Good question. Not sure I have a good answer other than ‘yes, and’
Kolie probably has a better mechanistic answer, but my handwavy coaching answer is yes, progressive motor unit recruitment would contribute to cardiac drift, but I don’t think cardiac drift is a good proxy for motor unit recruitment in the real-world. Other factors (blood volume changes, glycogen depletion, central fatigue, inter-muscular recruitment changes rather than intra-, etc.) I think have larger effects.
My current approach is to keep low intensity training targets low enough that the athlete doesn’t need to decrease power over time during prolonged training to accommodate either for a large cardiac drift or (more importantly, IMO) RPE drift, given their available training time and ability to recover between sessions.
If an athlete is prioritising training consistency, then IMO managing fatigue from a single training ride to allow the next session to be at an equally high quality is the top priority. Cardiac & RPE drift don’t need to be zero, but the athlete needs to understand how the prolonged fatigue from today’s ride will affect their training (and the rest of their life) tomorrow, next week, and next month.
One of the daily outcome goals I discuss with coaches & athletes is to get home from a training ride and not feel like they need to raid the fridge and melt into the couch for the rest of the day… that means managing fuelling, hydration, and fatigue (central and peripheral) appropriately during the ride, starting from minute 00. But I mostly consult with working athletes where melting into the couch for the afternoon isn’t an option.
I really like @kurt.braeckel’s answer using cardiac drift descriptively for monitoring, and focusing on individualised progression of duration. see: