You're asking the rectus femoris to go on a huge stretch there - and under very high velocities.  With a squat, you're not putting it on full stretch, as the hip and knee are both flexed.  So, with that in mind, it's not surprising at all that sprinting would bother your "quad" when squatting doesn't - especially since we know the overwhelming majority of folks out there are tight in the rectus femoris.  Why?

Well, first, you don't need to be a rocket scientist to know that, as a society, we sit far too much.  Second, though, is the fact that most people never really get above 90 degrees of hip flexion in anything that they do.  Mike Boyle has done a great job of outlining how we can develop imbalanced hip flexion patterns; essentially, we never use our psoas, the only hip flexor active above 90 degrees of hip flexion. The picture below is kind of rudimentary (and somewhat awkward), but it shows what I'm getting at with respect to the advantageous attachment points for psoas with respect to hip flexion above 90 degrees:

How many of the folks at your gym are getting 90+ degrees of hip flexion with their treadmill, stairclimber, and elliptical work?  None.  So, we underuse psoas, and overuse rectus - and it shortens up over time.  Take a short muscle through a maximal stretch at high-velocities, and it's going to hate you.  So, what to do?

Well, first, I'd recommend running through some warm-ups from Assess and Correct, and that'll cover a lot of the fundamentals (especially if you go through the assessments to figure out what else is going on).  One important thing that'll cover is activation work for psoas; Kevin Neeld demonstrates one option here:

Second, just add in some targeted static stretching for the rectus femoris a few times a day using this stretch (don't start using it until the "pulled quad" has settled down, though).

Third, and most importantly, ease your way into sprinting.  Not everyone is prepared to just jump right in full-throttle.  I discuss this in further detail in my contribution to the most recent Mythbusters article at T-Nation.  Basically, just get out there twice a week and do some 60-yd build-ups at 80% of your best on a grass field.

Hip pain - particularly of the anterior (front of the hip) variety - is a very common problem in the weight training population.

In her book, Diagnosis and Treatment of Movement Impairment Syndromes, Shirley Sahrmann discusses Femoral Anterior Glide Syndrome in excellent detail.  And, while it may seem like an obscure diagnosis, it's actually a really common inefficiency we see in a weight training population.

In order to understand this syndrome, you have to appreciate the attachment points and functions of the hamstrings and gluteus maximus.  With the hamstrings, you'll notice that they attach to the ischial tuberosity of the pelvis (with the exception of the short head of the biceps femoris, which attaches on the femur), and then run down to a point inferior to (below) the knee.  In other words, they are a two-joint muscle group.  All of the hamstrings aid in knee flexion, and all but the short head of the biceps femoris also aid in hip extension.

Conversely, the glutes attach on the pelvis and the femur; they're a one-joint muscle - and this is why they can so directly impact hip health.

You see, when the hamstrings extend the hip (imagine the hip motion that happens when one comes out of the bottom of a squat), they do so in a "gross" fashion.  In other words, the entire leg extends.  In the process, there is little control over the movement of the femoral head ("ball" in the "ball-and-socket" hip joint) - and it tends to migrate forward during hip extension, giving you a femoral anterior glide syndrome.  In the process, it can irritate the anterior joint capsule, and this irritation can give a sensation of tightness in the front of the hip.

Fortunately, the glutes can help prevent the problem.  Thanks to their point of attachment on the superior aspect of the femur (closer to the hip), they have more direct control over the femur as it extends on the hip.  As a result, they can posteriorly pull the femoral head during hip extension.  So, in an ideal world, you get effective co-contraction of the hamstrings and glutes as one extends the hip; they are a system of checks and balances on one another.  If you use the hamstrings too much in hip extension, you're just waiting to develop not only femoral anterior glide syndrome, but also hamstrings and adductor magnus (groin) strains and extension-based back pain.

As an aside, this hamstrings/glutes relationship is somewhat analogous to what you see at the shoulder with the subscapularis posteriorly pulling the humeral head as the infraspinatus and teres minor allow it to drift forward.  That's another newsletter altogether, though!

Once the femoral anterior glide issue is in place, the first course of action is to stop aggressively stretching the hip flexors.  While the issue gives a sensation of hip flexor "tightness," in reality, stretching the area only exacerbates the anterior hip pain.  A better bet is to just ditch the stretching for a few days, and instead incorporate extra glute activation work.  Eventually, though, one can reintegrate both static and dynamic hip flexor stretches.

Just as importantly, it's important to identify the causes.  We'll see this issue in runners who have no glute function, but more commonly, I'll see it in a weight training population that doesn't understand how to complete hip extension.  Here's what a hamstrings-dominant hip extension pattern would look like with squatting.

The final portion of hip extension is when the glutes are most active, so it's important to "pop the hips through" at lockout of deadlifts, squats, pull-throughs, and other exercises like these.  In the same squat example, it's really just as simple as standing tall:

Of course, this is just the tip of the iceberg when it comes to hip issues in athletes, but it's definitely something we see quite a bit. If you'd like to learn more, I'd highly recommend you check out our Functional Stability Training series, particularly the Lower Body and Optimizing Movement editions. They're on sale for 25% off through tonight (Cyber Monday) at midnight.

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In reality, this individual is a professional pitcher and actually has far more external rotation (roughly 130 degrees on his throwing side) than ordinary folks.  Stretching him into external rotation could actually cause injury.

So, why are his palms turned backward like that?  Well, it's very simple: his scapulae are abducted, or winged.  When the scapular stabilizers - particularly the serratus anterior and lower trapezius are weak - the shoulder blades sit further out to the sides.  The humerii are in normal, but their "foundation" (the scapulae) have been moved.

For more information on optimal assessment techniques, check out Assess and Correct.

As a random aside, off to the side in this video, you'll see how we tend to pair mobility/activation movements with power training, as Royals catching prospect Matt Morizio goes back and forth from clap push-ups to scapular wall slides.