Horses are capable of standing with limited muscle activity as a result of the so-called passive stay apparatus in the forelimbs and hind limbs.1–4 This is an important energy-saving mechanism for animals that stand for prolonged periods. The distal joints of the forelimb (carpal, metacarpophalangeal [fetlock], proximal interphalangeal [pastern], and distal interphalangeal [coffin] joints) and hind limb (fetlock, pastern, and coffin joints) are stabilized during standing by tendinous structures on the palmar and plantar sides of the limbs (Figure 1).2–6 The main structures contributing to the distal part of the passive stay apparatus are the SDFT, DDFT, TI (suspensory ligament), and associated distal ligaments of the proximal sesamoid bones (ie, the sesamoid ligaments). The sesamoid ligaments in combination with the TI prevent hyperextension of the metacarpophalangeal and metatarsophalangeal (fetlock) joints. The combination of the proximal accessory ligaments and the SDFT bridges the carpus, fetlock joint, and pastern joint and passively prevents overextension of these joints. The distal accessory ligament in combination with the DDFT forms a tendinous bridge that passively protects the fetlock, pastern, and coffin joints from overextension.
Although the passive stay apparatus is a well-known structure and is described in textbooks,2–4 little is known about its physical limitations or requirements for proper functioning. One reason for this lack of knowledge is that the distal tendons not only are elements of the passive stay apparatus, but also act as an energy-storage mechanism and contribute to vibration reduction during locomotion.7–9
Apart from trauma, only 2 types of clinical dysfunction of the passive stay apparatus are observed, and these are the result of muscle weakness10 or do not affect all the limbs equally.11 These clinical dysfunctions cannot be linked directly to changes in the passive stay apparatus and therefore provide no useful additional functional information on this apparatus.
In Friesians, a disorder (most likely genetic) resulting in dwarfism is characterized by reduced limb length and overextension of the distal limb joints, indicating a failing passive stay apparatus.12 These dwarf Friesians could therefore provide a good model to study the functional demands of the equine passive stay apparatus.
Overextension of the distal limb joints can be explained by 2 hypotheses based on differences in structural and mechanical properties of the tendons. The first hypothesis suggests that the tendons in dwarf Friesians are relatively longer, compared with the length of the bones, than those in clinically normal (nondwarf) horses. Because the tendons limit extension, longer tendons will result in greater extension of the distal limb joints. Obviously, long tendons (relative to the limb length) can result from elongation of the tendon or a reduction of the limb length.
A second hypothesis suggests that the tendons have the same proportions (equal resting length and cross-sectional area), but the stiffness of the tendons are different. When the tendon material is less stiff, loading of the tendon will result in greater elongation and therefore overextension of the joint.
The purpose of the study reported here was to determine differences in structural and mechanical tendon properties that are associated with changes in functionality of the passive stay apparatus in horses by examining clinically normal Friesians, dwarf Friesians, and ponies.
Deep digital flexor tendon
General linear model
Superficial digital flexor tendon
TCLP-2B, TML Sokki Kenkyujo Co Ltd, Tokyo, Japan.
LabView, National Instruments, Austin, Tex.
SPSS 15, SPSS Inc, Chicago, Ill.
QWin, Leica, Cambridge, England.
Schuurman SO, Kersten W, Weijs WA. The equine hind limb is actively stabilized during standing. J Anat 2003; 202:355–362.
Maierl J. Weissengruber G. Liebich HG. Statics and dynamics. In: König EK, Liebich HG, eds. Veterinary anatomy of domestic animals. Stuttgart, Germany: Schattauer, 2009;277–282.
Dyce KM, Sack WO, Wensing CJG. The forelimb of the horse. In: Dyce KM, Sack WO, Wensing CJG, eds. Textbook of veterinary anatomy. 3rd ed. Philadelphia: Saunders, 2002;568–605.
Dyce KM, Sack WO, Wensing CJG. The hindlimb of the horse. In: Dyce KM, Sack WO, Wensing CJG, eds. Textbook of veterinary anatomy. 3rd ed. Philadelphia: Saunders, 2002;606–625.
Batson EL, Paramour RJ, Smith TJ, et al. Are the material properties and matrix composition of equine flexor and extensor tendons determined by their functions? Equine Vet J 2003; 35:314–318.
Swanstrom MD, Zarucco L, Stover SM, et al. Passive and active mechanical properties of the superficial and deep digital flexor muscles in the forelimbs of anesthetized thoroughbred horses. J Biomech 2005; 38:579–586.
Minetti AE, Ardigo LP, Reinach E, et al. The relationship between mechanical work and energy expenditure of locomotion in horses. J Exp Biol 1999; 202:2329–2338.
Biewener AA. Muscle-tendon stresses and elastic energy storage during locomotion in the horse. Comp Biochem Physiol B Biochem Mol Biol 1998; 120:73–87.
Mero JL, Scarlett JM. Diagnostic criteria for degenerative suspensory ligament desmitis in Peruvian Paso horses. J Equine Vet Sci 2005; 25:224–228.
Back W, Lugt JJvd, Nikkels PGJ, et al. Phenotypic diagnosis of dwarfism in six Friesians. Equine Vet J 2008; 40:282–287.
Moon DK, Woo SLY, Takakura Y, et al. The effects of refreezing on the viscoelastic and tensile properties of ligaments. J Biomech 2006; 39:1153–1157.
Jansen MO, Vanbuiten A, Vandenbogert AJ, et al. Strain of the musculus interosseus-medius and its rami-extensorii in the horse, deduced from in-vivo kinematics. Acta Anat 1993; 147:118–124.
Hoyt DF, Wickler SJ, Cogger EA. Time of contact and step length: the effect of limb length, running speed, load carrying and incline. J Exp Biol 2000; 203:221–227.
Lee DV, Stakebake EF, Walter RM, et al. Effects of mass distribution on the mechanics of level trotting in dogs. J Exp Biol 2004; 207:1715–1728.
Riemersma DJ, Schamhardt HC. The cryo-jaw, a clamp designed for in vitro rheology studies of horse digital flexor tendons. J Biomech 1982; 15:619–620.
Sharkey NA, Smith TS, Lundmark DC. Freeze clamping musculo-tendineous junctions for in vitro simulation of joint mechanics. J Biomech 1995; 28:631–635.
Pollock M, Shadwick RE. Relationship between body-mass and biomechanical properties of limb tendons in adult mammals. Am J Physiol 1994; 266:R1016–R1021.
Swanstrom MD, Stover SM, Hubbard M, et al. Determination of passive mechanical properties of the superficial and deep digital flexor muscle-ligament-tendon complexes in the forelimbs of horses. Am J Vet Res 2004; 65:188–197.
Meershoek LS, Roepstorff L, Schamhardt HC, et al. Joint moments in the distal forelimbs of jumping horses during landing. Equine Vet J 2001; 33:410–415.
Shadwick RE. Elastic energy-storage in tendons—mechanical differences related to function and age. J Appl Physiol 1990; 68:1033–1040.
Abreu EL, Leigh D, Derwin KA. Effect of altered mechanical load conditions on the structure and function of cultured tendon fascicles. J Orthop Res 2008; 26:364–373.
Almeida-Silveira MI, Lambertz D, Perot C, et al. Changes in stiffness induced by hindlimb suspension in rat Achilles tendon. Eur J Appl Physiol 2000; 81:252–257.
Parry DAD. The molecular and fibrillar structure of collagen and its relationship to the mechanical-properties of connective-tissue. Biophys Chem 1988; 29:195–209.
Kjaer M. Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading. Physiol Rev 2004; 84:649–698.