In intercostal muscles, inspiratory EMG activity predominates in the upper part of the chest and decreases caudally. This activity gradient parallels the distribution of mechanical advantage, so that the most effective part is the most activated. The levator costae muscle inserts between each rib and the vertebra immediately rostral. In the cat, its activity was progressively stronger in the muscles located in the more caudal thoracic segments. This paradoxical distribution prompted us to study the mechanical advantage, muscle fibre type, EMG activity and metabolic activity of these muscles in the rabbit. In 7 anesthetized animals, the mechanical advantage was evaluated by measuring the fractional change in muscle length during passive inflation. The muscle shortened in each interspace and the mean shortening was -4.7±0.5, 4.0±0.4, -3.3±0.5, -0.8±0.6, 1.4±0.5%LFRC in the interspaces 2, 3, 5, 6, 7 and 8 respectively. Animals were then sacrificed and the rostro-caudal distribution of type I muscle fibre was evaluated immunohistologically. The proportion of type I fibre peaked at the top (interspace 2: 44±2%) and decreased caudally (interspace 4: 34±1%; 6: 28±3%; 8: 24±2%). In another group of animals, the raw EMG activity was recorded at rest and during resistive loading. The time lag between the air flow reversal and the onset of activity was measured. At rest and with resistive loading, EMG activity appeared earlier in the 2nd interspace than in the mid-thoracic segments and was delayed in the 7th and 8th interspaces. Muscles were freeze-clamped and the intensity of metabolic activity was then quantified using a 31P NMR evaluation. The ratio of inorganic phosphorus to phosphocreatine was highest in the 2nd interspace. We concluded that in levator costae the mechanical advantage and respiratory activity predominate in the upper interspaces, as in intercostal muscles. This neuromechanical matching is reflected in the muscle fibre type composition.