# Respiratory Muscle Strength as a Predictor of VO2 max and Aerobic Endurance in Competitive Athletes
## 1 Introduction  
The capacity for proper respiration, which includes supplying enough oxygen to the working muscles and removing carbon dioxide, depends on the efficiency of gas exchange during both external and internal respiration [1; 2]. The gold standard for evaluating functional response through gas analysis is the cardiopulmonary exercise test (CPET), which helps identify functional and pathophysiological limitations [1]. This test is particularly important for scuba divers, pilots, soldiers, and professional athletes who experience high levels of physical stress. During physical activity, lung ventilation increases to meet the oxygen needs of the skeletal muscles and activates the accessory respiratory muscles alongside the diaphragm [3]. Respiratory muscle function depends on the strength and endurance of these muscles [4]. Some studies have shown that inspiratory muscles can become fatigued after short periods of intense exercise. This phenomenon underscores the significant physiological demands placed on the respiratory system during vigorous physical activity [5; 6] and after long periods of moderate-intensity exercise [7]. Also, during these exercises, the respiratory muscles share the amount of oxygen consumed [8; 9]. It has been determined that fatigue occurs in the diaphragm muscle with increased respiratory need during high-intensity exercises of 85% VO2 max and above. Also, during these exercises, the respiratory muscles share the amount of oxygen consumed [10]. The findings from previous studies mentioned above underscore the shared demand for cardiac output and oxygen consumption between active skeletal muscles and respiratory muscles during exercise. Optimizing respiration plays a pivotal role in enhancing the efficiency of working skeletal muscles by facilitating the accelerated delivery of blood to the relevant regions. This interplay highlights the interconnectedness of respiratory and skeletal muscle function in supporting overall exercise performance and underscores the importance of respiratory optimization strategies in maximizing athletic potential [11]. However, if the endurance of the respiratory muscles is not sufficient, it is predicted that exercise performance may decrease due to early fatigue of the diaphragm [11]. High-intensity exercise causes peripheral vasoconstriction [12]. In addition, it triggers the respiratory muscle metaboreflex, a high sympathetic nerve activity that limits blood flow to the working muscles and thus the energy output and consumption required for breathing [12; 13]. The demanding work of the muscles used for inhalation has significant effects on the nervous system and the heart. In healthy people, voluntary resistance during inhalation that leads to muscle fatigue has been found to cause increases over time in muscle sympathetic nerve activity, heart rate, and mean arterial pressure. This is accompanied by a gradual decrease in arterial blood flow to inactive limbs [14]. Research also suggests that the reflex triggered by the fatigue of the inspiratory muscles is activated during full-body exercise. Specifically, blood flow to the legs is inversely related to the effort of breathing during high-intensity exercise, and changes in leg blood vessel resistance are directly related to the amount of noradrenaline (norepinephrine) released [15]. During prolonged intense full-body exercise, the response of the reflex associated with fatigue of the inspiratory muscles may limit exercise performance [16]. However, training the inspiratory muscles can delay the activation of the respiratory muscle reflex, potentially improving performance [12]. Our study represents a rare investigation that comprehensively evaluates both respiratory muscle parameters. The literature suggests that respiratory muscle fatigue typically does not manifest during exercise at intensities below approximately 80% of VO2 max [17].