Distribution of muscle fiber types and EMG activity in cat intercostal muscles.

The electromyogram (EMG) activity and histochemical properties of intercostal muscles in the anesthetized cat were studied. The parasternal muscles were consistently active during inspiration. The external intercostals in the rostral spaces and the ventral portions of the midthoracic spaces were also recruited during inspiration. The remaining external intercostals were typically silent, regardless of the level of respiratory drive. The internal intercostal muscles located in the caudal spaces were occasionally recruited during expiration. There was a clear correlation between recruitment patterns of the intercostals and the histochemically defined fiber type properties of the muscles. Intercostal muscles that were routinely recruited during inspiration had a significantly higher proportion of slow-oxidative muscle fibers.