###### Abstract  
Background: Understanding the responses of the cardio and respiratory systems during exercise, as well as their coupling in post-exercise recovery, is important for the prescription of exercise programs in physically recreative subjects. Aim: In this work, we aimed to set up an adjusted experiment to evaluate the relations and changes in parameters obtained from an analysis of cardiac and respiratory signals under three physiological conditions: relaxation, exercise, and post-exercise recovery. Material and Methods: Simultaneously recorded ECG (RR intervals) and respiratory signal during relaxation, bicycle ergometry exercise until submaximal heart rate (HR), and recovery in 10 healthy men were analyzed. The exercise included consecutive phases of 3 min in duration with a constant workload. Parasympathetic cardiac control (RMSSD), heart rate (HR), breathing frequency (BF), and respiratory cycle amplitude (RCA) were calculated. Anthropometric data were also collected. Results: Based on time series analysis, our results show that: (1) during exercise, an increase in HR was related to a reduction in HR variability and RMSSD, while an increase in BF was related to an increase in RCA, and (2) during recovery, HR and RCA significantly decreased, while RMSSD had a biphasic response. The results of multiple linear regressions showed that the averaged HR, RMSSD, and BF during 3 min segments of recovery were determined by several calculated and collected parameters. Conclusions: The parameters from the analysis of respiratory signals and RR interval time series under conditions of relaxation and exercise, along with anthropometric data, contributed to the complexity of the post-exercise recovery of cardiopulmonary parameters after submaximal HR exercise in healthy recreative males. b 20  
the complexity of CRC interactions is reflected in the non-linear dynamics between the systems, and these various forms of coupling can coexist at different time scales [1; 2]. Nowadays, it is clear that the regulation of cardio-respiratory coupling has consequences, not only on the level of blood gases and maintaining homeostasis, but also on other central functions such as arousal [3]. One of the areas of intensive research in quantifying CRC is the area of sports medicine. Exercise, which, according to [PERSON] et al. [4], represents \"planned, structured, repetitive, and purposive physical activity\", is an integral component of both sports and recreational activities. It has been shown that physical exercise contributes to increasing the ability of oxygen consumption and to a more economical cardiac response during exercise at submaximal heart rate intensity [5; 6; 7]. These aforementioned factors can consequently impact improvements in overall human work capacity, particularly through the enhancement of two vital systems whose interaction is not sufficiently researched, especially in exercise and recovery. In general, it is well established that physical activity affects cardio-respiratory coupling. It is shown that highly intensive training leads to an increase in CRC strength. However, it has still not been clarified whether this increase in CRC strength in physical activity has its origin in enhanced mechanics or enhanced neural circuits between the cardiac and respiratory centers in the medulla in trained people [1; 8]. Furthermore, inconsistent results are obtained, because various studies employ different protocols and modalities of physical activity, as well as its duration and intensity. Also, body posture in experimental protocols should not be neglected [9]. Little is known about the correlations between cardiac and respiratory parameters in different conditions: at rest, in exercise, and during post-exercise recovery. It is questionable as to what extent the resting heart rate and parasympathetic activity affect changes in both the cardiac and respiratory systems during exercise, i.e., to what extent they do not depend only on ongoing physical activity.