Intermuscular and Intramuscular Variations in Meat Quality Associated With Muscle Fiber Characteristics

Muscle Architecture and Physiologic Status

Skeletal muscles differ in anatomical location and functional role, and these differences are reflected in muscle-specific structure and metabolic environment (Listrat et al., 2016; Matarneh et al., 2021). As a result, meat quality can vary systematically among muscles and can also vary across locations within a single muscle (Jeremiah et al., 2003a; Denoyelle and Lebihan, 2004; Gariépy et al., 1990). Muscle morphological properties refer to muscle-level structural features such as overall architecture, fiber arrangement, and the connective tissue framework that supports and compartmentalizes fibers, whereas muscle physiologic properties refer to muscle-level metabolic capacity and energy status that condition early postmortem events. These upstream properties influence key postmortem processes, including the rate and extent of pH decline and the conditions during chilling and rigor development, thereby shaping quality traits such as color development, water-holding capacity, and tenderness (Huff-Lonergan and Lonergan, 2005; Warner, 2017; Warner et al., 2022). Collectively, muscle architecture and physiologic status set the initial structural constraints and metabolic conditions that shape early postmortem temperature-pH trajectories and microstructural outcomes, thereby preconditioning variation in water-holding capacity, color, and texture.

Muscle architecture provides a structural basis for muscle-specific differences in postmortem behavior and subsequent meat quality. At the muscle level, architecture encompasses the orientation and arrangement of muscle fibers within fascicles, including the degree of pennation and fascicle organization, which together determine how fibers are mechanically constrained in situ (Listrat et al., 2016; Park et al., 2022). These architectural features interact with the extracellular matrix and connective tissue framework that surround and compartmentalize fibers, providing structural support and force transmission and contributing to postmortem texture development through its influence on tissue stiffness and structural constraints (Purslow, 2005; Listrat et al., 2016; Warner et al., 2022). Importantly, architectural constraints can modulate early postmortem structural outcomes under chilling and rigor development (e.g., sarcomere shortening and associated microstructural changes), contributing to muscle-dependent differences in water-holding capacity and texture-related traits (Huff-Lonergan and Lonergan, 2005; Lundström and Malmfors, 1985; Warner et al., 2022).

Muscle physiologic status, particularly the balance between oxidative capacity and available energy reserves, is a major determinant of muscle-specific postmortem metabolism. Differences in glycogen availability influence the extent of postmortem pH decline, whereas the rate of early pH fall is dictated primarily by glycolytic flux, governed by temperature-dependent glycolytic enzyme activity and regulation of key rate-limiting steps, which together condition the biochemical environment governing protein functionality and structural change (Huff-Lonergan and Lonergan, 2005; Warner, 2017). When pH declines rapidly while muscle temperature remains relatively high, protein denaturation and altered myofibrillar functionality can reduce water-holding capacity and shift texture-related indices (e.g., higher shear force), whereas a more gradual pH decline is generally associated with improved water retention and tenderness development through more favorable conditions for postmortem aging processes (Wu et al., 2014; Warner et al., 2022). Importantly, physiologic heterogeneity can also occur within a single muscle because spatial differences in perfusion, local metabolic activity, and cooling rate during chilling can create region-dependent temperature-pH and metabolic trajectories and contribute to intramuscular variation in quality attributes (Park et al., 2024). Accordingly, variation in muscle-level oxidative capacity and energy reserves, and their spatial heterogeneity, can drive muscle- and region-specific postmortem pH trajectories that underlie differences in water-holding capacity and texture development.

Muscle Fiber Traits and Postmortem Mechanisms

Muscle fiber characteristics are commonly reported as the distribution of fiber phenotypes within a muscle, often accompanied by fiber morphometrics such as fiber CSA (Tables 1–3; Lee et al., 2010; Chang et al., 2003; Joo et al., 2013; Matarneh et al., 2021). Fiber phenotypes are typically classified based on myosin heavy chain (MyHC) isoform expression into slow-twitch oxidative (Type I) and fast-twitch phenotypes, including fast oxidative-glycolytic (Type IIA) and fast-glycolytic phenotypes reported as Type IIX and/or Type IIB. Hybrid fibers are also frequently observed (Song et al., 2020). Fiber-type nomenclature and corresponding assignments vary across species and studies; therefore, interpretation should consider the classification framework and typing method used (Schiaffino, 2018; Song et al., 2020; Lee et al., 2010). Notably, MyHC-2B (IIB) expression is species dependent, and fiber-type labels are not always directly comparable across methodologies. For example, MyHC isoform-based immunofluorescence in bovine muscles typically identifies Type I/IIA/IIX (and hybrid IIAX), whereas MyHC-IIB–defined fibers are more consistently observed in porcine muscles. Meanwhile, some histochemical (myosin adenosine triphosphatase based) studies in cattle report an “IIB” category, which should be interpreted cautiously in cross-species comparisons (Hwang et al., 2010; Joo et al., 2017; Song et al., 2020; Lang et al., 2020). For cross-study synthesis, fiber traits can be organized around key postmortem processes that link muscle biology to quality outcomes, including (1) pH-temperature kinetics driven by glycolytic potential, (2) oxygen consumption and redox balance that govern Mb state transitions and color stability, and (3) structural weakening during rigor and aging in which morphometrics and endogenous proteolysis contribute to texture-related variation (Huff-Lonergan and Lonergan, 2005; Faustman et al., 2010; Warner et al., 2022; Joo et al., 2013).

First, fiber metabolic properties shape postmortem pH-temperature kinetics by determining glycogen availability and glycolytic capacity, thereby conditioning protein functionality and water-holding capacity (Huff-Lonergan and Lonergan, 2005; Warner, 2017; Joo et al., 2013). Oxidative phenotypes are characterized by greater mitochondrial abundance and oxidative enzyme capacity and rely predominantly on aerobic metabolism, with a relatively greater contribution of fatty acid oxidation and oxidative phosphorylation to adenosine triphosphate (ATP) production in vivo (Lee et al., 2010; Joo et al., 2013; Schiaffino, 2018). Between these extremes, oxidative-glycolytic phenotypes exhibit intermediate metabolic characteristics, combining substantial oxidative capacity with relatively high glycolytic activity, and muscles often contain hybrid fibers that span this continuum (Joo et al., 2013; Schiaffino, 2018). In contrast, glycolytic phenotypes exhibit higher glycolytic enzyme activities, a greater capacity for rapid ATP generation via anaerobic glycolysis, and typically store more glycogen, thereby providing a larger substrate pool for postmortem lactate production (Ryu and Kim, 2005; Joo et al., 2013; Kim et al., 2013). Accordingly, glycogen availability and glycolytic potential are major determinants of the rate and extent of postmortem glycolysis and largely govern pH decline kinetics through lactate accumulation and associated proton production (Huff-Lonergan and Lonergan, 2005; Warner, 2017). Muscles with high glycolytic potential often show a faster early pH decline and, depending on preslaughter conditions, a lower ultimate pH, whereas more oxidative muscles commonly exhibit a more gradual pH decline and, in many contexts, a higher ultimate pH (Ryu and Kim, 2005; Joo et al., 2013; Matarneh et al., 2021). Because pH decline kinetics are strongly influenced by the early postmortem temperature trajectory, interpretation is best made in conjunction with the processing and measurement conditions reported in each study (Huff-Lonergan and Lonergan, 2005; Warner, 2017; Warner et al., 2022).

Second, fiber phenotype is linked to oxygen consumption and redox buffering through Mb content and mitochondrial abundance, which together govern Mb state transitions and color stability during storage (Mancini and Hunt, 2005; Faustman et al., 2010; Nair et al., 2016). Differences in Mb concentration and mitochondrial function can shift the balance among deoxymyoglobin, oxymyoglobin, and metmyoglobin in an oxygen-dependent manner, contributing to muscle- and condition-specific color dynamics (Wittenberg and Wittenberg, 2003; Mancini and Hunt, 2005; King et al., 2023). In more oxidative muscles, an Mb- and mitochondria-rich redox milieu may, under some conditions, support color stability; however, during extended storage or retail display, pro-oxidant pressure that exceeds endogenous antioxidant capacity can accelerate lipid oxidation and Mb oxidation, thereby compromising color stability and sensory-relevant quality attributes (Faustman et al., 2010; Mancini and Hunt, 2005; Sammel et al., 2002b). Because these outcomes depend on oxygen availability and handling conditions (e.g., packaging atmosphere, bloom time, and retail lighting), reported differences should be interpreted in relation to the specific measurement and storage context (King et al., 2023; Mancini and Hunt, 2005; Faustman et al., 2010).

Third, fiber morphometrics and rigor/aging-related structural weakening contribute to variation in texture-related traits, with fiber size and density influencing extracellular space and mechanical constraints within the tissue (Crouse et al., 1991; Listrat et al., 2016; Joo et al., 2013). During the early postmortem period, the combined temperature-pH conditions and muscle-specific architectural constraints modulate the extent of sarcomere shortening and the resulting microstructural state during rigor development, which, in turn, affects the subsequent response to aging (Huff-Lonergan and Lonergan, 2005; Purslow, 2005; Warner et al., 2022). In parallel, endogenous proteolysis during aging (e.g., calpain-mediated myofibrillar degradation and its regulation by calpastatin [CAST]) can differentially weaken the myofibrillar structure across muscles, contributing to variation in shear-force–based tenderness indices (Huff-Lonergan and Lonergan, 2005; Kim et al., 2021; Koohmaraie, 1988; Warner et al., 2022), and fiber phenotype may further contribute to this variability through differences in the calpain system (e.g., μ-calpain and/or CAST abundance or activity; Bhat et al., 2018; Muroya et al., 2012). Although intramuscular fat distribution and connective tissue characteristics can act as important codeterminants of eating quality, a fiber-centered view provides a useful basis for organizing how structural weakening mechanisms translate into texture outcomes under defined postmortem handling conditions (Totland et al., 1988; Purslow, 2005; Wood et al., 2008; Warner et al., 2022).

Breed and Genetic Modulation of Fiber Traits

Breed and genetic background can modulate muscle fiber phenotypes, substrate availability for postmortem glycolysis, and pathways governing structural change, thereby shifting the relative contribution of these postmortem processes (pH-temperature kinetics, oxygen consumption/redox balance, and structural weakening) to observed variation in meat quality traits (Chambaz et al., 2003; Joo et al., 2013; Matarneh et al., 2021; Rohrer et al., 2012; Suárez-Mesa et al., 2024). Ultimate pH is an integrative outcome of postmortem metabolism because it reflects the balance between glycogen availability at slaughter and the extent of anaerobic conversion of glycogen to lactate, with downstream effects on protein functionality, water-holding capacity, color development, and proteolytic activity (Huff-Lonergan and Lonergan, 2005; Ryu and Kim, 2005; Warner, 2017). Because dark, firm, and dry are attributes primarily associated with limited glycogen reserves and a high ultimate pH, whereas pale-, soft-, and exudative-like phenotypes are more strongly associated with a rapid early pH decline under relatively high muscle temperatures, interpretation benefits from considering both biological background and postmortem handling conditions (Matarneh et al., 2021; Huff-Lonergan and Lonergan, 2005; Warner, 2017). Consistent with species- and muscle-dependent metabolic patterns, cattle semimembranosus (SM) has been reported to exhibit higher ultimate pH than other muscles (Kim et al., 2016), whereas in goats longissimus lumborum (LL) can show higher pH than SM across breeds and sexes (Gawat et al., 2022). Breed and line effects can further shift these outcomes, exemplified by higher ultimate pH and redness in Xueshan chickens relative to the fast-growing Ross 308 line (Weng et al., 2022) and by breed-dependent differences in lightness between longissimus dorsi (LD) and gluteus medius (GM) muscles in goats (Hwang et al., 2019; Deng et al., 2024). Such breed/line differences likely reflect correlated responses to selection and production conditions (e.g., growth rate, muscle fiber/metabolic programming, stress responsiveness, and muscle energy reserves), which can shift postmortem metabolism and associated quality traits (Ismail and Joo, 2017; Huo et al., 2021). Compared with locomotor muscles, psoas major (PM) generally exhibits a distinct fiber-type distribution and metabolic profile, which can contribute to differences in early postmortem metabolism and aging-related tenderization; however, fiber-associated mechanisms are not the sole drivers of tenderness differences among muscles (Joo et al., 2013; Listrat et al., 2016; Purslow, 2005). In addition, connective tissue amount/cross-linking and postmortem handling factors that influence sarcomere length (e.g., carcass suspension) are well-recognized contributors to muscle-specific tenderness differences, including the consistently high tenderness of PM (Purslow, 2005). Collectively, these observations suggest that quality variation reflects hierarchic interactions among genetic background (breed/line), muscle type, and postmortem conditions, rather than uniform patterns across animals.

Genetic effects on meat quality can be interpreted through coordinated regulation of 3 mechanistic axes that align with the postmortem processes: (1) fiber-type specification and the oxidative program, which influences mitochondrial abundance and redox buffering capacity; (2) glycogen-related metabolism and glycolytic potential, which conditions postmortem lactate accumulation and pH-temperature kinetics; and (3) regulation of endogenous proteolysis during aging, which contributes to structural weakening and texture development (Joo et al., 2013; Matarneh et al., 2021; Warner et al., 2022). For the oxidative program, regulatory factors such as histone deacetylase 4 have been linked to expression patterns associated with slow/oxidative muscle phenotypes (Cohen et al., 2015; Schiaffino, 2018). For glycolytic potential, protein kinase adenosine–monophosphate-activated noncatalytic subunit γ 3 has been associated with pork quality traits through its role in glycogen metabolism and postmortem glycolysis (Rohrer et al., 2012). For proteolytic capacity, CAST acts as an inhibitor of calpain-mediated proteolysis and can, therefore, modulate the extent of tenderization during aging (Rohrer et al., 2012; Warner et al., 2022). Together, these examples illustrate that genetic associations with quality traits are most informative when framed as modulators of fiber-linked postmortem processes and interpreted in the context of muscle type and handling conditions.

Intermuscular Variations

Intermuscular variation refers to systematic differences in meat quality traits among distinct muscles that arise from differences in anatomical location and functional role (Figure 1, Tables 4–6; Jeremiah et al., 2003a; Jeremiah et al., 2003b; Joo et al., 2013). These functional differences are expressed as muscle-specific architecture and fiber profile and, in turn, shape early postmortem trajectories, including pH-temperature kinetics and rigor development that condition downstream quality outcomes (Lang et al., 2020; Listrat et al., 2016; Park et al., 2022; Matarneh et al., 2021). Muscles with different functions differ in fiber-type composition and metabolic organization, and the resulting intermuscular variation in postmortem pH decline, water-holding capacity, and color/texture development can be interpreted through the mechanistic lens of pH-temperature kinetics, oxygen consumption/redox balance, and structural weakening during rigor and aging (Huff-Lonergan and Lonergan, 2005; Ryu and Kim, 2005; Warner, 2017; Wittenberg and Wittenberg, 2003; Mancini and Hunt, 2005; Faustman et al., 2010; Purslow, 2005; Warner et al., 2022).

Figure 1.

Schematic representation of intramuscular and intermuscular variations in muscle fiber characteristics by major livestock species. Fiber-type identities are annotated next to each panel because staining/typing schemes differ across species and studies. Abbreviations: CL, cooking loss; CSA, cross-sectional area; DL, drip loss; FC, fiber composition; FD, fiber density; IMF, intramuscular fat content; MFC, muscle fiber characteristics; MQ, meat quality; PL, purge loss; RFA, relative fiber area; RFN, relative fiber number; WBSF, Warner-Bratzler shear force.

In cattle, intermuscular differences have been reported across breeds and populations and often follow recurring patterns among muscles (Table 4; Jeremiah et al., 2003a; Jeremiah et al., 2003b; Reuter et al., 2002; Searls et al., 2005; Joo et al., 2017). Across multiple cattle populations, the PM (tenderloin) has been associated with lower Warner-Bratzler shear force (WBSF) and higher sensory tenderness ratings, whereas locomotor muscles from the hindlimb are more often associated with higher WBSF (Figure 1; Hwang et al., 2010; Hwang et al., 2019). Consistently, in Canada AA beef cattle, comparative evaluations of major muscles indicated that the PM received higher sensory scores for tenderness- and juiciness-related attributes, whereas the rectus femoris, biceps femoris, and SM received lower scores for these attributes (Jeremiah et al., 2003a; Jeremiah et al., 2003b; Lawless and Heymann, 2010). Together, these findings suggest that intermuscular variation in eating-quality–related traits reflect a combination of fiber-associated metabolic characteristics and muscle-level structural constraints shaped by functional demand, rather than being restricted to a specific breed background (Joo et al., 2013; Lee et al., 2010; Purslow, 2005; Listrat et al., 2016). Intermuscular differences in color can also be interpreted within this framework, as variation in fiber metabolic profile is linked to differences in Mb and mitochondrial abundance and the postmortem redox environment that contribute to muscle-specific color dynamics. For example, beef LL is generally more color-stable, whereas PM is more color-labile during storage, which is consistent with muscle-specific differences in oxygen consumption and redox balance (Wittenberg and Wittenberg, 2003; Mancini and Hunt, 2005; Faustman et al., 2010; McKenna et al., 2005). Across species, muscle function-related intermuscular patterns have been reported in a broadly consistent direction, though the magnitude and expression of traits can vary with species-specific muscle biology and postmortem handling conditions (Figure 1, Tables 4–6; Joo et al., 2013; Listrat et al., 2016; Matarneh et al., 2021). For instance, in cattle, the PM is commonly associated with lower shear force than locomotor muscles (Hwang et al., 2010), whereas in poultry, the pectoralis major is frequently reported to exhibit lower water-holding capacity and higher shear force than leg muscles (Cheng et al., 2022a; Cheng et al., 2022b; Huo et al., 2022). This apparent contrast likely reflects species- and production-specific factors (e.g., differences in slaughter age and connective tissue maturation) as well as muscle-specific metabolic and postmortem pH-temperature trajectories that influence water-holding and texture outcomes, rather than a direct inversion of locomotor-vs.-support muscle effects across species. These within-species differences are generally consistent with a mechanistic framework in which fiber-type distribution, glycolytic potential, and muscle-level structural properties shape early postmortem trajectories and rigor development (Joo et al., 2013; Lee et al., 2010; Huff-Lonergan and Lonergan, 2005; Warner et al., 2022; Yue et al., 2024). In goats, muscle-specific quality differences are also observed, and breed background may modulate the pattern. van Wyk et al. (2022) compared 6 muscles from Boer and Indigenous Veld goats and reported muscle-dependent differences in tenderness-related indices, with the LL and infraspinatus tending to show lower shear force than hindlimb muscles such as the SM and semitendinosus (ST). Collectively, intermuscular variation reflects differences in muscle function and fiber profile and the resulting postmortem metabolic and structural trajectories, while intramuscular fat and connective tissue characteristics should be considered as codeterminants of eating quality and culinary suitability where relevant (Joo et al., 2013; Purslow, 2005).

Intramuscular Variations

Intramuscular variation refers to systematic differences in meat quality that occur across anatomically distinguishable locations within a single muscle (Figure 1, Tables 7 and 8). Such within-muscle heterogeneity reflects spatial nonuniformity in muscle architectural features, local fiber-type distribution, and perfusion- and metabolism-related status, which can create location-specific early postmortem conditions during chilling and rigor development (i.e., temperature-pH and metabolic trajectories; Apple et al., 2014; Kim et al., 2018; Park et al., 2024; Denoyelle and Lebihan, 2004). Accordingly, the relative contribution of pH-temperature kinetics, local oxygen availability/redox balance, and microstructural transitions during rigor and aging may differ across regions within the same muscle, yielding measurable differences in water-holding capacity, instrumental color, and texture-related traits (Huff-Lonergan and Lonergan, 2005; Faustman et al., 2010; Warner et al., 2022). Because intramuscular fat distribution and connective tissue characteristics can also vary spatially and contribute to eating quality, interpretation of intramuscular patterns benefits from considering these factors as covariates or codeterminants alongside fiber-centered mechanisms (Purslow, 2005; Wood et al., 2008; Warner et al., 2022).

The longissimus thoracis et lumborum (LTL) is frequently used as a reference muscle in meat quality research because of its economic importance and because sampling locations are often defined using relatively clear anatomical landmarks across studies (Joo et al., 2013). Within the LTL, position-dependent variation has been reported for tenderness-related indices, instrumental color, and water-holding capacity (Denoyelle and Lebihan, 2004; Gariépy et al., 1990; Lundström and Malmfors, 1985). WBSF is commonly used as an instrumental proxy for cooked-meat toughness under standardized testing conditions (Shackelford et al., 1997; Reuter et al., 2002; Warner et al., 2022). In both pigs and cattle, anterior regions of the LTL have been reported to exhibit lower WBSF than posterior regions (Kim et al., 2018; Park et al., 2024), a pattern discussed alongside regional heterogeneity in fiber-type composition and glycolytic potential (Table 9; Ryu and Kim, 2005; Joo et al., 2013; Kim et al., 2018; Park et al., 2024). Specifically, Kim et al. (2018) reported that “dark” portions had higher oxidative fibers and lower glycolytic (MyHC IIB) fibers than “light” portions (≈3-fold differences in relative area for MyHC I vs. MyHC IIB), whereas Park et al. (2024) showed an anterior-posterior gradient in bovine LTL with Type I relative area decreasing from approximately 45% to 50% (anterior) to approximately 25% to 30% (mid/posterior) and fast fiber categories (IIX and IIA/IIX) increasing toward posterior regions. Location-dependent differences in LTL color have also been observed, though the direction of change can differ by species and study context. Kim et al. (2018) reported the highest Commission Internationale de l’Éclairage (CIE) a* (redness) in the central region of the porcine LTL, whereas Park et al. (2024) reported relatively lower redness in the central region of the bovine LTL (Kim et al., 2018; Park et al., 2024). In the CIE system, L* indicates lightness, a* the red-green axis, and b* the yellow-blue axis (King et al., 2023). Such intramuscular color differences may be linked to spatial variation in Mb content, oxygen availability, and the local redox milieu that governs Mb chemistry during storage (Wittenberg and Wittenberg, 2003; Mancini and Hunt, 2005; Faustman et al., 2010). Small but detectable intramuscular variation in water-holding capacity has also been reported within the LTL (Kim et al., 2018; Kim et al., 2019; Park et al., 2024), consistent with the expectation that regional differences in pH decline, protein denaturation, and microstructure can modulate exudation and water distribution (Huff-Lonergan and Lonergan, 2005; Wu et al., 2014; Warner, 2017). Taken together, position-dependent expression of tenderness-related indices, color, and water-holding capacity within the LTL can be summarized as an outcome shaped by interacting determinants, including early postmortem temperature-pH trajectories, local redox conditions, structural constraints, and regional differences in intramuscular fat and connective tissue properties (Purslow, 2005; Wood et al., 2008; Warner et al., 2022; Huff-Lonergan and Lonergan, 2005; Faustman et al., 2010).

Beyond the LTL, intramuscular variation has also been documented in locomotion-associated muscles such as the SM and ST, in which fiber-type distribution and metabolic characteristics can differ across locations or compartments (Tables 9–10; Matney et al., 2021; Reuter et al., 2002; Denoyelle and Lebihan, 2004; Kim et al., 2018). For instance, Kim et al. (2018) partitioned porcine SM and ST according to differences in lightness (CIE L*) and reported that, in both muscles, the relatively low-L* region tended to have a higher proportion of oxidative fibers, whereas the relatively high-L* region tended to have a higher proportion of glycolytic fibers. The low-L* region was further described in relation to anatomical compartmentation associated with bony attachment and longitudinal tension and showed higher levels of oxidative metabolism-related proteins and Mb (Kim et al., 2018). These fiber- and metabolism-associated contrasts were reflected in color outcomes in a muscle-dependent manner. In the ST, the low-L* region exhibited significantly higher CIE a* than the high-L* region, whereas in the SM, CIE a* and CIE b* did not differ significantly between regions (Kim et al., 2018). In parallel, studies evaluating color development and stability during refrigerated storage in bovine SM have suggested that internal versus external regions can follow distinct color trajectories over time (Nair et al., 2016; Sammel et al., 2002a; Sammel et al., 2002b), likely reflecting region-dependent differences in oxygen diffusion, chilling rate, and localized postmortem metabolism (Wittenberg and Wittenberg, 2003; Mancini and Hunt, 2005; Faustman et al., 2010). Under comparable compartment definitions, texture-related indices have also shown muscle-dependent patterns. Kim et al. (2018) reported higher shear force in the high-L* region of porcine ST, whereas in porcine SM the high-L* region exhibited the lowest shear force. Position-dependent differences in shear force have also been reported within bovine SM (Table 10; Denoyelle and Lebihan, 2004; Phelps et al., 2016; Reuter et al., 2002; Shackelford et al., 1997). Overall, compartment-level heterogeneity in fiber and metabolic traits is readily reflected in intramuscular color variation, whereas texture-related outcomes likely reflect an additional integration of fiber traits with region-specific structural constraints and codeterminants (e.g., connective tissue and intramuscular fat content distribution), leading to muscle-dependent expression of within-muscle patterns (Purslow, 2005; Wood et al., 2008; Warner et al., 2022).

Cross-Species Perspectives

Across livestock species, muscle fiber phenotypic distributions along the oxidative-glycolytic axis and morphometric features such as fiber-type composition and CSA have been reported to differ, providing distinct starting points that can shape postmortem pH-temperature kinetics, redox environments, and patterns of microstructural change (Kim et al., 2016; Lebedová et al., 2019). For example, porcine longissimus is repeatedly characterized by a substantial contribution of fast-glycolytic fibers to relative area under histochemical and/or MyHC-based classifications (Lebedová et al., 2019). In avian pectoralis major, Type I fibers are often absent or present at very low proportions, consistent with a predominantly fast fiber profile (Choi et al., 2013). In ruminant longissimus, MyHC isoform-based data commonly describe coexistence of Type I, IIA, IIX, and hybrid fibers, indicating mixed oxidative-glycolytic profiles (Kim et al., 2016). Because fiber-type nomenclature and corresponding assignments can vary with the typing approach, cross-species synthesis is most informative when emphasizing recurring oxidative-glycolytic predominance patterns and morphometric features and organizing these patterns around the same postmortem processes, rather than attempting direct comparisons of absolute fiber-type values across taxa (Lebedová et al., 2019; Kim et al., 2016).

Building on these cross-species patterns in fiber phenotype and morphometrics, a practical insight is that they can help anticipate which postmortem process is likely to explain a larger share of observable quality variation under a given set of handling and measurement conditions (Joo et al., 2013; Matarneh et al., 2021; Listrat et al., 2016). In muscles with a pronounced fast-glycolytic profile, glycolytic potential and the kinetics of pH decline may be relatively more influential by shaping the pH-temperature trajectory, thereby modulating protein functionality and the extent of microstructural shrinkage, with downstream consequences for water-holding capacity (drip and cooking losses) and texture-related traits (Ryu and Kim, 2005; Huff-Lonergan and Lonergan, 2005). By contrast, in muscles with a more oxidative profile, Mb- and mitochondria-related features are more closely linked to color development and stability, consistent with a greater contribution of oxygen consumption and redox buffering (Mancini and Hunt, 2005; Faustman et al., 2010). Importantly, this process-based perspective does not imply a uniform direction of effect. Oxidative phenotypes may be associated with greater color stability under some packaging and retail-display contexts yet may also coincide with elevated oxidative burden under others (Faustman et al., 2010; Mancini and Hunt, 2005). In addition, codeterminants that can covary with fiber traits—such as intramuscular fat distribution and connective tissue characteristics—may partially mask or amplify texture-related outcomes depending on the context (Purslow, 2005; Wood et al., 2008). Accordingly, rather than listing absolute values of individual quality indicators, cross-species comparisons can be more coherently synthesized by interpreting fiber phenotypes in relation to (1) pH-temperature kinetics, (2) oxygen consumption/redox balance, and (3) structural weakening, so that apparent heterogeneity can be interpreted as conditional outcomes within a shared mechanistic framework.