Samples of the m. triceps brachii (caput longum), m. longissimus lumborum, m. psoas major, m. gluteobiceps, m. semitendinosus, m. semimembranosus and m. quadriceps femoris (m. rectus femoris) from 5 Pramenka breed male lambs aged 3 months, weighing 20-26 kg, were obtained immediately upon slaughter and dressing, frozen in liquid nitrogen, and cut on a Cryocut into 10-µm slices stained with hematoxylin-eosin. The diameter of 4456 muscle fibers was measured on microscopic images taken on a Nikon microscope at X50 magnification. Data were presented in tables. The muscle fiber diameter ranged from 10 to 50 µm and for m. psoas major, from 10 to 45 µm. The above listed muscle samples yielded the following mean muscle fiber diameters: 22.24, 23.85, 24.70, 25.38, 24.87, 22.26 and 25.85 µm, respectively. Small diameter fibers (> 30 µm) accounted for 64.88%-91.33%, and medium diameter fibers (31-50 µm) for 7.81%-35.12% of all fibers. There were no large diameter fibers (< 50 µm). The muscle fiber surface was predominated by small diameter fibers (48.23%- -80.70%), with the exception of m. triceps brachii (caput longum), where medium diameter fibers constituted up to 51.77% of the muscle fibers. Medium diameter fibers accounted for 19.30%-51.77% of the muscle fiber surface. A comparison with the previously reported observations in local Istrian Pramenka crossbreeds with Sardinian, East Friesian and domestic German Merino-Würtemberg rams revealed that the crossbreeds have all the muscular parameters considerably greater than purebred Pramenka lambs. Distribution frequency indicated the muscle fiber diameter to show a shift to the left, i.e. toward fibers of a smaller diameter in m. psoas major and m. semimembranosus, and a shift to the right, i.e. to fibers of greater diameter in m. triceps brachii (caput longum).
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