Choosing a Riding Horse Analysis of the Pelvic Region By Deb. Bennett, Ph. D June 2004 We begin with the most important part of the functional horse: the pelvic region or motor. You probably already knew that all the major propulsive muscles originate on the pelvis. Less obvious is the fact that the several possible shapes the equine pelvis may assume determine the potential sizes of these muscles and their effectiveness in propulsion. These photographs present extreme examples of pelvic conformation. Pelvic conformation is analysed in terms of the following features (Fig. 1): Length - measured from point of hip to point of buttock. Pelvic length (hip line length) divided by body length (Body line length) gives the proportional length of the pelvic region, which is a measure of the overall propulsive power of the horse. Fig. 1. Three possible >choices. On the basis of pelvic conformation, which one would you buy? Method of graphic analysis is shown for Horse 2. Slope - measured relative to the horizontal. Pelvic slope (slope of the hip line) interacts with the resting angle of the femur to determine the useful range of motion at the hip joint, and the point in the swing of the hind hoof where maximum power can be achieved. Acetabulum Placement: The position of the acetabulum, or hip socket, can be assumed for purposes of photographic analysis to lie directly beneath the bulge in the hip region caused by the great trochanter of the femur and its overlying bursa. This bulge does not show up on every photograph. When it does appear, it will be the most anterior highlighted region behind the point of the hip. In any case, the bulge over the acetabulum can always be palpated in a live horse, and any horse you consider buying should be so palpated. Biomechanically, the acetabulum separates the iliac part of the pelvis from the ischial part.(fig.2). Fig. 2. Position of Gluteus medius, semitendinosis and semimembraneous muscles in the horse. Fig. 2. On the iliac part originate the gluteals, use for sustained power, especially in the first half of hind leg retraction. On the ischael part originate the semitendonosis and semimembranosus muscles which power the second half or thrust phase of hind leg retraction. Furthermore, the ischium acts as a lever for the attachment of the sacro-sciatic ligament, which is one component of the “dorsal ligament system”. This system (we’ll discuss it more fully later) is what enables a horse to dorsiflex (bascule) the spine, to go well onto the bit, to achieve true collection, and to balance himself with the hind legs a la haute ecole. There are other levers in this system, but this one is important and often overlooked. Breadth, measured at three points: from left to right points of hip, from left to right acetabulum and from left to right points of buttock. Narrow hips are disadvantageous. This feature cannot be analysed in “standard position“ photographs used here but you should examine any purchase prospect for this feature also. Fig. 3. Effect of pelvic slope on hind leg function. The essence of scientific experimentation is to vary one factor while holding all others constant. thus, all lengths and angles in this figure are identical except pelvic slope. The differences between 1, 2, 3 and 4 are thus due to pelvic slope. There are other factors which mediate ‘suspension’ - we will vary other components of this system in future articles Depth, measured by dropping a perpendicular from the hip line to the centre point of the acetabulum. The pelvis should not be a flat plate. A deep pelvis can hold more muscle than a shallow one. This is especially significant when you consider that the power of a muscle is proportional to its cross-sectional area – not to its mass or its length. Sacro-lumbar joint placement (fig. 2). The pelvis joins the backbone at the sacro-lumbar (S-L) joint. In my opinion this is the single most important joint in the horse’s body for determining athletic ability and potential soundness. We will discuss these last two points further. Try a graphic analysis of each of these three horses. Eventually you will be able to assess conformation accurately by eye alone but for now the graphic method will force you to look at horse bodies systematically and objectively. The first horse, (fig.1) is done for you in order to illustrate the method which will be used throughout. 1. Get a tape, a very sharp pencil and tracing paper. On a table top, tape the tracing paper over the photographs. 2. Place a dot on the point of hip, point of buttock, point of shoulder, and acetabulum. 3. Construct hip, body, and horizontal lines as shown. 4. With a millimetre ruler or callipers, measure the hip and body lines. Apply the following formula to these measurements: 5. Measure the ischial length. Apply the following formula. Ischial Length x 100 Proportional Hip line length = Length of Ischium 6. With your eye measure the pelvic depth. Drop a perpendicular from the hip line to the acetabulum. Does the acetabulum lie on the hip line or below it? 7. Using a protractor, measure the angle between the horizontal line and the hip line. After you have completed your graphic analysis and answered these questions, you may then try to decide which pelvic structure is “best”. Guide your choice first by mentally swinging the hind legs of each. Which is most able to swing the legs in a long arc? Which horse has the longest ischium? Which horse has the greatest propulsive power? Now compare your analysis with mine to see which pelvic structure I prefer - and why. Analysis Horse 2. The chopped off appearance of this pelvis is due to lack of ischial length (only 29% of the total pelvic length). The acetabulum is placed far back in the total pelvic length, and thus this gelding lacks thrusting power. Also due to acetabulum placement , the whole of the hind leg is placed right at the back of the body – if it were not for his rather open hock and stifle angles this gelding would be “camped out behind”. The overall pelvic length is deficient being only 30% of the body length. In all wild equine, the pelvic length constitutes at least a third (33%) of the body length. I take nature’s motor to be the minimum acceptable size. The pelvic slope is however probably sufficient (17 degrees + or- 3) Measured in this way 15 dgrees is the absolute minimum I consider safe for all but expert riders and all riders over jockey weight. This gelding has been used in the worst possible way for him – as a saddle seat equitation horse, with regular use of a “bitting rig” complete with gag snaffle and 10 inch curb shanks to raise and “set” the head. He has never had any encouragement to bascule. He compensates form for structural weakness and back pain be going wide behind. Now in his teens he is more happily employed as a pleasure hack and school horse for beginners who have gained confidence by being able to ride the small movements made by his relatively stiff back. Horse 3. Here we see extreme pelvic slope (32 degrees) or ‘goose rump”. However this mare shows an acceptable acetabulum placement (32 degrees of the pelvic length is composed of ischium), and overall pelvic length (33% of body length ). Her limitations mainly lie in leg retraction. She cannot swing the femur very far behind the vertical without crushing it against the ischium. The gluteus medius muscles on such horses are commonly hypertrophied (over developed ), since the arc of useful hind leg movement is more or less restricted to that part which the gluteals power. Her thrusting power is diminished for another reason. The point of greatest hind leg leverage occurs when the femur is at 90 degrees to the hip line. If, when the femur is so angled, the hock and hoof lie under the ischium, the structure is ideal for an equal mix of upward and forward thrust. In this mare however , the hock and hoof lie in front of the acetabulum when the femur makes its 90 degree angle. “Goose rump” is thus a structure that promotes upward thrust at the expense of forward thrust – “level hips” is just the opposite. To accommodate powerful muscles – those having a large cross sectional area – the equine pelvis must be deep. Although I told you how to measure pelvic depth previously, I did not ask you to make a graphic analysis of it because, at the scale these photographs are printed, the error of your measurement might be fairly great. But I do not want you to forget to look for deep pelves in all prospects. Your next exercise is to place in order all the horses with respect to pelvic depth. You will find some are quite close, so you can call those a tie. Sacro-lumbar joint placement is extremely important in determining a horse’s ability to dorsiflex the spine ( bascule ). Because, as a beginning rider I wanted to more fully understand terms used by dressage judges, in the course of my research over the past several years I have developed some biomechanical definitions for those terms. These are the not same as are found in the FEI handbook, but I invite you to compare the two in order to derive greatest benefit from both: Collection starts from, and is always primarily the product of, flexion of the sacro-lumbar joint. Flexion of the sacro-lumbar joint means that the point of buttock ( the tuber ischii) moves downward and forward relative to the sacro-lumbar joint. This is not quite the same thing as the oft-spoken “lowering of the croup,” for the latter may or may not imply bending at the hip, hock and stifle joints. The biomechanical definition has two immediate benefits: It turns our attention away from the head and neck position, especially away from any ideas about a “head set” or a particular head position, as an indicator of collection. For the thinking rider, this definition immediately stimulates new questions: What other movements can a horse make to contribute to sacro-lumbar joint flexion? Making the horse bring the hind hooves as far forward as possible ( to track up ) ameliorates and contributes to sacro-lumbar (S-L) flexion. Conversely, letting the hind legs drag (disengagement) makes S-L flexion more difficult. What muscles primarily power this flexion? Contrary to the statements made by many riding instructors, the horses use of the back muscles (longissimus dorsi) does not cause or power the bascule! The position and attachment of the longissimus dorsi promotes spinal stabilisation during normal locomotion used abnormally, they may even cause ventroflexion (downward bending or anti-bascule) of the spine. During normal use, the two longissimus dorsi, one located on each side of the spine, are fired alternately. In fact, for dorsiflexion to occur at all, one of these muscles must be relaxed at all times. Ventroflexion results when the horse “clamps the back” then (fires both longissimus dorsi continuously). Dorsiflexion of the spine is produced when the horse flexes the rectus abdominus muscle (the one you use when doing sit ups). If a horse relaxes the longissimus dorsi alternately, in coordination with strong use of the rectus abdominus, he will, for example, be able to produce both the trotting leg movements and the strong dorsiflexion required to piaffe. If a horse uses the rectus abdominus and both sides of the longissimus dorsi simultaneously, one of two things will happen: Either he will move with a very stiff immobile back, and not be able to maintain pure paces (a poor piaffer), or he will rupture one or other set of muscles. Ruptured abdominals are rare – their mechanical position gives them the greater leverage, so they tend to win the contest. Ruptured longissimus dorsi are common, especially among gaited horses that track over at a high speed in competitions, and that are not encouraged to relax the back muscles or to bascule. Do any conformation features contribute to the ability to flex the S-L joint? Yes! The pelvic slope. The steeper the slope, the easier for the horse to pull the ischium downward. The shallower the pelvic slope, the farther the ischium can be rotated. The pelvic depth. The rectus abdominus attaches, by a strong tendon, to the pubis. The lower this point lies, the greater the mechanical advantage for the abdominals, no matter what the pelvic slope. Acetabulum placement. Horses with acetabula placed further back than about 70% of the total pelvic length (horses with a 30% ischia or shorter), finds that the force of gravity pushing upward at the acetabulum interferes with the downward rotation of the ischia. In other words, the further back the acetabulum lies the harder the abdominals must work to flex the sacro-lumbar joint. This is the one and only instance when great pelvic length is disadvantageous. Consider the mechanics in a long, but rather level hipped horse, whose acetabula are located at 70% of the pelvic length. Here the length makes the situation worse! Sacro-lumbar placement. It is highly desirable to have the sacro-lumbar joint as far forward as possible – ideally lying on a line connecting the left and right points of the hip. This has double benefits, for it shortens the functional back, particularly in the lumbar region. Secondly, it provides the greatest possible arc of rotation for the ischium, no matter what the pelvic slope. The further the ischium rotates downward, the greater the resulting bascule. Analysis In horse 2:3 – the overall pelvic length in this gelding is deficient (30% of body length), but that is not his greatest difficulty. He shows only 7 degrees, plus or minus 3 degrees, of pelvic slope. This is ameliorated somewhat by his acetabulum placement, with 35% of the pelvic length in the ischium. Nevertheless, when moving under a riders weight, if permitted to relax the abdominal muscles for even one stride, the lumbar region sags and the ischium rotates to a position higher than the point of the hip! Peakedness in the S-L joint, such as seen here, is caused by tearing of muscle and ligament fibres overlying the joint. Subsequently, the lumbar vertebrae in front of the joint sag downward and the tubera sacrale move upward relative to the lumbar vertebrae. This implies a moderate to considerable degree of dislocation of the S-L joint which were not born peaked (see horse 3 – that peakedness is congenital). 2.1 2.2 Peakedness as the result of structural damage often develops in brood factories – mares repeatedly used for producing foals, but never given enough conditioning. When examining the parents of a foal prospect, you may have to take this into account when assessing the dam – she may not be ongenitally peaked. In America, where riding horses are commonly broken at age 2, it is rare to find a horse in its teens that is not indeed broken at the S-L joint. I have also seen peakedness as a result of the tissue breakdown which accompanies azoturia. In all cases however the better the S-L placement, the less peakedness develops. Correct dressage and regular progressive conditioning may improve this condition. Horse 2:2 This mare has acceptable pelvic length (34%), slope (18degrees plus or minus 3), and acetabulum placement (31% of the pelvis is composed of ischium). Her drawback lies in S-L placement. This mare is a maiden, but was used early (before age 3 ) as a bareback bronc. The latter may have contributed to her peakedness. This is my own horse, who taught me to ride, so I am perhaps less objective about the functioning of this one than others. There is nothing in a pelvic structure like this to prevent the horse from doing adequately (if not brilliantly – more overall length wouldn’t hurt ) all the dressage exercises through to grand prix, without incurring injury. The mare has good suspension (pelvic slope not too shallow and acetabulum placement far enough forward), when working across country (pelvic slope not too steep). When frightened or excited I have observed her producing engaged passage, piaffe, and ballotade (unmounted). Her bascule, however, is never as great as that which 2:1 can produce, because the rearward S-L placement shortens the arc through which her ischia can travel. On top of this, her acquired peakedness diminishes what ever effect rotation of the ischium might have had in causing dorsiflexion of the back. Horse 2:1- this is a very strong S-L joint placement. The pelvic length is good (36% of the pelvic length is composed of ischium). It would take some mighty pounding to break this mare’s S-L joint, and even then, bending the joint would be more effective in basculing the back than in horse 2:2. This is best put to use where she is now – in competitive dressage. Sacro-lumbar placement – examples 1. Ideal placement, located on a line connecting left and right points of hip. 2. Fairly good placement, similar to horse 2:2, is the main saving grace of this pelvis. I believe this gelding has stood up better to his work than horse 2:3 because his S-L joint placement is better. 3. Again, a placement about like horse 2:2, but rendered ineffective by extreme pelvic slope, which also shortens the arc through which the ischis can travel. Order of all 6 pelves, in terms of depth. Deepest: Horse2:1 Intermediate depths, in order of depth (you may not have been able to detect these differences. If not that is okay): Horse 2:2 and horse 1:3 and horse 2. Shallowest horse 2:3. 2A 3A 1A Fig 1B The equine pelvis. A - front view. B - view form right side. The tuber sacrale is the pointiest part of the top of the pelvis. It is easy to see in most photogtaphs, and easy to palpate. True position of S-L joint is usually one or two inches in front of, and five or more inches beneath the skin overlaying tuber sacrale. When palpating, note the soft spot generally present between the first sacral vertebra and last lumbar vertebra.