How Xert Works

Understanding Fitness Signatures

Fitness Signature

Your fitness signature and how it changes over time, is the foundation of Xert.  Xert uses your fitness signature data in a variety of ways:

  1. Tracks how it changes over time.  This is referred to as your Progression.
  2. It is used to derive your Fitness Measure, such as 5 minute power.  This helps you see how you are progressing against a particular fitness goal.
  3. Second-by-second MPA for an activity, is calculated based on it,
  4. Using MPA and your signature, Xert calculates Focus, Stress, Strain and Specificity Rating.
  5. Workouts in the Workout Designer use it to determine MPA, Focus, Stress, Strain and Specificity Rating for the workout.
  6. Your Garmin Xert Code is based on it.
  7. Xert Mobile will query Xert Online for it and use it to program your workouts and for all the metrics that are displayed.

Establishing your fitness signature is key to ensuring all calculations and derived information are accurate.  Your Fitness Signature changes with training and exercise and will decline if you stop training.  Xert provides a number of tools to help you maintain your fitness signature.  In essence, Xert is a platform for managing your fitness signatures, how they change, how they compare with others and helping you see how the exercise you do affects them.

Understanding MPA

MPA Chart

At first blush, an MPA chart doesn’t appear to provide much information.  It shows your power data in red with another line – Maximal Power Available – in blue.  The blue line appears to meander up and down, sometimes coming down far enough, or power rising high enough for them to touch.  What’s the big deal?

What needs to be understood is that your fitness signature together with your power data provide a way to calculate MPA.  In simplest terms, MPA is your limit of power.  If your fitness signature is set correctly, it would be impossible for you to generate more power than what MPA indicates.  During maximal efforts, your power rises to touch MPA, sometimes stays there, and then drops below it.

When one thinks through this a bit further, one realizes that MPA is saying everything about your ability to generate power.  If we each hopped on our bikes and pedaled at 50W, most everyone would have no trouble with this.  Pedaling at 50W doesn’t say anything about you and your fitness.  However, as soon as the intensity starts to increase and longer durations efforts are sustained, those that fatigue less at higher intensities for longer durations, have better fitness.  Still, it is only when one athlete can’t keep up with the next athlete that one can assess as to who has the greater fitness.  “Can’t keep up” means they reached their MPA limit and it wasn’t enough.  This limit is what we have coined MPA and is calculated using a fitness signature together with your power data.  In essence, during maximal efforts when MPA and power come together on the chart, it means that we can determine your fitness.

Understanding Fitness Signature Extraction

Auto Extraction

When activities are synced/uploaded, Xert performs an analysis called extraction, whereby it looks to determine what three fitness signature parameters represent your MPA during the activity.  What this means is that Xert looks at a range of signatures and based on their MPA data, looks to see which combination of parameters best matches how you fatigued and recovered from fatigue during the activity.   The extracted fitness signature represents a set of numbers that provide an explanation for what your fitness was at the time of the activity.

Signature extraction is done automatically for you when you upload/sync an activity.  Automated signature extraction on an activity follows this process:

  1. Your current fitness signature is used to determine second-by-second MPA for the activity.
  2. By comparing MPA and power, Xert can determine how often you performed maximal efforts and how close your maximal efforts were to your maximums as calculated by MPA.
  3. If the activity has sufficient maximal efforts, Xert will attempt to extract the fitness signature for the activity.  (There is no sense in extracting a fitness signature for a recovery ride, for example.)
  4. If the extraction fails due to system error or if the activity doesn’t have sufficient maximal efforts, your current fitness signature will be saved with the activity.
  5. If the extraction is successful, using the extracted fitness signature, Xert will calculate:

The Trickle-Down Process

Trickle Down Notification

In the course of using Xert, at times, a new activity is inserted or prepended to your progression history.  Similarly, you may make a change to an existing activity’s fitness signature by performing MPA analysis and saving a new signature to that activity.  Sometimes an activity is deleted if it was added in error.

When an activity is inserted/prepended/delete or an activity’s fitness signature is changed, Xert performs a trickle-down process.  The trickle-down process is Xert’s way of applying the changes associated with the new activity or fitness change to your overall progression.  The trickle-down process runs until your last activity or to the next activity with a locked signature, whichever comes first.

For example, by inserting an activity what contained a fitness test into the middle of your progression, the fitness test may likely be a good expression of your fitness at that time.  Xert will extract a fitness signature for the activity and then recalculate all fitness signatures subsequent to that newly inserted activity.  This can affect breakthroughs and other metrics that are derived from your fitness signatures.

If you decide to change a signature for an activity by raising or lowering your Threshold Power, for example, and then save/lock the signature to the activity, Xert will then trickle-down the change to all subsequent activities until it reaches an activity with a locked signature or it reaches the end.

The trickle-down process is a convenient way to manage your entire fitness progression history by allowing you to focus on those activities with breakthroughs or tests that you may have performed and having Xert take care of the rest.

After completing the trickle-down process, your current fitness signature may be affected.  This will then be used used as your current fitness signature as described above.

Your First Batch of Activities

Establishing your fitness signature can be an involved process as Xert extracts each fitness signature for your the activities you upload/sync to establish your progression.  The more data you provide, the better Xert works.

When you first upload/sync your activities, Xert looks at the very first activity uploaded/synced and attempts to determine a starting fitness signature.  From there Xert will trickle-down any changes to your fitness signature from activity-to-activity as they are processed.

In general this has shown to work well but there are times, especially when maximal effort data is sparse or non-existent that Xert can get off track.  This is when you can use the Fitness Signature Calculator to identify a good working signature or perform MPA Analysis on an activity to put Xert on the correct path with your signature progression.

Manually Adjusting An Activity’s Fitness Signature

Save and Lock Signature

At times, you may want to fine-tune the fitness signature for an activity if:

  • There are data anomalies that resulted in an incorrect signature being automatically extracted
  • Zero offset of the power meter skewed the data
  • You changed power meters and the new data is higher/lower than previous activities
  • You wish to adjust the activity’s fitness signature to a set of a values that better explain your fitness
  • There are other aspects of your health/fitness that warrant an adjustment to the activity’s signature

As we have described, you can use MPA Analysis techniques to adjust your activity’s signature in order to determine the signature that best represents your fitness at the time of the activity.

Once you’ve settled on a fitness signature that you believe is better, you can Save and Lock your signature to the activity.  Xert will then initiate a trickle-down process which will update your activity fitness signatures that occur subsequent to this activity.

Save and locking an activity’s fitness signature is effectively telling Xert “This was my fitness at the time of the activity.  Don’t change it.”

The Progression Chart

Progression Chart

On your main My Fitness page, at the very bottom is your Progression Chart.  That chart shows a number of key pieces information about your fitness and training you’ve been doing, for the period selected:

  1. Medals
  2. Focus Line
  3. Accumulated Energy
  4. Threshold Power

A recent change has enabled the Progression Chart to either show data as Stress or as Strain.  In most cases these will be very similar, however it is possible for strain and stress to look different during training, where you may be looking to apply more strain on your fitness to improve the efficiency of the time you spend training.

Resetting Your Progression

If you wish to start everything all over again, you can use the Recalculate Fitness Progression feature in Account Settings / Profile tab.  This button will unlock all activities and use your current fitness signature to analyse your very first activity and perform a complete trickle-down process from there.   Use this feature if you wish to have Xert analyse your activities again.  This feature can be convenient to use if, for example, there is upgrade to the Xert algorithm that you wish to have applied to all your activities.   This can often lead to improved overall progression identification and fitness signature results.