Chose this formula when there is hind- leg weakness for the more cold and tired pet. Yang is function, so when the hind-legs begin to weaken we look to the TCM Kidney system. The Kidneys house the essence (jing), think of the word vitality. When function of the lower back, hind legs, and knees start to weaken, it is attributed to a decline of Yang-Qi. Yang is warming, thus when cold accumulates, we want to warm Yang to strengthen the hind-legs, lower back and knees.