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 ad" ministered, 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 ([HR.sub.max], calculated as 220 beats minus age): the athlete is considered ready to restart an effort when an empirically established percentage value of [HR.sub.max] 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 V[O.sub.2max] or [HR.sub.max] to set the exercise ([P.sub.peak]) and recovery ([P.sub.rec]) workloads and the use of an equation (equation 1) to calculate the mean load ([P.sub.mean]):
[P.sub.mean] = ([P.sub.peak] x [t.sub.peak] + [P.sub.rec] x [t.sub.rec])/([t.sub.peak] + [t.sub.rec]) Eq 1
where [t.sub.peak] and [t.sub.ree] 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 % [HR.sub.max] to prescribe exercise intensity in long IT resulted in different values for [P.sub.mean], [P.sub.peak] and [P.sub.rec] 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.
Thirteen national-level middle-distance runners (MDR) were recruited. The athletes' anthropometric and functional characteristics are summarized in Table 1. Before testing, all participants were informed of the risks associated with the study and written informed consent was obtained in accordance with the Helsinki Declaration. To ascertain the athletes' good health and to exclude any cardiovascular or respiratory disease, all participants underwent medical evaluation.
The subjects attended 2 separate testing sessions at our laboratory over a 3-week period. Initially, they completed an IET on a treadmill (RunRace, Technogym, Italy) to establish each individual's Ventilatory Threshold 2 (VT2, see also below) or "anaerobic threshold" according to the three-phase model (Binder et al., 2008) and maximum oxygen uptake (V[O.sub.2max]). The following day each subject performed two IT sessions at 80% and 120% of the VT2 speed in a randomized order and spaced at 6 hours.
Before all testing, subjects were required to abstain from any other unaccustomed strenuous exercise for at least 24 hours (aside from our protocol). Subjects were recruited in agreement with their coaches, who were training them to evaluate the level of training reached and to ameliorate the training program and establish their individual running times. Inclusion criteria were no less than 14 h training per week. Testing was performed at the same time of day ([+ or -] 2 h, to minimize diurnal variation), and on the same treadmill.
Incremental Exercise Test (IET)
The athletes initially undertook an IET until exhaustion. Each athlete began running at a speed of 10 km x [h.sup.-1] followed by a gradual 1 km x [h.sup.-1] increase every 2 minutes, until exhaustion. The total distance covered was between 5000 and 6000 m. The ventilatory equivalent method was used to determine VT2 i.e., the respiratory compensation point was identified as the point at which the [V.sub.E]/VC[O.sub.2] ratio began to increase (Meyer et al., 2004). In this way, it was possible to determine the two speeds for performing the IT sessions, i.e. at 80% and 120% of VT2.