For millions of years horses have had to adapt to their ever changing surroundings, and given the fact that horses are herbivores and herd animals making them prey they also had to evolve alongside many other animals who would hunt them. Most of how horses have evolved has been to give them and edge and the best chance of survival running away from predators (Olsen 2003). A large part of the reason horses were first domesticated is due to their speed and well adapted abilities to run long distances. These capabilities of the horse have been harnessed by humans for around 10″,000 years now, however, their roles have changed throughout history, despite this horses have displayed their innate athletic talents whether they are being used for hunting, pulling immense weights, on battlefields or in more recent years for entertainment and sport. A large part of the horses place in the modern world os due to their athleticism, perhaps this is what may lead many to believe them to be the superior athletes to humans. However, maybe when directly compared a different conclusion could be drawn. It is true that in both humans and horses the size of the heart can increase with fitness (Buhl et al. 2005) and it has been reported that heart size may be a factor in the success of athletes, both human and equine due to a process known as The Fick Principle (Poole 2004). The one clear advantage the horse has over the human as an athlete within the cardiovascular system is the function of the equine spleen. In horses the spleen is essentially a reserve of red blood cells that is built up when the horse is at rest which then allows these stores to be released when the horse exercises or requires the extra oxygen to the muscles. Their heart is adapted to handle this increase in red blood cells and can continue to pump thicker blood around the body just as efficiently (American Physiological Society 2006) this is exhibited very strongly in thoroughbreds who specialise in long distance races (Gunn 1989). These advantageous variations in the equine heart my give them an edge over humans as athletes as it allows for a significantly higher systole. Within humans the only advantageous adaptation is something commonly known as athletes heart or athletic heart syndrome, in which the heart is enlarged and usually exhibits a particularly low resting heart rate. This is something which is often displayed in endurance athletes (Ellison G et al. 2012). Splenic contraction is something that can be mimicked in humans although takes a lot of effort and is something only seen at the highest levels of sports. This can either be achieved through training at high altitudes of through an illegal practice called blood doping, in which blood is taken from an athletes body to then be put back into the body on perhaps the day of a crucial event giving them an increase in red blood cells. However this can come with serious consequences as the human heart is not able to continue the usual diastole due to a stroke volume which is much higher than normal. This is because the human heart is not adapted to the same degree as the equine heart as splenic contraction is a natural process within the horse. While the horse is arguably one of the most athletic animals seemingly designed to run long distances and high speeds, it is their respiratory system that is the limiting factor. Unlike in most other organisms including humans in which it is most likely the circulatory system or even the musculoskeletal anatomy (Franklin S 2012). One of the reasons that perhaps contributes to these respiratory issues, is that fact that unlike in many other animals horses are “obligate nasal breathers” (Negus V 1949) due to resistance and narrowing of the lower respiratory tract meaning it is perhaps not viable for the large volumes of air needed can be inhaled in this way. Where as in humans there is a “switching point” in which at a certain point in exercise breathing exclusively nasally is not enough and we must move on to oronasal respiration (Niinimaa V et al. 1980). In this specific example humans function better. The way that the equine respiratory system works means that some horses may have a substantial advantage over others; horses must conform to inhaling and exhaling depending on their gait and stride in gallop (Harris S 1996). In the suspension phase of a stride as the hind legs come forwards the abdominal internal organs are forced backwards away from the diaphragm allowing the horse to inhale and inflate the lungs, however, there has been some argument that this is not enough force for the ventilation. During the extended phase of the stride the abdominal organs are forced forwards causing the horse to exhale however it has been countered that the impact of the forelimbs on the ground would be enough to force air out of the lungs however there are also doubts to this theory and claimed by some that back flexion plays the key role in ventilation (Young I et al. 1992). This respiration can be referred to as Locomotor-Respiratory Coupling or LRC. Being bipedal humans can have many different phase locked patterns however research shows that 2:1 is favoured (Bramble D, Carrier D 1983). The reason LRC gives some horses advantage over others is that those with a longer stride will have a longer time to inhale or exhale compared to thoses with a short stride, demonstrated in world famous champion Secretariat who is reported to have one of the longest recorded strides (American Physiological Society 2006). In both humans and horses muscle is something that has to be built up over time through training, enhancing the metabolic pathways allows for maximum efficiency and strength (Clayton H 1991). Equine muscular systems are known to be highly responsive to training and and improvements can be seen within weeks (Knight P et al. 1991), and compared to humans horses are shown to be much better at maintaining peak fitness. It was found that horses with high levels of endurance training and success had very well developed I and IIa type muscle fibres shown through a higher percentage and increased size ( Rivero J-LL et al. 1993). I and IIa muscle fibres function on aerobic respiration so it of course makes sense that in endurance running this would be essential. They are also much slower to fatigue compared to IIb which runs anaerobically and fatigues much faster. When compared to humans horses have a higher percentage of muscle composition; in humans muscle makes up on average 40% of body weight, whereas in horses it can be up to 55% along with a higher distribution of type IIa muscle fibres showing that they really are built for running long distances. However, this isn’t really exclusive to horses as it has been shown that successful human endurance athletes have a high percentage of slow twitch muscles as aerobic respiration is essential in endurance exercises (Hamilton M and Booth F 2000). Overall I would argue that the horse is the superior athlete due to the significant adaptations within the horse specifically enabling them to travel long distances very fast and although their respiratory system may let them down it only limits their capabilities, not impeding on what they naturally can do. Horses can run extraordinary distances at impressive speeds so although they do have this limit, for the most part it is not as though it is a problem to most horses only those that are competing inendurance events at the highest levels.