An ACL injury refers to damage to the anterior cruciate ligament, a key ligament in the knee that helps stabilize the joint. This type of injury often occurs during sports activities that involve sudden stops, jumps, or changes in direction. Symptoms typically include pain, swelling, and a feeling of instability in the knee. Treatment for an ACL injury may involve rest, physical therapy, and in some cases, surgery to reconstruct the ligament. Recovery can take several months, and rehabilitation is essential to restore strength and function to the knee. Proper warm-up and conditioning can help prevent such injuries.