The influence of jaw-muscle fibre-type phenotypes on estimating maximum muscle and bite forces in primates

Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella ( N = 3), Cercocebus atys ( N = 4), Macaca fascicularis ( N = 3), Gorilla gorilla ( N = 1) and Pan troglodytes ( N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.

Comparison of two muscle fibre staining techniques and their relation to pork quality traits

This study compared two histochemical staining methods of muscle fibres and evaluated their relationship with the meat quality traits of two high-value porcine muscles. Immunohistochemical (IHC) and adenosine triphosphatase (ATPase) staining was used to assess the cross-sectional area and proportion of fibre-types I, IIa, IIx and IIb in the samples of longissimus lumborum (LL) and psoas major (PM) muscles collected one-hour post-mortem from 25 crossbred pigs [Large White_Sire × (Landrace × Large White_Dam)] at an average age of 152 days. Muscles differed in all fibre parameters, except the proportion and relative area of type IIx fibres. The LL muscle exhibited greater fibre cross-sectional areas of all fibre types, higher proportions of type IIb/IIB, and lower proportions of I and IIa fibres than the PM muscle in both staining techniques. These two muscles also differed marginally in moisture, crude protein and intramuscular fat content. The PM muscle showed a low correlation between fibre types and chemical composition, but the LL muscle showed moderate correlations between fibre CSA and area composition for moisture and ash content. After IHC staining, an increase in LL eye muscle area and drip loss were correlated with lower proportions of type I fibres, while a greater proportion of type IIx fibres resulted in increased LL eye muscle area and moisture content. Furthermore, a higher CSA of all fibre types in the LL decreased redness (a*) and moisture content of the muscle. Results showed that IHC is more appropriate than ATPase staining for the assessment of relationships between muscle fibre parameters and meat quality traits in pigs.