I am not too particular on tris as they grow fairly easy for me, however, to the best of my knowledge the last time I checked there was a differential, or should I say preferential recruitment of the??/middle or out head as you increased the angle to a more upright position. this has been verified by mri and emg studies. check out tudor bompas emg studies analysis for the specific numbers. but unless you have a specific favorite, I would say the next most important facet would be using a heavier weight to stimulate more motor units. this is exactly why an exercise like squats is prefered over leg extensions, or say barbell curls over concentration curls. Hope this helps.