Associate Professor, PhD Andrzej Klusiewicz
Józef Piłsudski University of Physical Education in Warsaw, Faculty of Physical Education and Health, Biała Podlaska, Poland

Corresponding author: andrzej.klusiewicz@insp.waw.pl

Abstract

Objective of the study was to determine the degree of respiratory muscle fatigue in relation to fitness level, and to determine the relationship between inspiratory muscle strength and exercise respiratory indices.

Methods and structure of the study. Elite rowers (n=10) and untrained healthy university students (n=28) performed progressive maximal tests. Respiratory indices were measured, including maximal inspiratory pressure (PImax) at rest and 3 min after exercise in both groups. In the literature, PImax is considered as an indicator of respiratory muscle strength.

Results and conclusion. In the group of rowers, significantly higher resting PImax (167±30 cmH₂O) was observed compared to the group of students (117±29 cmH₂O). However, after exercise, the percentage difference between resting and exercise pressures did not significantly differentiate between the groups. In both elite rowers and non-athlete students, respiratory muscle fatigue was recorded in about 60% of cases. There were no significant correlations between resting PImax and pulmonary ventilation, VO₂max, or oxygen equivalent; only in the group of students, respiratory muscle strength correlated significantly and positively with BMI. The present study confirmed that even a high level of fitness does not prevent respiratory muscle fatigue induced by a progressive maximal effort.

Keywords: elite rowers, healthy men, indices of inspiratory muscle function, response to exercise.

Background. The results of the studies published over the last several years have demonstrated that similar to other skeletal muscles, respiratory muscles are also fatigued during exercise [1, 9]. In general, the observed degree of respiratory muscle fatigue has been shown to depend on the testing effort used and the level of fitness of the study participants. A review of the literature suggests that symptoms of fatigue in these muscles may increase during both prolonged [11] and short-term high-intensity exercise [2]. In swimmers, the occurrence of respiratory muscle fatigue may take place even after swimming a single 200 m freestyle distance at maximum intensity [4].

Various testing procedures have been used to assess respiratory muscle fatigue. These included stimulation of the phrenic nerve, the use of electromyography (EMG), or measurement of maximal respiratory pressure [5]. Due to its non-invasive nature and simplicity, the evaluation of maximum inspiratory pressure (PImax) before and after exercise, considered an indicator of inspiratory muscle strength, is one of the most common methods [4, 12]. From the practical point of view, the problems of assessing respiratory muscle fatigue, which can reduce the exercise capacity of athletes, is particularly interesting. Therefore, there is a need for further characterization of the functional status of respiratory muscles after various types of exercise.

Objective of the study was to assess the degree of respiratory muscle fatigue depending on the level of fitness and to determine the relationships between inspiratory muscle strength and selected morphological and respiratory indices.

Research methods and structure. Subjects. The research involved 28 students of the University of Physical Education and 10 elite lightweight and heavyweight rowers (members of the U23 national team and the senior national team). The characterization of the study participants is presented in Table 1. The examinations of the rowers were conducted during the preparatory phase after obtaining the approval of the Research Bioethics Commission of the Institute for Sport, whereas those concerning the students were approved by the relevant Commission of the University of Physical Education.

Table 1. Basic morphological characteristics of students and elite rowers (mean±SD)

Exercise tests. The rowers performed a stepwise incremental test on the rowing ergometer (Concept 2, C model, Nottingham, UK). The exercise lasted 3 min and was interrupted by 30s intervals [3]. The first exercise load was 200 W in the lightweight and 220 W in the heavyweight rowers, increased in subsequent efforts by 30 or 50 W, respectively. The students performed an intermittent incremental stress test consisting of several trials lasting 3 minutes, with 1-minute intervals between the trials, on the Corival bicycle ergometer (Lode B.V., Netherlands). The power used in the initial trial was 50 W, and it was increased by 50 W in each successive trial to exhaustion.

Lactate concentrations (LA) were measured in the blood collected from the fingertip at rest, immediately after the subjects completed each trial, and 3 minutes after the entire test was completed, using the Super GL2 device (Dr Müller, Germany). During the tests, breath-by-breath (BxB) respiratory indices were continuously recorded using a MetaMax 3B ergospirometer (Cortex Biophysik GmbH, Germany) in students and a Vmax 29 series apparatus (Yorba Linda, CA, USA) in rowers.

Estimation of the maximal inspiratory mouth pressure (PImax). Measurements of PImax were performed at rest and 3 minutes after the test in both groups. This parameter, often used to estimate the strength of the inspiratory muscles, reflects the capacity to generate pressure by the combined maximal activity of these muscles during a short, almost static, contraction with the nearly total closure of the airways [3].

According to the procedure described earlier by other authors [8], all subjects performed 10 (minimum) to 15 (maximum) technically satisfactory breaths and the three highest measurements with less than 5% variability were regarded as maximum. The initial position of the inspiratory muscles was checked at the beginning of each effort with the residual volume (RV). All the tests were performed in the standing position. To attain maximal values, verbal encouragement was given to the tested subjects, who received visual feedback informing them of the applied inspiratory pressures. The measurements were recorded electronically utilizing the Lungtest 1000 software (MES, Kraków, Poland).

Statistical analysis. The statistical analyses were conducted with the Statistica 13.0 software. The normal distributions of variables were examined using the Shapiro-Wilk test. The U Mann-Whitney test was used to assess the relevance of differences. The strength of relationships between the variables was determined based on Spearman’s rank correlation or Pearson coefficients. The level of statistical significance was set at p ≤ 0.05.Results and discussion. Significantly higher resting PImax (167±30 cmH₂O) levels in the group of rowers were observed compared to the group of students (117±29 cmH₂O). However, the post-exercise percentage difference between resting and exercise pressures (PImax Δ) did not significantly differentiate between the groups, with -3.6±6.9 and -1.5±9.6%, respectively, Table 2. In both elite rowers and non-athletes students, respiratory muscle fatigue (negative PImax Δ levels) was recorded in about 60% of cases. No significant correlations were found between the levels of resting PImax and: pulmonary ventilation, VO₂max, and oxygen equivalent. Only in the group of students, respiratory muscle strength correlated significantly and positively with BMI, see Table 3.

Table 2. Test time, Peak power, maximal oxygen uptake (VO₂max), post-exercise lactate concentration (LA), and maximum inspiratory pressure (PImax) in groups of students and rowers (mean ± SD)

* - PImax Δ = (PImax Rest - PImax 3 min) / PImax Rest x 100

Table 3. Values of correlation coefficients between resting maximum inspiratory pressure (PImax Rest) and selected morphological and respiratory indices (PImax Δ - differences between resting and post-exercise values PImax, VE - pulmonary ventilation, VO₂max - maximal oxygen uptake, and VE/VO2max - oxygen equivalent) in groups of students and rowers

^* – p<0.05

Resting PImax values significantly differentiated between students and rowers and confirmed the beneficial effect of training on improving respiratory muscle strength. However, it should be noted that a reduction in post-exercise PImax was observed in ca. 60% of individual cases of both students and rowers. Similarly, Romer et al. [7] emphasized that respiratory muscle fatigue can also lead to reduced exercise capacity in highly trained cyclists. The high resistance to respiratory muscle fatigue demonstrated in individual cases is noteworthy, as the PImax increased by up to several percent after the completion of maximal exercise (in students, the highest increase was by up to 19.0%, whereas in rowers - by 6.3%). Similarly, in a study of moderately trained male individuals, the reduction in PImax following the shuttle run test to total failure averaged 8.0±7.5%, while individual cases showed a simultaneous increase of several percent in respiratory muscle strength [6].

There were also no significant relationships between respiratory muscle strength and breathing economy as assessed by oxygen equivalence. In previous studies, rowers with greater inspiratory muscle strength were characterized by improved breathing economy through increased tidal volume and decreased respiratory rate [12]. In our study, a significant positive correlation between resting PImax and BMI was found only in the group of students, which may point to a relationship between the inspiratory muscle strength and the total muscle mass.

In our study, no significant relationship was observed between inspiratory muscle strength and their susceptibility to fatigue (PImax changes Δ, %), (Table 3). In contrast to these data, some authors [6] have shown the presence of this relationship in moderately trained men. It was suggested that higher levels of inspiratory muscle strength may lead to lower relative force generation requirements during exercise and consequently reduce the occurrence of fatigue symptoms in these muscles.

Conclusion. The most significant finding of this study was that, regardless of the fitness level, respiratory muscle fatigue was observed after the test exercise in both study groups. The literature data [10, 12] and our findings presented here justify the need for the implementation of isolated respiratory muscle training in athletes as a method that can potentially reduce the occurrence of adverse effects of respiratory muscle fatigue.

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