump for example. This will be discussed in more detail during an upcoming part of the series.

Analysing a rep: Takeaways

To wrap up part two, a few takeaways for you to take into your next training session:

  • Range of motion can be a valuable piece of information for keeping technique consistent and monitoring the effect of fatigue on shortening or lengthening the distance covered.
  • Working phase velocity is a universally applicable metric for all exercise types. It provides the stability of mean velocity while filtering any float phase or passive deceleration on dynamic and ballistic lifts. By working with just a single velocity metric there is no need to change settings, making for one less point of friction when training with velocity tracking technology
  • Tempo is an under-utilised feature of velocity tracking, use this metric to ensure your athletes are controlling their eccentrics and counting their tempo sets accurately.
  • The same rules that apply to velocity variations can also be applied when training with power metrics.

Continue the fundamentals of VBT series with part three: analysing a set

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References and resources

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