In the external intercostal muscle of a given interspace, electromyographic (EMG) activity follows a mediolateral gradient and the posterior part of the muscle is the most active. The origin of this gradient, however, is unknown. The present studies investigated factors that could be involved in this phenomenon, namely the density of muscle spindles, the EMG sensitivity to rib motion and the distribution of type I (slow-twitch) muscle fibres. To do so, we evaluated in the rabbit the density of muscle spindles using a modified Duron's method. This density, expressed in number of spindles / unit cross-sectional area, was similar in the anterior, middle and posterior parts of the muscle (3^rd interspace) and averaged 0.20 0.03, 0.22 0.05, and 0.16 0.05 /mm², respectively. The sensitivity of the EMG activity to segmental reflexes was then assessed by altering the displacement of the lower rib during inspiration. The effects were quantified by using least square exponential regression. The slope of the relationship averaged -0.09 0.01, -0.13 0.02 and -0.10 0.02 in the anterior, middle and posterior parts of the muscle (3^rd interspace), respectively (NS). In contrast, parasternal muscle activity was less sensitive (slope = -0.03 0.01 ; P < 0.05 vs external bundles). Finally, immunostaining of the slow myosin was performed in the three bundles. Slow fibres are inhomogeneously distributed and the posterior bundle has a higher density of slow fibres (44.4 4.3%, P<0.05) than the other portions (29.1 3.5% and 33.1 2.2%). An inward gradient of muscle fibre types within the posterior bundle was also observed and the proportion of slow fibres was 21.8 1.7% and 42.8 1.6% at the external and internal surfaces of the muscle, respectively (P<0.0001). This could reflect heterogeneity in intrinsic properties of the a-motoneurons driving the different parts of the muscle and could therefore explain the heterogeneity of activity in this muscle.