Motion symmetry is believed to be the hallmark of an orthopedically normal gait and forms the basis of clinical gait assessment in quadrupeds. It depends on the observation of limb, trunk, and head motions at a walk and trot, and there is often disagreement between clinicians' gait scores for any given clinical case,1 particularly when lameness is mild. A study2 on dogs recovering from surgery for elbow joint dysplasia revealed no correlation between objective gait measurements, clinical gait score, and owner assessments. Objective gait analysis is increasing in popularity as methods developed largely in human gait analysis are adapted for equine and canine analyses.3,4 Before lame gaits can be meaningfully assessed, there is a need to characterize an orthopedically normal gait for reference purposes. To this point, most of the clinical gait studies in the literature have involved kinematic analysis or force plate analysis alone and have largely been based on the presumption that trotting is a symmetric gait in which diagonal strides are similar when the dog is sound (orthopedically normal).
Kinematic studies in sound trotting dogs have shown that any interlimb motion asymmetry is largely attributable to variation between trials,5–7 and researchers have concluded that such dogs are kinematically symmetric when trotting. Kinematic analyses of gait in horses have likewise shown that horses may have some natural motion asymmetry at a trot, but that there are overall insignificant differences in motion-time variables between limb pairs.8
In a study9 of horses walking over a force platform, ground reaction forces were approximately 90% symmetric between forelimb and hind limb pairs, and although there was some variation in this index among sound horses, repeated measurements within horses were consistent over a 3-year period. Vertical ground reaction force measurements in sound trotting horses is approximately 97% symmetric, and craniocaudal force symmetry is about 92%.10 At constant speed on an instrumented treadmill, sound trotting horses have small and clinically unimportant degrees of asymmetry in force and timing variables.11 Dogs trotting over multiple force platforms exert slightly higher peak vertical forces under their left forelimb and right hind limb than in the contralateral limbs.12 On the other hand, peak vertical force symmetry averages 96.3% beneath the hind limbs of walking dogs and 97.6% in the same dogs at a trot.13
Ground reaction forces measured by use of a force platform represent the external interaction between a limb and the ground. They measure the overall effects on the ground of muscle contractions that resist flexion of the limb joints under the influence of gravity acting on the superincumbent segments and trunk or that generate extension of the joints to raise the trunk's mass against gravity. Ground reaction forces also quantify the longitudinal and transverse shear forces produced as the limb muscles act to decelerate or accelerate the horizontal motion of the body. Knowledge of the segmental morphometry and limb segment kinematics allows these external forces between the foot and ground to be decomposed into joint reaction forces at each joint, and these are combined with segment positional information to calculate the net moment at each joint by use of inverse dynamics.14
All of the net joint moments can be summed to yield a so-called TSM for the limb,15,16 which can then be compared between limbs as an indication of mechanical gait symmetry. Individual joint moments can also be evaluated for their discrete contributions to the limb TSM, thus delivering more information about mechanical limb symmetry than the ground reaction forces and kinematic patterns in isolation. In this way, the peak extensor moment at the stifle joint during late stance contributes more to the TSM in Labrador Retrievers than in Greyhounds, despite the absence of active extension of the stifle joint in the Labrador Retrievers during push-off.15 A pilot study17 of mechanical asymmetry in a sound Labrador Retriever showed that despite similar ground reaction force amplitudes, the TSM at midstance was 37% larger for the right hind limb than for the left hind limb. The purpose of the study reported here was to identify whether sound Labrador Retrievers were mechanically symmetric by evaluating moment and power patterns of right and left hind limb joints and their TSMs in each dog.
Mechanically dominant in the left hind limb
Mechanically dominant in the right hind limb
Mechanically symmetric in the hind limbs
Total support moment
ProReflex, Qualisys AB, Gothenburg, Sweden.
Model 9287, Kistler Instrumente AG, Winterthur, Switzerland.
Keegan KG, Wilson DA, Wilson DJ, et al. Evaluation of mild lameness in horses trotting on a treadmill by clinicians and interns or residents and correlation of their assessments with kinematic gait analysis. Am J Vet Res 1998; 59:1370–1377.
Burton NJ, Owen MR, Colborne GR, et al. Can owners and clinicians assess outcome in dogs with fragmented medial coronoid process? Vet Comp Orthop Traumatol 2009; 22:183–189.
Budsberg SC, Jevens DJ, Brown J, et al. Evaluation of limb symmetry indices, using ground reaction forces in healthy dogs. Am J Vet Res 1993; 54:1569–1574.
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Voss K, Imhof J, Kaestner S, et al. Force plate gait analysis at the walk and trot in dogs with low-grade hindlimb lameness. Vet Comp Orthop Traumatol 2007; 20:299–304.
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Burton NJ, Dobney JA, Owen MR, et al. Joint angle, moment and power compensations in dogs with fragmented medial coronoid process. Vet Comp Orthop Traumatol 2008; 21:110–118.
Colborne GR, Walker AM, Tattersall AJ, et al. Effect of trotting velocity on work patterns of the hind limbs of Greyhounds. Am J Vet Res 2006; 67:1293–1298.
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