Rugby Union requires players to maintain high performance levels and imposes substantial physical workloads for prolonged periods of time across the playing season (Quarrie et al., 2017). To accomplish optimum results, it is necessary to develop a range of player capacities (e.g. low to high intensity running, strength training.) and skills (e.g. passing, kicking, wrestling) in a combination that may vary according to playing position, competitive level and match situation (Austin et al., 2011). In this context, a knowledge of the demands of the game (i.e. internal and external workload and energy expenditure (EE)) it is necessary to customize training practices and nutritional strategies, for optimal body composition and performance, for specific positional roles and competitive levels (Fontana et al., 2015; 2016).
The internal and external workload of rugby has been studied using different methodological approaches like the session-rating of perceived exertion (RPE), heart rate (HR) monitors and global positioning systems (GPS) (Cunniffe et al., 2009; Halson, 2014; Quarrie et al., 2017). On the contrary, few studies have focused on EE in rugby players: overall daily EE in rugby players was estimated using SenseWear armbands (Bradley, et al. 2015a; 2015b) or calculating the EE of accelerated running by using GPS in Rugby League players (Kempton et al., 2015). However, possibly due to the practical and technical limitations of the above methods in field conditions (Morehen et al., 2016), the EE of actual playing/training in Rugby Union is still unreported.
Indirect calorimetry, based on the direct measure of oxygen uptake (V[O.sub.2]), is the most common method to determine EE in exercise laboratories (Jequier et al, 1987). In addition, based on the well-known linear relationship between heart rate (HR) and V[O.sub.2], alternative indirect approaches for the V[O.sub.2] estimate and the subsequent estimate of EE have been developed (Astrand et al., 2003). These approaches allow accurate and precise estimates of EE in a variety of activities (Achten and Jeukendrup, 2003) and have been successfully applied in studies of elite Rugby Union (Da Lozzo and Pogliaghi, 2013); however, their practical applicability is reduced by the need of a preliminary incremental test in the laboratory (to establish the individual HR/V[O.sub.2] relationship), that increases the overall financial cost and reduces the time-efficiency of the approach.
In 2011 Wicks and colleagues developed a simple HR index ([HR.sub.index]) method to estimate V[O.sub.2] in healthy non-athletes and clinical populations without the need for a laboratory test to determine the individual HR/V[O.sub.2] relationship. The method was based on the observation that a valid linear relationship exists between [HR.sub.index] (calculated as actual HR/resting HR) and V[O.sub.2] expressed in multiples of the resting metabolic rate (V[O.sub.2METs]) (Wicks et al., 2011). Within the well-known limitations related to the use of HR measurement for V[O.sub.2] estimation (Achten and Jeukendrup, 2003), the [HR.sub.index] method may provide a new, low cost and easy to use alternative to existing methods to estimate V[O.sub.2] and EE during Rugby activities. In fact, HR monitors are relatively "cheap" equipment, already used by elite clubs and may also represent a convenient investment for lower level clubs offering important information on the absolute and relative intensity of exercise to a wide range of coaches and athletes. Furthermore, [HR.sub.index] has been investigated in its applicability in predicting aerobic fitness (V[O.sub.2max]) (Esco et al., 2012; Haller et al., 2013), a possibility that could be useful to determine and monitor players' fitness levels without requiring expensive metabolic-carts.
However, this appealing field method is based on a linear [HR.sub.index]/V[O.sub.2] relationship that has only been validated in a non-athletic population. Consequently, the application of [HR.sub.index] to individuals with a high fitness level requires preliminary validity verification.
Within this background, the current study tested the following hypotheses: i) a valid linear relationship between [HR.sub.index]/V[O.sub.2METs] can be confirmed in rugby players; ii) V[O.sub.2] and EE of incremental treadmill running can be accurately and precisely estimated based on [HR.sub.index] in rugby players; iii) players' V[O.sub.2max] can be estimated using [HR.sub.index].
Fifteen professional rugby players (24 [+ or -] 3 years, 8 forwards: 106 [+ or -] 10 kg, 1.89 [+ or -] 0.09 m, 18 [+ or -] 8 % fat mass; 7 backs: 93 [+ or -] 11 kg, 1.81 [+ or -] 0.08 m, 14 [+ or -] 6 % fat mass) playing in the Italian Rugby Union First Division, were recruited and included in the study after they gave their written, informed consent. The study was approved by the Departmental Ethics Committee and all experiments were done in accordance with the principles laid down in the Declaration of Helsinki. All participants were nonsmokers, free of any musculoskeletal, respiratory, cardiovascular and metabolic conditions that may influence the physiological responses during exercise testing. All the incremental tests were conducted in the exercise physiology laboratory of the sports sciences section of the University of Verona on an electromagnetically controlled treadmill (Runrace, Technogym, Italy).
Resting heart rate and anthropometry
Participants were instructed to avoid any physical exercise in the 48 hours before the testing session. In the morning, after medical clearance and after a 10-min rest in a seated position, HR was measured for a period of 3 min, using a heart rate monitor (Polar Electro Oy, Finland, acquisition frequency: 5 seconds). Resting HR was identified as the lowest HR of the three minutes of monitoring (Haller et al., 2013). Then, body mass (digital scale, Seca 877, Seca, Leicester, UK), height (vertical stadiometer, Seca, Leicester, UK) and percentage of body fat (based on the sum of the 6-skinfold thickness (Fontana et al., 2015)) were determined.
On the same day, participants performed a step-wise incremental test consisting of 1-minute baseline measurements, 3 minutes of warm-up at 8.0 km/h, followed by step-wise increments of 0.5 km/h every minute until volitional exhaustion. The treadmill was set at 1% gradient to replicate outdoor over-ground running (Jones and Doust, 1996). The above ramp incremental protocol was chosen to obtain a time to exhaustion around 8-12 minutes (American College of Sports Medicine, 2017) using a method extensively described elsewhere (Pogliaghi et al., 2014). In brief, based on previous data by our group and others (Duthie et al., 2003; Pogliaghi and De Roia, 2007) we anticipated V[O.sub.2max] values ranging between 46 and 52 ml x [kg.sup.-1] x [min.sup.-1]; considering the following energy cost of running (American College of Sports Medicine, 2017):
V[O.sub.2] (ml x [kg.sup.-1] x [min.sup.-1]) = 3.5 + [0.2 x speed (m x [min.sup.-1])] + [0.9 x speed (m x [min.sup.-1]) x grade (%)] (grade is in decimal form: 1%= 0.01)
we estimated a maximum running speed of 200-230 m/min (12-14 km/h). Starting from 8 km/h (i.e. the lowest speed at which all subjects would run) and...