Modern weights and morphometric measurements of the cheetah (Acinonyx Jubatus)

[Acinonyx sp.]

Anatomy, functional anatomy and morphometrical study of forelimb column in Asiatic cheetah (Acinonyx jubatus venaticus)  

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Ulna 
At the proximal end of the ulna, the olecranon projected beyond the radius. Its free end was expanded to form the olecranon tuber. There was a proximodistal groove on the lateral surface of the olecranon (Fig. 9). There was also an incisure at its base, where it lied against the radius. Proximal to the articular surface the sharpbordered anconeal process projected cranially (Fig. 11), while distally and on either side the lateral and medial coronoid processes also projected forwards (Figs. 9, 11). Between the two processes there was the trochlear notch which was articulated with the articular circumference of the radius (Fig. 9). There was an articular surface extended in lateromedial direction on the trochlear notch. Also a broad articular surface was under the trochlea notch (Figs. 9, 11). Distal to the trochlear notch the ulna was rough where it faced the radius.

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Figure 7. Cranial surface of radius: 1) Insertion of supinator, 2) Insertion of brachioradialis, 3) Radial (medial) styloid process, 4,5,6) Medial, middle and lateral groove respectively, 7) Radial tuberosity. Notice the diaphysis curvature

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Figure 8. Caudal surface of radius: 1) Radial head, 2) Radial tuberosity, 3) Insertion of biceps brachialis, 4) Insertion of brachialis, 5) Origin of radial head of deep digital flexor tendon, 6) Insertion of brachioradialis, 7) Radial (medial) styloid process, 8) Ulnar notch, 9) Transverse crest.

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Figure 9. Lateral surface of ulna: 1) Olecranon tuberosity and insertion of triceps, 2) Olecranon tuberosity and insertion of anconeus, 3) Trochlear notch, 4,5) Coronoid process, 6) Lateral styloid process.

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Figure 10. Medial surface of ulna: 1) Trochlear notch, 2) Radial notch, 3) Insertion of triceps, 4) Insertion of anconeus muscle, 5) Origin of ulnar head of flexor carpi ulnaris, 6) Ulnar head of deep digital flexor tendon, 7) Lateral styloid process.

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Figure 11. Cranial surface of the proximal half of ulna: 1) Olecranon tuberosity, 2) Anconeal process, 3,5) Coronoid process, 4) Insertion of pronator quadratus.

The shaft of the ulna was triangular in section and, like the radius, it was slightly convex cranially. The proximal half of the shaft in this bone was as thick as the distal one in caudal view. On the other hand, along the craniocaudal axis the upper part of the diaphysis was significantly longer than the distal part (Fig. 9, 10). There were two fossae on the lateral and medial surfaces of the shaft, the medial one was deeper than the other. Also, there was a broad nonarticular eminence on the lateral surface of the shaft (Fig. 9). The ulna was tapered distally. The olecranon tuber ended in three prominences. Two were cranial and with thin borders, while the third was caudal. The trochlear notch of the Asiatic cheetah was divided by a sagittal ridge into a larger lateral and a smaller medial surface. The medial coronoid process was broad, the lateral process was narrow and the anconeal process projected hook-like (Fig. 9, 11). The radial notch was concave and corresponded to the convex articular circumference of the radius (Fig. 10). The lateral styloid process projected distally and had a deeply convex articular surface for articulation with the carpal bones (Fig. 9, 10). Medially it had a convex articular circumference which joined with the radius. As mentioned before, the shaft was roughly prismatic, so it had three surfaces and three borders. The anterior surface was articulated with the posterior lateral aspect of the radius (Fig. 11). At the upper part of the anterior surface there were two articular facets for articulation with the corresponding facets of radius (Fig. 11). The medial surface was smooth and concave. The proximal end was expanded and comprised one large olecranon process and a semilunar notch (Fig. 10). The olecranon process had two surfaces and two borders. The lateral surface was convex and the medial one was concave (Fig. 9, 10). The anterior border was limited by a semilunar notch and thereby formed a beak like projection, known as anconeal process (Fig. 11).

B) Osteometric analysis The results are summarized in Table 1. Based on obtained data humerus was longer and thicker than radius.

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Discussion 
Quadrupeds typically support a greater proportion of their body weight with their forelimbs during steady state locomotion (Alexander and Jayes, 1983; Witte et al., 2004) and, with increasing speed, peak GRFs have been shown to increase (Witte et al., 2004). When travelling at top speed cheetah’s forelimbs are therefore likely to experience very high peak forces, and must be particularly adapt to resist large GRF joint torques. A long moment arm would increase the leverage that the muscle exerts at the joint (enabling a bigger joint torque for a given change in muscle length), maximizing the joint torque that can be achieved. Contrary to this, the forelimbs of quadrupeds are often thought of as springy struts (Blickhan, 1989; Blickhan and Full, 1993), where the GRF vector is aligned through the point of rotation of the forelimb on the body, resulting in small GRF joint torques, particularly at the shoulder (Carrier et al., 2008). Maintaining a longer stance time will help to limit the peak vertical forces that the cheetah’s limb experiences whilst maintaining the impulse required to support its own body weight when travelling at a given speed. Therefore, if peak force is a limit to an animal’s maximum speed, this may be a way for cheetah to maintain higher duty factors when travelling at low speeds, enabling it to attain higher maximal speeds. This will be of great importance in the forelimb, as the forelimbs tend to support a larger proportion of an animal’s body weight during steady state locomotion (Witte et al., 2004). According to results of Hudson et al. (2011) the cheetah’s humerus and radius are heavier than the greyhound’s, which will be essential for maintaining bone strength and safety factors (Alexander, 1993; Sorkin, 2008), but this will increase the inertia of the limb. Increased inertia would result in a longer swing time or more muscular work to accelerate and decelerate the limb through swing (Hudson et al., 2011). As mentioned in a previous study (Hudson et al., 2011) that is in agreement with our observations, the olecranon tuberosity in the cheetah is proportionally greater than that in the cat. Triceps is one of the muscles which insert to it. Cheetah’s musculoskeletal system must modulate and control the high speed maneuvering of its hunting style (Hudson et al., 2011). To prevent excessive joint torque, damage or instability at the elbow, the long head of triceps functions to extend the joint during stance (English, 1978). These hypotheses may justify the large olecranon tuberosity in the Asiatic cheetah According to a study in the cheetah, the forelimb musculature comprises 15.1 ± 1.2% of its total body mass, substantially less than its hindlimb which comprises 19.8 ± 2.2% of total body mass (Hudson et al., 2011). Pasi and Carrier (2003) suggested that the forelimbs of highly specialized runners would contain less muscle mass than the hindlimbs, as the forelimbs play a greater role in deceleration compared with the hindlimbs, which accelerate the centre of mass. This is because during deceleration muscles contract eccentrically (high force output), actively stretching to absorb energy, compared with the concentric (low force output) contractions used during accelerations, and therefore the forelimbs can contain muscles with smaller physiological cross-sectional areas to achieve the same force output (Hudson et al., 2011). It seems that the column of forelimb in the Asiatic cheetah is designed to reach this target because of its relatively extended length and small diameter. Increasing in length of a bone leads to increase the mass of the muscles around it. According to some studies many of the cheetah’s proximal intrinsic limb muscles are larger in mass than those in the greyhound (Hudson et al., 2011). They also have longer maximum moment arms in the cheetah when compared with felids, enabling to produce larger joint torques but reducing the capacity to produce high joint rotational velocities (Hudson et al., 2011; Williams et al., 2008). We hypothesize the large muscle mass and long bones leads to stronger levers to run, jump and hunt in this animal. Carrier et al. (2006) suggested that the serratus ventralis muscle functions for weight support while Hudson et al. (2011) suggested that the activity of this muscle causes the scapula translation and rotation that is observed in domestic cats. It was suggested that when both vertical and horizontal movements of the scapula during locomotion (Hildebrand, 1961, Hudson et al. 2011) combine with movements of long bones of the column of the forelimb in the cheetah they will enable longer strides, contact lengths and a more vertical limb at the extremes of stance, potentially aiding faster top speeds. It is required to withstand a larger joint torque, especially in the shoulder joint. The ability of some forelimb muscles to create larger joint torques in the cheetah will aid in this function, which will be of great importance at high speeds, when peak limb forces are likely to be higher (Witte et al., 2006, 2004). The high speed maneuvering that is characteristic of cheetah’s hunting style also results in high limb forces (Hudson et al., 2011). Furthermore, the deep and extensive articular surface increases resistance to these forces. According to some studies many of the cheetah’s proximal intrinsic limb muscles are larger in mass than them in the greyhound (Hudson et al., 2011). They also have longer maximum moment arms in the cheetah when compared with felids, enabling to produce larger joint torques but reducing the capacity to produce high joint rotational velocities (Hudson et al., 2011; Williams et al., 2008). We hypothesize the large muscle mass and long bones leads to stronger levers to run, jump and hunt in this animal. The cheetah possesses an additional muscle – the brachioradialis. It functions to supinate the paw, which is of crucial importance to the cheetah (Gorman and Londei, 2000; Russell and Bryant, 2001) and Asiatic cheetah for prey capture (Hudson et al., 2011). As shown in figures 3, 5, 7 and 8, the origin and insertion of this muscle is defined on both humerus and radius in Asiatic cheetah. It seems that the skeletal column in Asiatic cheetah is adapted to this muscle and its function. So the extensive articular surfaces, long shafts and small diameter of these two bones and the correlat-ed decrease in muscle mass results in increasing the flexibility of forelimb according to Asiatic cheetah´s characteristics such as running, hunting and jumping. According to origin and insertion of brachioradialis muscle (Fig. 3,5,6,7,8), cheetah can throw its forelimb to the front and stay in this position during jumping in high speeds. This action occurs due to movement of the shoulder joint and the activity of some extrinsic muscles of forelimb. As mentioned before, the length of the bones in this region acts as a lever and helps to long jumping. Tubers that relate with origin and insertion of brachioradialis muscle in cheetah are bigger than in the cat. It means that this muscle is more powerful in the cheetah than that in the cat and can exert a bigger force to move the forelimb and decrease swing time. Also this muscle is a supinator muscle and because of its long fascicles is apt for rapid joint rotation. According to Gorman and Londei (2000) and Russell and Bryant (2001) this muscle can cause at high velocity to rotate the joint through large angles. For this purpose this animal needs the extensive articular surfaces in the elbow, that we could show. Despite this, previous work on cheetah’s elbow has highlighted a reduced ability for supination when compared with other felids, with a conformation much like canides and other runner carnivores (Andersson, 2004), contradicting muscular anatomy (Hudson et al., 2011). There are some comparative studies between musculoskeletal anatomy of the cheetah forelimb and racing greyhound (Usherwood and Wilson, 2005; Williams et al. 2008). Williams et al. (2008) suggested that the large mass of muscle they observed in greyhound forelimbs may be used in propulsion or for bodyweight support. The fibers of forelimb muscles were considerably longer in the cheetah, which indicates a greater capacity for modulation of the muscle force–length relationship, and hence limb stiffness and mechanical work during stance. As a result of previous study, when scaled to body mass, the cheetah’s radius (P<0.01) and humerus (P<0.05) were found to be significantly longer than those of the greyhound. The length of the bones acts as a lever and provides a bigger joint torque. Our obtained data for both humerus and radius were similar to those given for cheetah. According to Alrtib et al. (2013) when the limb is landing either straight or towards the lateral side, the lateral condyle receives much more load per unit area than the medial condyle in a short period of time during the start of the weight bearing phase before the long lateral sides of the bones displace the load towards the medial condyle. Additionally, a significant difference in depth and width between the medial and lateral condyle was found in the current study, where the depth and width of the lateral condylee were significantly lower than those of the medial condyle. These results indicate that there is a difference in the surface area that receives the load in the contact phase. Eckstein et al. (2009) suggested that the increase in the surface area of bone can distribute the load over a wider area and consequently lead to a decrease in the mechanical stress on the surface. A similar condition occurs in racing horses. In racing horses, taking the relative size of the condyles into account, the high predisposition to lateral condylar fractures (Zekas et al., 1999; Radtke et al., 2003) might be related to a high load that may occur on the lateral condyle in the short period of time during the start of the weight-bearing phase. It also suggests that in the horses the thicker cartilage layer on the lateral condyle (Muir et al., 2008) may simply be a result of compensation associated with the relatively smaller surface area of the condyle itself. It seems likely that torsion of the bone can occur in some cir-cumstances during the weight-bearing phase and this would be likely to make the long lateral side more susceptible to fracture (Alrtib et al., 2013). According to this hypothesis, it may be that the lateral condyle of humerus in cheetah has a high risk of fractures. Reply