Specific Incremental Test for Aerobic Fitness in Trail Running: IncremenTrail

) is performed in a natural environment, including various ranges of slopes where maximal oxygen consumption is a major contributor to performance. The aim of this study is to investigate the validity of tests performed in uphill conditions named the “IncremenTrail” (IncT), based on the incremental ascending speed (AS) to evaluate trail runners' cardiorespiratory parameters. IncT protocol included a constant gradient slope set at 25% during the whole test; the starting speed was 500 m h 1 (25% slope and 2.06 km h 1 ) and increased by 100 m h 1 every minute (0.41 km h 1 ). Twenty trail runner specialists performed the IncT and a supramaximal exercise bout to exhaustion with intensity set at 105% of maximal AS (Tlim). Oxygen consumption,

25% during the whole test; the starting speed was 500 m h 1 (25% slope and 2.06 km h 1 ) and increased by 100 m h 1 every minute (0.41 km h 1 ). Twenty trail runner specialists performed the IncT and a supramaximal exercise bout to exhaustion with intensity set at 105% of maximal AS (Tlim). Oxygen consumption, breathing frequency, ventilation, respiratory exchange ratio (RER), and heart rate were continuously recorded during the exercises. The blood lactate concentration and rate of perceived exertion were collected at the end of the exercises. During the IncT test, 16 athletes (80%) reached a plateau of maximal oxygen uptake (65.5 7.6 mL kg 1 min 1 ), 19 athletes (95%) reached RER values over 1.10 (1.12 0.02) and all the athletes achieved blood lactate concentration over 8.0 mmol L 1 (17.1 3.5 mmol L 1 ) and a maximal heart rate 90% of the theoretical maximum (185 11 bpm). Maximal values were not signi cantly different between IncT and Tlim. In addition, ventilatory thresholds could be determined for all runners with an associated AS. IncT provided a suitable protocol to evaluate trail runners' cardiorespiratory limitations and allowed us to obtain speci c intensities based on the ascending speed useful for training purposes in speci c conditions. Keywords:trail running; testing; uphill; method; oxygen consumption; validation 1. Introduction Trail running (TR) is a generic term used to describe off-road running practice in a natural environment, including mountains, forests, deserts, and many other terrains [1]. TR competitions are organized with a minimum of asphalted road (20–25% maximum) and cover a large range of distance from 10 to more than 100 km with various ranges of elevation potentially over 10,000 m (e.g., the Ultra-Trail du Mont Blanc—UTMB ® , 170 km and over 10,000 m of elevation). In recent decades, TR has become extremely popular, and the number of participants and events has increased exponentially. Compared to level road events of the same length, TR includes uphill and downhill sections on various slopes, that are known to alter the energy cost of running and generate greater

Blanc—UTMB ® , 170 km and over 10,000 m of elevation). In recent decades, TR has become extremely popular, and the number of participants and events has increased exponentially. Compared to level road events of the same length, TR includes uphill and downhill sections on various slopes, that are known to alter the energy cost of running and generate greater muscular damage [2–4]. Moreover, uphill sections induce speci c running kinematics [5,6] necessary Sports2022,10, 174.

Sports2022,10, 174 2 of 8 to elevate the centre of mass at each step [7,8], and consequently greatly increase the energy expenditure [9]. Many recent studies have pointed out that TR performance is largely related to maxi- mal oxygen consumption capability [3,10–13], reinforcing the interest of . VO2max assessment for performance purposes. The effect of gradient slopes on . VO2max has been largely studied and results differ according to the slopes gradient, protocol, or the participants. Consider- ing the TR runner specialist population, few studies have investigated the effect of slope gradient on . VO2max reached during an incremental running test. Balducci et al. did not ob- serve the modi cation in . VO2max between level and slope conditions during an incremental running test on a treadmill at a 12.5 and a 25% gradient [14]. More recently, Schöf et al. reported similar . VO2max for a trail runner specialist and a road runner when comparing outdoor tests on a natural slope at 16% and a treadmill running at a 1% gradient [15]. In addition, De Lucas et al. proposed a new test based on a slope gradient increase instead of a horizontal speed increase and did not point to any difference between the two typologies of test [16]. In contrast to these results, Scheer et al. reported signi cantly higher . VO2max in a speci c trail test performed on a treadmill with an increase in speed and slope over the test (1 km h 1 and 1% gradient slope per minute) [17]. In line with these results, Cassirame et al. investigated physiological variables at ventilatory thresholds and at exhaustion during incremental eld tests at the levels 15%, 25% and 40% of gradient slope [18]. This study indicated that uphill running allows highly trained TR specialists ( . VO2max > 65 mL kg 1 min 1 ) to reach higher . VO2max than those running in level conditions. Neverthe- less, this study also demonstrated that . VO2max is not signi cantly different between a 25 and a 40% slope gradient and this gradient allows runners to achieve physiological limits equally.

running allows highly trained TR specialists ( . VO2max > 65 mL kg 1 min 1 ) to reach higher . VO2max than those running in level conditions. Neverthe- less, this study also demonstrated that . VO2max is not signi cantly different between a 25 and a 40% slope gradient and this gradient allows runners to achieve physiological limits equally. Based on this statement, it appears of interest to investigate and evaluate TR specialists in uphill conditions to obtain physiological limitations and adaptations in situations closer than those faced during TR practice. In addition, we noted that the range of slopes and eld typology required more speci c intensity markers to prescribe training intensities or calculate training load [19]. In their study, Cassirame et al. [18] indicated that ascending speed (AS) can be a relevant indicator of the intensity for slopes between a 25 and a 40% gradient. In this study, . VO2 reached at exhaustion or ventilatory thresholds in gradient tests set at 25 and 40% did not differ. For several years, AS has been calculated with GPS and or barometer technology and TR specialists can use this variable in real time on their heart rate monitor and analyze it after training. This AS concept is also largely in uenced by challenge on social networks where athletes highlighted their maximum elevation performed in time from 1 h to 24 h. Traditionally, running performance capabilities including . VO2max assessment are per- formed in level conditions with existing or adjusted tests as “VamEval” or the “Universit² de Montr²al track” tests [20], or with a minor slope gradient (1–2%) when performed in treadmill conditions [21]. Classic tests cannot be performed in a slopes condition without adjusting the speed protocol to ensure physiological exhaustion [22] and lead to maximal oxygen consumption instead of peak values [23,24]. Previously, Scheer et al. [17] presented a trail test based on both slope and speed increase. Unfortunately, a starting protocol with a slope gradient lower than 25% does not ensure linear progression of AS and a simultaneous increase in slope and speed induces an abrupt increase in

exhaustion [22] and lead to maximal oxygen consumption instead of peak values [23,24]. Previously, Scheer et al. [17] presented a trail test based on both slope and speed increase. Unfortunately, a starting protocol with a slope gradient lower than 25% does not ensure linear progression of AS and a simultaneous increase in slope and speed induces an abrupt increase in energy expenditure [9,25] and, consequently, an AS determined during the test could not be used for training. For these reasons, a speci c test named “IncremenTrail” (IncT) [26] has been developed in order to evaluate . VO2max and the AS. Contrary to classical tests, the IncT is based on an incremental AS instead of a horizontal speed with a constant slope set at a 25% gradient. This gradient was selected based on a previous study indicating that a maximal physiological response can be observed at 25% [18]. The IncT protocol starts at 500 m h 1 of AS (25% slope and 2.06 km h 1 ) and increases by 100 m h 1 every minute (0.41 km h 1 ) keeping the slope

Sports2022,10, 174 3 of 8 gradient constant during the entire test. Given that the IncT aims to evaluate maximal physiological capabilities as . VO2max , it is important to verify that the protocol was designed for lead athletes to reach their maximal capabilities and to avoid a stop to the test induced by muscular fatigue or non-physiological reasons [23,27]. Firstly, the aim of this study was to investigate whether maximal cardiorespiratory variables are reached by TR runners during the IncT by comparing variables obtained with a veri cation phase performed thereafter [28,29]. A secondary aim was to evaluate if venti- latory thresholds (VT1 and VT2, respectively) can be determined during the incremental protocol using the IncT. 2. Materials and Methods 2.1. Participants Twenty well-trained male TR runners (training volume >8 h per week) were included in the present study (age: 30.1 8.4 years; height: 179.7 6.5 cm; body mass: 68.7 6.6 kg). Their training volume was 7.5 2.1 h per week for 8.8 6.7 years. The participants were healthy, without injuries in the previous 6 months, and were not taking any medication. All subjects did not undergo any strenuous exercise in the three days before the test. Written informed consent was obtained from the subjects, and the study was conducted according to the principles laid down in the Declaration of Helsinski. This study has been approved by “COMITE DE PROTECTION DES PERSONNES SUD MEDITERRANEE IV (ID-RCB: 2019-A03012-55). 2.2. Experimental Design All the participants performed the IncT test on a large and motorized treadmill (S 1930, HEF Techmachine, Andrezieux-Boutheon, France) with a 25% slope. The AS at the beginning of the test was set at 500 m h 1 and increased by 100 m h 1 every minute until exhaustion as described previously. Maximal AS (ASmax) was considered as the speed in the last stage completed by the athlete. After 30 min of passive recovery, the participants performed a second bout of exercise at a supramaximal intensity to verify the maximal values obtained during the IncT test [24,28,30,31]. In this second bout of exercise, athletes started to run on the treadmill

described previously. Maximal AS (ASmax) was considered as the speed in the last stage completed by the athlete. After 30 min of passive recovery, the participants performed a second bout of exercise at a supramaximal intensity to verify the maximal values obtained during the IncT test [24,28,30,31]. In this second bout of exercise, athletes started to run on the treadmill set at the same gradient slope (25 ) for a 5-min warm-up at a speed set at 60% of the ASmax. After this phase, athletes ran at 105% of the ASmaxwith the objective of maintaining the speed until exhaustion (Tlim). The experimental design can be observed in Figure. Figure 1. Experimental design description. In the graphic (A), the grey area represents the ascending speed in km h 1 where black solid line represents the slope gradient in degrees; in the graphic (B), the grey area represents the ascending in percentage of maximum ascending speed obtained during A where black solid line represents the slope gradient in degrees.

Sports2022,10, 174 4 of 8 During both situations, respiratory gas exchanges and heart rate were collected breath by breath during 2 min at rest and during the exercise by a Metalyzer 3B-R3 system (Cortex Biophysics, Leipzig, Germany). The breathing ow was measured through a bi-directional digital turbine and a 220-cm sample line tube collected inspired and expired air to measure O2and CO2concentrations. All data were transmitted wirelessly to a computer using MetaSoft Studio © studio software 5.5.2 (Cortex Biophysics, Leipzig, Germany) to calculate the O2consumption ( . VO2 , L min 1 ) and CO2output ( . VCO2 , L min 1 ). Before each test, a ow sensor was calibrated with a 3 L syringe and gas sensors were calibrated with air ambient and reference gas (15% O2, 5% CO2) as recommended by the manufacturer. . VO2max (mL kg 1 min 1 ) was calculated using the highest . VO2 measurement average in 30 s for IncT and Tlim tests. In the same time period, the average of the following physiological variables was processed to obtain maximal values: the respiratory exchange ratio (RER), heart rate (HRmax) in bpm, minute ventilation ( . VEmax ) in L.min 1 , and breath frequency (BF) in cycles.min 1 . VT1 and VT2 were determined using the Wasserman and Beaver methods [30] by three unsighted operators. The mean of the two closest values (out of three) was reported. In addition, the presence of the plateau of . VO2 was investigated and con rmed when the . VO2 remained stable (less than 150 mL O2variation) during at least 30 s despite the workload increase [31]. For both tests, blood lactate concentration[La]was measured 1, 3, 5, and 7 min after the end of exercise and the highest value was retained. A capillary blood sample was extracted from the nger (0.5 L) and analysed using the Lactate Scout+ (EKF Diagnostic, Cardiff, United Kingdom). At the end of each step, the subjective perception (RPE) of effort was quoted by each participant using the rating of perceived exertion scale from 0 to 10. 2.3. Data and Statistical Analyses

the highest value was retained. A capillary blood sample was extracted from the nger (0.5 L) and analysed using the Lactate Scout+ (EKF Diagnostic, Cardiff, United Kingdom). At the end of each step, the subjective perception (RPE) of effort was quoted by each participant using the rating of perceived exertion scale from 0 to 10. 2.3. Data and Statistical Analyses Results are presented as means standard deviation (SD). For both IncT and Tlim, the data were average over 15 s [28]. For all participants, criteria to conclude that maximal data were reached during IncT were a combination of the following variables: (i) plateau of . VO2 ; (ii) RER 1.1; (iii)[La]> 8 mmol L 1 after the test; (iv) the participants stopped the exercise despite strong verbal encouragement; and (v) HR greater than 90% of the predicted maximum (220 bpm age) [23,28,31]. We calculated the percentage of tests that validated each criterion. The Kolmogorov–Smirnov test was used to investigate data distribution, and it con rmed that all variables were normally distributed. Thus, the Student t test for paired values was performed to compare the maximal data obtained during the IncT and Tlim. For all statistical comparisons, statistical signi cance was established asp< 0.05. 3. Results Results of maximal data during the IncT and Tlim are presented in Table. During the IncT test, 16 athletes (80%) reached a plateau of . VO2 , 19 athletes (95%) obtained RER values higher than 1.10, and all athletes obtained[La]over 8.0 mmol L 1 andHRmax 90% of the predicted maximum value. This means that 16 athletes (80%) ful lled the four pre-determined maximal criteria, 19 ful lled more than three criteria (95%) and all athletes ful lled more than two criteria. Table 1.Physiological values in IncremenTrail (IncT) and time limits exercise (Tlim) tests (n= 20). IncT Tlim pValue Test duration (s) 943 92 162 68 * 0.0012 . VO2max(mL kg 1 min 1 ) 65.6 7.6 65.5 9.0 0.83 Duration of . VO2maxplateau (s)58.5 39.2 58.3 29.9 0.79 HRmax(b min 1 ) 185 11 183 10 0.63

IncremenTrail (IncT) and time limits exercise (Tlim) tests (n= 20). IncT Tlim pValue Test duration (s) 943 92 162 68 * 0.0012 . VO2max(mL kg 1 min 1 ) 65.6 7.6 65.5 9.0 0.83 Duration of . VO2maxplateau (s)58.5 39.2 58.3 29.9 0.79 HRmax(b min 1 ) 185 11 183 10 0.63

Sports2022,10, 174 5 of 8 Table 1.Cont. IncT Tlim pValue . VEmax(L min 1 ) 170.4 20.3 171.9 23.1 0.72 BF (cycles min 1 ) 55 7 57 7 0.68 [La](mmol L 1 ) 17.1 3.5 15.9 2.6 0.41 RER 1.12 0.02 1.13 0.02 0.55 RPE 8.1 0.6 8.6 0.6 0.21 Data are presented as mean SD; * Signi cant difference between IncT and Tlim exercise,p< 0.05; . VO2max , maximal oxygen consumption;HRmax, maximal heart rate; . VEmax , maximal minute ventilation; BF, breathing frequency;[La], blood lactate concentration; RER, respiratory exchange ratio; RPE, rating of perceived exertion. Maximal values obtained during the Tlim were not statistically different from maximal data obtained during the IncT (Table). VT1 and VT2 were successfully determined by the three operators during the IncT for every subject and the corresponding values are reported in Table. Table 2. Physiological values corresponding to the ventilatory thresholds (VT1, VT2) in IncremenTrail test (n= 20). VT1 VT2 . VO2(mL kg 1 min 1 ) 39.8 8.2 57.4 7.9 . VO2(% . VO2max) 60.4 9.3 87.6 7.6 HR(b min 1 ) 143 19 173 12 HR(% HRmax) 77.1 7.1 93.6 3.1 AS (m h 1 ) 1115 206 1565 200 AS (% ASmax) 61.1 8.7 86.0 8.0 . VE(L min 1 ) 67.1 14.8 117.0 19.1 BF (cycles min 1 ) 26 6 37 8 Data are presented as mean SD; VT1: rst ventilatory threshold, VT2: second ventilatory threshold, . VO2 : oxygen consumption volume,HR: heart rate, . VE : minute ventilation, BF: breath frequency, AS: ascending speed. 4. Discussion It is well known that the determination of . VO2max is highly in uenced by the mode of testing (i.e., treadmill, cycle ergometer, rowing ergometer, etc.) [28]. For a given locomotion, the characteristics of the test (duration of the step, increment in intensity) can also alter the different determined parameters at submaximal (ventilatory thresholds), and maximal levels. Previous studies already demonstrated that . VO2max is in uenced by the gradient slope used during an incremental running test [17,18,27,32]. As TR is characterized by uphill and downhill sections and triggers speci c

The IncremenTrail test evaluates aerobic fitness in trail runners under specific uphill conditions.