Mechanical properties of respiratory muscles in primates

We examined the in-vitro mechanical characteristics of the diaphragm and parasternal intercostal muscles in adult baboons. At optimal length (L₀) the parasternal intercostal muscles were consistently faster than the diaphragm as judged by a shorter time-to-peak tension and a reduced twitch-to-tetanus ratio. However, maximal absolute force at L₀, correlated for cross-sectional area, revealed that the inherent strengths of both inspiratory muscles were identical (31.5 ± 3.2 N/cm² for diaphragm vs 29.5 ± 1.1 N/cm² for parasternal intercostals). Moreover, at submaximal stimulation rates, the parasternal intercostals produced significantly less tension at L₀ than the diaphragm. At lengths less than L₀, however, the parasternal intercostal muscles significantly less tetanic tension than the diaphragm. These differences are similar to previously reported values in dogs (Farkas et al. (1985) J. Appl. Physiol. 59: 528-535). Thus, based on our findings, it appears that within a given species, the parasternal intercostals are quite different in mechanical terms from the diaphragm. Since both muscles are recruited at all levels of ventilation, including quiet breathing, these differences do not appear to be related to the activation levels. Moreover, since the present results were similar to those in dogs, the differences between both muscles do not appear to be affected or altered by body habitus.