A new study on Ultra-Endurance triathletes found...

...when heart rates (HR) from laboratory based swim, bike and run assesments, to determine heart rate at Ventilatory Threshold (VT), were compared to heart rates during an Ultra Endurance triathlon race, it was interesting, but not unexpected, to see the following:

Swim: 
Tested HR @ VT: 137.1b/min
Race HR: 149.2b/min

Bike: 
Tested HR @ VT: 142.8b/min
Race HR: 139.2b/min

Run : 
Tested HR @ VT: 147.7b/min
Race HR: 136.2b/min

Lab  assessment

Racing 

The research found that the greater the time between the swim and the cycle, the slower the time achieved in the run.

The researchers suggested that the best predictor of performance for this group of triathlets was:

1. Weight adjusted VO2max
2. The difference in heart rate between the Swim and bike legs

This study was conducted by INEFC-Barcelona Sports Sciences Research Group Barcelona, Barcelona, Spain and Departament d’Enginyeria Informàtica i Matemàtiques, Universitat Rovira i Virgili, Tarragona, Spain. Reserachers: A. Barrero1, D. Chaverri1, P. Erola2, X. Iglesias1, F. A. Rodríguez1

Abstract