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The influence of boot design on exercise associated surface temperature of
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tendons in horses
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L. Hopegood1, L. Sander2, A. D. Ellis3
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1 Nottingham Trent University, School of Animal, Rural and Environmental Sciences,
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Brackenhurst, Southwell, Nottinghamshire, NG25 0QF, UK,
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2Hofgut Beutig 1, 04749, Ostrau, Germany.
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3UNEQUI, Research, Education, Innovation; Southwell, NG25 0DS, UK.
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Corresponding author: lyn.hopegood@ntu.ac.uk
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Abstract
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Sport horses frequently injure tendons of the lower limb. Tendon boots are commonly applied
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for structural support and trauma prevention during competitions. However these boots may
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increase heat stress in the area. Two separate studies were carried out with the aim to
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improve understanding of the effect of boots on heat around the tendon area. Study 1
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measured heat emitted from two types of boots (traditional and perforated, cross over design)
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covering the superficial digital flexor tendon (SDFT) in 4 horses during a set ridden and
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lunged exercise test. Study 2, a Field test, measured the effect of boot style (traditional,
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perforated and open fronted) on skin surface temperature in 131 horses, after completing a
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cross country event test (either a BE 100 three day event or a CCI* - two day short format
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event). The Raytek Raynger ST20 (infrared thermometer) was used to measure temperatures
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during both studies. The MobIR®M4 Thermal Imager was also used in Study 1 to compare
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measurement methods. A significant correlation was found between both measurement types
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(p<0.001; R2=0.94). Boots designed with perforations demonstrated greater heat emissions
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than traditional (non-perforated) boots (+ 3.5º C, p<0.01). In Study 2 mean tendon surface
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temperature for perforated type boots (28.0°C) was significantly lower than for traditional
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boots (32.3°C) and for open fronted tendon boots (31.1°C) (P<0.001). As this was an applied
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field study, additional environmental factors, such as speed and fitness level of horses, may
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have influenced results.
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Take home message
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Although exact mechanisms leading to these findings and the link between heat and tendon
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injury need to be researched further, it is advisable to design boots to minimise tendon
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exposure to high temperatures, which may contribute to tendon injury.
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Keywords: heat, equine, protective equipment, thermal imaging
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Introduction
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Injuries of the superficial digital flexor tendon are one of the most common causes of
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lameness in the Thoroughbred race horse (Patterson-Kane and Firth 2009) and in athletic
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sports horses (Murray et al. 2006). In National Hunt racing (over jumps) 89% of all ligament
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and tendon injuries are to the SDFT (Ely et al. 2009).
Singer et al. (2008) found 24% of all
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injuries (0.45% of starts recorded at events) during the cross country phase in eventing are
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related to tendons and ligaments. In addition 43 % of injuries sustained during training for
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eventing are tendon or ligament injuries and 36% of these involved the SDFT (Singer et al.
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2008). The human Achilles tendon (AT) is considered functionally equivalent to the equine
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SDFT and in humans the AT has a pivotal role in saving energy during high-speed
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locomotion, by reducing muscular work, which leads to increased heat in the area (Malvankar
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and Khan, 2011).
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Wilson and Goodship (1994) measured heat produced inside tendons in vivo and found
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temperatures of 45°C in the SDFT and concluded that this heat could be a major contributor
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to degenerative changes in tendons of equine and human athletes.
At temperatures of 45°C -
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48°C over a period of 10 minutes a rapid decline of tendon fibroblast activity takes place,
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resulting in cell death (Birch et al. 1997; Burrows et al. 2008). Yamasaki et al. (2001) also
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showed in vitro that when tendons were exposed to 45°C for 10 minutes only 27% of
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tenocytes survived and showed in vivo that Temperatures of 45°C were reached after a short
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gallop on the track. Although tenocytes have a higher heat resistance than other cells these
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temperatures at shorter time periods are likely to influence tendon matrix quality leading to
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some damage of tenocytes (Smith 2004; Patterson-Kane and Firth 2009). The extrapolation of
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the in vitro and in vivo data on core temperatures to in vivo tendon injuries should be made
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with caution, as this has not yet been directly linked.
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In order to prevent mechanical injury to the lower limb from over reach, hitting jumps or
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during a fall, various types of boots are worn routinely during jumping competitions (Murphy,
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2008). The encasement of the lower limb by the boot may increase heat stress to soft tissue
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but to date in vivo research in this area is scarce.
The design and structure of horse boots
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varies according to their purpose. Traditional boots, such as brushing boots enclosing the
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limb distal to the carpus and proximal to the metacarpophalangeal joint used to be made of
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leather with leather straps. Open-fronted tendon boots were generally worn by show jumpers,
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providing extra padding around the tendons but being open dorsally to ‘remind horses to pick
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up their feet’ over jumps (Murphy, 2008). Modern boots are often made of a mix of more
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pliable and softer cushioning materials which are cheaper, easier to clean and maintain.
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Polyvinyl Chloride (PVC) is a water-proof rigid thermoplastic polymer with added
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plasticizers for flexibility. It has a thermal conductivity of 0.14-0.17 W/m-K (the lower the
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number the more insulation it provides). Polycarbonate, another thermoplastic polymer, with
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a thermal conductivity of 0.19-0.22 W/m-K is used because of its light weight, impact
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strength (Izod 600-850 J/m) and temperature resistance (Rouabah et al. 2007). Most modern
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horse boots combine a softer inner layer such as neoprene (polychloroprene, thermal
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conductivity of 0.054 W/m-K - high insulation) and a synthetic rubber which also provides
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insulation (e.g used in wetsuits), and aids prevention of rubbing (Bardy et al. 2006).
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Thermoplastic elastomers (generally a mixture of plastic and rubber) are used for their
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softness and durability (Holden et al. 2000) imparting high elastic properties and these have a
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thermal conductivity of 0.209-0.251 W/m-K. Combining these materials in a boot will
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prevent endogenous heat dispersal. Heat dissipation from the surface of the limb without a
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boot occurs through evaporation and radiation. With a boot further conduction is required
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through the boot material followed by radiation from the boot surface.
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In recent years a focus on prevention of injury and a deeper understanding of tendon injuries
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has led to the development of boots with perforation holes (often called ‘air-cooled’ boots),
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which are marketed as ‘allowing better dissipation of heat through air circulation to the limb’.
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However, no research, to the knowledge of these authors, has been published which tests this
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theory.
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Aims
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The overall aim of the studies presented here was to measure the effect of fully closed versus
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perforated boots on SDFT skin surface temperature in exercising horses.
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The aim of Study 1 was to measure heat emitted from either traditional closed boots or novel
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air-perforated boots after two controlled exercise tests in 4 horses.
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The aim of Study 2 was to measure the effect of boot style used for 131 horses on tendon skin
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surface temperature following a cross country Field test.
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Materials and Methods
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Experimental Design
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The study passed the procedures of the Ethical Review Committee at Nottingham Trent
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University. Study 1 employed a controlled cross over design with 4 horses. The Treatment
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consisted of traditional designed tendon boots or a perforated (air-flow) boot. Study 2 was a
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field study and measured SDFT skin surface temperature of 131 horses immediately after
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completing a high intensity exercise (Cross Country Event).
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Thermometer
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The Raytek Raynger ST20 Laser Thermometer (Berlin, Germany) (non-contact infrared
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thermometer with a temperature range of -32°C to 535°C) was used to measure temperatures.
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The Raytek Raynger ST20 is usable in all weather conditions and was used in both studies for
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speed and ease of measurement. Temperatures were taken approximately 5 cm from the limb
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according to manufacturers’ guidelines (Raytek Raynger).
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Study 1
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Four sound German Warmbloods (550 ± 50 kg Bodyweight, 6-13 years old) were selected.
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The study consisted of two parts, a lunging test and a ridden exercise test which occurred over
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four days with two different phases (cross-over design). The Eskadron (Werther, Germany)
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more ‘traditional’ cross country boot and the New Equine Wear ‘perforated’ boots
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(Chippenham, UK) were tested on all four limbs. Both boots were chosen as they were made
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similarly with a PVC upper surface and Neoprene cushioning material underneath. Each test
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was performed twice and horses wore one set of boots (e.g. traditional) on left limbs while
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wearing the other set (e.g. perforated) on the opposite leg in a further cross-over design.
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Therefore, each leg was used as a separate unit with the opposite leg as control treatment so
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that measures were taken simultaneously reducing any environmental effects over time.
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Measurements were always performed in the same order to eliminate timing results (i.e.
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temperature was taken from left fore and hind legs first, then from right fore and hind
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resulting in a further cross-over between boot types). The set exercise tests (same rider or
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handler) were carried out in an arena with a sand surface and included 10 minutes warm up in
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walk followed by 5 minutes of trot in each direction, and a canter for a) Ridden test: 2.5
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minutes on each rein and b) Lunging test: 1 minute on each rein.
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After the exercise, temperatures were taken in three areas to assess heat emissions with the
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boot still in place at the lateral aspect of the boot (area of SDFT): just 1 cm below the top
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edge of the boot (top), the mid-point (middle – midway between the carpus and the
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metacarpal-phalangeal joint) and 1 cm above the bottom edge of the boot (bottom) on all four
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legs with both methods of temperature measurement.
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Study 2
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The field test was carried out at the International Horse Trials at Aldon. Horses from two
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classes were used: A British Eventing (BE100) – three day event: length: 3000 m;
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height/No.of jumps: 1.5 m/25, n=61; and a Fédération Equestrienne International (FEI)
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Concours Internationale Combined – One Star (CIC)* - two day short format event – length:
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3000 m; height/No.of jumps: 1.3 m/24, n=69). Horses in the three day BE 100 event also
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have completed the field and track phases (incl. 4000 m Endurance) on the same day prior to
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the cross country event. Twenty-one types of boots could be distinguished according to style
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and manufacturer. Results for boots were pooled into three groups according to style: a
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traditional boot design (closed all around the leg = traditional, n=93), boots with a design
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using holes, perforations or mesh design to allow for air to cool the leg (perforated, n=24),
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open fronted tendon boots (tendon boots, n=12) and no boots (n=2).
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Temperature of the left front leg only was taken, mid-way between the carpus and the
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metacarpo-phalangeal joint on the SDFT, immediately after horses had crossed the finish line
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after the boot had been removed (one leg only was used purely because of speed and the
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horses needing to be cooled down). A few horse owners insisted on removal of boots
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immediately after the finish line of the Field test and therefore temperature had to be
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measured in all horses after removal of the boot.
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Statistical Analysis
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Statistical analyses was carried out using IBM SPSS (v17.00). The significance level was set
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as P<0.05. Data were analysed for normal distribution (Kolmogorov-Smirnov) and Analysis
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of Variance (ANOVA) was applied, testing for differences between boots and for effects and
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interactions between left and right, fore and hind, exercise and phases (Study 1). For study 2
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ANOVA was used to test for effect of boots and cross country class. Unless stated otherwise,
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means are reported with standard errors.
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Results
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Study 1
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There were no differences between left and right legs, fore and hind legs or according to
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phases and no interaction was identified. Therefore independent leg data were pooled and
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when all temperature areas (top, middle and bottom) were considered together there was no
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significant difference in heat emissions between the two boots but there was a significant
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difference between areas irrelevant of boots (P<0.001; n=8; Figure 1).
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Figure 1. Distribution of mean heat emissions of outer surface of boots for horses undergoing
two exercise tests depending on measurement area (ab – significantly different P=0.000,
F=18; ANOVA, n=8)
There was a significant difference in heat emissions from the middle area of boots between
exercises and between boots (P<0.01) (Table 1).
Table 1. Mean heat emissions from the outer surface in the middle of the boot according to
boots and exercise for 4 horses
Traditional
Perforated
Treatment
Lunging
Ridden
Lunging
Ridden
Temperature
(°C)
11.63
8.54
15.46
12.00
s.d.
1.9
1.9
2.0
2.9
P-value1
n
P=0.064, F=5.1
P=0.074, F=4.7
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Boots
Exercise
Traditional
Perforated
Lunging
Ridden
10.08
13.73
13.54
10.27
2.4
3.0
2.7
3.0
P=0.005, F=11
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P=0.009, F=9.6
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1Univariate Anova 2x 2 Exercises 3x 2 Boots
Study 2
There was no significant difference in limb temperatures between the two eventing classes
although the perforated boots showed a much narrower distribution of measures for the CIC*
Event (Figure 2).
Traditional
Boot Type
Perforated
Open-fronted
38-
36"
34"
o
32-
E 30-
28-
26"
BE100
Class
Figure 2. Distribution of temperatures under the tendon boots at the finish line of two Cross
Country Events (BE 100 – 3 day event with roads and tracks, n=61; CCI* = 2 day show
jumping and cross country short format event, n=69)
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There was a significantly lower temperature under perforated boots compared to open-fronted
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tendon boots (p<0.001) and compared to traditional boots (p<0.001, Table 2). The difference
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between open-fronted and traditional boots was slight but significant (p<0.05) and much
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lower temperatures were recorded for the two horses, who did not wear boots.
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Table 2. Mean, minimum and maximum temperatures of skin surface according to type of
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boot for 129 horses upon completion of the cross country phase of a one day event
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Boot Type
Mean1
s.e.
Min
Max
Traditional
32.33 a
±0.17
29.3
36.5
Perforated
28.66 b
±0.32
25.6
32.6
Open-fronted
31.10c
±0.47
28.9
33.4
No Boot 2
21.70
21.2
22.2
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1Anova, Bonferroni: Superscripts ab, bc P<0.001; ac P<0.05
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2 n=2, not included in statistical analysis
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Discussion
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The aims of this study were to measure the effect of boot style on heat emissions from the
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surface of boots following light exercise and on skin surface temperature under the boots
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following heavy exercise. The RayTek Laser Infra-red thermometer was ideal for temperature
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measurements in the competition environment, as it could be used quickly and easily. The
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slightly unconventional application of different boots on opposing legs was used in Study 1 to
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help eliminate environmental influences.
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As the thermal image showed a large change in temperatures between the centre of the boots
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and the top and bottom, it was decided to take mean measurements from these three points for
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both measurement devices. In Study 1 there were significantly lower heat emissions from the
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middle of both boots (p<0.01) compared to the top and bottom showing a stronger insulation
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effect of boots in this area which may be due to a greater insulating effect or it could be due to
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less heat in the leg underneath those areas relative to the more proximal and distal areas.
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Although this area of the of the SDFT has been found at higher risk of developing lesions,