Procedures
Swimmers warmed up with some flexibility exercise on land and 800 m swimming at lower intensity. Then, they finished it with one maximal 25 m front crawl swimming. Following warming up, swimmers performed maximal 200 m front crawl swimming. Each swimmer swam alone. They were instructed to swim as fast as possible without any predefined swimming strategy. They chose their own patterns of velocity, SR and BF during swimming. The swimming test was performed in a 25 m indoor pool with a water temperature of 27^oC.

Measurements
Swimming time per each 25 m distances was measured by using digital CASIO stopwatch (Casio Electronics Co., London, United Kingdon). The swimming test was filmed from the side using video camera (DCR-TRV 410E, PAL standard recorder, Sony, Tokyo, Japan) operating at 25 Hz. Velocity for each 25 m was calculated by dividing the time with 25 m. The measures of stroke parameters were taken from videotapes. The elapsed time for five complete one arm stroke cycles during about 12 m section of each pool length was measured to calculate SR (stroke cycles × s^-1). BF was calculated by dividing the number of breaths with the time, which were both measured during the swimming test. SR and BF were measured for each 25 m.

Measures of blood parameters included lactate concentration ([LA]), partial pressure of blood O₂ (Po₂) and CO₂ (Pco₂) and pH before and during the first and third minute after the swimming test. Capillary blood samples (60 - 80 l) were taken by micro puncture from a hyperemied earlobe. Blood samples for measuring LA were diluted in a LKM41 lactate solution (Dr. Lange, Berlin, Germany) and analysed using the MINI8 (Dr. Lange, Berlin, Germany) photometer. Blood samples for measuring Pco₂, Po₂, and pH were collected in heparinized glass capillaries and introduced into the blood gas analyser ABL5 (Radiometer, Copenhagen, Denmark).

Observations
Different swimming strategies during high intensity swimming may influence on metabolic parameters in a different manner (Thompson et al., 2004). Considering that, the subjects within each group were divided into sub-groups according to their pattern of velocity. The individual data of velocity for every 25 m were plotted against distance. The plot velocity/distance for each subject was examined visually to select subjects with a similar pattern of velocity within each group. The main criterion for group selection was a change (increase or decrease) of velocity at final 50 m. After this selection, the majority within each group represented the CS and RS group, respectively. The swimmers in each testing group who used a different pattern of velocity (the CSa and RSa group; as mentioned in the following chapter, the swimmers in these groups increased their velocity during the final 50 m) were analyzed and compared with the CS and RS group separately.

Calculations
In order to describe the changes of velocity, SR and BF during the swimming test the data of these parameters were fitted by linear interpolation for each subject. Considering that velocity and SR significantly changed after the first 50 m during 200 m front crawl event (Sidney et al., 1999) the linear regression model was used without the data measured at the first 25 m. Therefore the pattern of velocity was described individually by using a linear model:

where v_s is velocity at distance s (v₅₀ is velocity at 50 m distance), b represents the slope coefficient of regression line or rate of velocity decrease (∆v/s) and s is the selected distance (m). The same method was used for estimating the patterns of SR and BF.

The rate of [LA] increase that occurred during the swimming test (RLA) was additionally calculated for each subject by using the following equation:

(Equation 2)

where ∆[LA] is the difference between [LA] measured during the first minute after and before the swimming test and t is the time (t) needed for completing the swimming test. The result was expressed as a change of [LA] per minute (mmol·l^-1·min^-1). The same method was used for calculating the rate of pH decrease that occurred during the swimming test (RpH).

Statistical analyses
The results were presented as average values and standard deviations (SD). Normal distribution for each variable was tested using Kolmogorov Smirnov test. The independent t test was used to compare the data between groups. For statistical analysis of blood parameters the ANOVA for repeated measurements was carried out using swimming level (former competitive - recreational) and swimming test (before - during the first minute after) as factors (between and within factors, respectively). The Pearson's coefficient was computed to test linear correlation between SR and BF during the swimming test. A 95% level of confidence was accepted for all comparisons. All statistical parameters were calculated using the statistics package SPSS (SPSS Inc., Chicago, United States of America) and the graphical statistics package Sigma Plot (Jandel, Tübingen, Germany).

Most of the former competitive and recreational swimmers continuously decreased their velocity during 200 m distance. These swimmers made up the CS group (ten former competitive swimmers) and the RS group (six recreational swimmers). However, two former competitive swimmers and three recreational swimmers dramatically increased their velocity during the final 50 m. Therefore, these swimmers made up the CSa and RSa group, respectively. Because of an acceptable number of subjects only the data of the CS and RS group were statistically analysed in full.

After the selection, the pattern of velocity became more homogenous: from the first 50 m, where velocity was the highest (except in the starting phase), it continuously decreased to the end of swimming (Figure 1). There were significant differences in velocity at every 50 m between the CS and RS group.

When individual data of velocity were fitted by a linear model, then v50 showed clear difference between both groups (Table 1). However, the rate of velocity decrease through the swimming test was similar in both groups.

There were no significant differences in SR during maximal 200 m front crawl swimming between the CS and RS group (Figure 2). Furthermore the patterns of SR did not differ between both groups (Table 1).

There was a significant difference in BF50 between the CS and RS group (Table 1). All swimmers in the RS group maintained their BF close to steady fluctuations or they increased it very slightly throughout the swimming test (Figure 3). On the contrary, there was much variability in the pattern of BF in the CS group (as shown in Table 1 there was a high standard deviation in ∆BF/s calculated at this group).

A comparison of BF and SR during the swimming test showed a clear difference between both groups (Figure 4 on the left and on the right). Significant correlation existed between SR and BF in the RS group (r = 0.89, p < 0.01). On the contrary, BF independently changed from SR in the CS group (r = 0.01, p > 0.05).

Parameters of blood-gas (Po₂ and Pco₂), acid-base status (pH) and lactate concentration ([LA]) obtained before and in the first and third minute after the swimming test are given in Table 2 (the CS group) and Table 3 (the RS group).

As shown in Table 2 and Table 3 Po₂ and Pco₂ did not change significantly during the swimming test. There were no significant differences between both groups in these two parameters. During the swimming test [LA] and pH were significantly increased and decreased (p < 0.01), respectively. Changes reached similar values at both groups. However, these similar changes occurred at different velocities, therefore at shorter periods of time in the CS group. According to this, the RLA was significantly higher (4.9 ± 0.9 mmol·l^-1·min^-1) in the CS group than in the RS group (4.0 ± 0.6 mmol·l^-1·min^-1) (p < 0.05) (Figure 5 on the left). On the contrary, there were no significant differences in RpH between groups (-0.08 0.03 min^-1 in the CS group and -0.09 ± 0.02 min^-1 in the RS group) (Figure 5 on the right).

The data obtained from the CSa and RSa group are presented in the Table 4. These data were not statistically analysed by inferential procedures. Two swimmers in the CSa group decreased their velocity during the first 100 m, but they dramatically increased it at the end of the swimming test. According to final results, they were the fastest swimmers. They had a similar BF during the swimming test as swimmers in the CS group. They also increased their BF throughout the swimming test in a similar pattern as other former competitive swimmers. However, they swam with the lowest SR. Despite of the highest velocity they maintained their blood gas parameters within the same limits as their colleagues in the CS group (Table 4). Acid-base and [LA] parameters changed in a similar manner and magnitude as in the CS group. Three swimmers in the RSa group decreased their velocity through the first 100 m and then increased it at the last 50 m. However, their average velocity was not higher than in the RS group. They swam with lower SR and BF then the RS group. Their Pco₂ showed a clear tendency to be higher after the swimming test then before it (Table 4).

The aim of the present study was to ascertain whether maximal 200 m front crawl swimming strategies and breathing patterns influenced blood gas and acid-base parameters in a manner which gives advantage to former competitive swimmers in comparison with their recreational colleagues. As expected, the CS group swam maximal 200 m front crawl swimming significantly faster than the RC group (Table 1). There were no significant differences in SR during the swimming test between both groups (Figure 2). However, high standard deviations calculated in this stroke parameter at both groups showed large variation between swimmers in the pattern of SRduring the swimming test. Some swimmers decreased it, in others it remained constant and for a few others, SR increased as the swimming test progresed. Results of some previous studies (Chatard et al., 2003; Kjendlie et al., 2006; Sidney et al., 1999) also did not show consistent pattern of SR during different competition races. In addition, swimming velocity is product of SR and stroke length. Considering that, it could be assumed that the CS group had longer stroke length than the RS group. According to Keskinen, 1993, this could be primarily due to higher swimming efficiency in the CS group in comparison with the RS group. However, this could be only an assumption, considering that clean velocity was not calculated in present study. The measurements of velocity should be taken over a distance of 15 m in order to ignore the changes in velocity at start and turns.

Despite a higher velocity former competitive swimmers (the CS and CSa group) swam with a lower BF in the first half of the swimming test then the RS group. In addition, they increased their BF in the second half of the swimming test probably to maintain the breathing dependent blood gases in a certain range where they did not stimulate the breathing too intensively. These results were in accordance with the results of Strumbelj et al., 1999 who also obtained similar Pco₂ values before and after maximal 400 m front crawl swimming at competitive swimmers. Therefore, in contrast to expectations, the former competitive swimmers maintained their parameters influenced by breathing successfully in the certain range, away from hypercapnia and/or hypoxia despite of maximal intensity swimming. They achieved better regulation of blood gas and pH parameters by more dynamic regulation of velocity and breathing. The RS group seemed to reach a similar goal too, however, at lower swimming velocities. The rate of velocity decrease through the 200 m distance was similar in the CS and RS group. However, three recreational swimmers (the RSa group) used a different pattern of velocity as in the RS group. Swimmers in the RSa group increased their velocity during the final 50 m; however, they were not the fastest among recreational swimmers. Furthermore, the RSa group also swam with the lowest SR and BF with clear consequences: their Pco₂ changed more dramatically and differently as in other groups despite their similar pH and [LA]. It seemed that they were not adapted for a precise and dynamic regulation of breathing during maximal effort. In trying to answer the question whether the former competitive swimmers would show any advantage to recreational swimmers according to their breathing it seemed that the former competitive groups of swimmers (the CS and CSa group) developed a more precise and dynamic regulation of breathing during maximal 200 m front crawl swimming than the recreational group of swimmers (the RS and the RSa group). Former competitive swimmers started with a low BF because they did not need to breathe frequently. However, they increased their BF dramatically when they needed to. They acquired this pattern of BF through many years of intense training and competitions. On the contrary, recreational swimmers did not show this pattern. Their BF matched their SR (Figure 4 on the right). This pattern enabled a successful regulation of blood gases in the RS group. Furthermore, swimmers in the RSa group used the lowest BF without additional corrections during the swimming test. These conditions probably resulted in an increase of their Pco₂. Cardelli et al., 1999 obtained similar differences in pattern of BF during maximal 100 m front crawl between more and less expert swimmers. They suggested several physiological and biomechanical interpretations for such conditions. However, we argued against their conclusion that avoiding too strong oxygen deficit is the reason for earlier use of a high breathing frequency at less expert swimmers in comparison with more expert swimmers. Results of the preset study suggested that elevated Pco₂ rather than lower Po2 was the main stimulus for higher breathing frequency during the swimming test at the RS group than at the CS group.

In our study the high velocity was accompanied by an increased [LA] and decreased pH during the swimming test. The values of [LA], measured during the first and third minute after the swimming test, indicate the maximality of the swimming exercise. These results were close to results of Bonifazi et al., 1993 who measured [LA] after competition. Between the CS and RS group there were no significant differences in the values of [LA] and pH measured after the swimming test as well as in the changes of these parameters during the swimming test. However, the changes of [LA] and pH during the swimming test happened in a shorter period of time in the CS group in comparison with the RS group. Considering that, we also compared the rates of [LA] increase and pH decrease between both groups. The rates of [LA] increase were higher in the CS group and lower in the RS group. However, the rates of pH decrease were similar in both groups. This indicates that the CS group had probably more sensitive, dynamic and powerful regulation of the bicarbonate buffering system. This regulation mainly depends on breathing. This seemed to be one of advantages of the CS group.

Additionally, the pattern of BF at the CS group (initially low and increased during the swimming test) showed a probably better integrative adaptation of this group to higher swimming velocities by using several mechanisms, as follows. First, lower BF during swimming is partly compensated with larger tidal volumes (Peyrebrune et al., 2003; Town and Vanness, 1990; West et al., 2005). Larger tidal volumes may represent a larger initial reserve for increasing pulmonary ventilation during maximal swimming. This was probably realized during the second half of the swimming test by using a higher BF at the CS group. We assume that former competitive swimmers (the CS and CSa group) can achieve this goal at faster swimming because of a larger vital capacity of lungs and consequently larger tidal volume during swimming. Second, a lower BF during swimming may have some biomechanical advantage for a swimmer's performance. Lerda et al., 2001 analysed the interactions of breathing and arm actions in the front crawl. They found that breathing while swimming increased the discontinuity in the propulsive action of the arms. This greater continuity, in addition to the improved gliding position of the body obtained in a front crawl without breathing could result in greater swimming efficiency by reducing energy cost (Chatard et al., 1990) and hydrodynamic resistance (Kolmogorov and Duplisheva, 1992). These facts could enable faster swimming when lower BF is used (Pedersen and Kjendlie, 2006).

These were characteristics that may represent possible advantage factors of the CS group. However, it can not be concluded that the RS group is not adequately adapted for front crawl swimming. The RS group was training for at least five years, never more than three training sessions per week. The main goal of their training was to improve their swimming technique. Therefore, they trained at a lower intensity of swimming without any reduced breathing frequency and preparations for competitions. Consequently, they became adapted to such a type of training and not to maximal effort during 200 m front crawl swimming. On the contrary, the CS group was better adapted to maximal effort during 200 m front crawl swimming. Their training program often included a higher and maximal intensity of swimming and preparations for competitions. They also frequently trained with reduced breathing frequency. This kind of training (also often referred to as hypoxic training or controlled breathing frequency training) adapts swimmer to swim with fewer breaths (Kapus et al., 2005). Adaptation to hypercapnia and respiratory acidosis could also be the result of such training (Dicker et al., 1980; Kapus et al., 2003; Town and Vanness, 1990).

Presented differences between both groups were mostly related to more powerful energetic processes and to more dynamic and precise regulation of breathing in the CS group. This adaptation could be realized only by an intense and long period of swimming training at selected (not all) subjects. Therefore, this was the reason why there were age differences between both groups in the present study. For a swimmer it is not possible to reach the international level and to adapt in such a manner that corresponds to the CS group in a short period of time of training. On the contrary, the adaptations characteristic of the recreational level can be obtained in a much shorter time.

• Training programs for competitive swimmers should promote adaptations to maximal efforts.
• Those adaptations should include high and maximal intensity swims with controlled breathing frequency (taking breath every fourth, fifth, sixth or eighth stroke cycle for front crawl swimming).
• Such training will improve breathing regulation in order to impose a better synchronization between breathing needs and breathing response during maximal swimming.