Here is how the stress would break down from the previous 7x10 example:
These stress metrics are an attempt to quantify the total stress felt by the individual, based on the Stress Index developed and pioneered by Coach Mike Tuchscherer. A detailed explanation of the Stress Index is outside the scope of this article, but for now, you can think of it as a unitless measure that we use to guide training volume and intensity within the app. It is based on several lines of theoretical and empirical research, as well as decades of coaching experience. And because the Stress Index factors in both the volume and intensity of a set, it is more granular than older methods that only include sets and this allows us to make intensity more adaptable to user preferences.
Now, to give you an idea of what 2.9 total stress is on the Stress Index, it’s roughly equivalent to doing three sets @8.5-9 RPE, or three hard sets. What really makes the above 7x10 @ 6 RPE scenario unpalatable to some (myself included) is that the proportion of metabolic stress (think “lifting for cardio”), when compared to central stress (think “central nervous system fatigue”), is a whopping 3:1.Note, powerlifters usually train closer to a 2:1 or 1:1 ratio of metabolic to central stress. This does not mean that a 3:1 is wrong or bad but just that we will be sucking air during the workout and may be tempted to skip sets. However, since each set is submaximal, if you do this you are missing out on the benefits of effective reps that may come in later sets.
To address this issue with submaximal training, Mike and I decided we need to have a way to allow a user more control over their intensity. Increasing the relative intensity of some sets leads to central stress increases, and in order to hold total stress equal, metabolic stress would necessarily decrease. You see, metabolic stress is directly related to RPE and number of reps, whereas central stress is directly related to RPE and inversely related to the number of reps in most cases. Therefore, when altering a training protocol by reps, the delta in metabolic and central stress will be inversely correlated. Alternatively, a change in RPE leads to an increase in both, though at differing rates. Our resident sport scientist Dr. Jacob Goodin calls this the Constrained Training Stress Hypothesis, which states that when total stress is held constant, any adjustment to central stress must be accompanied by an inversely equivalent adjustment to metabolic stress in almost every case, and vice versa. You can think of it like a balanced math equation:
Integrating the Stress Index into our inference engine allowed the A.I. to interpret changes to the set volume and intensity according to the rules of the Constrained Training Stress Hypothesis. The result is that the total stress of a given workout is now resilient to volume or intensity changes, making it possible to adjust the weights higher, the reps lower, and changing the number of sets prescribed while still accomplishing the same amount of stimulus for adaptation. Remember that the SRA principle states that when we experience a training stimulus (S) recovers (R) and then adapts (A) to that stimulus in order to handle it the next time around.
Now, by incorporating the Stress Index, the Constrained Training Stress Hypothesis, and the principle of SRA , we can change the manner that a similar stimulus load of stress is administered. In essence, we can empower the user to choose how they consume training volume.