Comparative functional anatomy of the epaxial musculature of dogs (Canis familiaris) bred for sprinting vs. fighting

Abstract

The axial musculoskeletal system of quadrupedal mammals is not currently well understood despite its functional importance in terms of facilitating postural stability and locomotion. Here we examined the detailed architecture of the muscles of the vertebral column of two breeds of dog, the Staffordshire bull terrier (SBT) and the racing greyhound, which have been selectively bred for physical combat and high speed sprint performance, respectively. Dissections of the epaxial musculature of nine racing greyhounds and six SBTs were carried out; muscle mass, length, and fascicle lengths were measured and used to calculate muscle physiological cross-sectional area (PCSA), and to estimate maximum muscle potential for force, work and power production. The longissimus dorsi muscle was found to have a high propensity for force production in both breeds of dog; however, when considered in combination with the iliocostalis lumborum muscle it showed enhanced potential for production of power and facilitating spinal extension during galloping gaits. This was particularly the case in the greyhound, where the m. longissimus dorsi and the m. iliocostalis lumborum were estimated to have the potential to augment hindlimb muscle power by ca. 12%. Breed differences were found within various other muscles of the axial musculoskeletal system, particularly in the cranial cervical muscles and also the deep muscles of the thorax which insert on the ribs. These may also highlight key functional adaptations between the two breeds of dog, which have been selectively bred for particular purposes. Additionally, in both breeds of dog, we illustrate specialisation of muscle function by spinal region, with differences in both mass and PCSA found between muscles at varying levels of the axial musculoskeletal system, and between muscle functional groups.

Keywords: Canis; axial; back; greyhound; locomotion; muscle; vertebral column.

Figure 1

Diagrammatic representation of muscles of the vertebral column coloured by muscle group: red – longissimus; blue – transversospinalis; green – iliocostalis; orange – other.

Figure 2

(A) Scaled muscle mass. (B) Scaled PCSA (physiological cross-sectional area). (C) Scaled fascicle length. (D) Architectural Index for muscles of the vertebral column. Bars represent mean ± 95% confidence intervals for Staffordshire bull terriers (dark red; n = 6) and greyhounds (red; n = 9). Muscles are arranged from cranial to caudal along the vertebral column. Significant differences (P < 0.05) between dog breeds are indicated by asterisks.

Figure 3

Functional regionalisation of greyhound muscle architecture by region of the vertebral column (cervical – dark, thoracic – pale and lumbar – mid) and by muscle functional group (red – longissimus; blue – transversospinalis; green – iliocostalis; orange – other) for (A) scaled muscle mass, (B) scaled PCSA (physiological cross-sectional area), (C) scaled fascicle length and (D) Architectural Index. Bars represent mean ± SE.

Figure 4

Functional regionalisation of Staffordshire bull terrier muscle architecture by region of the vertebral column (cervical – dark, thoracic – pale and lumbar – mid) and by muscle functional group (red – longissimus; blue – transversospinalis; green – iliocostalis; orange – other) for (A) scaled muscle mass, (B) scaled PCSA (physiological cross-sectional area), (C) scaled fascicle length and (D) Architectural Index. Bars represent mean ± SE.

Figure 5

(A) Scaled mean fascicle length against scaled physiological cross-sectional area (PCSA) for epaxial muscles of greyhounds (circular symbols) and Staffordshire bull terriers (triangular symbols). Bars represent SEM. Muscles towards the bottom right of the plot are suited to performing work over a wide range of motion; muscles to the top right are suited to work and power production; muscles to the top left of the plot have ability to generate high amounts of force. (B) Re-scaled plot, with the longissimus dorsi muscles removed to show, with improved clarity, the functional regionalisation of other back muscles. Point colours represent muscle group (red – longissimus; blue – transversospinalis; green – iliocostalis; orange – other). Muscle abbreviations are given in Table 1.