A contusion is a bruising.  The most common contusion in football, despite padding, is a quadriceps contusion.

Quadriceps contusion
Football is considered by many to be a "bone-bruising" sport. For the most part, this is very true. The quadriceps muscles are particularly susceptible to contusions (bruising). A quadriceps contusion is literally a deep bruise of the muscle(s) of the thigh closest to the femur bone. As a result, scar tissue can form and restrict muscle function. Symptoms of thigh contusions include pain, swelling, limping, and loss of knee motion.

Mechanism of injury
Thigh contusions are a result of direct blunt trauma. This refers to sudden forceful injury caused by a blunt object, such as a helmet or shoulder pad. This injury creates a force that is transmitted through the substance of the muscle. The damage, however, usually occurs in the layer closest to the bone. The quadriceps muscles are in contact with bone throughout the length of the thigh, and thus, are especially vulnerable. The muscle damage sustained can be either minor (involving only swelling on a cellular level) or major (involving rupture of the capillaries and leading to heavy bleeding). Heavy bleeding can prolong symptoms and delay your return to play.

Treatment goals
The treatment plan should include limiting swelling and bleeding; minimizing the amount of scar formation; and preserving the elasticity, contractility, and strength of the injured and uninjured muscle tissue.

Return to play
Athletes may return to non-contact participation when there is 120 degrees of pain-free active knee motion. Full participation may occur when the athlete has full range of motion and 90% of contralateral (opposite) leg strength, usually within 1 to 2 weeks. This obviously depends upon the person and the injury.  Each one is different.  Upon return to contact sports after a quad contusion, the athlete, if they wanted, could have the basic thigh pad modified with a ring-shaped pad to prevent further injury.  But as a football player, this injury is so common, you get used to playing through the pain.