Hi Jeremy,

Thanks for your comments and insights. Although I do agree with the concept of a metabolic limit to one's performance in a marathon, my issue is that none of the studies do an adequate job of calibrating for racing during high temperatures relative to training temperature. I have been using a Stryd pod and last year it predicted my finish times very well in the Boston Marathon and London Marathon six days later when I stuck to its calculated CP (Critical Power), which is like but not identical to critical speed. But since then I have run 6 more marathons most of which were at hotter temperatures and the prediction was way off. For example, my 2024 Boston Marathon time was ~37 minutes slower than my 2023 time. But in 2023 the temperature was 48 degrees throughout the race (ideal!) whereas this year it started at ~50 and ended at ~70. The carnage among runners was evident towards the latter half of the race. Stryd has a temperature correction capability it the app but it doesn't work well for me and I believe that is because it doesn't have enough data to adequately compensate for 1) salty sweaters and 2) runners with Parkinson's disease. From my data and my experience over 24 marathons in the last 4.5 years, I am certain that sodium deficit causes severe cramping for me and many other, though not all, runners that limits performance in the marathon on warm days. By taking in enough sodium to compensate for my loss rate, I can prevent the debillitating cramps but, unfortunately, I still haven't uncovered the algorithm to consistently have a great race each time out.

-Joe