A concussion is brain injury caused by a blow to the head or a violent shaking of the head and body. **A sprain is a stretching or tearing of ligaments, the fibrous tissue that connects bones and joints. A contusion is blood or bleeding under the skin due to trauma of any kind; typically black and blue at first, with color changes as healing progresses (bruise). A strain is a stretching or tearing of a muscle or a tissue connecting muscle to bone (tendon).
