Functional specialisation of pelvic limb anatomy in horses (Equus caballus)

Abstract

We provide quantitative anatomical data on the muscle-tendon units of the equine pelvic limb. Specifically, we recorded muscle mass, fascicle length, pennation angle, tendon mass and tendon rest length. Physiological cross sectional area was then determined and maximum isometric force estimated. There was proximal-to-distal reduction in muscle volume and fascicle length. Proximal limb tendons were few and, where present, were relatively short. By contrast, distal limb tendons were numerous and long in comparison to mean muscle fascicle length, increasing potential for elastic energy storage. When compared with published data on thoracic limb muscles, proximal pelvic limb muscles were larger in volume and had shorter fascicles. Distal limb muscle architecture was similar in thoracic and pelvic limbs with the exception of flexor digitorum lateralis (lateral head of the deep digital flexor), the architecture of which was similar to that of the pelvic and thoracic limb superficial digital flexors, suggesting a functional similarity.

Fig. 1

(A)Lateral superficial, (B)lateral deep and (C) medial deep view of the anatomy of equine pelvic limb. *Sartorius and gracilis are transparent so that deeper muscles can be visualised (Figures adapted from König & Liebich, 2004).

Fig. 2

Muscle volume (A & B) and mean muscle fascicle length (C & D) for pelvic and thoracic limb muscles. Muscles are plotted starting with the proximal muscle with the largest mean volume/fascicle length and ending with the distal muscle with the smallest mean volume/fascicle length. Abbreviations for pelvic limb muscles are given in Tables 1 and 2. Thoracic limb muscle abbreviations are as follows: LOT (long head of triceps brachii), SVT (serratus ventralis thoracis), PP (pectoralis profundus), BO (brachiocephalicus and omotransversarius); SVC (serratus ventralis cervicis), LD (latissimus dorsi), PTD (pectoralis transvs. and pectoralis descendens), SC (subclavius), SS (supraspinatus), LAT (lateral head of triceps brachii), TP (trapezius), RC (rhomboideus cervicis), BBM (medial head of biceps brachii), RT (rhomboideus thoracis), BBL (lateral head of biceps brachii), MT (medial head of triceps brachii), ECR (extensor carpi radialis), DDFH (humeral head of flexor digitorum profundus), UL (ulnaris lateralis), CDE (extensor digitorum communis), FCU (flexor carpi ulnaris), SDF (flexor digitorum superficialis), FCR (flexor carpi radialis), DDFU (ulna head of flexor digitorum profundus), LDE (long digital extensor), DDFR (radial head of flexor digitorum profundus), APL (abductor pollicis longus).

Fig. 2

Muscle volume (A & B) and mean muscle fascicle length (C & D) for pelvic and thoracic limb muscles. Muscles are plotted starting with the proximal muscle with the largest mean volume/fascicle length and ending with the distal muscle with the smallest mean volume/fascicle length. Abbreviations for pelvic limb muscles are given in Tables 1 and 2. Thoracic limb muscle abbreviations are as follows: LOT (long head of triceps brachii), SVT (serratus ventralis thoracis), PP (pectoralis profundus), BO (brachiocephalicus and omotransversarius); SVC (serratus ventralis cervicis), LD (latissimus dorsi), PTD (pectoralis transvs. and pectoralis descendens), SC (subclavius), SS (supraspinatus), LAT (lateral head of triceps brachii), TP (trapezius), RC (rhomboideus cervicis), BBM (medial head of biceps brachii), RT (rhomboideus thoracis), BBL (lateral head of biceps brachii), MT (medial head of triceps brachii), ECR (extensor carpi radialis), DDFH (humeral head of flexor digitorum profundus), UL (ulnaris lateralis), CDE (extensor digitorum communis), FCU (flexor carpi ulnaris), SDF (flexor digitorum superficialis), FCR (flexor carpi radialis), DDFU (ulna head of flexor digitorum profundus), LDE (long digital extensor), DDFR (radial head of flexor digitorum profundus), APL (abductor pollicis longus).

Fig. 3

Internal architecture of flexor digitorum superficialis [(A), SDF] and tibialis cranialis [(B), TCN]. The muscles have been sectioned through the muscle belly to reveal the arrangement of the fascicle bundles. Note the striped appearance of the internal aponeurotic tendon of SDF and contrast that with the single internal tendon of TCN.

Fig. 4

(A) Ratio of tendon rest length to muscle fascicle length in pelvic and thoracic limb muscles. A vertical broken line separates pelvic limb MTUs from thoracic limb MTUs. Pelvic limb abbreviations follow those given in Tables 1 and 2. Pelvic and thoracic limb muscles with the same name are distinguished by the suffixes _P and _T, respectively. Thoracic limb abbreviations are as follows: FCU (flexor carpi ulnaris); FCR (flexor carpi radialis); UL (ulnaris lateralis); SDF_T (flexor digitorum superficialis); DDFH (humeral head of flexor digitorum profundus); DDFR (radial head of flexor digitorum profundus); DDFU (ulna head of flexor digitorum profundus); EDLA_T (extensor digitorum lateralis); EDC (extensor digitorum communis), ECR (extensor carpi radialis). Thoracic limb muscle tendon unit data are from Brown et al. (2003a). In order that muscle tendons with smaller TRL : MFL ratios can be seen, the y-axis is restricted to 70 and the value for SDF_P is given at the top of the column. (B)Ratio of estimated tendon stretch when the muscle exerts its maximum isometric force to mean muscle fascicle length in pelvic limb muscles. Mean muscle fascicle lengths were taken directly from Table 4. For those muscles with several bellies but a single tendon of insertion (i.e. biceps femoris and gastrocnemius), mean fascicle length was weighted according to muscle belly mass following the methods of Alexander et al. (1981).

Fig. 5

A comparison of maximum force and mean fascicle length for pelvic and thoracic limb muscles. Data are of proximal (solid squares) and distal (open squares) pelvic limb and proximal (solid triangles) and distal (open triangles) thoracic limb muscles. Thoracic limb extrinsic muscle data are from Payne et al. (2005); supraspinatus, biceps brachii and triceps brachii are from Watson (2004) and all other intrinsic muscle data are from Brown et al. (2003a). Thoracic limb muscle abbreviations: SVT (serratus ventralis thoracis); BBL (lateral head of biceps brachii); SDF_T (thoracic limb flexor digitorum superficialis); SC (subclavius); LD (latissimus dorsi); BO (brachiocephalicus and omotransversarius); LOT (long head of triceps brachii); PP (pectoralis profundus). Force (F_max) was estimated from muscle PCSA and maximum isometric stress of skeletal muscle (0.3 MPa, Woledge et al. 1985; Zajac, 1989; Medler, 2002). Only those muscles with high force and/or long fascicle lengths are labelled.