F, Beginning of hoof acceleration. G, Mid-swing. H, End of hoof deceleration. Adapted from Fredricson I, et al: A method of three-dimensional analysis of kinematics and co-ordination of equine extremity joints. A photogrammetric approach applying high-speed cinematography, Acta Vet Scand 37 Suppl :1, , with permission.
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Current technology allows equine locomotion to be quantified in two main ways: 1 kinematic —the geometry of movement or description of the way body parts move in space without regard for the forces producing it; and 2 kinetic —examining the action of forces that produce movement without regard to the quality of movement. These are complementary ways of exploring the intricate combination of factors resulting in equine locomotion and may be used in isolation, in combination, or for comparison.
Three primary orthogonal axes of coordinates are used for biomechanical analysis, and terminology differs among studies:. Transverse—horizontal, mediolateral, or yaw. Vertical or pitch Figure Axes denote the axis of rotation of the orthogonal plane movement.
Pitch —downward movement of nose with upward movement of tail and vice versa. Roll —rotation around the longitudinal axis of the plane z. The axis designation as x , y , or z is arbitrary and must be defined in any data presentation or reporting. Standardizing velocity is of utmost importance, as both kinematic and kinetic parameters have been demonstrated to be directly velocity dependent Khumsap et al.
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When kinematic and kinetic data are synchronized with morphometric data mass, center of mass, and mass moment of inertia of body segments and mathematical formulas, more detailed gait analysis such as net joint moments, joint power profiles, and changes in center of mass position can be performed see inverse dynamic analysis below. Statistical analysis of these data may or may not allow significant statistical conclusions to be drawn; however, trends toward or away from a biologically significant effect may be revealed.
Biomechanical studies using these techniques can help in analysis of the characteristics of normal gait in multiple planes; detection of subtle gait asymmetries and compensatory changes, which has practical application for lameness evaluation; and assistance in performance assessment and quantification of the efficacy of intervention therapies and regimes. Inverse dynamic analysis is a mathematical technique based on a biomechanical model of the limb as a linked series of rigid segments without friction or translation.
Using this technique, interactions between the segments during the stance are summarized as net joint forces and net joint moments, which enables noninvasive in vivo evaluation of the net internal forces of peripheral joints and tendons at any time interval during this gait phase. A quasi-static equilibrium between internal and external forces and the moments acting on the joint is assumed, and inertial parameters are often disregarded as being negligible during the stance. Comparison of sequential time frame points enable the calculation of the work done net torque and power generated and absorbed about each joint.
The application of this technique to the swing phase gives joint moments representing the net effect of muscular forces acting on segmental inertial properties to produce angular and linear acceleration of body segments against gravity Lanovaz and Clayton , The design of the rigid segment model necessitates a simplification of musculoskeletal mechanics, thus limiting the extent of data interpretation. Friction and joint structure integrity are not considered; semirigid segments of the body cannot be accurately analyzed because of force attenuation ability e.
Isolation of individual muscle activity and characterization of the co-contraction of agonistic—antagonistic muscles cannot be determined as muscle action is represented as a net moment. Inverse dynamics analysis is also particularly sensitive to input data, so any inaccuracy will have important ramifications for resulting joint moment data.
More recently, equine locomotion studies have employed forward dynamic analysis , which uses the input of internally applied joint forces and torques to simulate body section movement, thus predicting movement patterns. This analysis allows customization of modeling for individual subjects and prediction of movement patterns minimizing overall muscle effort. Computer modeling by indirect mathematical reconstruction of expression of movement based on easily obtainable input parameters may, to some extent, replace the often tedious and inconvenient current direct measuring techniques.
The generalization inherent in such calculations will, however, limit the application and interpretation of the conclusions. Kinematic analysis of the horse in motion allows quantification of the components of equine gait which can be visually or subjectively assessed. Martens et al.
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Videography and optoelectronic systems used for kinematic analysis can evaluate temporal timing variables such as stance times, swing times, duty factors; linear distance variables; and angular measurements, thus describing trajectories of body segments and joint angles in space over time. These data also allow the relationship between time and displacement velocity and acceleration or deceleration to be calculated.
With appropriate software, this is a particularly valuable way to measure equine spinal mechanics, as vertebral access is difficult and joint movement amplitudes are small, which limits visual appraisal. Imaging can be either two-dimensional or three-dimensional, depending on the number of cameras and the orientation of camera angles. Data interpretation and comparison are dependent on the precise definition of the parameters measured and in the expression of statistical terms. Two-dimensional imaging is easily performed; however, data analysis is limited, and errors will be introduced if significant primary and coupled movements are not captured by the camera lens angle.
While motion at the distal limb is primarily restricted to flexion or extension in the sagittal plane, the small movement components of rotation, circumduction, translation, or shearing, which may be relevant to locomotion or lameness studies, are not discernible in two dimensions. Furthermore, body parts may move temporarily out of camera view, and marker tracts may cross each other. Three-dimensional studies overcome these failings to a certain degree but are more complex in operation and analysis, and the setup expense limits its use to only a few specialty practices and academic institutions.
Data presentation of kinematic gait analysis can be done by using spreadsheets, graphics, or stick figures Clayton and Schamhardt, The most commonly employed plotting formats use trajectory versus time, which allows the comparison of different kinematic features occurring simultaneously or the comparison of simultaneous kinematic, kinetic, and biophysical data.
To allow serial videographic or optoelectric recording of skeletal range of motion through different planes of space and time, surface skin markers are applied to prominent, palpable anatomic usually bony landmarks, highlighting particular limb or trunk segments and joint angles. To standardize the position, markers should be located at the estimated center of joint rotation and the distal and proximal ends of each segment under investigation. Markers can be nonreflective material delineated and recognized in color by the kinematic system; retroreflective material, which reflects light back to the imaging source; or strobing light-emitting diode LED markers, which produce light the kinematic system can track.
The respective x , y , and z positional values gained from kinematic recording allow mathematically based software programs to calculate linear and, more particularly, angular velocities and accelerations in some systems almost spontaneously. A skin marker system will potentially introduce error to experimental design, as some landmarks are difficult to repeatedly and precisely identify because of the depth of overlying musculature or fat and the specificity of a small marker placement on a large bony landmark.
Inadequate preparation of the marker site e. More importantly, the task-dependent movement of overlying skin or soft tissue relative to the underlying bone, which occurs with positional change, will introduce error. Skin movement relative to bone is more apparent proximally than distally Weller et al. Two-dimensional mathematical correction algorithms to adjust for skin displacement have been developed on Dutch Warmbloods at the walk and trot van Weeren et al. The corrections are for a limited number of body segments and are only applicable to horses with comparable conformation, gait style, and velocity.
A preliminary three-dimensional method for skin displacement correction at the equine radius has been designed; however, further studies are needed to extend the application to the entire forelimb and hindlimb, which would allow more sensitive kinematic gait analysis Sha et al. The late s saw the beginning of the quest to scientifically analyze equine locomotion. Muybridge is cited comprehensively in the literature as being the pioneer of kinematic movement analysis in horses, which involves the use of photographic stills to record relative stance durations during gait Figure The use of serial photography required the horse to move through the field of view of 24 cameras triggered in succession.
Serial still photography only captures one instant of time per frame and records three-dimensional movement in two dimensions, so total movement analysis is not possible. Marey improved on this work by developing chronophotography, which captured several different images on one photographic plate; however, accurate kinematic measurements were difficult because of image overlap.
High-speed cinematography, videography, and optoelectric systems evolved from these original concepts. Cinematography was used extensively in the past and employed a fine resolution 10 to frames per second and a wide range of shutter and camera speeds.
The process was expensive and labor intensive in methodology and analysis e. Currently, videography, combined with commercial software packages and optoelectric systems, is most often used for kinematic equine gait analysis. These systems all require calibration; automatic digitization of raw data, manual digitization of raw data, or both; integration of the calibration and digitized information transformation so that image coordinates can be correctly scaled to size; and smoothing to reduce the electric noise of digitalization caused by random fluctuation of electrical currents usually performed using a low-pass filter, although piecewise quintic splines and Fourier series reconstruction may be used.
This allows real-time data capture and analysis Clayton and Schamhardt, Videography: Videography can be in either analog or digital format. The hardware and software are comparatively inexpensive, easy to operate, and portable; and data collection is immediately available for analysis.
Care must be exercised to prevent errors caused by skin marker placement and skin displacement as discussed previously. Additionally, the camera must be accurately orientated perpendicular to the segments of interest, and lighting conditions are critical for clarity of footage. Although videographic data collection can be performed outdoors in sunlight, some digital videographic systems allow manual identification of markers, which may be useful even in less-than-optimal lighting environments.
The disadvantages of analog recordings are the likelihood of the VHS video home system recording medium degrading over time, unless loaded onto a computer, and the processing or digitizing of the video being time consuming. Options in digital processing include online or postprocessing autodigitizing of data collected which reduces time cost.
Optoelectric systems: Optoelectric systems based on the emission or detection of infrared light are the most effective collection systems for kinematic data. Current infrared based systems, used in conjunction with reflective markers or LEDs, have high framing rates and high resolutions, enabling the capture of whole body movement even as precisely as measuring the deformation of the hoof wall during stance Thomason and Peterson, Multicamera setups can localize markers to within 0.
The frame-by-frame representation of the changing marker position can be presented as computer-generated stick figures or graphs of joint position plotted against time. The major pitfall in using optoelectric systems is that aberrant markers may result from the sensitivity of the system to the infrared spectrum of daylight or any other reflective surface within the experimental area. Low-pass filtering to minimize marker trajectory noise may cause temporal shifts in key parameters of raw signals, thus distorting or dampening mean phase and magnitude values in the resulting data Molloy et al.
Kinetic analysis is the study of the mass distribution and dimensions of forces acting on the horse during the stance and motion, which are indiscernible to human sensory perception Weishaupt , This movement analysis is concerned with the effect of forces on such factors as inertial properties, acceleration, energetics, and work.
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The main aims of analyzing force production during locomotion are to explain the influences of force on movement sound or lame ; maximal performance; the metabolic cost of locomotion; the possible triggers for gait transition; and the safety margins of musculoskeletal structures during high-speed motion indicating predisposition to injury.
Marey is credited throughout the literature as being the pioneer of equine kinetic analysis. Using pneumatic principles, Marey developed various pressure sensor devices that were attached under the hoof and around the distal limb, measuring and recording pressure changes in cannon bone during hoof—ground contact at various gaits Figure Collaboration with other scientists produced a pneumatic force plate, which was used to measure force in the vertical axis.
The same principle is used today, although modern technology allows much more detailed measurements of force and pressure at the hoof—ground interface to be recorded and analyzed. A variety of electronic force sensors have been used in walk, trot, canter, jumping, and lameness studies, and these will be summarized below. Force plates: The force plate provides a dynamic, noninvasive, quantitative assessment of the amplitude and orientation of ground contact forces transmitted through one limb during the stance phase. This assessment represents the sum of trunk and limb forces generated by and resulting from the stance and also reflects the acceleration of the body mass of the horse.
The orientation of these forces is measured by deflection of sensing elements strain gauges or piezoelectric quartz crystals in the three orthogonal components of the ground reaction forces GRF : 1 vertical, 2 longitudinal, and 3 transverse Figure Vertical GRF is of the greatest magnitude; it most directly measures limb specific weightbearing and sensitivity in grading lameness Weishaupt , and so is the most commonly used measurement in force plate studies. Craniocaudal GRF quantitates forces affecting forward progression—braking deceleration and propulsion acceleration. Mediolateral GRF has the smallest amplitude, so few studies have used this variable.
FIGURE Limb positioning at the time of characteristic ground reaction force amplitudes of the right forelimbs and hindlimbs of a clinically sound Dutch Warmblood horse at normal walk. The phases of the concurrently loaded limbs are presented in a bar diagram. Reproduced from Merkens H: Quantitative evaluation of equine locomotion using force plate data [dissertation]. Utrecht, Netherlands, , State University, with permission. After recorded data are analyzed in all three parameters, peak forces, total impulses total forces integrated over time , and average force or impulse can be determined, which helps describe the rate and pattern of limb loading and unloading.
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