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Validation of a Three Dimensional Motion Capture System for Use in Identifying Characteristics of the Running Walk Roberson PE*, Zhang S, Kojima CJ University of Tennessee, Knoxville, TN, USA

Summary The objective of this experiment was to assess differences between the walk (W) and running walk (RW) of the Tennessee Walking Horse (TWH) through the use of a three-dimensional (3D) system. Four TWH were ridden through a six-camera motion analysis system at the W and RW. Reflective markers were used to track the movement of body segments and joint centers. Four specific gait events were used to calculate the temporal stride characteristics of W and RW. From these many other gait characteristics were calculated such as stride length and overstride. Joint angles of the lower leg were also characterized for each gait. Hind stance duration, advance placement, and advance lift off (all expressed as a percentage of total stride time) were similar between W and RW. At the carpal joint, the relative angles that differed between W and RW at the gait events are the same angles that are strongly correlated to stride length, overstride and advance placement. Velocity-independent characteristics were observed: hind stance duration as a percentage of total stride time, advance placement as a percent of total stride time, and advance lift off as a percentage of total stride time. Use of velocity-independent variables may make comparisons of gaits among horses possible even when the velocity among the horses is significantly different or cannot be controlled. Adaptation of 3-D kinematic analysis techniques will enhance our current understanding of equine locomotion and aid in the diagnosis and treatment of abnormal gait. Introduction

The running walk (RW) is a unique symmetrical four beat gait performed by the Tennessee Walking Horse (TWH). The timing of hoof placements during a stride and gait formulas have been used to describe the RW. Stance duration, advance lift off, advance placement and several other temporal characteristics of the RW have been described based on two-dimensional (2-D) data (4,5). The velocity at which the RW is performed has been shown to have a significant effect on some kinematic variables of the gait (5). The RW has not been described 3-Dimensionally (3-D). Temporal stride characteristics of the RW have not previously been compared between velocities or to the walk of the TWH. In a 3-D human kinematics study, runners ran at three velocities and three timing patterns. The ankle, knee and hip joint angle rates of change were more affected by changes in velocity than was angular displacement of those joints (3). Three-dimensional analysis of equine gaits may identify those stride characteristics that are not velocity dependent. Identification of velocity-independent stride characteristics may aid in the evaluation of gait quality and lameness. Validation of the methodology for 3-D equine gait analysis can be accomplished by comparing 2-D temporal stride characteristics of the RW obtained during a 3-D data collection process to the values of previously reported 2-D stride characteristics.

Materials and Methods A three-dimensional (3-D) motion capture system was adapted for use in characterizing the biomechanics of the RW. Registered TWH (n = 4) were ridden through an arrangement of high-speed digital infrared cameras at the walk (W) and RW. Five trials per gait per horse were recorded. A dynamic 3-D model was created and used to label and track body segments as described by Capello et al., (1). Temporal stride characteristics and carpal joint angle values were extracted by a custom script file and gait formulas were calculated for each gait. Gait formulas are comprised of hind stance duration as a percentage of total stride time and advance placement time as a percent of total stride time (2). The first number of the formula is hind stance as a percent of stride duration, and the second number of the formula is the lag time of the front hoof in relation to the hind hoof as a percentage of total stride time. A large number in the first position compared to a smaller number in the first position of the gait formula is an indication of a slower velocity; so a horse with a gait formula of 66-33 has a slower velocity than a horse with a gait formula of 65-33. The second number in a gait formula of 66-33 indicates the hind hoof is 33% of the stride duration ahead the front hoof. Hind stance as a percent of stride duration and advance placement as a percentage of stride duration data obtained from two previous studies of the RW as well as data from this study were used to calculate gait formulas associated with the RW. A right handed global coordinate system (GCS) was used to describe carpal joint angles. The X axis is medial/lateral in direction and is the pivot point of flexion and extension; movement to the left of the origin is positive, and to the right, negative. The Z axis runs vertically and is the pivot point of adduction and abduction; upward movement from the point of the origin is in the positive direction. The Y axis is in the anterior/posterior direction and is the

pivot point of inversion and eversion; forward movement from the point of origin is in the positive direction. A local coordinate system (LCS; x, y, z) for each body segment is defined in a similar manner. Carpal joint angles were calculated by relating the LCS of the radius to the LCS of the third metacarpal bone. Statistical analysis of stride characteristics was performed using the analysis of variance model in SAS 9.1.3 (SAS, 2002). A randomized block design with replication was used. Each block was a horse and each block received two treatments (walk and running walk) with four replications of each treatment collected. The statistical findings were compared with two previous studies of the RW. Results In the present study, hind stance duration as a percent of total stride time, advance placement as a percent of total stride time, and advance liftoff as a percent of total stride time were not significantly between W and RW (P > 0.05). Gait formulas for the W and RW in this study were 66-22 and 57-22 respectively. Three-dimensional analysis of the carpal joint angles at four gait events (front heel strike, front toe off, hind heel strike and hind toe off) during the walk and running walk indicated that five of the 12 angles were significantly different between the two gaits (Table 1). Hind heel strike (HHS) z-axis, front heel strike (FHS) x-axis, FHS z-axis, hind toe off (HTO) x-axis and front toe off (FTO) x-axis differed between the W and RW. No y-axis angles differed significantly at any of the four gait events between the walk and running walk. Carpal joint yaxis angles ranged from (mean ± SEM) 17.3 ± 4.0 degrees at HHS for the RW to 22.1 ± 4.3 degrees at FTO for the walk.

Event Hind Heel Strike Hind Heel Strike Hind Heel Strike Front Heel Strike Front Heel Strike Front Heel Strike Hind Toe Off Hind Toe Off Hind Toe Off Front Toe Off Front Toe Off Front Toe Off

Axis x y z x y z x y z x y z

N 30 30 30 31 31 31 30 30 30 26 26 26

Walk -66.2 ± 4.3 18.3 ± 4.1 -2.6 ± 3.4 -10.5 ± 1.4 21.7 ± 5.1 -4.6 ± 2.2 3.8 ± 5.8 21.7 ± 5.9 2.1 ± 2.4 -30.7 ± 3.6 22.1 ± 4.3 8.6 ± 2.8

Running Walk -69.2 ± 4.9 17.3 ± 4.0 4.0 ± 3.4 -18.6 ± 1.7 19.8 ± 5.2 0.1 ± 2.2 -4.5 ± 5.6 21.9 ± 5.6 4.6 ± 2.4 45.6 ± 4.2 20.8 ± 4.5 8.2 ± 3.0

P ns ns 0.0068 0.0009 ns 0.0003 0.0141 ns ns 0.0141 ns ns

Table 1. Comparisons of carpal joint angles at specific gait events and along 3 axes between the walk and running walk. Means ± SEM are shown. Probability values > 0.05 are denoted by “ns” (not significant).

The mean ± SEM for lateral advance liftoff as percent of stride, lateral advance placement as percent of stride, stride duration, hind stance as percent of stride and front stance as percent of stride were found to be similar to those previously reported for the RW (Table 2). Velocity of the RW varied between all studies. In the current study, velocity, stride time in sec, and front stand time as percent of stride were significantly different between the Walk and the RW.

Current Study Walk Velocity, m/s

A

Nicodemus et al. (2002)

Nicodemus and Clayton (2003)

Running Walk

Running Walk

Running Walk B

1.77 ± 0.127

3.41 ± 0.138

3.8 ± 0.18

2.66 ± 0.34

Not Reported

Lateral advance placement as % of stride time

22 ± 2

22 ± 2

12 ± 3

22 ± 2

17 ± 7

Lateral advance lift off as % of stride time

25 ± 2

19 ± 3

18 ± 4

10 ± 2

10 ± 5

0.683 ± 0.012

0.753 ± 0.038

0.678 ± 0.044

Stride time, sec

A

1.065 ± 0.036

B

0.651 ± 0.04

Hind stance as % of stride time

66 ± 3

56 ± 2

58 ± 3

53 ± 5

Front stance as % of stride time

68 ± 2

49 ± 3

51 ± 5

48 ± 1

46 ± 6 53-17

A

57 ± 4 B

Gait formulas

66-22

57-22

56-12

58-22

Strides/horse

4

4

6

6

5

Horses

4

3

6

6

3

Table 2. Comparison of temporal stride characteristics and gait formulas from four studies of the walk and running walk. Means + SEM are shown for the current study; results from the previous studies were reported as means + SD and are shown as such. Unreported data are represented by “nr”. Gait formulas were computed based on the published data. Different superscripts in a row within the same study are significantly different (P < 0.05; calculated for the present study only).

Discussion The 2-D results from this study are consistent with results from previous studies on the RW. This means that the 3-D data from the present study should provide a more complete characterization of the RW than the 2-D data. Additionally, these results support the concept that the kinematics of the TWH running walk differ from those of the walk. The comprehensive gait study that developed the original gait formulas of the running walk described three gait formulas common to the TWH: 54-31, 32-22, and 30-29 (2). Nicodemus et al. (5) evaluated the running walk at a fast gait and at a slow gait with the mean velocities of 3.8 m/s and 2.66 m/s for fast and slow respectively. Using the temporal characteristics of the two velocities in that study, we calculated a gait formula for each velocity; fast: 56-12 and slow: 58-22. Nicodemus and Clayton (4) studied the running walk to determine temporal stride characteristics but did not consider velocity; instead, the horse’s trainer and breed association criteria were used to determine if the horse was performing the proper gait. We

calculated the gait formula from the results of that study to be 53-17. In the current study, riders were instructed to direct their horses to perform the two gaits at the riders’ preferred velocity and quality. The velocity at the running walk was 3.41 ± 0.13 m/s and generated the gait formula 5722, similar to the results of these previous studies of the running walk. Based on the comparison of data from all four studies, hind stance duration as a percentage of total stride time, advance placement as a percentage of total stride time, and advance lift off as a percent of total stride time may be suitable characteristics to be used for comparison in a clinical setting. If a clinical exam finds that the velocity-independent variables of a horse differ from these reported values, it may be an indication of lameness. These variables are not breed specific since the walk of the TWH is similar to the walk of the Quarter Horse (Hildebrand 1965). It is unknown how lameness would affect the velocity-independent stride characteristics. The front limb characteristics could have a greater difference or be more similar between the walk and the running walk. Or, the hind limb stride characteristics could become different from the walk to the running walk. The y-axis represents inversion and eversion of the carpal joint. No significant differences were found in y-axis angles of the carpal joint at any of the four gait events. Observation of increased inversion or eversion during locomotion may be an indication of abnormal motion pattern. This study suggests that in order to develop clinically relevant biomechanical models of equine locomotion, joint specific velocity-independent stride characteristics should be identified. Use of velocity- independent variables may make comparisons of gaits among horses possible even when the velocity among the horses is significantly different or cannot be controlled. The adaptation of 3-D kinematic analysis techniques to this field of study will allow for enhancement

of our current understanding of equine locomotion, aiding in diagnosis and treatment of lameness at the level of an individual joint.

References (1)

Capello A, Cappozzo A, La Palombara PF, Lucchetti L, Leardini A. Multiple anatomical landmark calibration for optimal bone pose estimation. Human Movement Science 1997;16: 259-274.

(2)

Hildebrand M. Symmetrical gaits of horses. Science 1965;150:701-708.

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Karamanidis KA, Arampatzis, Bruggemann GP. Symmetry and reproducibility of kinematic parameters during various running techniques. Med Sci Sports Exerc 2003;35:1009-1016.

(4)

Nicodemus MC, Clayton HM. Temporal variables of four-beat, stepping gaits of gaited horses. Appl Anim Behav Sci 2003;80:133-142.

(5)

Nicodemus MC, Holt KM, Swartz K. Relationship between velocity and temporal variables of the flat shod running walk. Equine Vet J Suppl 2002;34:340-343.