PHYS-13 · Principle
After a week of heat training my heart rate is lower at the same pace. Am I fitter?
Partly it is more blood: plasma expands within days of heat or new training, so the heart fills better and beats slower. Weeks off shrink it again.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
After a week of heat training my heart rate is lower at the same pace. Am I fitter? Partly it is more blood: plasma expands within days of heat or new training, so the heart fills better and beats slower. Weeks off shrink it again.
Mechanism
Plasma expansion accounts for nearly all training-induced hypervolaemia over the first 2-4 weeks, then plasma and red cells share it; it is a net gain of body water and solutes, held by renal sodium retention through a more sensitive aldosterone action, and it brings a larger stroke volume and lower heart rate during exercise (Convertino 1991).
Plasma expands at once with training and red cells over weeks; repeated heat expands plasma, and altitude causes plasma loss within hours and dehydration lowers it (Sawka et al. 2000); that review saw no red-cell change with heat, but across 211 later studies more heat exposures went with more haemoglobin mass (REF-3491).Across 211 heat-acclimation studies plasma volume rose 5.6% and end-exercise heart rate fell 17 bpm, with most adaptations in the first week (full text) (REF-3491).
With under four weeks off, highly trained athletes lose blood volume and VO2max fast, and exercise heart rate rises without restoring maximal cardiac output (REF-374); single studies put plasma volume about 5% down at 14 days (REF-1133) and 6% down after 4 weeks of much reduced running in 21 recreational marathoners (REF-942, B).
Common misreading
"My heart rate dropped after a camp, so my threshold is up"; "a week off and my heart rate is high, so my fitness is gone".