Numerous conditions or injuries can cause sudden, unilateral or one-sided leg pain. Leg pain may be constant or intermittent, and it can develop gradually or suddenly, depending on the cause of the pain, the tissues affected and the extent of the injury or condition. Some types of leg pain -- especially leg pain that's caused by trauma or a musculoskeletal injury -- may be unilateral or felt on only one side of the body.
Contusions
Contusions, also known as muscle bruises, are a common cause of one-sided leg pain. According to the American Academy of Orthopaedic Surgeons, or AAOS, muscle contusions are common among athletes participating in contact sports. In fact, only muscle strains cause more sports-related injuries than contusions. Leg contusions occur when a person experiences a direct blow or several blows to the muscles of the lower extremity, which crushes underlying tissues such as muscles and connective tissue without causing a laceration or open wound. Common signs and symptoms associated with contusions include pain, swelling and discoloration in the affected area. The injured area is usually tender when pressed. The AAOS states that if a person places too much stress on the injured tissues during recovery, excessive scar tissue can develop, which may lead to further problems.
Quadriceps Strain
A quadriceps muscle strain is a common cause of unilateral leg pain. The Aurora Health Care website states that a quadriceps strain is a partial tear of the muscle fibers that compose the quadriceps muscle group. The quadriceps muscle group consists of four muscles -- vastus lateralis, vastus medialis, vastus intermedius and rectus femoris. The quadriceps muscles are located on the front portion of the thigh, and they help flex the thigh at the waist and extend the shin at the knee. The most common cause of a quadriceps muscle strain is an over-stretching of the muscle, although a direct blow to the front of the thigh and overuse injuries can also cause a quadriceps strain. Common signs and symptoms associated with a quadriceps strain include pain and tenderness in the front of the thigh, quadriceps stiffness and weakness and bruising in the affected area.
ACL Tear
A tear of the anterior cruciate ligament, or ACL, can cause immediate, one-sided knee and leg pain. According to the MedlinePlus website, an ACL injury involves the over-stretching or tearing of the ACL -- an important ligament within the knee joint that helps stabilize the knee. ACL tears may be partial or complete. ACL tears may be caused by contact or non-contact sports. Blunt-force trauma to the side of the knee -- which can occur while playing football -- is one of the most common causes of an ACL tear. Other sports that require rapid stopping and starting or pivoting, such as basketball, soccer and skiing, may also cause an ACL tear. Common signs and symptoms associated with an ACL tear include unilateral knee and leg pain, pain that is worse during weight-bearing activity, a popping sound at the time of injury and knee swelling that manifests within 6 hours of the injury.
