Fascia has been described as the sweater that surrounds the individual muscle and then muscle groups that have a common action. Think about how it feels to wear a shirt that is tight. Reaching for something or making large movements is difficult and feels restrictive. The same is true when the fascia around a muscle is tight, movement is restricted and pain may be present. Again the result can be an unwillingness or inability to perform at previous levels.
Lets look at a specific muscle group in regards to flexibility and massage.
The hamstrings are a group of three muscles that go from the very top back of your horse's leg to the stifle. The hamstrings primary action is to extend the hip. They also aid in lateral movement and kicking. Inflexibility in this muscle group can cause a shortened forward stride, a resistance to lateral work and joint discomfort in the hindquarters. Inflexibility within a muscle or muscle group will also increase the risk of injury.