Modelling the effects of age and sex on the resistive and viscoelastic components of the work of breathing during exercise.

Healthy ageing and biological sex each affect the work of breathing (Wb ) for a given minute ventilation ( V ̇ E ). Age-related structural changes to the respiratory system lead to an increase in both the resistive and viscoelastic components of Wb ; however, it is unclear whether healthy ageing differentially alters the mechanics of breathing in males and females. We analysed data from 22 older (60-80 years, n = 12 females) and 22 younger (20-30 years, n = 11 females) males and females that underwent an incremental cycle exercise test to exhaustion. V ̇ E and Wb were assessed at rest and throughout exercise. Wb - V ̇ E data for each participant were fitted to a non-linear equation (i.e. Wb  = a V ̇ E 3 + b V ̇ E 2 ) that partitions Wb into resistive (i.e. a V ̇ E 3 ) and viscoelastic (i.e. b V ̇ E 2 ) components. We then modelled the effects of healthy ageing and biological sex on each component of Wb . Overall, the model fit was excellent (r2 : 0.99 ± 0.01). There was a significant main effect of age and sex on the resistive component of Wb (both P < 0.05), and a significant main effect of age (P < 0.001), but not sex (P = 0.309), on the viscoelastic component of Wb . No significant interactions between age and sex on a V ̇ E 3 or b V ̇ E 2 were noted (both P > 0.05). Our findings indicate that during exercise: (i) the higher total Wb in females relative to males is due to a higher resistive, but not viscoelastic, component of Wb , and (ii) regardless of sex, the higher Wb in older adults relative to younger adults is due to higher resistive and viscoelastic components of Wb .