The biomechanics of suspension

research on the gait of a galloping horse spans several decades. As early as 1959 Milton Hildebrand wrote, “Motions of the running cheetah and horse,” a study actually inspired by a Walt Disney film, “African Lion”. The study compares the running styles between the cheetah and the horse, and distinguished the differences in the gait that make the cheetah faster. Using the common understanding that speed = stride length x stride rate, Hildebrand theorises that flexion and extension in the back contribute to speed by increasing the swing of the limbs, thus increasing the duration of the suspended phase of the stride. He also found that advancing the limbs more rapidly required a combined effort from not only the intrinsic muscles of the limb but also the muscles supporting the spine. Today professionals refer to these supporting muscles as “the core.” More current research supports these early theories. According to Hilary Clayton, a leading expert on equine gait analysis today, “Most of the increase in stride length is achieved as a result of prolongation of the aerial phase.” We know that speed can be increased by increasing the stride length. Therefore, increasing the suspension phase of the gate will increase the stride length and increase overall speed. Now going back to Hildebrand’s research, changes in two variables lead to prolonging the suspension phase of the gallop. The first modification is to increase or maintain the flexion and extension in the equine spine, and the second is to strengthen the muscles that support the spine or “core” musculature. So, how do you increase motion in the equine spine? Although, maybe the more important question is, “How you do prevent it from diminishing?” First let’s consider the spine and joint physiology of the horse’s back. The thoracic and lumbar spine is a relatively ridged unit. The rib cage limits motion in all direction. Therefore, most flexion and extension occurs in the thoracolumbar junction and in the lumbosacral junction. These are also the areas where most injury occurs because they are not protected by the rib cage. Trainers need to pay special attention to this area of the spine as it is prone to injury and is critical to correct biomechanics. You can check for signs of injury by running your thumb and finger with moderate pressure down the horse’s back on either sides of the spine. If the horse drops their back away from the pressure and/or of you see the muscles of the back tighten this is a good indication that there is an injury that needs to be addressed. When an injury occurs, it could be from a fall, slip, saddle, or rolling in turnout. Whatever the case may be, a cascade of events ensue, beginning with locally or regionally restricted vertebral motion which leads to a compensatory hypermobility in the segments above and below the restricted area. The body responds to hypermobility as instability by tensing the surrounding muscles in an attempt to stabilise the area. Inflammation and pain are often associated with this type of injury. However, because the horse is an animal of prey, it is not inclined to reveal its discomfort but injury will most likely cause an alteration in gait and an increase in concussive forces and even lead to lameness. Additionally, if the injury is not addressed the body will actually start to deposit calcium at the hypermobile areas also in an attempt to stabilise them. This is how bone spurs occur and calcium deposits end up in tendons and muscles. You can easily see the potential to limit flexion and extension in the spine that this scenario poses. Therefore, to optimise the suspension phase it is critical to address and monitor the ability of the horse to flex and extend its thoracolumbar spine. To more deeply examine the second variable for increasing speed, let’s look specifically at developing the spine stabilising muscles or “core.” Core training was original ly the focus of rehabilitating back problems. Now sports medicine doctors and professionals have been using core training not only for rehabilitation, but also to prevent injury and improve performance. The same strategy can be applied to the racing horse. The biomechanics of the horse’s core musculature is referred to as a bow and string model. The spine and associated ligaments make up the bow and three muscle groups make up the strings. The dorsal epaxial muscles help to straighten the bow and stabilise the curve. The muscles of the ventral surface of the spinal column – the longus coli and psoas – help to flex the bow and stabilise the column. The major muscle group that facilitates flexion at the thoracolumbar junction of the spine is the abdominal muscle group. Only when there is unrestricted movement in the spine and tension in this string, can the bow flex. Therefore, if abdominal muscles are weak there will not be enough tension to flex the back and draw the pelvis to the sternum. It is this combined movement of flexion in the back and bridging between the pelvis and the sternum that needs to occur during the suspension phase of the gait. In my experience, core strength, in particular the strength of the abdominal muscles, is often overlooked and can easily be added as part of the daily training regime. Teaching your grooms two simple exercises can have far-reaching (no pun intended) results. The Rounding Reflex There are two ways to do this exercise. Stand to the side facing the horse at his croup. Start by locating the spinus processes of the vertebrae from the dock of the tail up. Apply pressure with your thumb or fingers to the tips of the processes until the horse responds. Or you can locate the “valley” of the hamstring muscles and draw an imaginary line from the dock of the tail on both sides of the horse towards the hip, approximately 4 inches. Mark two coin-sized spots and using your fingers, thumbs, coins, or the dull end of two hoof picks, make a little scratching motion. Start with light pressure, like scratching a mosquito bite, and increase until the horse engages its abdominals, arches its back and tucks its hind end down. The response should be smooth and slow; if the horse responds in a jerky manner the pressure should be reduced. Hold this for approximately 10-15 seconds, then release and repeat. There should be little to no movement in the stifle and lower leg. This exercise can be performed every time the horse is groomed. The Back Lift Place your fingers or the dull end of a hoof pick(s) under the horse's chest and belly. Press into the sternum and upper abdomen. The horse’s abdominal muscles should tighten and the back should rise. You may need to scratch its belly a little while applying upward pressure. Again, hold this position for 10 – 15 seconds, then release and repeat. This too can be performed every time you groom the horse. Incidentally, both of these exercises can be used as assessments for the horse’s ability to flex its thoracolumbar spine. If the horse is unable to perform these exercises or displays a reluctance to do so it is most likely due to restrictions in the spine and pain present in the area. In addition to these two specific exercises, I have found it helpful to use Kinesio taping techniques. [For more on Kinesio tape, please refer to “Working the Kinesio tape therapy magic on equine athletes” by Ken Snyder in Issue 32 of European Trainer.] Kinesio Tape is a unique therapeutic tool that has provided new options in rehabilitation and sports medicine throughout the world. The tape has many applications depending on how it is utilised. In this case, it would be applied to provide structural support to joints and muscles without restricting a healthy range of motion and correct improper muscle function by providing neurosensory feedback during movement. I encourage preventative “Current veterinary research is indicating that the effects and benefits of chiropractic care for horses are similar to that of people” care through regular chiropractic treatments. If you’re not familiar with chiropractic care it involves the mobilisation of the spinal joints and extremities to restore proper biomechanics and nervous system function. Chiropractors apply a specific force in a specific vector to a joint to restore normal function. The intent is to elicit a therapeutic response via induced changes in articulations, muscle function, and neurologic reflexes. While chiropractic is considered to be an alternative medicine, research has proven that it is the most effective treatment option for low back pain in humans. Current veterinary research is indicating that the effects and benefits of chiropractic care for horses are similar to that of people. In 1997 a study by Gal, Herzog, and Kawchuk found that “segmental spinal motion characteristics induced during chiropractic manipulations in horses are similar to those reported in humans.” Dr. Kevin Haussler, DVM, DC, PhD and faculty at the College of Veterinary Medicine, Cornell University from 1999 - 2005, studied the effects of spinal manipulative therapy on horses. According to Haussler’s research, “the spinal manipulative technique increased vertebral flexibility and increased tolerance to pressure in the thoracolumbar portion of the spine.” Other studies conducted by Dr. Haussler find that “direct measures of chiropractic manipulations demonstrated substantial induced segmental spinal motion in these horses...the induced spinal motion supports current theories on the effects of chiropractic manipulations on joint physiology.” These are important findings not only for increasing the suspension phase of the gallop, but also for equine athletes in general because the prevalence of back problems in horses is reported as high as 94%. Walt Disney probably did not intend to inspire such a discussion with his true life adventure picture but it is interesting how findings from 1959 hold the same significance today. Even with all the new technology that has been developed over the de cades, it still takes good biomechanics to make great strides.

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(European Trainer - issue 36 - Winter 2011)

VETERINARYWebmaster25 November 2011Biomechanics, EUT36, Equine kinematics, Equine biomechanics