Background: The diaphragm, our body’s principal breathing muscle, plays a central role in sustaining life with every breath we take. Its ability to generate force across different lengths known as the force-length relationship is crucial for efficient respiration. However, despite its importance, we still lack a clear understanding of how this relationship varies across age and between sexes in healthy individuals. Most existing data are drawn from patients with respiratory illnesses or select populations, leaving a gap in baseline physiological knowledge.
Aim: This study set out to investigate how age and sex influence the diaphragm’s force-length characteristics in healthy adults, using standard chest X-rays. We also aimed to explore how body composition, particularly body mass index (BMI) interacts with diaphragmatic performance.
Materials and Methods: A total of 100 healthy adults (46 males and 54 females), aged between 19 and 79 years, were enrolled in this prospective cross-sectional study. All participants had normal lung functions. Standardized digital posteroanterior chest radiographs were obtained during full inspiration and expiration. Key diaphragmatic measurements were made using calibrated imaging software, and force estimates were calculated using validated biomechanical models. Statistical analyses included multivariate regression, controlling for height, weight, and thoracic dimensions.
Results: The findings revealed three major insights. First, there were no significant differences between males and females in diaphragmatic force production (mean 24.3±3.8 N for females vs. 23.6±3.9 N for males; p=0.38) or in diaphragm movement during breathing. Second, diaphragmatic force remained stable across the adult lifespan, with no evidence of age-related decline (β=-0.004, p=0.86). Third, BMI showed a strong positive correlation with diaphragmatic force (r=0.81, p<0.001), explaining over 70% of the variance in the force estimates. On average, the diaphragm moved 3.9±2.1 mm more during inspiration than expiration (p<0.001), reaffirming its dynamic contractile role.
Conclusion: This study offers reassuring news: in healthy adults, diaphragmatic strength does not wane with age and is not significantly different between sexes. Instead, body mass appears to play the most important role in diaphragmatic performance. These findings emphasize the value of considering body composition in clinical respiratory assessments and provide a set of normative values that can inform future research and healthcare practice.