The Changing Face of Gait Analysis Dr. Hilary M. Clayton, BVMS, PhD, MRCVS McPhail Equine Performance Center College of Veterinary Medicine Michigan State University East Lansing, MI 48824 A Brief History of Motion Analysis The gaits and movements of horses have long been of interest to artists and scientists. From the early cave paintings to the present day, artists have depicted horses in motion. Meanwhile scientists have strived to describe equine locomotion and to decipher the limb movements in different gaits. Horses have evolved in accordance with the requirements of a cursorial lifestyle. This has resulted in the ability to cycle the limbs more rapidly than the human eye can see and, consequently, artistic impressions did not accurately represent the kinematics and limb coordination patterns of the gaits. Early photographic techniques required long exposure times that produced blurry images of moving events. The development of photographic techniques and their influence on the science of gait analysis is a fascinating story Leland Stanford, who is perhaps best known as a railroad magnate and founder of Stanford University, was also a racing enthusiast. He owned a horse called Occident, who held the world record for trotting a mile. Stanford believed that there was an aerial phase in the trot, and employed Eadweard Muybridge, a landscape photographer, to prove his point. By developing photographic plates with faster exposure times, Muybridge was able to produce a rather blurry photograph of Occident in an airborne phase. Muybridge went on to record tens of thousands of sequential still photographs of people and animals engaged in various tasks and gaits, that provided an invaluable reference source. They have been republished in three fascinating volumes (1). Muybridge used a battery of still cameras triggered in sequence to obtain his series of photographs. His contemporary, Etienne Jules Marey, developed a photographic gun, in which the film actually revolved to record a series of pictures. This was the precursor of the cine camera. Although developed for scientific purposes, cinematography soon became more important as a source of entertainment. Further advances in technique produced high speed cinematography and short exposure times that allowed the collection of high quality scientific films that could be analyzed to study gaits and locomotion. But high speed cameras were finicky to use and there was a considerable time lag between making the recording and viewing the results. The development of videography solved many of these problems, but, at least initially, at the expense of temporal and spatial resolution. Analysis of cine films and videotapes was tedious in the early years, since the process was entirely manual. Computerization has facilitated this process and, today, fully automated systems are available. The motion analysis system in the McPhail Center tracks reflective markers on the subject using six infra-red cameras. The cameras are strobed and can be set to record at 60, 120 or 240 Hz. The markers are tracked in three-dimensional space in real time. A computer-generated stick figure is produced that can be rotated and zoomed to give a detailed view of the motion from any perspective. Interestingly, the primary market for these systems is now the entertainment industry, where they are used in computer animations and video games. With regard to motion analysis, computerization has enabled us to gather vast amounts of data in a short space of time and to perform computational tasks that would have been impossible only a few years ago. In the 1970s, the research group from the Swedish University of Agricultural Sciences pioneered the use of high speed cinematography in equine gait analysis, and their work on trotting Standardbreds continues to the present day. The Swedish group also introduced the equine treadmill, which has had a profound effect on research on equine exercise and gait analysis. Other research centers dedicated to equine gait analysis have been established in Europe and North America. The range of techniques has expanded to include analysis of ground reaction forces using a large force plate, and other techniques such as electromyography and accelerometry. The past 20 years has certainly been an exciting era to be involved in gait research and the future holds enormous promise. Research in the Mary Anne McPhail Equine Performance Center The Mary Anne McPhail Equine Performance Center opened in 2000 as a state-of-the-art center for equine locomotion analysis. It is equipped with a real time motion analysis system, a 60 x 120 cm2 force plate, telemetered electromyography system, and transducers customized for specific applications, such as measuring rein tension during riding. One of the active areas of research interest in the McPhail Center is in the function of the equine tarsal joint. The tarsus is interesting because it is a complex joint with four levels of articulations (figure 1), and it is frequently a site of lameness in performance horses. Anatomically, it is a complex joint with four levels of joints. Most of the motion at the tarsus is flexion and extension at the tibiotarsal joint, which is the most proximal joint. The three distal joints (proximal intertarsal, distal intertarsal and tarsometatarsal) are low motion joints, but it is these joints that are the site of bone spavin, which makes them of particular interest mechanically. We have performed a series of studies designed to characterize the motion of the tarsus in three-dimensions, to determine how much of the motion is occurring at the distal joints, and to evaluate how the motion changes due to lameness. The results showed that, during the stance phase of the stride, the tarsal joint flexes through 11°, abducts (rotates away from the midline) through 3o and internally rotates through 1.5° (figure 2). At the same time, the cannon bone slides a forward, laterally and distally relative to the tibia. During the swing phase of the stride, the tarsal joint undergoes a considerably larger range of motion than during the stance phase. The joint flexes through 45°, abducts through 10° and externally rotates through 5° (figure 2), and the cannon bone slides forward, laterally and distally relative to the tibia. These findings have been analyzed further by applying a combination of anatomical measurements and computer modeling to determine how much of each type of motion is occurring at the tibiotarsal joint, and how much is occurring at the distal tarsal joints. The results indicate that both rotational and sliding movements are occurring at the distal joints. During the stance phase, these movements comprise internal rotation, together with forward and lateral sliding of the cannon bone; during the swing phase there is forward and lateral sliding of the cannon bone. The bones that form the distal tarsal joints are flat and slab-like, and the joints between them are arranged, more or less, in a horizontal plane (figure 1). This orientation of the bone surfaces would allow the bones to slide and swivel, which is in accordance with our experimental findings. Since the distal tarsal joints are the site of bone spavin, the ability to differentiate motion at these joints from the overall joint motion represents a significant step forward. The motion recorded at the distal tarsal joints is larger than has been reported previously in static loading tests of cadaver tarsal joints, probably because static loading does not take account of shear forces that are present during locomotion and are the cause of sliding and rotational movements. Gait alterations in association with distal tarsal joint synovitis, which is an early inflammatory change that may precede the development of degenerative joint disease, involve a decrease in tarsal joint flexion during stance, and in cranial (forward) sliding of the metatarsus relative to the tibia during stance. The distal tarsal joints were the source of approximately 25% of the reduction in sliding motion. Evaluation of the compensations for mild tarsal lameness by the other limbs indicate that weight-bearing by the diagonal front limb is also decreased. This is indicative of a general unloading of the lame diagonal, not just the lame hind limb. It might be anticipated that there would be a compensatory increase in weight-bearing in the limbs of the opposite diagonal, but this was not found to be the case. We interpret this finding as being indicative of the horses moving with a “flatter” or less bouncy gait, that has less vertical displacement of the horse’s body during the stride. Thus, by moving with less ‘lift’ in the trot stride, the horses were able to reduce the load on the tarsal joints (and the entire limb). Therefore, the earliest signs of tarsal pain may involve a reduction in gait quality, rather than an overt asymmetry or lameness. Figure 1: Anatomy of the tarsal joint. Figure 2: Three dimensional motion of the tarsal joint. Flexion/extension (green line), abduction/adduction (dark blue line), internal/external rotation (pink line).