Figure 3.1: The A-‐frame setup. A) The right camera positioned perpendicular to the A-‐ frame. B) The custom-‐built A-‐frame in the middle of the runway with build-‐in force plate. C) The runway and jump.
3.3.4 Trials
Each dog performed three trials over the A-‐frame at each angle of incline (40°, 35° and 30°), off leash but guided by their owners, for a total of nine trials. Only trials where dogs contacted the force plate with the first limb that contacted the A-‐frame were included. The sequence of the A-‐frame angle of incline for each dog was determined by block randomization. The dogs were allowed to rest for at least 20 minutes before the A-‐frame was repositioned to the next angle of incline.
3.3.5 Data collected
The stance time (ST) in seconds was recorded. Ground reaction forces perpendicular to the inclined surface was investigated and is termed to vertical forces. These forces included the
peak vertical force (PVF), the passive impact peak (PIP; first peak in the force time curve), the rate of force development (rising slope) to the peak vertical force (RFD_PVF) and to the passive impact peak (RFD_PIP), and the time point (percentile) in the stance phase at which the peak vertical force (tPVF) and passive impact peak (tPIP) occurred. Cranio-‐caudal forces investigated included the peak braking (PBF) and peak propulsive forces (PPF), the time point (percentile) in the stance phase at which peak braking (tBF) and propulsive forces (tPF) occurred, and the proportion of time for limb contact spent in braking (% braking) and propulsion (% propulsion) (Figures 3.2 and 3.3).
Figure 3.2: Typical force time curve depicting the measurements acquired. A) Passive impact peak. B) peak vertical force. C) peak breaking force. D) peak propulsive force.
Figure 3.3: Typical force time curve depicting the measurements acquired. a) time point of passive impact peak b) time point of peak vertical force c) time point of peak braking force d) time point of peak propulsive force. Red arrow) time spent within braking. Green arrow) time spent within propulsion.
3.3.6 Data Processing
The kinetic data was processed using Matlab (Mathworks version 7.10.0, Natick, Massachusetts). Data was normalized to body weight. The videos were assessed using Kinovea software (http://www.kinovea.org/) to identify foot landing. The center of mass marker allowed vector calculation of the approach speed (numerical differentiation of displacement data) using vertical and horizontal velocity components before impact. Vertical (vz) and horizontal (vy) velocity was calculated from displacement and time (velocity = change
the first central distance method by using one video frame before impact, at impact and following impact. Speed was defined as the magnitude of the resultant velocity vector.
3.3.7. Statistical Analysis
For each dog, the mean of three trials for PVF, PIP, RFD_PVF, RFD_PIP, tPVF, tPIP, PBF, PPF, tBF, tPF, % braking and % propulsion was calculated for each A-‐frame angle and was the response of interest. The data for these parameters were considered continuous and found to be normally distributed, based on failure to reject the null hypothesis of normality using the Shapiro-‐Wilk test. Data was summarized as mean (95% confidence interval [CI]) for each A-‐frame angle. The mean (95% CI) velocity for the successful runs was calculated for each A-‐ frame angle and reported. Velocity was compared between A-‐frame angles using a one-‐way ANOVA followed by posthoc comparisons of means against a Tukey-‐adjusted P<.05 if the F test was significant at P<.05.
The mean PVF, PIP, RFD_PVF, RFD_PIP, tPVF, tPIP, PBF, PPF, tBF, tPF, % braking and % propulsion were each evaluated for a fixed effect of A-‐frame angle using a mixed linear model that included the random variance of dog, and covariates dog height and velocity. The A-‐frame angle of incline sequence was initially included but was not significant and subsequently excluded from the model. The adjusted least square means and 95% CI for each parameter is reported. Where there was a significant effect of A-‐frame angle, pair-‐wise comparisons were made between A-‐frame angles using least square means against a Tukey adjusted P-‐value of ≤0.05. The inter-‐dog and intra-‐dog coefficient of variation (CV) was
calculated for each parameter (CV=SD/mean x 100%). The intra-‐dog CV was calculated for the three trials at each angle of incline and the overall mean intra-‐dog CV for each A-‐frame angle is reported. The inter-‐dog CV was calculated from the adjusted least squares means that accounted for the covariates, and are reported for each A-‐frame angle. All statistical analysis was performed using SAS v9.4 (SAS Institute, Cary, NC).
3.4. Results
All dogs successfully completed nine trials for 72 successful trials. The intra-‐ and inter-‐dog CV was acceptable with most parameters having a coefficient of variation less than 20% (Tables 1 and 2). 40 35 30 Contact 4.8 3.1 3.2 PIP 9.8 9.5 17.1 tPIP 13.6 13 16.5 RFD-‐PIP 7.9 10.7 11.1 PVF 1.9 6.1 3.4 tPVF 7.7 7.9 6.3 RFD-‐PVF 2.6 4.8 5.6 Braking force 16.9 8.4 17.5 tBF 8 5 8.3 Propulsive force 8.9 7.7 9.7 tPF 4.7 7.7 4.6 % Braking 1.6 4.8 3 % Propulsion 1.6 4.8 3
Table 1: Intra-‐dog coefficient of variation (%). [PIP (passive impact peak), RFD (rate of force development), PVF (peak vertical force), tPIP (time passive impact peak), tPVF (time peak vertical force), tBF (time braking force), tPF (time propulsive force)]
40 35 30 Contact 1.8 1.8 1.9 PIP 13.6 16.4 14.9 tPIP 18.8 21.7 19.6 RFD-‐PIP 9.2 11.9 10.8 PVF 5.1 5.6 5.1 tPVF 6.8 7.3 6.7 RFD-‐PVF 7.6 9.6 8.4 Braking force 11.3 12.9 10.4 tBF 3.7 4.1 3.5 Propulsive force 11.3 14.8 17.5 tPF 4.7 4.7 4.2 % Braking 5.1 5.5 6.1 % Propulsion 5.1 5.5 6.1
Table 2: Inter-‐dog coefficient of variation (%). [PIP (passive impact peak), RFD (rate of force development), PVF (peak vertical force), tPIP (time passive impact peak), tPVF (time peak vertical force), tBF (time braking force), tPF (time propulsive force)]
The mean (95% CI) velocity for all trials was 5.6 m/s (5.4-‐5.9 m/s). All dogs were galloping prior to entry of the A-‐frame. The mean velocity at 40° was 5.5 m/s (5.0-‐5.9), at 35° was 5.5 m/s (4.9-‐6.0) and at 30° was 5.8 m/s (5.4-‐6.4). The velocity was not significantly different between the three angles of incline (P=0.452).
Contact with the A-‐frame at the 40° angle of incline resulted in a mean (95% CI) passive impact peak of 1.2N/N (0.85 -‐ 1.53) which occurred at the 9th percentile (5.26 – 12.13) of the stance phase. The mean (95% CI) peak vertical force was 2.4 N/N (2.2 -‐ 2.7) which occurred at the
38th percentile (32.7-‐43.7) of the stance phase. There was no significant effect of A-‐frame angle on the vertical ground reaction forces at the three A-‐frame angles of incline (Table 3).
40 35 30 P
Contact (s) 0.11 (0.11 to 0.12) 0.12 (0.11 to 0.12) 0.11 (0.11 to 0.12) 0.1043 PIP (N/N) 1.19 (0.85 to 1.53) 1.09 (0.71 to 1.46) 1.1 (0.75 to 1.44) 0.7416 tPIP 9 (5.26 to 12.13) 8 (4.5 to 12.1) 8 (4.96 to 11.9) 0.1436 RFD-‐PIP
(N/N/s) 145.3 (117.3 to 173.3) 122.57 (91.96 to 153.17) 124.4 (96.18 to 152.64) 0.1635 PVF (N/N) 2.4 (2.2 to 2.7) 2.2 (2 to 2.5) 2.4 (2.1 to 2.6) 0.5694 tPVF 38 (32.7 to 43.7) 39 (32.8 to 44.7) 39 (33.9 to 44.9) 0.8956 RFD-‐PVF
(N/N/s) 63.6 (53.5 to 73.7) 50.9 (40.6 to 61.3) 57.2 (47.1 to 67.3) 0.2217 Braking force
(N/N) -‐ 0.5 (-‐0.63 to -‐0.39) -‐0.46 (-‐0.58 to -‐0.34) -‐0.6 (-‐0.7 to -‐0.44) 0.5145 tBF 12 (10.96 to 12.8) 12 (10.62 to 12.61) 12 (11.55 to 13.4) 0.1630 Propulsive
force (N/N) 0.44 (0.34 to 0.55)* 0.37 ( 0.26 to 0.49) 0.29 (0.18 to 0.39)* 0.0130 tPF 57 (51.8 to 63) 58 (51.9 to 63.4) 64 (58.6 to 69.8) 0.1543 % Braking 24.7 (16.6 to 32.8)* 25.3 (16.6 to 34) 36.7 (28.6 to 44.9)* 0.0165 % Propulsion 75.3 (67.2 to 83.4)* 74.7 (66 to 83.4) 63.3 (55.1 to 71.4)* 0.0165
Table 3: Adjusted least square means and 95% confidence intervals. [PIP (passive impact peak), RFD (rate of force development), PVF (peak vertical force), tPIP (time passive impact peak), tPVF (time peak vertical force), tBF (time braking force), tPF (time propulsive force)]. (* significantly different)
Contact with the A-‐frame at the 40° angle of incline resulted in a mean peak propulsive force of 0.44 N/N (0.34 to 0.55) which occurred at the 57th percentile (51.8 to 63) of the stance phase. The peak braking force was -‐0.5 N/N (-‐0.63 to -‐0.39) which occurred at the 11th percentile (10.96 to 12.8) of the stance phase. Contact with the A-‐frame at the 40° angle of
incline resulted in % propulsion of 75.3% (51.8 to 63) and % braking of 24.7% (16.6 to 32.8). The peak propulsive force and the % propulsion were significantly higher at the 40° angle of incline compared to 30° (P=0.013, P=0.0165, respectively) and the % braking was significantly lower at the 40° angle of incline compared to 30° (Table 3). There was no significant difference between the 40° and 35° angle of incline and 35° and 30° angle of incline for these parameters.
3.5. Discussion
Contact with the standard A-‐frame angle of incline at 5.6m/s (5.4-‐5.9m/s) was characterized by a mean passive impact peak of 1.2 N/N and a mean peak vertical force of 2.4 N/N. The occurrence of the passive impact peak coincided with the peak braking force but the peak vertical force did not coincide with the peak propulsive force. A reduction in the A-‐frame angle of incline from 40° to 30° resulted in a decrease in the peak propulsive force with less time spent in the propulsive phase (75.3% versus 63.3%) and consequently, more time spent in the braking phase (24.7% versus 36.7%). The peak propulsive force recorded for the standard (40°) incline angle of the A-‐frame was 0.44 N/N. The results for all other kinetic parameters did not support our hypothesis of an incremental decrease with a reduction in the angle of the A-‐frame incline.
Ascent of the A-‐frame at 40° angle of incline resulted in a higher propulsive force with more time spent in propulsion compared to the 30° A-‐frame angle. A higher propulsive force and time spent in propulsion at the 40° angle of incline may be required to facilitate a change in direction while maintaining velocity 12. An increase in propulsive force with an increase in incline angle was also identified in dogs ascending a 15° angle of incline, where the forelimbs exerted mainly propulsive forces during uphill trotting 12. The effect of incline requiring more propulsion explains the significant difference identified between the time spent in propulsion at the 40° incline angle (75.3%) versus the time spent in propulsion at 30° incline angle (63.3%). We did not find a significant effect of A-‐frame angle of incline on the peak braking force. The first landing forelimb during a gallop acts to brake the speed 104 and since their
speed was similar for the three angles of incline this likely explains our result.
The PVF identified in our cohort had a 95% confidence interval of 2.2-‐2.4 N/N, identified at a mean velocity of 5.6 m/s. In comparison, a similar mean PVF of 2.12 N/N was reported for the first landing forelimb in a homogenous cohort of dogs, of similar body morphology to dogs in our study, recorded at a gallop over level ground at a mean (SD) of 9.2 (0.3) m/s 104. Thus, although there was no incline in that study, the higher velocity results in a increase within the vertical ground reaction forces as expected 83. If our dogs had entered the A-‐frame at a higher velocity, perhaps under trial conditions, the vertical ground reaction forces may have been even higher.
Characterization of the forces on entry to the A-‐frame revealed a repeatable presence of the passive impact peak. The first peak in vertical forces represents the passive impact peak associated with initial paw strike in the early stance phase. In people, the passive impact peak (termed the heel strike transient) is associated with the etiology and progression of a variety of conditions, such as osteoarthritis 76. The second peak represents maximal vertical forces generated in late stance. The drop in vertical force between the peaks represents the rollover of the foot between paw strike and toe off 35. The vertical forces of a dog at a trot have been typically described as a single, sharper peak for both forelimbs and hind limbs 35. It has been assumed that faster gaits do not have the first peak within the force-‐time curve with two peaks identified, presumably because the events in the stance phase are happening more quickly 35 or alternatively it may be missed due to data processing or an inability of the force
plate to detect high frequency data 73. Two peaks have been previously identified in dogs during galloping, although it was termed the mean peak vertical force, and recorded a mean of 1.25 N/N 104. This is comparable to the mean passive impact peak of 1.19 N/N we recorded in dogs entering the A-‐frame at 40° incline. During ascent of the A-‐frame 40° incline, the peak braking force occurred at the 12th percentile of the stance phase while the passive impact peak occurred at the 9th percentile, consistent with an explanation of paw-‐strike dynamics. The peak propulsive force occurred later during the stance phase at the 57th percentile, and was not associated with the earlier peak vertical force at the 38th percentile. This is expected as the dogs make a transition from a vertically directed force to a cranio-‐caudal directed force to facilitate push-‐off from the ground.
Factors that contribute to variability in kinetic gait analysis include trial velocity, dog size and conformation, trial repetition, and day-‐to-‐day variation, 64,70,82,83 and therefore, reference intervals for ground reaction forces of clinically normal dogs remain elusive. The standard process of normalizing ground reaction forces with bodyweight alone appears insufficient to control for all size-‐dependent variability 84. When additional dog-‐specific morphometric measurements such as functional limb length or height at the withers are used for data normalization, significant differences between breeds are still recognized but variance is markedly reduced 85,86. Despite having dogs of similar morphology in our cohort, we included height at the withers as a covariate to improve our data analysis by reducing variance. Height at the withers is commonly used to distinguish between height differences in agility dogs 2 and is associated with functional limb length 62. It is easy to measure and may be more reliable than functional limb length. Height at the withers has been used to normalize data of dogs of
varying morphology and although morphometric normalization of ground reaction forces does reduce variance 84, it does not eliminate it 85,86 and inter-‐ and intra-‐dog variation should also be considered in interpretation of results.
Although the velocity of our dogs remained within a tight 95% confidence interval (5.6 – 5.9 m/s), we did include velocity as a covariate to improve our data analysis by again, reducing variance. Most studies have restricted the velocity range to +/-‐ 0.3m/s 35 but by restricting the range of velocity, trials may be discarded and the duration of data collection is lengthened. Including trials over a moderate velocity range can improve the efficiency of trial capture and although this has shown minimal effect on the variance of peak vertical force and vertical impulse in heterogeneous populations; 87 the velocity should still be considered as a possible source of variation.
Velocity measurement using the first central difference method is superior compared to velocity measurement utilising photocells, since it measures velocity over a shorter distance, only at the step of interest, which was an important consideration in our study since the dogs entering the A-‐frame had a direction change. Our measurement of velocity allowed recording when the dog contacted the A-‐frame. This may be a better representation of (deleted limb) velocity, as the velocity is measured mid-‐stance and thus, the number of strides taken before entering the A-‐frame has limited impact on this measurement. Most previous veterinary kinetic investigations use multiple photocells (three to five), spaced at a specified distance, to measure the velocity of a dog passing over the force plates. The subject’s velocity is calculated by dividing the known distance between photocells by the time between triggering
consecutive photocells. Inaccurate measurements of velocity can occur when the photocells are triggered by different parts of the subject’s body or by the handler 97. Kinematic analysis lends itself to more accurate velocity measurement of anatomical locations since the location of multiple markers can be identified in every frame 53, although as with kinematic gait analysis the reflective marker is placed over the presumed center of mass without clipping the hair and this can induce error 108.
In conclusion, a change in the A-‐frame angle of incline from 40° to 30° resulted in a decrease in the peak propulsive force with less time spent within the propulsive phase. During ascent up the A-‐frame at a 40° incline, a higher propulsive force, combined with extended time spent in propulsion, is required to maintain forward movement and convert potential energy into forward kinetic energy. The passive impact peak was identified in all force-‐time curves and its relevance requires further investigation. Further research is required to assess the effect of repetitive loading during agility and whether a further reduction in the A-‐frame angle of incline will benefit agility dogs.
ACKNOWLEDGMENT
Funding, in part, provided by the Canine Research Fund of the Australian National Kennel Council and the College of Veterinary Medicine, Murdoch University
DISCLOSURE
The authors declare no conflicts of interest related to this report
3.6. References
1. Levy I, Hall C, Trentacosta N, et al. A preliminary retrospective survey of injuries occurring in dogs participating in canine agility. Vet Comp Orthop Traumatol 2009; 22: 321–324.
2. Cullen KL, Dickey JP, Bent LR, et al. Internet-‐based survey of the nature and perceived causes of injury to dogs participating in agility training and competition events. J Am Vet Med Assoc 2013; 243: 1010–1018.
3. Appelgrein C, Glyde MR, Hosgood G, et al. Reduction of the A-‐Frame angle of incline does not change the maximum carpal joint extension angle in agility dogs entering the A-‐frame. Vet Comp Orthop Traumatol 2018; 60–64.
4. Cullen KL, Dickey JP, Brown SHM, et al. The magnitude of muscular activation of four canine forelimb muscles in dogs performing two agility-‐specific tasks. BMC Vet Res 2016; 13: 68.
5. DeCamp CE. Kinetic and kinematic gait analysis and the assessment of lameness in the dog. Vet Clin North Am Small Anim Pract 1997; 27: 825–840.
6. Budsberg SC, Verstraete MC, Soutas-‐Little RW. Force plate analysis of the walking gait in healthy dogs. Am J Vet Res 1987; 48: 915–918.
7. Carrier DR, Gregersen CS, Silverton NA. Dynamic gearing in running dogs. J Exp Biol 1998; 201: 3185–3195.
8. Pfau T, Garland de Rivaz A, Brighton S, et al. Kinetics of jump landing in agility dogs. Vet J 2011; 190: 278–283.
9. Yanoff SR, Hulse DA, Hogan HA, et al. Measurements of vertical ground reaction force in jumping dogs. Vet Comp Orthop Traumatol 1992; 5: 44–50.
10. Australian national kennel council Rules for the conduct of agility
trialshttp://ankc.org.au/media/4369/agilityrules-‐2016.pdf (2015, accessed 10 September 2017).
11. Lee D V. Effects of grade and mass distribution on the mechanics of trotting in dogs. J Exp Biol 2011; 214: 402–411.
12. Walter RM, Carrier DR. Ground forces applied by galloping dogs. J Exp Biol 2007; 210: 208–216.
13. McLaughlin R, Roush J. Effects of subject stance time and velocity on ground reaction forces in clinically normal greyhounds at the trot. Am J Vet Res 1994; 55: 1666–1671. 14. Verdini F, Marcucci M, Benedetti MG, et al. Identification and characterisation of heel
strike transient. Gait Posture 2006; 24: 77–84.
15. Gillespie KA, Dickey JP. Determination of the effectiveness of materials in attenuating high frequency shock during gait using filterbank analysis. Clin Biomech 2003; 18: 50– 59.
16. Jevens DJ, Hauptman JG, DeCamp CE, et al. Contributions to variance in force-‐plate analysis of gait in dogs. Am J Vet Res 1993; 54: 612–615.
17. Riggs C, DeCamp C, Soutas-‐Little R, et al. Effects of subject velocity on force plate-‐ measured ground reaction forces in healthy greyhounds at the trot. Am J Vet Res 1993; 54: 1523–1526.
18. Voss K, Galeandro L, Wiestner T, et al. Relationships of body weight, body size, subject velocity, and vertical ground reaction forces in trotting dogs. Vet Surg 2010; 39: 863–869.
19. Voss K, Wiestner T, Galeandro L, et al. Effect of dog breed and body conformation on vertical ground reaction forces, impulses, and stance times. Vet Comp Orthop
Traumatol 2011; 24: 106–112.
20. Krotscheck U, Todhunter RJ, Nelson SA, et al. Precision and accuracy of ground reaction force normalization in a heterogeneous population of dogs. Vet Surg 2014; 43: 437–445.
21. Mölsä SH, Hielm-‐Björkman AK, Laitinen-‐Vapaavuori OM. Force platform analysis in clinically healthy Rottweilers: comparison with Labrador Retrievers. Vet Surg 2010; 39: 701–707.
22. Hans EC, Zwarthoed B, Seliski J, et al. Variance associated with subject velocity and