PHYS-08 · Principle
My heart rate never reaches the zone in 1-minute reps. Was the rep too easy?
Heart rate cannot say. Oxygen uptake and heart rate lag a change in work, so a short rep ends before they arrive. Judge it by power, pace or effort.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
My heart rate never reaches the zone in 1-minute reps. Was the rep too easy? Heart rate cannot say. Oxygen uptake and heart rate lag a change in work, so a short rep ends before they arrive. Judge it by power, pace or effort.
What it does not mean: that a slow fall afterwards means low fitness: heat and the work before it change the fall (Daanen 2012), and recovery got faster in the 10 triathletes who became functionally overreached (REF-208, B+).
Mechanism
Muscle VO2 lags ATP use with near-exponential kinetics and phosphocreatine and glycolysis cover the deficit; above the lactate threshold a slow component emerges 90-180 s in, giving a delayed steady state in the heavy domain and a climb toward VO2max in the severe (Goulding 2023, full text).About 86% of it arises in the working limbs (Xu & Rhodes 1999), first tied to recruiting less efficient type II fibres and now chiefly to fatigue of the fibres at work (Jones et al. 2011); fast kinetics mean a smaller deficit and better tolerance (Poole & Jones 2012).
In intervals breathing rate followed the work-rest alternation while VO2 and heart rate did not change abruptly (the authors' studies, REF-1084, C+); at critical power 7 trained men's heart rate, VO2 and lactate were still rising when they stopped after 20-41 min (REF-750, B). Heart rate rises steadily through prolonged exercise in most studies (REF-1341; PHYS-11).
Afterwards heart rate falls as vagal activity returns and sympathetic drive withdraws, faster in the trained (Borresen & Lambert 2008); age, heat and the work before confound the reading (Daanen 2012).
Common misreading
"Heart rate in the target zone is what makes a 30/15 session work"; "my heart rate is slow to drop, so I am unfit".