Heart rate unreliability during interval training recovery in middle distance runners

Heart rate (HR) was tested as a reliable index for recovery management during interval training (IT), considering its relationship with the several factors involved in respiratory, metabolic and cardiovascular homeostasis. Thirteen runners underwent two different IT sessions: at 80% and 120% of the second ventilatory threshold (VT2). Throughout both sessions HR, oxygen uptake (VO2), carbon dioxide production (VCO2) and pulmonary ventilation (VE), were measured by means of a portable gas analyzer. Carbon dioxide production excess (CO2excess), respiratory exchange ratio (RER), oxygen pulse (OP) and oxygen debt (O2debt) were also estimated. A significant increase in HR values (144 versus 150 beats·min(-1) between the first recovery and the last, p < 0.001) was observed at 80% of the VT2 speed. At the over-threshold intensity, HR rose from 159 to 168 beats·min(-1) from the first recovery to the last (p < 0.001). OP showed a declining trend from the first to the last recovery at 80% at the VT2 speed (18.3 versus 16.4 mL·beats(-1), p < 0.05) and between the first and the last recovery in tests performed at 120% of the VT2 speed (17.8 versus 16.3 mL·beats(-1), p < 0.05). No change occurred in CO2excess, VO2, RER, VE and O2debt. On the basis of our research, the use of fixed HR as a reliable index of the established recovery is inaccurate and unfit for training. The phenomenon of cardiac drift to set the restart timing after the repetitions, i.e. by progressively increasing HR values, should be taken into account by coaches. Key pointsDuring an IT session, if recovery time after repetitions is fixed, HR supplies a different indication compared to all the respiratory parameters: HR indicates an incomplete recovery while the other parameters do not.The use of fixed HR values as a reliable index of the established recovery during IT is inaccurate and it may be the cause of under-training.To set the restart timing after repetitions the phenomenon of cardiac drift should be taken into account by coaches.HR drift during recoveries did not appear linked to the CO2excess.