Something to remember is the differnet muscles that make up the hamstrings. The long head of the biceps femoris is involved in both hip extension and knee flexion. The short head is only involved in knee flexion (like a hamstring curl) and lateral rotation of the tibia. The short head doesn't get worked in non knee flexion activities (Stiff legged deadlifts, Stiff legged good mornings) and works in a very limited range of motion in activites like romanian deadlifts etc.. Anyway the jist of what I'm trying to say is that you haven't really hammered that short head much and your leg curls did just that.
To answer a question like this, just take a look of the function of the muscle in question. In this case, the hams are knee flexors AND hip extensors. Now a lot of people will advise that the hams are heavily involved in rock-bottom squats (and I'm not necessarily disputing this) because in the bottom position of a squat, you're in a high degree of hip flexion, which of course places a lot of load on the hip extensor (namely the hams).
However, a more sophisticated observer will also notice that, in the bottom position of a rock-bottom squat, the knees are also in a high degree of flexion. So what does that mean?
Very simply this: a good deal of the tension that the hams experience from the hip flexion is countered by the relaxation of the hams due to the flexed knee.
So that's the ideal positional configuration to place the greatest possible tension on the hams? A large degree of hip flexion combined with a relatively extended knee. That leans stiff-leg deadlifts, RDL's, good-mornings, back extensions, pull-throughs, reverse hypers, etc.
Now don't miss my point... the hams are certainly stimulated in almost any type of squat. Just thought I'd add a bit of biomechanical perspective to the discussion...
During the full squat, the hamstrings go through a countercurrent contraction. One end contracts while the other lengthens. Like someone said above, maybe the short head just got hammered with the leg curls. Also, the standing leg curl is peak contraction movement if I am not mistaken. That may have something to do with it.
You're probably using a relatively narrow stance. Generally a narrow stance tends to favor the quads over the hammies. In my opinion when you use a relatively narrow stance (relative to a power squat) and go deep into the hole, you do work your hammies, but you are also putting a lot of stress on 2 other major muscle groups, the glutes and the hips. So you are generally splitting energy or stress between the quads, the glutes and the hips with the hammies and core muscles (back and abs) recieving less stress. This is just my gereral opinion of an ass to grass oly style squat.
Now when you compare this to a power lifting squat, you basicly can't go ass to grass because that takes incredible flexibility due to the extra wide stance. The extra wide stance places less stress on the quads, making the hips hammies and glutes recieving the bulk of the stress.
So I feel that an oly style narrow stance puts minimal stress on the hammies when going deep. I think that if you went only parallel on not deeper you may put more stress on the hams than if you went ass 2 grass.
I think that if you incorporated power lifting squats into your program as well as the oly/body building squats, you will start strengthening your hammies a lot more. Then the next time you do standing leg curls you probably wouldn't feel it the next day.
I think that my point is that when you go ass to grass, you use your glutes more than hammies, when you go to about parallel, I think the stress is a little opposite.
I do not necessary agree with this! There was some research done on it (on the group of competitive o/lifers somewhere in Sweden). Activation of hamstrings still increases the lower you go.
However, the lower you go, the greater, SIGNIFICANTLY greater involvement of gluts! It is gluts that drive you really out of hole PERIOD! I think that Charles Staley in his post was the closest to "the truth" here
I agree with all you write about spreading/sharing the load between the other muscles and increased activation of hamstrings and adductors in Power Squat...but... It does not mean that really narrow or OL low squat does not do anything for hamstrings... that's simply biomechanics - hams are hip's extensors and they do assist in straightening you out when your ass touches the green - regardless the stand, variation of squat, toes out or straight.
One last point here - do it under control, CONTROL the weight, do NOT just fall down with a bar on your shoulders- you'll ALWAYS feel your hamstrings that's A promise!!