Effects of Endurance Training Intensity on Pulmonary Diffusing Capacity at Rest and after Maximal Aerobic Exercise in Young Athletes

compared the effects of varying aerobic training programs on pulmonary diffusing capacity (TL CO), pulmonary diffusing capacity for nitric oxide (TL NO), lung capillary blood volume (Vc) and alveolar–capillary membrane diffusing capacity (DM) of gases at rest and just after maximal exercise in young athletes. Sixteen healthy young runners (16–18 years) were randomly assigned to an intense endurance training program (IET,n= 8) or to a moderate endurance training program (MET,n= 8). The training volume was similar in IET and MET but with different work intensities, and each lasted for 8 weeks. Participants performed a maximal graded cycle bicycle ergometer test to measure maximal oxygen consumption (VO 2max) and maximal aerobic power (MAP) before and after the training programs. Moreover, TL CO, TL NOand Vc were measured during a single breath maneuver. After eight weeks of training, all pulmonary parameters with the exception of alveolar volume (VA) and inspiratory volume (VI) (0.104 <p< 0889; 0.001 < ES < 0.091), measured at rest and at the end of maximal exercise, showed signi cant group time interactions (p< 0.05, 0.2 < ES < 4.0). Post hoc analyses revealed signi cant pre-to-post decreases for maximal heart rates (p< 0.0001, ES = 3.1) and improvements for VO 2max (p= 0.006, ES = 2.22) in the IET group. Moreover, post hoc analyses revealed signi cant pre-to-post improvements in the IET for DM, TL NO, TL COand Vc (0.001 <p< 0.0022; 2.68 < ES < 6.45). In addition, there were increases in Vc at rest, VO 2max, TL NO and DM in the IET but not in the MET participants after eight weeks of training with varying exercise intensities. Our ndings

hoc analyses revealed signi cant pre-to-post improvements in the IET for DM, TL NO, TL COand Vc (0.001 <p< 0.0022; 2.68 < ES < 6.45). In addition, there were increases in Vc at rest, VO 2max, TL NO and DM in the IET but not in the MET participants after eight weeks of training with varying exercise intensities. Our ndings suggest that the intensity of training may represent the most important factor in increasing pulmonary vascular function in young athletes. Keywords: aerobic training; pulmonary capillary blood volume; alveolar–capillary membrane; alveolar volume; NO/CO transfer Int. J. Environ. Res. Public Health2021,18, 12359.

Int. J. Environ. Res. Public Health2021,18, 12359 2 of 12 1. Introduction Pulmonary diffusing (or transfer) capacity for carbon monoxide (TLCO) from the alveoli to blood is used to determine the function of the alveolar–capillary membrane [1]. Measurements of TLCOcan predict arterial oxygen desaturation [2] and evaluate the prog- nosis of conditions such as emphysema and lung resection surgery [3]. The diffusing capacity of lungs is determined by the alveolar–capillary membrane diffusing capacity (DMCO) and pulmonary capillary blood volume (Vc) [4]. Exercise improves TLCOin chil- dren [5] and adults [6] due to increases in Vc and DMco [7,8]. Running increases ventilatory performance in children [9], suggesting that intermittent exercise enhances respiratory demand [10] and that endurance activity alters the properties of the lung alveolar–capillary membrane by improving alveolar membrane conductance [11]. However, the magnitude of training-related cardiopulmonary adaptation depends on both the intensity and duration of the exercise training programs [6]. The speci city of the training stimulus is related to the exercise modality used (endurance, strength or speed). Several studies report improve- ments in TLCOfollowing chronic exercise training in healthy adults [12]. Furthermore, it is generally accepted that high-intensity endurance training improves pulmonary function more than low-intensity endurance training [13]. In addition, linear relationships exist between DMCOmeasured at rest, aerobic capac- ity [14] and exercise performance in adults [15]. These ndings suggest that improvements in endurance performance (or aerobic capacity) should also improve TLCO. The effects of an intense aerobic training program on pulmonary diffusing capacity (TL) at rest and during maximal exercise in young athletes are unknown. Moreover, the mechanisms by which intense endurance training, compared to a moderate endurance training program, improves pulmonary diffusion capacity in young athletes are speculative. In fact, several studies involving young participants suggest that such effects are due either to improve- ments in the pulmonary exchange surface or to better vascularization of the pulmonary capillary bed [11,16,17]. The aim of this study was to investigate the effects of an intense endurance training program compared to a moderate endurance training program on the DMCOand Vc during an eight-week study in young athletic males.

suggest that such effects are due either to improve- ments in the pulmonary exchange surface or to better vascularization of the pulmonary capillary bed [11,16,17]. The aim of this study was to investigate the effects of an intense endurance training program compared to a moderate endurance training program on the DMCOand Vc during an eight-week study in young athletic males. We hypothesized that (i) an intense endurance training program, compared to the moderate endurance training one, could produce greater increases in TLCO, Vc and DMco and (ii) increases in these parameters (TL, Vc and DM) may be associated with increased pulmonary vascular development and lead to greater distensibility of the pulmonary circulation in young athletes. 2. Materials and Methods 2.1. Participants Sixteen healthy young male athletes (16–18 years old) participated in the study and were randomly assigned to two different training groups: an intense endurance training program (IET,n= 8) and a moderate training program (MET,n= 8). The participants were middle-distance runners recruited from an athletic center in Nabeul (Tunisia). All participants had been engaged in systematic training programs and in national compe- titions during the previous six years. The participants were non-smokers with normal vital capacities and no histories of cardiopulmonary diseases or allergies. A schematic representation of the experimental design is illustrated in Figure. An a priori power analysis (expected SD of residuals, desired power = 0.90 and alpha error = 0.01) was computed using GPower 3.1 software (Version 3.1, University of Dusseldorf, Germany) to simulate a statistically signi cant group-by-time interaction for TL, our primary outcome [18]. The analysis indicated that a total sample size of 16 would be suf cient to achieve medium-sized group-by-time interaction effects. Written informed consent for participation was obtained from each subject and their parents or guardians prior to the study after receiving verbal and written explanations on the risks and bene ts of the experimental protocol. The ethics committee of the Sousse Medical University (Tunisia) approved the study, which was in accordance with the latest version

their parents or guardians prior to the study after receiving verbal and written explanations on the risks and bene ts of the experimental protocol. The ethics committee of the Sousse Medical University (Tunisia) approved the study, which was in accordance with the latest version

Int. J. Environ. Res. Public Health2021,18, 12359 3 of 12 of the Declaration of Helsinki. The physical characteristics of the study participants at the time of inclusion are listed in Table. There was no difference between groups for any of these parameters at baseline. Figure 1.Experimental design. Table 1. Subject characteristics and maximal exercise performances of athletes at the start of the study (T1). Data are mean standard deviation (SD). IET group: intense endurance training group; MET: moderate endurance training group. IET Group (n = 8) MET Group p-Value (n = 8) Age (years) 17.0 1.0 17.1 0.6 0.76 Weight (kg) 64 2 65 2 0.46 Height (cm) 172 7 172 4 1 VO 2max (mL kg 1 min 1 ) 47.2 1.4 47.5 1.2 0.67 Maximum Work Load (w) 245 30 250 37 0.77 Heart Rate (beats/min) rest 63 2 64 2 0.35 max 197 2 196 2 0.32

Int. J. Environ. Res. Public Health2021,18, 12359 4 of 12 2.2. Procedures Baseline (T1) anthropometric data (height to the nearest 0.1cm and weight to the nearest 100 g) were collected using standard stadiometers (Seca™, Hamburg, Germany) and scales (Tefal, France). Maximal oxygen consumption (VO2max) and maximal aer- obic power (MAP) were determined using standard protocols with exercise performed on a bicycle ergometer (Monark cycle). The subjects performed unloaded cycling at 60–65 revolutions/min (rpm)for the rst minute after which the work rate was increased every minute according to the Cooper and Weiler-Ravell procedure until VO2max was reached [19]. Oxygen consumption (VO2) and carbon dioxide (VCO2) production were determined using a calibrated metabolic measurement system (MedGraphics CPX St Paul, MN, USA). The transfers of nitric monoxide (NO) and carbon monoxide (CO) were measured on the same day. Each participant performed three validated transfer measurements: two at rest (before exercise) and another at the end of maximal exercise. Exercise was performed similarly to the last standard protocol but with incremental increases in workload. The validity of the maneuver for the transfer measurement was rst checked by the participant performing the maneuver without hesitation, with his mouth tightly closed around the mouthpiece, and holding his breath steadily during the pre-set time. The validity was then checked by examining the trace depicting volume changes during the maneuver, i.e., the computer-generated trace should lack a pause during the fast inspiration, be at during the breath hold and be continuous during expiration. The results were considered valid if these criteria were met. All subjects were trained previously in these maneuvers. Transfers of NO (TLNO) and CO (TLCO) measurements were realized simultaneously during a single breath maneuver using an automated apparatus (Medisoft, Dinant, Namur, Belgium) following the latest ERS Guidelines [20]. DM and Vc values were determined from TLNOand TLCOvalues as previously described [21]. Since the reactivity of NO and hemoglobin was considered very high and its inverse negligible, TLNOwas considered equivalent to DMNO. DMCOwas deter- mined using the coef cient of proportionality (a) and the DM values of the two gases (aDMNO= aDMCO= 1.97 ) following Graham's

ERS Guidelines [20]. DM and Vc values were determined from TLNOand TLCOvalues as previously described [21]. Since the reactivity of NO and hemoglobin was considered very high and its inverse negligible, TLNOwas considered equivalent to DMNO. DMCOwas deter- mined using the coef cient of proportionality (a) and the DM values of the two gases (aDMNO= aDMCO= 1.97 ) following Graham's law. The reactivity of CO with hemoglobin at a PO2 of 110 mmHg was derived from the relationship published by Forster [22] in which measurements were carried out at physiological pH values [23]. No corrections were made for hemoglobin concentrations as Stam et al. [24] reported that corrections have a limited effect on TLNO, TLCOand DM values in healthy individuals [25]. The same tests (maximal oxygen consumption, maximal aerobic power and NO/CO transfer) were repeated 8 weeks later (T2). All resting tests and exercise measurements were performed using the same equipment, calibrated using identical methods and measured with identical laboratory techniques during the initial (T1) and follow-up (T2) tests. More- over, to minimize any effects of diurnal variation, the two testing sessions were conducted within 2 h at the same time of the day (Figure). Changes in alveolar volume (VA), pulmonary diffusing capacity for CO (TLCO) and NO (TLNO), membrane factor for CO (DMCO), lung capillary blood volume (Vc), inspiratory volume (VI) and residual volume (VR) were measured in resting subjects before (T1) and after the training programs (T2) and also immediately after maximal exercise in the IET (following intense endurance training program) and MET (following moderate endurance training protocol) groups. 2.3. Exercise Training Program Experienced coaches and sports scientists trained the IET and MET groups during the eight-week intervention period. Only the IET group followed a maximal training program while the MET group followed a moderate training program (Table). All athletes were free of injuries during the training and testing periods. Each training session was supervised, and heart rates were measured with portable heart rate monitors (Sport-Tester PE4000). Subjects did not participate in any other physical training activities during the

while the MET group followed a moderate training program (Table). All athletes were free of injuries during the training and testing periods. Each training session was supervised, and heart rates were measured with portable heart rate monitors (Sport-Tester PE4000). Subjects did not participate in any other physical training activities during the

Int. J. Environ. Res. Public Health2021,18, 12359 5 of 12 study. Details of the training programs are summarized in Table. Participants exercised 3 daysper week (Monday, Thursday and Saturday) during weeks 1 to 4 of the intervention and exercised 4 days per week (Monday, Tuesday, Thursday and Friday) from week 5 to 8. Each exercise session lasted for 90 min and started and ended with a 10 min stretching period. The sessions included running distances of 400 m, 600 m, 800 m and 1500 m that were separated by active recovery periods. All training sessions were performed on an athletic track (400 m). Training volumes (running distances) were identical in the two groups, but the intensities were different and determined according to maximum heart rate (HRmax): low intensity 60% of HRmax, moderate endurance training 70–80% of HRmax and intense endurance training 85–95% HRmax [26,27]. Training sessions were performed during the afternoon (3:30 to 5:00 p.m.). Athletes received detailed instructions on performing each series of protocols and were always supervised. Table 2.Training programs for intense endurance training group (IET) and moderate endurance training group (MET). Intense Endurance Training Group (IET) Moderate Endurance Training Group (MET) Weeks 1 to 4 Weekly Training Program Monday Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: moderate intensity 70–80% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: low intensity (aerobic exercises) 60% of HRmax Active recovery = 2–3 min Tuesday Recovery Recovery Wednesday Recovery Recovery Thursday Warm-up/Drills: 30 min Running 400 m/600 m/800 m: heavy endurance training 85–95% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 minRunning 400 m/600 m/800 m: moderate endurance training 70–80% of HRmaxActive recovery = 2–3 min Friday Recovery Recovery Saturday Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: moderate intensity 70–80% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: low to moderate intensity 60–70 of HRmax Active recovery = 2–3 min Sunday Recovery Recovery Weeks 5 to 8 Weekly Training Program Monday Warm-up/Drills: 30 min Running 400 m/600 m/800 m: moderate to

30 min Running 600 m/800 m/1000 m/1500 m: moderate intensity 70–80% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: low to moderate intensity 60–70 of HRmax Active recovery = 2–3 min Sunday Recovery Recovery Weeks 5 to 8 Weekly Training Program Monday Warm-up/Drills: 30 min Running 400 m/600 m/800 m: moderate to heavy endurance training 70–95% of HRmax Warm-up/Drills: 30 min Running 400 m/600 m/800 m: low intensity 60% of HRmax Tuesday Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m): moderate endurance training 70–80% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: low endurance training 60% of HRmax Active recovery = 2–3 min Wednesday Recovery Recovery Thursday Warm-up/Drills: 30 min Running 800 m/1000 m/1500 m: 85–95% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 min Running 800 m/1000 m/1500 m: 70–80% of HRmax Active recovery = 2–3 min Friday Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: moderate to heavy intensity 70–95% of HRmax Active recovery = 2–3 min Warm-up/Drills: 30 min Running 600 m/800 m/1000 m/1500 m: low intensity 60% of HRmax Active recovery = 2–3 min Saturday Recovery Recovery Sunday Recovery Recovery

The study investigates how different intensities of endurance training affect pulmonary function in young athletes.