Legs, Why Won't You Move?

I like to consider myself a relatively in shape person. I exercise almost every day, eat right, and try (most nights) to get the amount of sleep required for a teenager to function well. However, despite all my efforts to stay fit and be able to endure strenuous runs and workouts, I find that my body seems to have a limit. For example, I was at conditioning the other day and we were running up the hill. About halfway up when I was "reaching deep within" to find my final push that would send me faster, I found nothing. That was it. I quite literally could not make my legs go any faster. I'm not in any way a lazy person and don't do things half effort and became incredibly frustrated at myself, anger directed towards my useless legs, as they seemed to be yelling back at me, "Yeah right, I'm not moving anymore." So is there a point where the body simply just gives, despite the determination of the person in control?

Most of the forward force in a running stride is supplied by the quadricep muscles. These are attached to the knee by the quadriceps tendon.

How fast can your legs carry you?

Ok, so this quote doesn't answer my question. It was hard to find an answer online that did. What I did learn from the attached link is that perhaps my quadriceps tendon fails to be as strong as I wish it to be, therefore, more strength building will be needed on my part. More conditioning...exciting. Basically it comes down to the mind and mental state you place yourself in when trying to accomplish any task. I guess I just wasn't in "the zone" that day. However, I still believe that there is a point when you have reached your max and failure to acknowledge this could lead to damaging your body.