It was disconcerting when Baltimore Ravens running back Ray Rice went down untouched in the fourth quarter of Sunday's game against Cleveland and immediately reached for the front of his left hip, which, as it turns out, is precisely where he suffered an injury.
Despite the fear that non-contact injuries have inspired among football audiences (largely due to the fact that most season-ending ACL tears occur in this manner), Rice's injury does not appear to be nearly that serious. Although he was on his back for a few moments while the medical staff evaluated him, Rice did get up and walk off the field under his own power (a good sign), albeit limping slightly.
The Ravens later announced that Rice had suffered a hip flexor strain, and he was not immediately scheduled for further testing, such as an MRI -- another good sign. Coach John Harbaugh said after the game Sunday that he is "optimistic" about Rice, if you want to factor in his opinion.
So how concerning is a hip flexor injury for a running back? There are several unknown variables, most notably the precise location and the degree of injury to the tissue, but generally speaking, even a mild hip flexor strain has the potential to affect a runner's power.
The hip flexors are a group of muscles that do exactly as the name implies -- flex the hip by lifting the leg toward the chest -- anchoring at the pelvis (with some attachments running as deep as the vertebrae of the lower back!) and attaching to the thigh or leg. Their primary function when the leg is moving in space may be to flex the hip, but they also serve to stabilize the pelvis when standing. Think of them as helping to advance the leg of a runner but also providing stability to help that runner generate power.
When we see a running back who is strong enough to "drive with his legs" while dragging a pile of defenders forward, he is often drawing heavily on power from his hip flexors (among multiple other muscle groups). Any injury to the hip flexors thus has the ability to compromise power. While a mild strain may not be enough to sideline Rice, it's possible that it could limit his ability to run effectively.
Unfortunately, there's no real way to measure that effectiveness until we can actually observe him running in a game situation. Later in the week, we should have an idea of whether he'll be testing that power in Week 3 against the Houston Texans.