Background: Rehabilitation of tendon injuries in horses often involves cryotherapy to reduce inflammation and occasionally tissue heating to increase collagen extensibility. The application of alternating cold and hot (ie contrast therapy) is widely used in human physical therapy; however, its utility in equine rehabilitation is largely unknown. Objectives: The objectives of this study were to (a) assess if the equipment could achieve therapeutic tissue temperatures (<15 and >40°C) at different tissue depths relative to the digital flexor tendons and (b) evaluate the time-temperature profiles during serial heating and cooling cycles using a contrast therapy device. Study design: In vivo experiment. Methods: In 4 adult horses with normal forelimb digital flexor tendons, fine-wire temperature probes were placed superficially on the skin and implanted subcutaneously, deep to the superficial digital flexor tendon (SDFT) and deep to the deep digital flexor tendon (DDFT). Temperatures were recorded over three complete thermal (hot-cold) cycles. Minimum and maximum temperatures were recorded and the rate of temperature changes and the areas underneath the time-temperature curves (ie thermal load) were calculated. Results: Minimum and maximum tissue temperatures (°C) included: superficial skin [12.6 ± 1.0; 42.4 ± 2.4], subcutaneous tissues [14.1 ± 0.8; 42.3 ± 2.2], deep to the SDFT [15.6 ± 0.8; 41.7 ± 2.6] and deep to DDFT [25.1 ± 2.0; 38.0 ± 3.5]. An initial rapid rate of tissue temperature change between 3.2 and 4.3°C/min occurred within tissues to the depth of the DDFT. Tissue thermal loads during heating ranged from 255 to 607°C*second and from 309 to 780°C*second during tissue cooling, with the lower values noted deep to the DDFT. Main limitations: Unknown clinical efficacy in diseased tissues. Conclusions: The applied contrast therapy was consistently able to induce cooling and heating of tissues to the depth of the DDFT.

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