Training Stress Score (TSS) is a way for us to understand the physical demand a workout placed on our body. By taking into consideration both duration and intensity TSS allows us to more effectively track training load on a day to day, week to week and month to month basis for proper programming/progression.
We can look at TSS on a daily basis to see how much training stress we put on our body that day which will allow us to understand how much time we may need to recover before our next key workout. Tracking TSS on a weekly/monthly basis allows us to program properly by truly understanding the cumulative training load we are pilling onto ourselves over time.
Through day to day tracking you can begin to understand how much TSS you can personally handle and still be able to train the next day. Can you handle 80, 100 or even 150 TSS a day and still be ready to do it again tomorrow?
From a programming perspective TSS allows us to understand how much training stimulus we put on our body each week/month. As a coach it is my job to walk that fine line of making sure I do not put too much training load onto the athletes body that it causes overtraining syndrome but also being sure to put enough stress on it to cause a positive adaptation. Training Stress Score allows me to track that training load in a quantifiable manner rather than just tracking hours or miles. One of the main goals of a training plan is to condition the body to handle more TSS month to month, year to year.
Before we get into how TSS is calculated we need to also understand Intensity Factor (IF) which is simply the percent your resulting Normalized Power (NP) is to your Functional Threshold Power (FTP) for a given workout. So for an athlete with an FTP of 265 who does a workout with a resulting NP of 225 would produce an IF of 0.85 (85% of FTP) for that workout. If you are unsure what NP is you can refer to my previous post.
Training Stress Score (TSS) is calculated by taking into consideration both the intensity (IF) and duration (time) of a workout. As a baseline a score of 100 TSS would be equal to riding your FTP for 60 minutes; which is shown below.
Example - 1 hour ride at 265 NP with a 265 FTP.
Time = 1 hour = 3,600 seconds
NP = 265 watts
IF = 1.00 (NP divided by FTP)
FTP = 265 watts
TSS = (Time x NP x IF) / (FTP x 36)
TSS= (3,600 x 265 x 1.00) / (265 x 36)
TSS = (954,000) / (9,540)
TSS = 100
It is pretty easy to imagine what a 1 hour ride at 100% FTP would feel like; we would feel trashed afterwards. Would we feel the same way after 2 hours at 71% (low Zone 3) of our FTP? Most of us would feel like that would be an easy ride but then again we are always shocked later in the afternoon when we feel a bit wiped out. That is because your body still did the same amount of work; your body still dealt with the same amount of stress.
2 hr ride, NP 188, IF 0.709, FTP 265
TSS = (7,200 seconds x 188 NP x 0.709 IF) / (265 FTP x 36)
TSS = 100
Based on the resulting TSS from the two workouts above can see that the body will require the same amount of recovery from the 2 hour ride at 71% effort as it would from going full boat for 1 hour at 100% effort as both workouts result in a TSS of 100. But keep in mind that just because the stress on the body is the same does not mean that the benefits are. As you can imagine riding for 1 hour at 100% of FTP vs 2 hours at 71% of FTP will lead to very different physiological adaptations.
From a programming perspective TSS is one of the most important metrics to track because it gives you a quantitative way to measure actual training load instead of the traditional modes of tracking distance and/or time. Tracking miles and/or time gives us a sense of volume but not intensity which is also very important when programming. Below I have tabled three of my training weeks that show how both time and miles do not lead to a true picture of training load compared to using TSS.
If we look at the weeks above and use “Weekly Training Hours” or “Weekly Training Miles” as our metric of training load it would lead you to believe that week 1 had the biggest training load. Although when we look at TSS we know that week 2 actually had the greatest training load because of the higher TSS; less time and miles than week 1 but more intensity resulted in a harder week of training. It is also interesting to note that week 1 and week 3 had the same exact training load based on TSS but there was a difference of 2h:28m of training time as well as 50 miles, meaning the intensity during the week 3 rides were also higher overall than the intensity of rides in week 1. Again If you used either time or miles as your metric for training load you would certainly come to the wrong conclusion regarding which weeks were more stressful.
Programming wise It would be easy to come to the conclusion based on those three weeks that I was tapering down training load from week 1 to week 3 based on time/miles but based on TSS you can see that was certainly not the case. Based on TSS we know for a fact that the training load did not go down at all from week 1; I was certainly not tapering.
In the examples above of the workouts and weekly training load we can see why TSS is such an important metric for you to pay attention to and track. From a workout standpoint it will allow you to understand how much recovery time you might need from day to day or how to plan workouts during the week to be effective. From a program standpoint it is so important to understand not only the duration of training you are doing but also the overall intensity as well; more time or more miles does not always equate to more training!
Bottom line is when building a training program it is all about how you progress in duration and intensity. TSS allows for you to do that objectively. Simply tracking miles or hours does not allow you truly understand the impact those miles have on your body.
Shortcut equation to figure out projected TSS of a ride
(IF x IF) x Hours x 100 = TSS
4 hour ride at 80% would be???
(0.80 x 0.80) x 4 x 100 = 256 TSS
Ironman athletes should target 300 or less TSS for 112 mile bike leg.