Pain in back part of thigh on plant leg

My leg will be feeling fine during practices and games when I run, play catch, etc. But when I go through my pitching motion, on flat or mound, and when, or close to, releasing the pitch a sharp pain in the back of my upper leg. It instantly goes away when I return to my regular upright position and doesnt hurt to run or anything like that. In games it isnt a problem for some reason (adrenaline maybe?).

Id like to know whats wrong because I believe I am holding back a little bit in terms of throwing my fastball at 100% effectiveness.

Which leg? Stride leg? If so, you could have a lack of flexibility in the hamstrings and maybe a slight pull/strain. As the throwing arm extends forward, the upper body flexes forward and that stretches the hamstring of the stride leg.

Yes, it is my plant/stride leg. If it were a strain or pull wouldnt I feel it even when I am not pitching though? I mean, I can pitch, feel that pain, then stop pitching and sprint and it wont hurt one bit.

Flexibility certainly could be an issue. I am out of playing shape. I currently stand 6’2", 192 lbs. and turned 17 in March. I dont appear overweight but my body fat % is definitly over 20% and I am not very muscular (although my legs are quite strong from a decade of soccer.) That is another thing, I throw easily mid-70s right now, and I really think with some fat-to-muscle conversion and general weight loss I could be throwing in the 80s.

I agree that if it was a strain then you would probably feel it when you sprint. Maybe if you’re a pitcher who straightens out your stride leg and then bends forward during the follow-through and really stretches the heck out of that hamstring. I don’t know what else to tell you except that if it doesn’t get better in a reasonable amount of time then you should get it checked out by a doctor.

I have almost the same exact problem, except the pain is in my lower back/buttocks kind of area. Does it have to do with my overextending it? Should I stride less? or put less pressure on my landing?