Fibre types, capillary supply and enzyme activities in human intercostal muscles

The relative occurrence of slow twitch (ST) and fast twitch (FTa and FTb) fibres, fibre size, capillary supply, and glycolytic and oxidative enzyme activities in external (EXT) and internal (INT) intercostal muscles was determined on biopsies obtained during thoracotomy from the mid-axillary line in six patients with normal lung function. EXT and INT showed a similar occurrence of ST fibres. Both FTa and FTb fibres were seen in EXT while only FTa fibres were seen in INT. The size of the three fibre types in EXT was similar. In INT the fibre size was larger for FTa than for ST (P less than 0.05). INT appeared to have a larger number of capillaries per fibre than EXT (P less than 0.05). A positive correlation was demonstrated between variables measured during spirometry and fibre size in INT only. No difference in glycolytic and oxidative enzyme activities between EXT and INT was observed. The histochemical characteristics suggest that, in the mid-axillary line, INT are more used than EXT, though this conclusion did not appear to be confirmed by the enzyme activities determined.