*2.4. Results and Discussion*

## *2.5. A Drop Test to Study the Dynamic Behaviour of the Sport Sand Surfaces*

Figure 6A–I shows the impact acceleration versus time of the sand sample with three different moisture levels and rates of compaction. The peak of each impact acceleration is the maximum acceleration (G*max*). The red, blue and black lines represent 400 mm, 500 mm and 600 mm drop heights, respectively.

The main observation from Figure 6 and impact data given in Table 2 is that, regardless of the moisture content, increasing the compaction rate of the sand sample, has resulted in an significant increase in the G*max* [*p* = 0.0003 F = 12.9], J*max* [*p* = 0.00001 F = 22.6] and Contact time [*p* = 0.023 F = 4.6].

**Figure 5.** Accelerometry used on racing greyhounds. (**a**) (**A**) A greyhound galloping on the straight section of a track with sand surface and wearing the modified jacket with IMU pocket. (**B**) An integrated kinematic measurement system (ikms), developed in house, was used to record the acceleration signals. Data validation of the ikms was achieved using a commercial IMU (GPSport) device. (**b**) Forward (Black trace) and vertical (blue trace) acceleration vs time of three consecutive strides of a greyhound and the corresponding galloping gait events. Rotary gallop is a four-beat gait with two flight phases [41]. The limb impact in this gait moves from limb to limb in a circular pattern that is, left fore-leg (LF), right fore-leg (RF), compressed flight phase (CFL), right hind-leg (RH), left hind-leg (LH) and extended flight phase (EFL).

The same effect is also seen when the moisture content is increased (while the sand density is kept constant), mainly when the moisture level increased from 12% to 17%, which was statistically significant [*p* = 0.054 F = 4.21].

Increasing the drop height increased the velocity at the time of the impact and the higher the initial impact velocity, the higher the value of the G*max*. This reveals the rate dependency of the sand [29].

**Figure 6.** The G*max* versus time of the sand samples with different moisture levels and rates of compaction. The red, blue and black lines represent the drop height of 400 mm, 500 mm and 600 mm, sequentially.

Figure 7A–I shows load-deformation plots of the sand sample with three different moisture levels and rates of compaction. The slopes of the fitted dashed lines to the loading phases of the superimposed plots is the stiffness coefficient of the sand sample (based on the method adopted by Aerts & Clercq [42] in analysing the performance of athletic shoes with hard and soft soles). The red, blue and black lines represent 400 mm, 500 mm and 600 mm drop heights, respectively.

To see whether the moisture content affects the stiffness coefficient of the sand, sand sample with the same density but different moisture content were compared with each other. It is observed that increasing the moisture contents (Here after moisture content) within a 12–20% range, increases the stiffness coefficient.

In the low traffic condition, this increase is 87% when the water content is changed from 12% to 17%, and only 24% when it is changed from 17% to 20%. In the medium traffic condition, this increase is 26% when the water content is changed from 12% to 17% and 55% when the moisture content increases from 17% to 20%. Similarly, in the high traffic condition altering the moisture content from 12% to 17% increases the stiffness by 47% and increasing the water content from 17% to 20%, increases the stiffness coefficient by 16%. This behaviour suggest there is a nonlinear positive relationship between the moisture content and the stiffness coefficient.

To see whether the sand density affect the stiffness coefficient of the sand sample, the samples stiffness coefficient are compared with each other while keeping the moisture content constant.

For a sand sample with 12% moisture content, the stiffness coefficient increases up to 95% and 84%, as the rate of compaction is altered from the low to medium traffic condition and from medium to high traffic condition, respectively. For a sand sample with 17% moisture, this increase is up to 41% and 92%, as the sand samples are compacted from the low to medium traffic condition and from the medium to high traffic condition, respectively. For a sand sample with 20% moisture content, this increase is up to 76% and 45% increase, as the sand samples are compacted from the low to medium traffic condition and from medium to high traffic condition, respectively.

**Figure 7.** The load-deformation plots of the sand samples with different moisture levels and rates of compaction. The red, blue and black lines represent a drop height of 400 mm, 500 mm and 600 mm, respectively.

Increasing the sand density increases the stiffness coefficient of the samples. This is because when increasing the sand density, the interlock between sand particles will increase, hence increasing the stiffness [43].

The moisture content and traffic conditions of the sand samples, G*max*, J*max*, contact time (ms), energy loss (by calculating the area under the load-deformation plots), and the calculated stiffness coefficients (K), are tabulated in below Table 2.


**Table 2.** Impactdata from conducting a drop test on the sand sample.

In the provided results in Table 2, the contact time was not affected by the moisture level of the sand samples, but it significantly decreased with increases of the density of the sample. Thus, low to medium density of the sand sample was found to provide the favourable range of contact time with regards to injury prevention.

It is observed that altering the moisture content, significantly increased the G*max* and J*max* with no substantial change seen in the contact time. Moreover, the rates of compaction significantly increased all the impact data. It is also argued that the high G*max* and J*max* and short contact time were associated with high injury rate. Accordingly, comparing all the impact data it seems that the sand sample with 20% moisture content in a low traffic condition resulted in the most favourable behaviour with regards to both the injury prevention and race performance. The sand sample in this condition had the lowest energy loss compared to all other cases. The contact time was also in the favourable range as mentioned above. Finally, the G*max* and J*max* values were relatively low.
