Determinants of cardiorespiratory fitness in amateur male cross-country skiers

The aim of the study was to present the performance parameters, their assessment and their predictors in the group of amateur cross-country skiers at the beginning of the preparatory period. For this purpose, incremental exercise tests were carried out on the treadmill. Body composition measurement was performed using impedance analyzer. The main findings of the study were that the most correlated with VO 2 relative (max) were percentages of body fat, running speed (maximum) (km/h), running speed (final) (km/h), ventilation (L/min) and physiological cost of running (ml/kg/km). The appointment of the predictors can be an effective tool to assess the efficiency of this group and prepare a proper training plan for them.

Sports training is a long-term process. It is focused on achieving the maximum capacity of the body. This applies to all aspects in the mental and physical sphere of a sportsman. They are caused by numerous adaptation mechanisms. They occur in the cardiovascular, muscular, nervous, hormonal and an immune system. The weight and composition of the body are also important. They are most visible in athletes of endurance disciplines (for example: cross-country skiing, triathlon, cycling etc.). Monitoring and evaluation of physiological changes is essential in the training process, also in nonprofessional athletes.
The aim of the study was to determine and evaluate the baseline of cardiovascular capacity in amateur athletes who have started in long distance races. In addition, predictors for maximum oxygen uptake were determined.
It is an indicator of maximum exercise capacity. Systematic physical activity can protect such people against an early heart attack and civilization diseases such as obesity and overweight with their consequences. On the other hand, long distance races are becoming more and more popular among amateurs. This is easily seen in the steadily increasing number of participants in such competitions.
Cross-country skiing is an endurance effort that involves almost all muscle groups and requires sustained endurance strength training preparations. Adjusting the right training plan is therefore the key to achieve the goals in the base-building period. It is also the main factor that can protect against excessive strain on the body by physical effort. The level of efficiency of highly qualified athletes is well known and described in the literature, but there is still lack of information about these parameters at amateur athletes. Due to the specificity of sport and physical effort, as well as trends in society, the issues raised are extremely important. Setting predictors for maximum oxygen uptake can give important data for trainers, nutritionists and physicians cooperating with such persons. This paper takes into consideration those issues and presents the results of the ergospirometry test and the evaluation of the results of the cross-country skiers after the recovery period, at the beginning of the macrocycle, and their predictors.

Material and methods
The study was conducted in accordance with the guidelines of Good Clinical Practice and the Helsinki Declaration. The test report was approved by the Bioethics Committee. All participants submitted a written consent for research. The research group consisted of 16 welltrained amateur skiers. The competitors worked professionally in a big city and could spend up to 90 minutes daily for physical training.
The research was carried out at the end of the recovery period in May. The goal was to determine and evaluate the parameters of circulatory-respiratory fitness and predictors of maximum oxygen uptake. The criteria for inclusion in the study were: consent to participate in the study, possession of current medical approval, completion of at least three long distance races in the last season. Exclusion criteria were: lack of consent for participation in the study, poor health, lack of medical consent.
Body weight and the weight were measured on Tanita Body Composition Analyzer BODY IN MC-980 MA consisting of an 8-point touch electrode system. The test was carried out just before the incremental exercise tests began. The following were determined: body weight, water content, minerals, vitamins, fat content in the body (% and kg) and slim mass muscle (muscle mass in % and kg), WHR (waist to hip ratio) and BMI (body mass index). Body weight and body composition are important in endurance sports. They have a significant impact on the level of maximum oxygen uptake, which is the best measure of the ability of endurance athletes (especially content of adipose tissue and body muscle). Changes toward lower values of body fat are characteristic of a healthy inf luence of endurance training. Detailed data characteristics of the study group are shown in Table 1.
Incremental exercise test. To assess the aerobic capacity expressed by the level of maximum oxygen uptake (VO 2 max) an incremental exercise test was used with gradually increasing intensity. This test was performed on a treadmill using HP Cosmos CPET equipment and Cosmed Quark/k4B2. The test started at speed of 6 km/h and 0% treadmill inclination. Then every 3 minutes, the speed was increased by 1 km/h, and the inclination by 1%. The test was continued until the subjective feeling of exhaustion by the competitor (to refuse). The frequency of heart contractions at rest and during exercise was recorded using the Garmin ANT+ heart rate monitor. The paper presents below the maximum results of the test.
Statistics. Variables were analyzed using basic descriptive statistics: number of persons (N), arithmetic mean, median, minimum (Min), maximum (Max) and standard deviation (SD). The Pearson correlation coefficients were used, whose values -in the case of Note: all data are presented as means ± standard deviation and the difference (Δ -delta).
Abbreviations: BMI -Body Mass Index, N -number of patients, SD -Standard deviation. statistical significance -can be interpreted as follows in Table 2. The value of 0.05 was assumed as the significance level (denoted by * p<0.05, ** p<0.01, *** p<0.001).
The calculations were made in the statistical software (ver. 3.6.0).

Results
The Table 3 shows results of descriptive statistics for maximum capacity during the treadmill exercise test in the group of athletes.
Correlations for independent variables. The VO 2 variable relative (max) had significant correlations with 7 variables. Most of these correlations were moderately strong or strong, and positive. The results are shown in Table 4.

Discussion
The main findings of the study were that the most correlated with VO 2 relative (max) were percentages of body fat, running speed (maximum) (km/h), running speed (final) (km/h), ventilation (L/min) and physiological cost of running (ml/kg/km).
The studies of body composition showed that the average value of this parameter was 78.52 kg and the adipose tissue content was at the level of 15.5% (12 kg). The level of body fat was at medium level. The desirable range of adipose tissue content ranges for amateurs from 11.0-14.00% (rated as good) to 20.0% (rated as acceptable). Elite athletes have significantly lower score. In previous research [1] we noted lower fat mass 13.85%. A reduction in body fat in the body by about 3-5% could improve the efficiency of movement and increase VO 2 max. Too high body weight has an adverse effect on the musculoskeletal system. This increases the possibility of the occurrence of overload injuries. By reducing body mass and accordingly by reducing body fat, the level of oxygen intake per kilogram of body weight increases. This translates into better exercise capacity. The decrease of this parameters are desirable. This was also confirmed by the results of correlations, in which fat tissue was the strongest variable for VO 2 max.
Cross-country (XC) skiing is one of the most demanding sport disciplines. It involves protracted competitions on varying terrain employing a variety of skiing techniques that require whole body work to different extent. Sportspecific peak aerobic power (VO 2 peak) is one of the main determinants of performance in sprint and distance crosscountry skiing [2]. Several studies [3,4] showed that world-class skiers are among the endurance athletes with the highest VO 2 max. It has been associated in both sex with maximal values above 70 and 80 ml/kg/min or 4.0 and 6.0 l/min.
Other scientists [2] monitored physiological differences between sprint and distance-specialized cross-country skiers. In this research the elite male sprint skiers showed different anthropometric and physiological qualities than the distance skiers, with these differences being directly related to body mass (with sprinters weighting more than distance skiers). The elite distance skiers had higher VO 2 max than our amateur (83.0±3.2 vs 48.37±5.06) and they were lighter (71.8±7.2 vs 78.52±6.18 kg) and shorter (178±7 cm 181.44±6.53). They BMI was smaller than in amateur (22.5±1.3 kg/m 2 vs 23.84±1.35). But in the older groups of long-distance cross-country skier the results were lower than in other elite athletes, but still higher than in our research (VO 2 max: 64±5 vs our result 48.37±5.06) [5]. In other studies [6] elite cross-country skiers were younger (aged 25±4 years) and they had a higher VO 2 max (65±4 ml/kg/min). The highest values were recorded at Note: all data are presented as means±standard deviation and the difference (Δ -delta).
Abbreviations: HRmax -maximum heart rate, VO 2 max -maximal oxygen uptake, n -the number of patients, SD -standard deviation. one Norwegian skier (7.48 liters per minute, which was 94 ml per kilogram of body weight [7]), but they were similar to results of best professional marathon runners and cyclists. We should emphasize that the differences are noted between specialized long-distance versus all-round skiers. The first group was characterized by lower VO 2 max, but they had a reduced oxygen cost [8].
The age in our group (38.69±7.95) was significantly higher than among the best long-distance skiers. According to Knechtle and Nikolaidis [9] the age of peak performance of cross-country skiers competing in long distance races was much lower and closer to the peak of aerobic capacity than what was found in previous studies in marathon road runners. In our study (VO 2 max) it was 48.37 ml/kg/min and 3.83 l/min. These results were lower than those reported among professionals in all disciplines and specializations, but higher than those found in the population.
The previous results showed that exercising at very vigorous intensity may be beneficial for oxygen uptake [10]. In person who trained systematically (but not as a professional athlete) the VO 2 max level was 47.5 ml/kg/ min. In group without impact of exercise VO 2 max was lower (40.1 ml/kg/min). In our research the average VO 2 max was similar to people who reported 75-149 min activity per week.
WHO guidelines on pro-health activities indicate the desired level of VO 2 max as 35 ml/kg/min. In this case, the athlete substantially complies with these guidelines. The high level of maximum oxygen uptake supports the circulatory-respiratory system, quality of life and health protection. This is particularly important in older people who have sarcopenia and the problem of overweight and obesity. It is estimated that every year the level of maximum oxygen uptake decreases about 0.5-0.6 ml/kg/ min. In active people that decrease is smaller. Parameters measured in the study were higher, which may affect the state of health. It is worth emphasizing the three-year risk for mortality based on achieved VO 2 max. Patients who achieve a VO 2 max greater than 17 ml/kg/min have an associated three-year risk of about 20%, significantly lower than in patients who achieve a peak value less than 14 ml/kg/min.
In the group VO 2 max level was significantly higher than in population. This could indicate the healthpromoting effect of such a lifestyle. In recent decades, methods of cardiac rehabilitation have been widely used in patients after acute myocardial infarction, coronary syndrome and cardiac operations [11]. It is well known that short-term high-intensity interval training (HIIT) to moderate-intensity continuous training (MICT) have impact on improving cardiorespiratory fitness, markers of inflammation, and glucose control in previously inactive adults with elevated risk of developing type 2 diabetes [12]. That type of physical activity could be added to the program of patients' pre-rehabilitation programs before on-pump coronary artery bypass grafting (CABG) [13]. This result shows that cross country skiing could be successful physical activity in prevention this disease.
Maximum ventilation (VE), which is one of the indicators determining the efficiency of the respiratory system, was at the level of 144.32 l/min. These are mediocre values. The reason for this may be poor biomechanics of breathing, respiratory disease or too much body weight. The maximum rate of heart contractions (HRmax) was 183.38 bpm. This value is genetically determined. At this age, there are no statistically significant changes as a result of physical training. Prevention of cardiovascular disease has to be integrated into primary health care.

Predictors
The main findings of the study were that the most correlated with VO 2 relative (max) were percentages of body fat, running speed (maximum) (km/h), running speed (final) (km/h), ventilation (L/min) and physiological cost of running (ml/kg/km). To this moment no VO 2 max predictors in amateur sport have been determined. This makes their reference and assessment difficult. Nevertheless, this information suggests that to improve the athletes' level of sports, body fat should be reduced. This will significantly affect the cardiovascular capacity and increase the remaining parameters. This can be achieved mainly thanks to the use of aerobic effort and an appropriate, balanced diet and supplementation. In addition, these behaviors will affect the improvement and protection of health. This study took into account the results of body composition and maximum exercise parameters. In the future, they should be extended, for example, by analysis of morphology, acidity and performance parameters at the anaerobic threshold.

Conclusion
The results of the study present the cardiovascular capacity of amateur cross-country skiers and it's predictors. According to our knowledge, this paper is the first one in the literature that presents the assessment of such a group. Previously, only a case studies were presented.
In the literature, the attention is paid to improve mental tolerance of exercises under the inf luence of physical training, but only in professional athletes. Evaluation of cardiopulmonary resuscitation devices in middle-aged persons is the starting point in the preparation of competitors for repeated participation in long-distance races in the winter season and protection against civilization diseases. The use of systematic physical training results in lower adipose tissue content and a higher level of maximum oxygen uptake as compared to the recommended for the population. It also has a significant impact on increasing the efficiency of the cardiovascular and respiratory system and protection of the heart against early infarcts, as well as