Effect of Inspiratory Muscle Training on Diaphragm Muscle Thickness, Fatty Liver Density and Aerobic Capacity in Elderly Men: A Randomized Controlled Trial

1. Introduction Modern lifestyles present a multitude of challenges, many of which differ markedly from those encountered by previous generations. The nature of these challenges has evolved over time, leading to significant shifts in the ways individuals maintain and protect their health. Consequently, primary health risk factors have also transformed. While the importance of preserving health to maintain quality of life is well established, the determinants influencing this process have undergone a substantial evolution. In contemporary contexts, the identification of key health determinants and the long-term monitoring of their effects have become essential for the development of effective health promotion strategies.

Within this framework, physical activity and fitness have emerged as pivotal contributors to overall health and well-being in modern societies. Adequate levels of physical activity are positively correlated with favorable biomarkers and higher quality of life. This is particularly critical in aging populations, where maintaining physical activity levels supports functional capacity and mitigates age-related physiological decline.As aging progresses, health concerns such as sarcopenia, reduced muscle strength, and diminished functional capacity become increasingly prevalent.

Respiratory muscle strength is especially affected by aging, declining by approximately 8-15% per decade after the age of 50, in both sedentary and active older adults. Muscle fiber counts in elderly individuals may decrease to only 20% of the total observed in young adults, and diaphragm and intercostal muscle strength can decline by up to 25% due to sarcopenia and muscle atrophy. This reduction in respiratory function can contribute to a decline in exercise tolerance and physical activity, thereby accelerating overall physiological deterioration. In this context, respiratory muscle function particularly diaphragm thickness serves as a key biomarker for ventilatory capacity and functional health in aging populations.

Inspiratory muscle training (IMT) has gained increasing attention as a strategy to enhance functional capacity and support quality of life in both clinical and aging populations. However, prior research indicates that short-term protocols (e.g., 8 weeks) produce only modest and clinically insignificant improvements in maximal exercise capacity. In contrast, submaximal exercise assessments, such as the six-minute walk test (6MWT), better reflect daily functional activities and are therefore more suitable for evaluating clinically relevant improvements. This consideration informed the selection of 6MWT as a primary outcome in the present study.

IMT exerts its physiological benefits primarily through delaying or attenuating the respiratory metaboreflex, thereby reducing respiratory fatigue, lowering blood lactate accumulation, and improving oxygen availability to locomotor muscles. It can induce adaptations such as diaphragm hypertrophy, enhanced respiratory efficiency, decreased dyspnea, improved endurance, favorable shifts in muscle fiber composition, and optimized neuromuscular control of respiratory muscles. Collectively, these mechanisms suggest that IMT has the potential to improve both functional performance and overall quality of life in aging adults, while potentially contributing to metabolic health by modulating factors such as fatty liver density. As the global population ages, international studies have emphasized the benefits of inspiratory muscle training (IMT) as a cost-effective and low-risk intervention for improving respiratory function, exercise performance, and quality of life in older adults.