The Hind Limb
WHIST THE FORELIMB is mainly concerned with support, the hind limb is for propulsion. It is the structure that drives the horse forwards. Straightened out, the hind limb of the horse is functionally longer than the forelimb because it is the ‘powerhouse’ that drives the horse forwards. Also the hind hooves are a different shape to those of the front limbs. Whilst the front hooves are wider and rounder to help support the weight of the forehand, the hind hooves are longer and more pointed to enable the toes to penetrate the ground giving the horse a firm base against which to propel the body forwards. In this chapter we will illustrate and describe the major muscles of the hind limb and the structures that enable the horse to sleep standing up and rest a hind limb whilst so doing. These are the reciprocal apparatus and the stifle locking mechanism.
Muscles of the hind limb
MUSCLES OF THE HIP JOINT
TENSOR FASCIAE LATAE Figure 3.1
ORIGIN Tuber coxa
INSERTION a. Third trochanter (with superficial gluteal muscle); b. with fascia lata on patella, lateral patellar ligament and cranial border of tibia
INNERVATION Cranial gluteal nerve
FUNCTION a. Flexes hip; b. extends stifle; c. protracts hind limb; d. tenses fascia lata
DEVELOPMENT ISSUES Relatively few, most trauma based
Figure 3.2 Superficial gluteal
SUPERFICIAL GLUTEAL (Figure 3.2)
ORIGIN Tuber coxa
INSERTION Third trochanter and fascia lata
INNERVATION Cranial and caudal gluteal nerve
FUNCTION a. Flexes hip; b. abducts (moves away from body) and protracts hind limb
DEVELOPMENT ISSUES Reduced step length; resists lateral movement
MIDDLE GLUTEAL (Figure 3.3)
ORIGIN a. Longissimus lumborum; b. gluteal surface of ilium; c. sacrum; d. sacroiliac and sacrosciatic ligaments
INSERTION Greater trochanter at head of femur
INNERVATION Cranial gluteal nerve
FUNCTION a. Extends hip; b. abducts hind limb
DEVELOPMENT ISSUES As above
Figure 3.4 Accessory gluteal
ACCESSORY GLUTEAL (Figure 3.4)
ORIGIN Gluteal surface of ilium
INSERTION Distal to greater trochanter
INNERVATION Cranial gluteal nerve
FUNCTION a. Extends hip; b. abducts hind limb
DEVELOPMENT ISSUES As above
Figure 3.5 Deep gluteal
DEEP GLUTEAL (Figure 3.5)
ORIGIN Spine of ischia
INSERTION Greater trochanter at head of femur
INNERVATION Cranial gluteal nerve
FUNCTION Abducts hind limb
DEVELOPMENT ISSUES As above
CAUDAL THIGH MUSCLES (HAMSTRINGS GROUP)
BICEPS FEMORIS (Figure 3.6)
ORIGIN a. Vertebral head: spinous and transverse processes of last three sacral vertebrae, sacrosciatic ligament and tail fascia; b. pelvic head: tuber ischia
INSERTION a. Patella, lateral and middle patella ligaments; b. cranial border of tibia and crural fascia; c. via long calcanean tendon to calcaneus
INNERVATION Caudal gluteal and sciatic nerves
FUNCTION a. Extends hip and stifle; b. with caudal division flexes stifle; c. abducts hind limb
DEVELOPMENT ISSUES Shortening of cranial portion of stride; resists lateral movement; discomfort in hind joints
Figure 3.7 Semitendinosus
SEMITENDINOSUS (Figure 3.7)
ORIGIN a. Vertebral head: last sacral and first two caudal vertebrae, tail fascia and sacrosciatic ligament; b. pelvic head: ventral tuber ischia
INSERTION a. Cranial border of tibia, crural fascia; b. via long calcanean tendon to calcanius
INNERVATION Caudal gluteal and sciatic nerves
FUNCTION a. During weight bearing: extends hip, stifle and hock; b. During non-weight bearing: retracts and adducts limb
DEVELOPMENT ISSUES As above
SEMIMEMBRANOSUS (Figure 3.8)
ORIGIN a. Vertebral head: first caudal vertebra, sacrosciatic ligament; b. pelvic head: ventromedial aspect of tuber ischia
INSERTION Medial condyles of femur and tibia
INNERVATION Caudal gluteal and sciatic nerves
FUNCTION a. During weight bearing: extends hip and stifle; b. during nonweight bearing: retracts and adducts limb
DEVELOPMENT ISSUES As above
DEEP MUSCLES OF THE HIP
Figure 3.9 Gemelli
GEMELLI (Figure 3.9)
ORIGIN Dorsal border of ischium
INSERTION Trocanteric fossa of femur
INNERVATION Sciatic nerve
FUNCTION Rotates femur laterally
DEVELOPMENT ISSUES Overreaching injuries on front limb; moving on three tracks
Figure 3.10 Internal obturator
INTERNAL OBTURATOR (Figure 3.10)
ORIGIN Internal surface of ischium and pubis from border of obturator foramen to pelvic symphysis
INSERTION Trocanteric fossa of femur
INNERVATION Sciatic nerve
FUNCTION Rotates femur laterally
DEVELOPMENT ISSUES Overreaching injuries on front limb; moving on three tracks
Figure 3.11 External obturator
EXTERNAL OBTURATOR (Figure 3.11)
ORIGIN Ventral surface of pelvis from border of obturator foramen
INSERTION Trocanteric fossa of femur
INNERVATION Obturator nerve
FUNCTION Rotates femur laterally and adducts limb
DEVELOPMENT ISSUES As above plus horse resists lateral work
Figure 3.12 Gracilis
GRACILIS (Figure 3.12)
ORIGIN Pelvic symphysis via symphysial tendon
INSERTION Crural fascia, medial patella ligament, cranial border of tibia
INNERVATION Obturator nerve
FUNCTION Adducts limb
DEVELOPMENT Resists lateral work
Figure 3.13 Adductor
ADDUCTOR (Figure 3.13)
ORIGIN Ventral surface of pelvis; symphysial tendon
INSERTION Caudal surface and medial epicondyle of femur
INNERVATION Obturator nerve
FUNCTION Adducts and retracts limb
DEVELOPMENT ISSUES As above
Figure 3.14 Pectineus
PECTINEUS (Figure 3.14)
ORIGIN Pubis and iliopubic eminence
INSERTION Medial surface of femur
INNERVATION Obturator nerve and femoral nerve
FUNCTION Adducts limb; flexes hip joint
DEVELOPMENT ISSUES Resists lateral work
STIFLE EXTENSORS
Figure 3.15 Sartorius
SARTORIUS (Figure 3.15)
ORIGIN Iliac fascia and insertion tendon of psoas minor
INSERTION Medial aspect of stifle
INNERVATION Femoral nerve
FUNCTION Flexes hip; protracts and adducts limb
DEVELOPMENT ISSUES Unlevelness; resistance to lateral work
Figure 3.16a and b Quadriceps femoris: a) rectus femoris; vastus lateralis; b) vastus medialis; rectus femoris. The vastus intermedius is not shown because it is deep to the other structures.
QUADRICEPS FEMORIS (Figures 3.16a and b)
a. rectus femoris (Figure 3.16a)
ORIGIN Shaft of ilium cranial to acetabulum
b. vastus lateralis (Figure 3.16a)
ORIGIN Proximolateral on femur
c. vastus medialis (Figure 3.16b)
ORIGIN Proximomedial on femur
d. vastus intermedius (not shown)
ORIGIN Proximodorsal on femur
INSERTION Via intermediate patellar ligament on tibial tuberosity
INNERVATION Femoral nerve
FUNCTION Flexes hip, extends and stabilises stifle
DEVELOPMENT ISSUES Unlevelness; resistance to lateral work
POPLITEUS (Figure 3.17)
ORIGIN Lateral condyle of femur
INSERTION Caudomedial border of tibia
INNERVATION Tibial nerve
FUNCTION Flexes stifle
DEVELOPMENT ISSUES Unlevelness; dragging of toe
HOCK EXTENSORS AND DIGIT FLEXORS (CAUDAL)
Figure 3.18 Gastrocnemius
GASTROCNEMIUS (Figure 3.18)
ORIGIN Supracondylar tuberosities of the femur
INSERTION Via long calcanean tendon onto calcaneus
INNERVATION Tibial nerve
FUNCTION Extends hock and flexes stifle
DEVELOPMENT ISSUES Unlevelness; shortness of step; dragging of toe
Figure 3.19 Superficial digital flexor
SUPERFICIAL DIGITAL FLEXOR (Figure 3.19 – see also reciprocal apparatus on page 58 and Figure 3.26)
ORIGIN Supracondylar fossa of femur
INSERTION Plantar on distal end of proximal phalanx and proximal collateral tubercles of middle phalanx
INNERVATION Tibial nerve
FUNCTION Proximally acts as part of reciprocal apparatus
DEVELOPMENT ISSUES Little if any muscle tissue: evolved as passive spring; injury mainly as a result of trauma
Figure 3.20 Deep digital flexor: a) lateral digital flexor; b) tibialis caudalis; c) medial digital flexor.
DEEP DIGITAL FLEXOR (Figure 3.20)
a. lateral digital flexor
ORIGIN Caudal surface of tibia with tibialis caudalis
b. tibialis caudalis
ORIGIN Caudal surface of tibia with lateral digital flexor
ORIGIN Lateral tibial condyle
INSERTION Plantar on distal phalanx
INNERVATION Tibial nerve
FUNCTION Extends hock and flexes digit
DEVELOPMENT ISSUES Unlevelness; dragging of toe
HOCK FLEXORS AND DIGIT EXTENSORS
TIBIALIS CRANIALIS (Figure 3.21)
ORIGIN Lateral condyle and tuberosity of tibia
INSERTION a. Dorsal branch on proximal end of 3rd metatarsal and 3rd tarsal bone b. medial branch on 1st and 2nd tarsal bone
INNERVATION Peroneal nerve
FUNCTION Flexes hock
DEVELOPMENT ISSUES Short stride cranially
Figure 3.22 Peroneus tertius
PERONEUS TERTIUS (Figure 3.22)
ORIGIN Lateral condyle of femur
INSERTION Tarsal bones (except 1 and 2) and on 3rd metatarsal
INNERVATION Peroneal nerve
FUNCTION Entirely tendinous and forms reciprocal apparatus with superficial digital flexor
DEVELOPMENT ISSUES Usually traumatic only
Figure 3.23 Lateral digital extensor
LATERAL DIGITAL EXTENSOR (Figure 3.23)
ORIGIN Lateral collateral ligament of stifle and adjacent tibia and fibula
INSERTION Joins tendon of long extensor
INNERVATION Peroneal nerve
FUNCTION Extends digit and flexes hock
DEVELOPMENT ISSUES Short stride cranially and dragging of toe
INTERROSEUS (SUSPENSORY LIGAMENT) (Figure 3.24)
ORIGIN 3rd metacarpal, calcaneus and 4th tarsal bone
INSERTION Proximal sesamoid bones
INNERVATION Tibial nerve
FUNCTION Counteracts overextension of fetlock
DEVELOPMENT ISSUES Usually at origin in horses worked on deep rubber surfaces
In the hind limb, the structures below the fetlock have the same configuration as the forelimb including the sesamoidean ligaments (see Chapter 2).
Like the forelimb, the hind limb also contains suspensory apparatus using the same structures as in the forelimb. However, the hind limb also contains two other specialised energy-saving systems which fix the hind limb to enable the horse to sleep standing up: the stifle locking mechanism and reciprocal apparatus.
Stifle locking mechanism
Figure 3.25 The stifle locking mechanism, left hind dorsal view.
The stifle locking mechanism (Figure 3.25) enables the horse to lock the stifle out completely, preventing any flexion during muscle relaxation on rest. The group of muscles that perform this action comprise the quadriceps femoris and tensor fasciae latae, which have insertions on the intermediate patellar ligament. When contracted, with the limb weight bearing, they pull the patella, the parapatellar cartilage and the medial patellar ligament inwards and upwards and ‘hook’ them over the medial trochlear ridge of the femur. This prevents the stifle joint from flexing. The horse will rest the other hind limb by slightly flexing the joints and resting on the toe. To return the stifle to normal movement, the horse contracts the quadriceps and lifts the patella off the trochlear ridge. You can observe this in the resting, standing horse when he is resting one hind limb, and locking the stifle of the other hind limb. However, some muscular effort must be required for this process as the horse will tire of standing on one hind limb after a few minutes and will change to lock the opposite hind limb whilst resting the one that had previously been locked.
Sometimes (particularly in fast-growing warmblood breeds) this can happen during movement. This leads to the condition known as ‘locking stifle’, which is also known as upward patellar fixation, in which the hind limb locks out during normal movement, without conscious effort by the horse. Usually, by making the horse walk backwards, the stifle will unlock, but it can be a distressing condition for horse and owner. Normally, as the horse gets stronger, the syndrome will resolve completely without any intervention, but in some cases surgery may be indicated. Locking stifle is a condition that very often responds to skilled physiotherapy treatment.
Reciprocal apparatus
Figure 3.26 The reciprocal apparatus
In the absence of trauma, the horse cannot flex or extend the stifle without flexing or extending the hock – the two are inextricably linked by a system known as the reciprocal apparatus (Figure 3.26). This comprises two muscles, the superficial digital flexor muscle (see page 52), and the peroneus tertius muscle (see page 54). Because of the action of these two muscles the movement of the two joints mirror each other. However, neither of these structures are strictly muscles. The peroneus tertius is entirely tendinous and the superficial digital flexor has very little muscle tissue within it; it has evolved to become a passive spring. Therefore the stifle and the hock must move in unison. It follows that if the stifle is locked by the locking mechanism then, because of the reciprocal apparatus, the hock is also locked.
Chapters 2 and 3 should have ensured you now have an appreciation of how the limbs of the appendicular skeleton work, and just how fragile they can be, especially below the knee and hock.