There are two differences with the hamstring curl.
- To target the two hamstring groups separately (the lateral biceps femoris or the medial semitendinosus and semimembranosus) you don't vary the width of your feet, but the angle of your feet.
If you want to engage the biceps femoris more, then angle your feet outward, while keeping the legs parallel.
If you want to engage the semi-muscles more, then angle your feet inward.
- Also, in order to isolate the hamstrings during the curl, you can disengage the gastrocnemius by plantarflexing your feet. By dorsiflexing the feet, the hamstring curl becomes a calf-exercise as well.