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 points During 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. Key words: Carbon dioxide excess, oxygen pulse, oxygen debt Introduction Although athletes spend a greater amount of time resting than performing, research has dedicated little attention to the practical management of functional recovery after training (Bishop et al., 2008). Complete restoration of the integrity of the organs stressed by workloads is essential to attain optimal performance and this can be achieved only through an appropriate balance between loads administered, physical and psychological stress deriving from the competition and functional recovery. Thus there is a wide variety of recovery techniques as part of training programs targeting this goal (Barnett, 2006). For such reasons, some scientific updates on this topic have recently highlighted the need to expand studies on various aspects of functional recovery in athletes (Bishop et al., 2008; Barnett, 2006; Achten and Jeukendrup, 2003). Functional recovery can be divided into three main categories: a) immediate relief after a workout, b) short recovery between repetitions during a session of interval training, c) recovery practices between workouts (stretching, massage, sauna etc). The focus of this research was to address point b) of the aforementioned classification in an attempt to add to current knowledge on metabolic adjustments that occur during the short recovery between repetitions in interval training (IT) in athletics. In most cases coaches manage the recovery between repetitions by selecting one of three empirical procedures: (i) a ratio between exercise and recovery times. If one has a 3min workout, according to the energetic pathway to elicit, coaches set a recovery (e.g. 1:1, meaning that recovery will take 3min as well); (ii) monitoring the RPE or any other subjective way to learn how tired the athlete is; (iii) using an HR value as a percentage of the theoretical maximum (HRmax, calculated as 220 beats minus age): the athlete is considered ready to restart an effort when an empirically established percentage value of HRmax is restored. An itemized focus on the prescription of IT with a fourth solution for determining duration and intensity of recovery time was recently pointed out by Tschakert and Hofmann (2013). In their brief review the authors suggested the use of a threshold model rather than percentages of VO2max or HRmax to set the exercise (Ppeak) and recovery (Prec) workloads and the use of an equation (equation 1) to calculate the mean load (Pmean): Pmean = (Ppeak · tpeak + Prec · trec)/(tpeak + trec) Eq 1 where tpeak and trec are the peak workloads and recovery durations respectively. By using this method the authors of the aforementioned review made a good attempt to modulate the IT in accordance with the training periods and cardiorespiratory strain. Significant in this regard are the recent results by Tschakert et al. (2015) which showed that the use of % HRmax to prescribe exercise intensity in long IT resulted in different values for Pmean, Ppeak and Prec across athletes with respect to their individual workload at the second threshold measured in an incremental exercise test (IET). Therefore, with the combined use of a threshold model and equation 1 it would be possible to provide more accurate HR management. In any case, several practitioners, even when aware that HR is not an accurate way to manage recovery during IT, probably use HR because it is an affordable and straightforward way. However, recovery time management of this kind is experiencing difficulties: the time of the recovery of fixed HR values can increase depending on several factors (intensity and number of repetitions, weather conditions, hydration status etc.). This is to say that we are dealing with a phenomenon known as cardiac drift (Coyle, 1998; Goodie et al., 2000; Gonzales-Alonso et al., 1997). Furthermore, both recovery load and recovery duration affect muscle metabolic recovery which is crucial in maximizing work capacity during the subsequent intervals (Bucheit and Laursen, 2013). Our research aimed to analyze the use of HR as a reliable index for recovery management during IT, considering its relationship with the several factors that can influence respiratory, metabolic and cardiovascular homeostasis and their implications in the cardiac drift phenomenon. Taking into account that the IT method was introduced to extend training duration at intensities higher than those achievable with continuous training, the most precise indication possible on recovery time should be established during IT sessions.