Because a performance horse spends a great deal of time in training and strenuous activity, its joints may become strained and damaged. Synovial joints bear the greatest amount of weight and move often as your horse's muscles work to propel him forward. Ac Pharma Flex RX cording to Jerry Black, President of the American Association of Equine Practitioners, horses can develop joint injuries and diseases when overworked, especially when engaged in equine competitions. In addition to being overworked, even average joint usage has the potential for joint damage.