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Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Belgium.

1. Because the inspiratory mechanical advantage of the canine parasternal intercostal muscles is greatest in the third interspace and decreases gradually in the caudal direction, the electromyograms of these muscles in interspaces 3, 5 and 7 have been recorded in anaesthetized, spontaneously breathing dogs. Each activity was expressed as a percentage of the activity measured during tetanic, supramaximal stimulation of the internal intercostal nerve (maximal activity). 2. Parasternal inspiratory activity during resting, room air breathing was invariably greater in the third than in the fifth interspace (62.0 +/- 6.0 vs. 41.3 +/- 4.6% of maximal activity; P < 0.001) and smallest in the seventh interspace (22.8 +/- 2.7% of maximal activity; P < 0.001). This distribution of activity persisted during hyperoxic hypercapnia and during breathing against increased inspiratory airflow resistance. 3. This rostrocaudal distribution of activity also persisted after complete paralysis of the diaphragm as well as after deafferentation of the ribcage. 4. Studies of the distribution of the muscle fibre types indicated that the parasternal intercostals in all interspaces had a higher proportion of slow-twitch oxidative (SO; type I) fibres than fast-twitch oxidative-glycolytic (FOG; type II a) fibres. 5. Thus the topographic distribution of parasternal inspiratory activity along the rostrocaudal axis of the ribcage is precisely matched with the topographic distribution of mechanical advantage. This extraordinarily effective pattern of activation probably results from the unequal distribution of central inputs throughout the parasternal motoneurone pool.

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