Eastern grey kangaroo ('Macropus giganteus') myofibres: 2. Characteristics of eight skeletal muscles

Abstract

The myofibre characteristics of eight skeletal muscles of economic importance, comprising six muscles from the upper hindlimb, one from the lumbar and one from the sacral region, from five eastern grey kangaroos (Macropus giganteus) were determined. Differential staining of myosin heavy chains allowed myofibres to be classified as Types 1 (slow oxidative), 2A (fast oxidative-glycolytic) and 2X/2B (fast glycolytic), as well as the intermediate or transitional Types 2C (Type 1–Type 2A intermediate) and 2AX/B (Type 2A–Type 2X/2B intermediate). The m. psoas minor had a higher area comprising Type 1 myofibres (41.4%) relative to total myofibre area than did any of the other muscles studied (each <5%). This was due to the m. psoas minor having a higher percentage (31.9%) and larger average cross-sectional area (CSA; 4211 µm²) of Type 1 myofibres. Type 2X/2B myofibres comprised over 70% of the relative area in the mm. semimembranosus, semitendinosus and gluteus medius, compared with 34.2% in the m. psoas minor, with the other muscles intermediate. The proportion of Type 2A myofibres ranged from 19.1% (m. gluteus medius) to 34.6% (m. caudal dorsolateral sacrocaudalis) of the relative myofibre area. The m. caudal dorsolateral sacrocaudalis had the largest average myofibre CSA and the m. adductor the smallest (5539 and 2455 µm², respectively). Among the intermediate myofibre types, Type 2AX/B myofibres were more prevalent (range 4.3%–13.0% of myofibres) than Type 2C myofibres (≤0.5%). Overall, the correlations between carcass weight and the percentage and relative areas of myofibres were positive for Type 2A and negative for Type 2X/2B myofibres. The results provide a detailed characterisation of myofibres in kangaroo skeletal muscles of economic importance. Furthermore, they enhance our understanding of factors influencing kangaroo muscle structure and post-mortem metabolism and provide potential indicators of eating quality of kangaroo meat.