Abstract
Physiological cross‐sectional area (PCSA) is an estimate of a muscle’s contractile force potential, usually derived by dividing muscle mass by the product of a muscle’s average fascicle length and the density of mammalian skeletal muscle. The most commonly used density constants (~1.06 g/cm3) have been derived from experimental studies of tissue samples collected predominantly from the lower limbs of several model taxa, which have included cats, dogs, rabbits and guinea pigs. However, the generalized application of this constant to broader analyses of mammalian myology assumes: that muscle density is consistent regardless of 1) anatomical region and 2) ontogenetic age of the specimen. To investigate these assumptions, we measured the density of whole muscles from specific anatomical regions (head: masseter, digastric, quadratus labii; forearm: pronator teres, extensor carpi radialis longus; leg: gastrocnemius, soleus, plantaris; trunk: psoas minor) from three different age cohorts (3 weeks, 8 months and 2 years) of 48 (n= 12, 24, 12 respectively) New Zealand white rabbits (Oryctolagus cuniculus). The data were analyzed for statistically significant differences between muscle types and specimen ages using Tukey HSD tests. Our findings demonstrate no regional differences in muscle density within the 3‐week‐old cohort. Contrastingly, within the older age cohorts, statisitically significant differences in muscle density were observed between anatomical regions. The plantaris was consistently the most dense muscle studied (1.07 g/cm3) while quadratus labii was the least dense (1.04 g/cm3). The most prominent region specific variation was seen within the triceps surae, with soleus being consistently and significantly less dense than gastrocnemius and plantaris in the older age cohorts. These data suggest that age and region dependent measurements of muscle density may increase the validity of PCSA estimations.
Highlights
Physiological cross-sectional area (PCSA), an important biomechanical variable, is an estimate of a muscle’s contractile force potential and is derived from dividing muscle mass by the product of a muscle’s average fascicle length and a theoretical constant representing the density of mammalian skeletal muscle
The distributions of muscle density for the three age-cohorts and each muscle indicates variation based on both age and anatomical region (Fig. 2)
The quadratus labii and the soleus were the only two muscles that did not show any significant differences in muscle density between the three age-cohorts (Table 2)
Summary
Physiological cross-sectional area (PCSA), an important biomechanical variable, is an estimate of a muscle’s contractile force potential and is derived from dividing muscle mass by the product of a muscle’s average fascicle length and a theoretical constant representing the density of mammalian skeletal muscle This density constant is usually taken from experimental studies of small samples of several model taxa using tissues collected predominantly from the lower limbs of adult animals. This convergence may reflect similarities in study design between these experiments, which were all comprised exclusively of adult individuals, and limited variation among anatomical regions Given these similarities, using a constant for muscle density makes some inherent assumptions. Overarching trends within this literature suggest that muscle density will increase until approximately middle-age and decline throughout senescence
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