First, one must understand the two classes of tissue found in the human body: Bone, and Soft Tissue. This means that every tissue in the body, apart from bones, such as muscles, tendons, & ligaments is Soft Tissue. A Soft Tissue Injury is generally an injury to ligaments (known as a sprain), and to muscles and tendons (known as strain). For example, a neck sprain/strain is generally known as a "whiplash soft tissue injury."
In contrast, Hard Tissue injuries relate to injuries to the bone like fractures, and they tend to heal better with no motion which is why hard casts are often used in treating them. Soft Tissue injuries, on the other hand, tend to heal better with proper motion which is why they respond well to physical therapy and chiropractic. Bones tend to take longer to heal because the blood supply is lower compared to Soft Tissue injuries which have potential to heal quicker. Bone breaks tend to heal stronger than they were before the break, but not so for Soft Tissue injuries which almost never heal back as they were before. Soft tissue injuries cause scar tissue to build up and they need to be treated right away with proper motion. This will help to prevent the scar tissue causing stagnation of the blood supply to joints of the spine, or body.