Next month's article on jasoncholewa.com will discuss the biomechanics of why machine based hamstring curls are a poor exercise. Here is an alternative to the machine based hamstring curl, and, finally a use for the smith machine!
The supine HS curl ups requires active hip extension while the hamstrings flex the knee. This 1. keeps the spine in neutral alignment (vs. shifting it into APT with machine based lying leg curls) and 2. requires hamstring (and glute) activation at the hip at the same time as the knee, which makes it a much more functional movement for anyone who engages in land based movement (i.e.: running, sprinting, jumping.)