Typology of the respiratory muscles in normal men and in patients with moderate chronic respiratory diseases.

Histochemical muscle fibre composition was studied in biopsies from the diaphragm, the external and internal intercostal muscles in the fifth intercostal space and horizontal and vertical parts of the serratus in 8 subjects with normal lung function and in 18 patients with abnormal lung function (6 restrictive and 12 obstructive). Muscle fibres were classified as type I (slow twitch) or type II (fast twitch) on the basis of their myofibrillar ATPase pH lability. All the muscles studied showed a mosaic pattern with the two fibre types. In every respiratory muscle, the percentage of type I fibres was higher than 50%. There was no significant difference between the three groups (normal, restrictive and obstructive) for each of the six muscles studied. The fibre diameters were similar for all types and muscles (range of means: 41-63 micron), except for the diaphragm in which the diameters of type I and type II fibres were significantly higher in the normal subjects as compared to the obstructive and restrictive patients. Moreover, there was a significant linear correlation between the diameter of the diaphragmatic fibres and vital capacity and FEV1. A low "atrophy factor" was found for each muscle studied with a large intra-individual variation. The results support the idea that the increased respiratory loading due to disease do not hypertrophy the respiratory muscles and suggest that the effect of the disease on the respiratory muscles is more related to the increased mechanical impedance of the respiratory system rather than to the changes in thoracic configuration.