Sprains and strains of the back, and particularly the lower back, are the most common types of injuries that cause back pain. Because sprains and strains of the back are often very painful, people often seek medical attention for them, though they usually only cause short term back pain, and they usually improve on their own. Back sprains and strains are terms that are often interchangeably, but they are caused by injuries to different structures of the back. Back sprains usually refer to overstretching or tearing of one or more ligaments in the back. Back strains are usually caused by overstretching or tearing of the muscles that move and support the back.
A muscle injury caused by back strain will usually heal on its own with no treatment, but people may often seek treatment anyway because of the amount of pain and immobility involved with the injury. The back muscles and the nerve fibers threaded through them may affect one another, with muscle tears affecting the nerves, and the affected nerves causing the muscles to contract. When the affected nerves that are branched throughout a muscles signal a continuous contraction of the muscle, the resulting condition is known as a muscle spasm. Muscle spasms can be very painful, and limit motion in their constant state of contraction.
Chronic muscle pain is often referred to as myofascial pain syndrome. Myofascial pain syndrome affect one or more muscles, and cause the muscle and the sheath covering it (the fascia) to be in a perpetual state of contraction or tension. This type of chronic tension of the muscle or the fascia covering the muscle may be caused by direct injury to the affected muscle(s), or indirectly due to pressure or injury to the nerve supplying that muscle or muscles. We know that when the nerve roots exit bilaterally through the side of the spine, they branch several more times, eventually reaching the muscle fibers and other tissues of the body. If the nerve is damaged or squeezed due to a back problem such as a bulging disc, any structures beyond the site of injury for that nerve could be affected. So damage or injury to one of the nerve roots or nerve branches that supply a particular muscle could cause the muscle to become tense or go into spasm.
Back strains of the muscles can also result from trauma or repetitive use, or it can be secondary to an underlying disease. Any underlying disease, or arthritic changes in the joint that cause instability in that joint may cause the muscles that move and protect that joint to stiffen up or spasm, in an attempt to that body to stabilize that area of the body. Muscular tension is one of several coping mechanisms of the body for stabilizing arthritis in joints or instability of the spine. With chronic joint abnormalities, certain muscles may tighten to try to stabilize a joint junction.
Muscle may also go into spasm when muscles are stretched beyond a normal range of motion. The body responds to this extreme stretching of the muscle by undergoing an involuntary contraction. This immediate contraction of the muscle to protect itself served to prevent against doing any additional overstretching to the muscle, but the resulting tension in that muscle may press the nerves inside the muscle to cause upper back pain and lower back pain.