Started NSM, then my performance dropped and need help figuring out why
Hello, I've started NSM in June and for the first few weeks it felt great. Then something suddenly changed and I'm now much slower at both easy and subT effort. I’m trying to understand whether I mismanaged the training somehow.
A bit of history for context.
I'm male. I have no sporting background and spent most of my childhood sedentary. After starting university I gained weight and was close to 100 kg at 188 cm in 2020. I lost about 20 kg and started running in July 2022 at age 24.
- 2023: ~1,500 km
- 2024: ~2,200 km
- 2025: ~3,500 km
- 2026 so far: 2,388 km, projected ~3,800 km
My running has generally been a cycle of a few months of progress followed by an injury and losing some of that progress.
I ran a 1:40:46 PB for HM in April using 12 week Pfitz HM plan (73 - 102 km). I have 5K PB 21:55, during the half marathon I also split for 46:35 10K.
I had my LTHR measured in 2025 at 179 with max HR of 196.
After reading lots of success stories here and reading the book, I started NSM June 22. During my HM block I had averaged around 85 km/week, so I decided to start with 8 hours.
Using paces for 22k 5k as guidelines I also adjusted for heat according to the book. Easy runs were around 6:50-7:10. I was running 77k a week. I also do around 40 minutes of strength training three times a week. Nothing heavy, only using dumbbells up to 20 kg.
I setup intervals. icu (screenshot here) so I'd be able to track my load. Since starting NSM on June 22 I haven't left grey. My sleep quality has been great, I have never seen my HRV be this high. I felt fresh coming into every subT session. It was very different to what I was used to, I was very happy with it. Even during the heatwaves in Europe, HR and effort felt great.
After roughly six weeks things suddenly changed. On easy runs I previously ran around 6:50–7:10/km while staying under 70% max HR. Now I sometimes have to slow to over 8:00/km to stay under the same HR.
My previous subT paces have also suddenly become impossible. For example 6-minute intervals:
- Two weeks ago: max HR 176 bpm, effort ~6/10
- Now: max HR reaching 187 bpm, effort more like 9/10
HR also rises much faster during the intervals and doesn’t recover nearly as well between reps.
At roughly the same time, I’ve also had an issue with my left leg feeling weak/off, mainly around the outside of the knee, outer shin, outer calf, and the ball of the foot. Weirdly, I barely notice it during subT, but I definitely notice it during easy runs. There isn’t any pain or tenderness to touch, and calf raises, single leg squats, and one leg hops are essentially symmetrical between sides.
It feels like my running economy and ability to handle subT effort suddenly fell off a cliff.
During summer I start my runs run early at like 6-7 in the morning but I never run fasted if that's relevant. I track my calorie intake and I don’t think I’m underfueling. I've read iron deficiency may be a cause so I had it tested last week and its well within normal range.
TL;DR (AI-generated from my post): Started NSM June 22 after averaging ~85 km/week during a Pfitz HM block. For ~6 weeks everything felt great: good sleep, high HRV, low RHR, easy runs ~6:50–7:10/km under 70% max HR, subT around 6/10 effort. Then performance suddenly dropped. Easy pace sometimes needs to be >8:00/km to stay under the same HR, and 6-min subT reps that previously peaked around 176 bpm now reach 187 bpm and feel like 9/10 effort. At the same time my left leg started feeling weak/off around the outer knee/shin/forefoot, although basic strength tests are symmetrical. Recovery metrics still look excellent. Wondering if this could be accumulated fatigue from starting NSM too aggressively, or if I should be looking for another cause.
My question is:
Did I start with too much volume/intensity despite feeling fresh for the first six weeks? Could this be accumulated fatigue that simply isn’t showing up in HRV/RHR, or would you be looking for something else entirely?
Also, given my current PBs, does 8 hours seem unnecessarily high? Would I likely get similar or better results from a lower-volume version of NSM, especially considering my history of recurring injuries?