Causes / typical injury mechanism: Forceful, eccentric contraction of the thigh muscles, usually during sports:

• Landing from a high jump (basketball and volleyball players)
• Sudden change in direction (football and soccer players)
• Long distance downhill running
• Falling backwards while feet are caught or fixed to the ground (elderly patients)

Classic history and presentation:

• Pain and dysfunction at the anterior thigh either immediately or within several days after injury; pain is worse with weight-bearing.
• Swelling and ecchymosis can be seen in more severe strains.

Prevalence:

• Age – Mild to moderate strains most often occur in active young adults younger than 40 years. Ruptures are more common in patients older than 50 who have comorbidities such as diabetes mellitus, obesity, and hyperparathyroidism.
• Sex – Female patients are at 3 times increased risk compared to male patients.

Risk factors:

• Current musculoskeletal injury of the leg, especially of the knee (eg, anterior cruciate ligament [ACL] tears) or hamstring, causing lack of coordinated movement between muscle groups.
• Use of fatigued muscles (rectus femoris is 65% type II muscle fibers). The dominant leg is more often affected.

Pathophysiology: The main function of the quadriceps is to decelerate the lower limb, acting as an antagonist to the hamstring. During rapid deceleration, the quadriceps is at risk of injury as the normal coordination pattern may be disrupted by a tight hamstring, fatigue, previous injury, or other factors.

The 4 muscles that comprise the quadriceps are the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. The rectus femoris is the only one that crosses 2 joints. The rectus is the most commonly injured quadriceps muscle; this is believed to stem from it spanning both the hip and knee joints.

Grading system:

• Grade 1 (mild) – No loss or minimal loss of strength, no palpable muscle defect on physical examination.
• Grade 2 (moderate) – Moderate loss of strength, may feel a small palpable muscle defect on physical examination of the anterior thigh.
• Grade 3 (severe) – Usually complete loss of strength, can often feel a palpable muscle defect.