There are two types of injury to muscle. A muscle strain occurs when muscle fibres are pulled apart during a very strenuous activity such as jumping or all-out sprinting. A muscle contusion is caused by a direct blow to the muscle, usually during a contact sport such as football, soccer, or hockey.
Both a strain and a contusion involve the disruption of muscle fibres, although contusions can be worse than strains.
The severity of an injury to the quadriceps muscles can vary from minor bruising to a complete tear. Injuries are easily hidden by the large size of the thigh muscle so that bleeding and swelling in the deep part of the muscle are often under-estimated.
The severity of the muscle injury is determined by assessing the active range of motion of the knee joint; that is the range of motion that you can perform on your own, unassisted.
Other factors used to grade injury severity include: