Conditions for the 2018 Commonwealth Games men's marathon were far from optimal with temperatures reaching 29oC and humidity levels up to 74%, so clearly pacing and a good sense of individual thermoregulatory effectiveness were going to be key for all competitors.
Callum Hawkins led the field from an early point in the race (see chart below) breaking away from the other front runners at around 25km and, as is clear from the final result, his pace was too fast for the conditions. In effect he was pacing for a 40km race rather than the full 42km, as evidenced by his collapse with 2km to go. There were no doubt multiple physiological changes that caused the collapse (multifactorial fatugue) but the underlying issue will have been inadequate thermoregulation.
Callum's sweating efficiency was insufficient to cope with rise in core body temperature resulting from Callum's increased workload. There will have been a number of knock-on effects - reduced blood volume and elevated blood viscosity leading to increased heart rate, a loss of key electrolytes leading to reduced muscle efficiency, and eventually, as these effects continue, a reduction in sweat rate which will clearly lead to a further increase in core body temperature. Ultimately central fatigue sets in and the central nervous system will prioritise survival over performance, reducing blood flow to the muscles. At this point Callum would have experienced disorientation, a reduction in mental accuity and decision making ability and finally, physical collapse.
So what are the implications for dealing with this situation? In the very short term the only solution is to reduce race pace in order to help maintain a stable core body temperature. From the chart we can see that the winning runners reduced their pace at the 25km mark, enabling them to successfully complete the full 42km distance. Medium to long term a greater focus on measuring thermoregulatory efficiency is recommended. Train warm and perform cool (or as cool as conditions permit...) is the mantra. Even 2 weeks of monitored acclimation training will have a real impact, and even in the absence of professional facilities, including environmental chambers, simple techniques including wearing additional layers of clothing, and longer, low intensity training sessions, will help.