## 4 Discussion  
### Horse Impact Data  
This study successfully collected quantitative force data from a horse falling onto a surrogate headform. To the best of the authors' knowledge, this is the first time that such data has been collected and presented. In general, the repeatability of the force-time curves is good, despite movement of the headform in some cases. Similarities in the shape of the loading curves can be seen for each impact location and standard deviations about the mean values are small. Statistically significant differences (\(p<0.05\)) in peak loads were observed between the left hind quarter and sacral vertebrae, the lumbosacral junction and the sacral vertebrae, and between the sacral vertebrae and the right hind quarter (see Table 5).  
The sacral vertebrae impact location applied the highest peak loads to the headform in all cases. This is likely due to the stiff, bony nature of this location with minimal soft tissue covering. Additionally, the location is at the centre of the animal and, during the impact, there was minimal movement of the headform, which ensured that it properly registered most of the associated load.  
Quite the opposite was true of the hind quarter impact locations. There was much less headform stability here as initial contact between the horse and the headform could cause the headform to be pushed away from the horse. The lumbosacral junction vertebrae impact location proved to be very stable and shows a very different loading curve when compared to the other impact locations. A steep initial rise is followed by a much smaller slope, and in some cases a plateau. Due to the nature of this location, this two-stage loading phase occurs when the initial contact between the horse and the headform is first made and then, as the impact continues, the front and rear of the animal come into contact with the ground, effectively reducing the effective mass applied to the headform. Additionally, during each impact, the horse rotated as the legs came to rest on the concrete floor. It is possible that the rotation of the animal caused some additional instability of the headform.  
Horse mass appears to be a factor. A 7.3% increase in the mass of the falling horse resulted in, on average, an 11.8% increase in the peak load applied to the headform. The horses used in this study were not large and would typically fall into the pony category [18]. Much heavier animals are ridden  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Helmeted vs. Un-Helmeted Impacts** & \(p\)**-Value** & **\% Difference** \\ \hline Combined Locations & **<0.001** & 43.60 \\ Lumbosacral Junction & **<0.001** & 55.40 \\ Sacral vertebrae & **<0.05** & 36.73 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Helmeted vs. un-helmeted peak load comparisons for both combined and individual impact locations. \(p\)-Values in bold indicate statistically significant differences. Note: These values are calculated using un-helmeted data from horse 2 only as this was the horse used for helmeted impacts.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Impact Location Comparison** & \(p\)**-Value** & **\% Difference** \\ \hline LHQ vs. RHQ & \(>\)0.005 & 18.41 \\ LHQ vs. Lumbosacral Junction & \(>\)0.005 & 12.98 \\ LHQ vs. Sacral Vertebrae & **<0.005** & 26.49 \\ Lumbosacral Junction vs. Sacral Vertebrae & **<0.001** & 39.13 \\ Lumbosacral Junction vs. RHQ & \(>\)0.005 & 5.46 \\ Sacral Vertebrae vs. RHQ & **<0.005** & 44.36 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Pooled peak load comparisons for all impact locations. \(p\)-Values in bold indicate statistically significant differences. LHQ and RHQ denote left and right hind quarters, respectively.  
by equestrians and most sport horses weigh around 500-600 kg. A similar drop test with an animal of such a larger mass would result in a significant increase in loads applied to the headform.