Challenging the loss of complexity theory: Insights from isometric force output in chronic obstructive pulmonary disease

According to the “loss of complexity” theory, aging and disease are expected to reduce complexity of physiological outputs, thereby limiting the system’s adaptability. However, it remains unclear whether this concept applies to the neuromuscular system in people with chronic obstructive pulmonary disease (pwCOPD). This study aimed to challenge the loss of complexity hypothesis by assessing the regularity, as well as the steadiness and the accuracy, of force production during submaximal isometric contractions in pwCOPD compared to healthy individuals. Seventeen pwCOPD and seventeen age- and sex-matched healthy participants performed submaximal isometric contractions of the knee extensors at six target forces from 10 to 60% of their maximal voluntary contraction (MVC). Regularity of force signals, considered here as one dimension of physiological complexity, was assessed using sample entropy (SampEn) and percentage of determinism (DET) from the recurrence quantification analysis. Steadiness and accuracy were quantified using the coefficient of variation (CV) and the root-mean-square error (RMSE), respectively. PwCOPD exhibited 26.5% lower MVC than healthy individuals. No significant main effect of group or interaction effect (group × contraction intensity) was observed for SampEn, DET, CV and RMSE, suggesting a preserved force control in pwCOPD at all assessed force levels. Our results indicate that the loss of complexity theory may not apply in moderate COPD, at least for the dimension of complexity captured by regularity-based measures, suggesting that neuromuscular alterations associated with COPD may not be sufficient to impair the temporal regularity of force output, questioning the universality of the loss of complexity theory.