I'm an "older" cyclist and am training for a Century in Utah in August. Several dyas ago, I did a 35 miler of which more than 25 is tough uphill grades. I just kept pluggin along the entire way, but it was a little tough. About halfway thru, my right thigh had some severe pain right on the top, primary thigh muscle. (not knee related, seat hight,etc.) after rubbing it for a bit, I contiued on and finished ok. However, today, on a long. easy ride, I had periods when it would hurt and then completely go away. When I am on flat or easy grades, I can spin along with absolutely not even a twinge in the thigh.
I would think that if it was strained, etc., I would feel something all the time!
sounds like the rector femoris. actually not the "main" quad, really. the other three are bulkier. the rectus femoris is important, though, in that not only does it extend the leg at the knee, but it crosses the hip and acts as a significant hip flexor. did it primarily bother you on the upstroke (assuming you were clipped in)? could be a minor strain in which the affected fibers may be only occasionally used. I had one once, and I felt it relatively high on my leg, near the hip, but only when I performed a specific motion with my hip at a certain angle. went away in a few days.
It doesn't hurt on the upstroke at all. It hurts on hard grades where I push down hard and slow. I've been in need of a hip replacement for a bout 5 years, but I keep putting ot off. Due to the bad hip, I have to push my knee "outward" at the top of the stroke to avoid hip pain...then inward again as I push down toward the bottom of the stroke. Actually, the riding helps my hip and it is the reason I have been able to avoid replacement so far. I think the pedling grinds off the bad spots?
Anyway, I would say the hip is the cause, but I've been riding 100-120 miles a week with it for the last 5 years and no thigh pain. I would simply take a break for a week or two, but I'm going to ride the Century with my son-in-law (who lives in Utah) and being from California, the elevation always is an obstacle for me so I don't want to have to take a break unless absolutely necessary. I try to keep up with stretching before and after, but, again, the right hip makes it impossible to stretch properly.
I have a 9 degree cleat on my left shoe and a stationary cleat (zero degree) on the right to maintain my hip angle on the right. Like I say, I'd know my hip is the problem, but I've been set up screwy like this for a long time.