In a catapult, a large force is applied to store energy, which is then released rapidly to accelerate a small mass. This shows that the bones are arranged in a column, directly on top of each other, giving strength and ensuring that concussive forces spread evenly up the limb. Bony landmarks underlying skin-fixed markers in two marker-placement schemes. The joint is designed to absorb shock. The digits include a medial thumb (when viewed with the palm down), containing two phalanges, and four fingers, each containing three phalanges. Short, highly pennated fibers in combination with a long, elastic tendon are characteristic of muscles involved in elastic energy storage and release. In the canter, overall limb loading decreases with less elastic energy being stored in the SDF tendon, and the DDF tendon being more loaded (Butcher et al., 2007). The trot is the most important gait for evaluation of the quality of a horse’s movement and for detection of lameness. Lacertus fibrosus has a much smaller mass than the internal tendon of biceps brachii and is capable of storing much less energy (10–28 J) (Watson & Wilson, 2007). In general, muscle size (volume and mass) and fiber length decrease in a proximal to distal direction within the equine limbs. In the horse’s forelimb, the biceps tendon is stretched by forward movement of the trunk and the changing orientation of the ground reaction force vector relative to the shoulder and elbow joints during the stance phase. This chapter reviews the structure and functions of the equine forelimbs in relation to locomotor activity, including kinematics (movements) and kinetics (forces) during the stride. 2 - st. letter L. 3 - st. letter N. 4 - st. letter A. From the front, a plumb-line dropped from the point of shoulder should bisect the limb and hoof. Long fibers arranged in parallel with the long axis of the muscle belly have the greatest capacity to shorten the muscle. The absence of a clavicle allows the scapula more freedom to rotate and translate relative to the ribcage, which may contribute to an increase in stride length. During galloping, the proximal limb from scapula to elbow shortens by about 12 mm, whereas the limb distal to the elbow shortens by around 127 mm. 27, 31–38, with permission from the Equine Veterinary Journal. The fiber direction suggests a primary role in forelimb protraction. The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. GRF data can be combined with kinematic data using a link segment model to calculate internal forces within the limb that cannot be measured directly (see Chapter 19 for details). In the equine distal limb, the suspensory ligament and SDF tendon are primarily responsible for storing and releasing elastic energy. Energy absorption and generation on the extensor (cranial) aspect of the shoulder joint are similar in magnitude to those at the elbow joint but occur slightly later in the stance phase. Left: measurement of the angle between the proximal and distal segments on the anatomical flexor aspect. Common Structures of the Proximal Forelimb and Shoulder Scapula The Scapula forms the basis of the shoulder region, providing points of attachment of extrinsic and intrinsic muscles. During standing, the suspensory ligament is fully capable of supporting the horse’s weight passively (Dyce et al., 1996). Fig 6.1 Two methods of measuring joint angles of the forelimbs with the measured angles being represented by black arcs. A stride is regarded as the unit of measurement. Left: markers placed over centers of joint rotation with limb segments being represented by lines joining the markers. Tendons can recoil elastically much faster than muscles shorten, which is beneficial in situations where rapid movement is required. (1995a) used standardized procedures to describe sagittal plane kinematics of the trot in a large group of Warmblood horses. Locomotor muscles account for about 42% of the horse’s body mass (Gunn, 1978) with the large, powerful muscles concentrated in the proximal limb, while the distal forelimb makes use of long, elastic tendons to reduce the metabolic cost of locomotion. By contrast, the small medial head of triceps (mass, 85–271 g; fiber length, 9–17 cm) and the anconeus muscle each account for only 2% of elbow extensor muscle mass and both are composed almost entirely of slow-twitch fibers, suggesting their role is to support the elbow in extension during stance (Ryan et al., 1992). Traditionally the ideal shoulder has a 45° slope to the horizontal with a similar hoof–pastern angle. With an increase in galloping speed, the DDF muscle and its associated tendon assume a greater role in support of the MCP joint, thus relieving some stress on the SDF tendon. Extensive research has been done to find similarities and differences in the DNA sequences of different an-imals. During standing, the suspensory ligament is fully capable of supporting the horse’s weight passively (Dyce et al., 1996). Today, muscle fibers comprise about 10% of the suspensory ligament. In general, muscle size (volume and mass) and fiber length decrease in a proximal to distal direction within the equine limbs. Flexor and extensor muscles are prominent in most mysticetes (exceptMegaptera), and sperm whales and beaked whales (physeterids, kogiids, and ziphiids), but are lacking in other fami- The forearm should be long and well muscled, and the cannon bone should be short with adequate flat bone. The CDE muscle is active in terminal swing at the walk, when it extends the digit in preparation for ground contact, (Jansen et al., 1992). The fatigue-resistant, slow-twitch fibers of the SDF tendon act eccentrically or isometrically during stance with changes in length of the musculotendinous unit being due almost entirely to stretching of the elastic tendon (Butcher et al., 2007). Only gold members can continue reading. Right: two markers placed along the long axis of each segment are joined to represent the segment with adjacent segments intersecting at the joints. Architectural properties of the muscles of the equine antebrachium Reprinted from Willemen, M.A., Savelberg, H.H.C.M., Barneveld, A., 1997, The improvement of the gait quality of sound trotting warmblood horses by normal shoeing and its effect on the load on the lower forelimb, Livestock Production Science, 52 (2), 145–153, with permission from Elsevier. The architectural properties of these muscles have been described (Hermanson, 1997; The deep digital flexor (DDF) has three distinct muscle bellies, humeral, ulnar and radial, each of which is innervated by a separate branch of the median nerve suggestive of neuromuscular compartmentalization (Zarucco et al., 2004). The functions of the DDF are to flex the digital joints during the swing phase and to generate a propulsive force during the second half of stance. Both of these muscles insert on the accessory carpal bone, which increases their moment arm and facilitates their ability to stabilize the carpus during stance. Since biceps brachii and the long head of triceps brachii are biarticular, their interaction affects motion and stability of both shoulder and elbow joints. It is active in late swing and early stance (Tokuriki et al., 1989; The muscles of the forearm move and stabilize the carpal and digital joints. When the DDF muscle becomes fatigued, the SDF tendon is over-loaded and predisposed to strain injury (Butcher et al., 2007), which occurs frequently in equine athletes, especially in Thoroughbred racehorses (Peloso et al., 1994). The rhomboideus lies underneath the trapezius and ties the scapula into the sides of the spinous processes of the thoracic vertebrae and the nuchal ligament. ), Architectural properties of the muscles of the equine antebrachium. Serratus ventralis cervicis (Fig. This muscle is more variable in its mass than the other extrinsic muscles, which may reflect adaptation in response to the amount and type of training. 6.4). About this Quiz. Trapezius (Fig. 6.5) (van Weeren et al., 1990a; Back et al., 1995a). In contrast, the fibers in the radial head are short and highly pennated (pennation angle close to 30°). It is a two-beat gait with the limbs coordinated by diagonal pairs. Read This! 6.2, Table 6.1). The tree shrew skeleton closely resembles that of early mammals and represents the ancestral forelimb skeleton. It is enveloped by an aponeurotic sheath that is part of the thoracolumbar fascia. Compared with supraspinatus, biceps brachii has a larger force generating capacity and a larger moment arm at the shoulder joint, which suggests that it may be a more effective extensor of the shoulder (Watson & Wilson, 2007). Tags: Equine Locomotion Forelimb. In general, the limb spring is stiffer in larger animals but, within an individual, stiffness of the limb spring is nearly independent of speed. Summary of forelimb motoneuron pool organization Forelimb motoneuron groups in mammals can be divided into a rostral group that includes deltoid, supraspinatus, infraspinatus, and biceps brachii, and a caudal group that includes the forearm flexors and extensors, triceps, pectoralis, and the intrinsic muscles of the hand. The DDF muscle, however, has fast-twitch fibers that contract concentrically and are susceptible to fatigue. Large, cursorial animals use this musculotendinous arrangement to move at high speeds with a relatively economical metabolic cost (, The extrinsic muscles of the forelimb, which have an attachment to the bones of the limb and an attachment to the trunk, are responsible for suspending the trunk between the forelimbs and for moving the forelimbs relative to the trunk. 6.3, Table 6.2) is somewhat smaller than the thoracic part of the muscle and differs in having relatively long fibers and a smaller cross-sectional area (Payne et al., 2004), that confer the ability to support the base of the neck or to retract the limb by rotating the proximal scapula cranially. Scapula The instability seen with paralysis of the supraspinous nerve (Sweeney) supports this presumptive function. Skin displacement relative to the underlying bones is always a concern when kinematic studies are based on skin-fixed markers. Label the bones in each animal forelimb in Model 2. They form a large, powerful muscle, with long fibers oriented parallel to the muscle belly (Payne et al., 2004). The humeral head, which comprises about 74% of total muscle volume, has the longest fibers of any muscle in the antebrachium, but also has some short and medium length fibers. 6.3, Table 6.2) are large with relatively long fibers that suggest a primary role in adducting the forelimb (Payne et al., 2004). Subclavius (Table 6.2) has long fibers (519 mm) that allow generation of large forces to assist in adduction and retraction of the forelimb or stabilization of the scapula (Payne et al., 2004). At this time, the net joint moment is on the cranial side of the joints (Lanovaz et al., 1999) indicating that the biarticular extensor carpi radialis is flexing the elbow to protract the limb and is controlling the inertially driven carpal flexion (Colborne et al., 1997a, b). belonging to or near the back or upper surface of animal. Distances are expressed as percentage segment length between the two reference markers. 6.5) (van Weeren et al., 1990a; Back et al., 1995a). They are subject to high tendon strains as the limb is loaded during stance, especially at the trot. The digital flexor and extensor muscles (Table 6.3) are characterized by having long tendons relative to their muscle length. It is enveloped by an aponeurotic sheath that is part of the thoracolumbar fascia. Typical marker configurations involve either placing a marker over the center of rotation of each joint or aligning two markers along the long axis of each segment (Fig. The ulna is very small except for the olecranon process, which forms part of the elbow. The thorax is slung between the two scapulae by an arrangement of muscles, tendons and ligaments known as the thoracic sling. The shape and proportions of the feet should be suitable for the limb, a pair and ‘in balance’. At faster speeds, vertical excursions of the center of mass are reduced and the limb sweeps through a larger angle during its stance phase causing the horse to bounce off the ground more quickly (Farley et al., 1993). In a catapult, a large force is applied to store energy, which is then released rapidly to accelerate a small mass. Fiber length and pennation angle affect the range of motion through which the muscle contracts and its ability to generate force. This fiber composition is well suited for its support role as part of the stay apparatus (Swanstrom et al., 2005), and for attenuating high-frequency forces associated with impact (Wilson et al., 2001). The functions of the DDF are to flex the digital joints during the swing phase and to generate a propulsive force during the second half of stance. MFL, mean fiber length; PCSA, physiological cross sectional area; MPA, mean pennation angle. et al. The deltoid muscle arises from the scapula spine. Furthermore, the angle may be expressed in absolute terms or it may be normalized to the standing angle, the angle at ground contact or the average angle during the stride (Mullineaux et al., 2004). 6.1). Common faults include the following: The fetlock joints should be well defined and bony rather than puffy. The functions of the musculotendinous system of the equine forelimb include connecting the forelimb to the trunk; supporting the body mass; stabilizing the joints in opposition to the force of gravity during the stance phase; generating forces that are used for propulsion, braking and turning; and flexing the joints to lift the hoof clear of the ground during the swing phase. In a galloping horse, the biceps tendon has been estimated to release 243 J in 0.11 s, which would require the power output of 50 kg of non-elastic muscle (Wilson & Watson, 2003). Today, muscle fibers comprise about 10% of the suspensory ligament. However, there are marked differences in muscle architecture and tendon properties of the deep and superficial digital flexor muscles that are indicative of the different roles played by these musculotendinous units in locomotion. 6.3) and omotransversarius are considered together since their fibers cannot be separated close to their origins in the shoulder region. Serratus ventralis thoracis is the primary muscular component of the thoracic sling, which suspends the trunk between the forelimbs and controls the position of the thorax and withers relative to the scapulae when the forelimbs are weight-bearing. Compared with supraspinatus, biceps brachii has a larger force generating capacity and a larger moment arm at the shoulder joint, which suggests that it may be a more effective extensor of the shoulder (Watson & Wilson, 2007). In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of the clavicle, scapula, and coracoid. It supports the metacarpophalangeal (MCP) joint during the stance phase, which is critical to the function of the equine limb. In practice, as long as the shoulder is flat and long enough to ensure a good stride length, it does not matter if it is a little upright. 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