To our knowledge, this is the first study to assess the thermophysiological effects of pre-cooling on horses. Despite the moderate intensity exercise and moderate environmental conditions elicited, we observed rises in thermophysiological variables (Figure 2
, Table 3
). Most importantly, pre-cooling by cold-water rinsing directly after warming-up led to a smaller rise in Tre
, and Trump
during training compared to the control condition. These attenuated rises can be used to increase the margin for heat storage, allowing a longer exercise time before a critical Tre
(for horses most likely Tre
~41 °C [1
]) is reached. This may help to optimize equine welfare during eventing competitions and may potentially be beneficial for performance on the cross-country test, even in moderate environmental conditions. In addition, the environmental conditions and individual horse characteristics (like sex, age, and breed) should be taken into account as these affected Tre
, and/or Trump
4.1. Testing Day
Minimal effects of testing day were expected because a cross-over design was used. However, for the second and warmer testing day (WBGT day 1: 15.9 ± 0.4 °C; day 2: 21.1 ± 3.9 °C), we found significantly lower HR and Trump
values but higher Tre
values (Supplementary Tables S1–S4
). Based on visual observations, the lower HR appeared to be caused by the fact that the horses were more habituated to the situation the second time, which caused less arousal. The higher environmental temperature at the second testing day likely impaired the heat dissipation by the smaller temperature difference between the skin and environment, causing larger heat storage and consequently a higher overall Tre
. It is plausible to assume that, on the cooler day, more heat was lost via skin conduction, heating up the skin.
In the present study, there was a (non-significant) difference between the testing days in GSL; 5.89 ± 1.66 L at the second and warmer testing day and 4.83 ± 1.83 L on day 1 (n
= 10). This likely supported cooling the skin down and attenuating the rise in Trump
. Such differences in physiological responses between testing days are not surprising considering previous equine research, concluding that the number of exertional heat illness cases was significantly higher when WBGT was 20–23.9 °C (~day 2 of the present study) compared to <20 °C (~day 1 of the present study) [30
In addition, physiological responses were compared in different environmental conditions [4
]. McCutcheon et al. [17
] concluded that LSR and sweat electrolyte concentrations were largely reflected by a higher environmental temperature (20 °C to 32–34 °C). Likewise, Geor et al. [4
] observed significantly higher rates of heat storage in hot-dry (WBGT: 24.6 ± 0.3 °C) and hot-humid conditions (WBGT: 24.7 ± 0.3 °C) compared to cold-dry conditions (WBGT: 16.6 ± 0.2 °C). Therefore, the ~5 °C difference in WBGT shown here, even in cool to moderate environmental conditions, can have a large effect on rates of heat storage in eventing horses at competition days or during moderate to intense training sessions.
Despite balancing the order of testing, such temperature-dependent observations should be taken into consideration when interpreting our results, which were collected on two days that differed regarding the environmental conditions. Interestingly, in previous research the required evaporation (Ereq
) already exceeded the maximal evaporative heat loss (Emax
) in cool-dry conditions (WBGT: 16.6 ± 0.2 °C) [4
]. By definition, this represents un-compensable heat stress. Assuming that the un-compensable heat stress also applies to the present study on both testing days, it is likely that the conditions were at least similar regarding compensability (i.e., the effectiveness of thermoregulation). From a practical point of view, this should be highlighted to eventing riders and trainers, as it is commonly believed that these conditions are not challenging for eventing horses.
4.2. Thermophysiological Responses
In the present study, the rise in Tre
during training was smaller following pre-cooling than without a pre-cooling strategy (control; Figure 2
). As pre-cooling is intended to increase the margin for the rise in core temperature [20
], our pre-cooling strategy appeared to be effective [1
]. As expected, the largest absolute effect of pre-cooling was observed for both Tshoulder
(~2–3 °C; Figure 2
), which is due to the cold-water rinsing of the skin. A larger difference between core and skin temperature allows for more heat dissipation by conduction, which may reduce heat strain [1
]. The largest effect of the pre-cooling strategy on Tre
, an indicator of core temperature, which is considered the critical determinant for performance in the heat [7
], was only 1% or 0.3 °C here (Supplementary Table S2
). Although, the normal Tre
range of a horse is narrow (variation ~1 °C [34
]), from a physiological point of view, such a reduction in Tre
may seem small. In particular, when considering that alongside pre-cooling, multiple factors, like environmental conditions, affected Tre
to the same extent.
However, only a few studies evaluated the horses under field conditions in a comparable situation where they actually train and compete. In relation to cooling methods, this may be important as the mass of the rider and saddle, and loss of cooling surface area due to the saddle can affect the thermophysiological responses of a horse [31
]. In a field study by Hargreaves et al. [31
], submaximal exercise in horses was evaluated in hot-humid (HH; environmental temperature 31.3 ± 0.9 °C) and cool-dry (CD; 17.6 ± 0.4 °C) circumstances. While in controlled laboratory settings, the environmental conditions largely affected the physiological parameters as discussed above, Hargreaves et al. [31
] observed that, directly after cantering, Tre
was 39.0 °C in HH and 38.5 °C in CD conditions. In light of this relatively small Tre
difference (~0.5 °C) between conditions, lowering Tre
by 0.3 °C as in the present study may be physiologically relevant.
In another study by Marlin et al. [6
] following high-intensity exercise in hot-humid environments, end-exercise Tre
was 39.3 ± 0.3 °C, while simultaneously pulmonary artery temperature (Tpa
) was 42.3 ± 0.4 °C. After the first 30-s post-cooling period (water temperature ~6 °C), Tpa
decreased directly while Tre
continued to increase (to 40.1 ± 0.2 °C). After 2 min of cooling, Tre
showed a gradual decrease during the rest of the total cooling period of 6 min (end Tre
39.3 ± 0.3 °C, while end Tpa
was 38.2 ± 0.2 °C). The decreases correspond to 12 °C/h (0.2 °C/min) for Tre
, while Tpa
decreased with 0.8 ± 0.1 °C/min.
In a more recent study, five cooling methods in a hot-humid environment (WBGT temperature: 31.8 ± 0.1 °C) were evaluated. These horses showed the largest decrease in Tpa
using a shower method (tap water of ~26 °C for 30 min), compared to the intermittent application of cold-water (16 L of ~10 °C water every 3 min for a total of 30 min) [8
]. When the horses were showered after the intense training session, Tre
decreased from 39.7 ± 0.6 °C to 38.6 ± 0.4 °C in 30 min. This is a decrease in Tre
of 0.04 °C/min, corresponding to a total decrease of 0.29 °C after 8 min of cooling (which is the average pre-cooling time of the present study). Utilizing the showering method, Tpa
decreased just below 39.0 °C within 2.1 ± 0.6 min, and, after 9 min, Tpa
decreased to baseline temperatures (Tpa
~38 °C). This would reflect a Tpa
decrease of 0.44 °C/min.
In the study of Kohn et al. [10
], where the horses were washed with cold-water post-exercise a 0.05 °C/min decrease in Tre
was seen in the first 15 min. At the same time, Tpa
decreased with a rate of 0.25 °C/min. The lag in reduction of Tre
compared to Tpa
represents a delay in distribution of heat to peripheral body tissues or may reflect the heat of the hind limbs transferred to the rectum [1
]. It is important for horse riders, trainers, and owners to understand that Tre
is not as indicative of the actual core temperature (i.e., temperature of the internal organs) as is sometimes assumed, due to the lag in Tre
. An increase in Tre
commonly corresponds to a larger increase in Tpa
and muscle temperature [4
], and this should be taken into account when predicting the core temperature and assessing heat strain in horses.
As shown in the studies mentioned above, post-cooling methods facilitate more heat dissipation compared to the conditions without cooling. Effective post-cooling methods appear to result in a Tre
decrease of between 0.04–0.2 °C/min [6
]. The present study was performed under field conditions and the horses were cooled prior to exercise instead of post-exercise, resulting in an increase in Tre
as heat is still produced due to exercise. The cooling durations elicited in the abovementioned studies largely exceed the time available for pre-cooling horses at equestrians events. In addition, saddled horses have a restricted cooling surface area, compared to non-ridden horses.
Here, we observed an attenuated increase of 0.3 °C following ~8 min of pre-cooling (~0.04 °C/min) in saddled horses compared to the control condition. This seemingly small difference in Tre
can still be biological effective, especially as it shows a similar cooling rate in Tre
compared to other post-cooling studies [6
]. It would have been interesting to measure Tpa
in these horses as larger temperature effects are typically visible in Tpa
compared to Tre
. Unfortunately this was not possible in these horses under field conditions. As there is evidence of a dose-response relationship between pre-cooling volume and ensuing physiological outcomes [36
], future research should investigate a more aggressive pre-cooling type (i.e., larger volume or longer duration). Nevertheless, small reductions in heat strain could already help improve horse welfare during equestrian events.
On the other hand, as the potential thermophysiological benefit of pre-cooling may be relatively small, the advantages and disadvantages of pre-cooling should be considered carefully. The potential detrimental effects of pre-cooling could arise from counteracting the increase in muscle temperature, elicited by warming-up. Elevated muscle temperatures allow for increases in the muscle metabolism, that positively affect performance. However, a Tre
of 41 °C is the upper limit, at that moment muscle temperature is already much higher, and it is likely that muscle denaturation will occur [37
]. This should absolutely be avoided at all times.
Pre-cooling may not be beneficial prior to short-duration exercise, which heavily relies on muscle metabolism. However, pre-cooling may be beneficial for longer-duration exercise (>5 min), with a higher thermoregulatory burden. We observed a significant effect of pre-cooling on Tre
from 6 min into exercise (Supplementary Table S2
) and the largest effect after 20 min of exercise, supporting this theory.
Anecdotal evidence indicates that in practice riders and trainers already pre-cool their eventing horses before the cross-country test, while others believe that pre-cooling causes myopathies and muscle cramps. Previous post-cooling studies have not observed muscle stiffness or discomfort of the horses [6
]. Neither were any adverse effects of pre-cooling found during nor following this study. As core temperature is the main driver of sweating [19
], the non-significant different GSL is most likely explained by the small differences in Tre
during training following or without cooling (Figure 2
). Locally, pre-cooling did not affect the sweat production or sweat composition (Table 3
). Since LSR affects sweat composition [17
] and was not different between conditions, this likely explains the absence of differences in sweat composition. Future research is still required to fully understand the thermophysiological impact of pre-cooling on horses.