If it does tilt, which way? This is on a flat bench.
The reason I ask is that I am recruiting lower back muscles and glute muscles instead of just hamstrings, and a pelvic tilt shifts the focus to different parts of the hamstrings.
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Under standing circumstances I would say look at your posture, if you're naturally inclined to a more donald-duck-like posture (your but sticking out) you should squeeze your glutes and thus facillitate backwards tilting of the pelvis.
When you're more of a pink-panther-type (your but tucking under) you should stick your but out more, ?squeezing your lower back more I think? You should really look at some Elliott Hulse/strengthcamp videos.
However since you're lying I presume flat on your stomach i would think squeezing your glutes is inevitable thus tilting the pelvis backwards (pulling your but in)
According to rdlfitness.com, you should not tilt your pelvis. Tilting the pelvis forward risks hyperextending the spine if the hips come off the bench. Tilting the pelvis backwards shortens the hamstrings at the hip, so when you flex the knee as well you can put the hamstrings into an overshortened position of active insufficiency. If you do lying leg curls, which they do not recommend, keep the pelvis untilted and the spine neutral to protect your lower back.