Many workout and activity trackers ahve some measure that tries to determine how your fitness varies over time. Across different wearables and trackers this has a variety of names. Health mate, or Withings, which makes smart scales and watches uses a Health Mate Fitness Score. Apple notably measures VO2 max as a proxy for Cardio fitness. Strava has a training load value that is noted in three metrics Fitness Score.
Training Peaks, is no different from these but they mainly call their fitness measure Training Stress Score (TSS) as well as Fitness Fatigue, and Form.
It is worth highlighting that although many of these scores seemingly track overall fitness, they do so in very different ways. The V02 max and Health Mate Fitness both look at proxies that can be objectively measured physiologically. That is to say, they track to things that could be tested in a lab.
TSS and Fitness however are calculated metrics that follow a defined calculations, but can not be measured on a point basis. Two people with identical TSS scores and resulting Fitness scores from Training Peaks may preform very differently on any objective measure of fitness.
What is TSS?
TSS stands for Training Stress Score. Ultimately it is a proprietary calculation used by Training Peaks to determine a value of how intense an individual workout is. The concept is based on a variety of research that links the duration of activity, power generation per weight, and heart rate data into an single value.
Research into the usefulness of TSS makes a point to note that “Training load can provide important information to athletes and coaches; however, monitoring systems should be intuitive, provide efficient data analysis and interpretation, and enable efficient reporting of simple, yet scientifically valid, feedback.” (citation).
For the most part this recommendation means that in order to get anything out of TSS, you need it to point to an action to take. This typically means that TSS of an activity should be used as a way to tell you something like “Was that ride, that went over a very different course and in different weather, harder than the one I did last week?” or “Should I be working out hard tomorrow”.
What Do Fitness Fatigue and Form Mean?
For the second question posed above (Should I workout hard tomorrow) Training Peaks uses TSS to calculate another series of values, Fitness, Fatigue, and Form that can be used to determine accumulated training stress over time.
Fitness as defined by Training Peaks is, at its core, a simple addition problem. The total value of all workouts TSS over the past 42 days effectively make up a the Fitness Value.
Fatigue is a bit of the opposite, but built to go up as well. As training stress builds up over time, so does fatigue. If there is no time off and stress keeps accumulating the fatigue rating keeps growing. Fatigue however can go down on a day with a workout, so long as the TSS of the workout was low enough. This in effect becomes a recovery day. For tapering during a long training plan the goal is to ramp down the stress of workouts without doing so quickly enough to also lose fitness.
Form generally is measured as the difference between the two values. Unlike the others the natural or desirable state of it may not necessarily mean more is better. If fitness and fatigue are equal then form will be 0. Generally speaking a negative form value means you are slightly “over trained” and still building. A positive score on form means that you have recovered sufficiently to have a chance at outputting a great effort.
What is missing from Fitness, Fatigue, and Form is any external insights including injuries, weight gain, or sickness. On a few days taken off due to the flu your fatigue may show as reduced and your form may look increased, while it is obvious to any who have tried to run after being sick that sometimes there are lingering effects.
What are Good Fitness Fatigue and Form Scores on Training Peaks?
The difficulty with any of these values is to understand how they compare to others. This is difficult to say since the TSS winds up being so personal. In part this is because TSS gets calculated based on the inputs for maximum heart rate as well as power and heart rate zones.
For Fitness on Training Peaks, higher is better. Of course this only looks at the past 42 days, or six weeks. If you ran an epic long distance high stress race 42 days ago, then a day of rest or even a recovery event may still result in a drop in fitness. Those who are following a multi-week 7 day schedule can expect to see their fitness climb in excess of 80, with some reporting Fitness scores well over 100.
On Fatigue, lower is better. Fitness is more like Form though in that there is a lower limit to what to consider good. Too low of Fatigue may indicate that you have not had any hard workouts in a while. Anecdotally those who are training for marathons or triathlons and follow a multi-week 7 day schedule can expect to see a Fatigue score as high 100 or more. Fatigue scores below 20-30 likely are from the only occasional activity.
For Form, generally speaking a positive number is good. A positive value means generally indicates that you have been recovering or tapering. If planning to go for a big event or race, having a slightly positive score is ideal although a hugely positive score may actually be worse as it could be a drop in both fitness and fatigue.
How does Fitness Fatigue and Form Compare to Strava, HealthMates, and Apple Cardio Scores?
As discussed above each of these wearable and activity trackings has it’s own way of displaying cardio fitness or fitness scores. For the most part however Training Peaks Fitness will NOT match the value of V02 max values (namely Apple or Health Mates) since they are do not even have the same unit of measure. While there may be some correlation with V02 max scores, the correlation is very rough and comparing the two is not advised.
Strava and others that calculate a sustained training load value, can however be compared. Training Peaks uses a proprietary calculation to determine Fitness scores though so there is no reason to think that the same activity loaded to both platforms will result in the same score. Even so, if you ahve loaded all the same activities to both Strava and Training Peaks, and the settings of both have the same heart rate zones and max heart rates, then you should expect that the values to be very similar.
One other notably similarity is that for both Strava and Training Peaks, the Fitness curves and values are hidden behind the premium subscriptions. If you want to find other ways to calculate these, without spending on subscription fees, here are some related articles: