In athletes with a low training age, I would say that training knee flexion is important, in terms of building up the overall strength of the muscle. Personally, i always include some sort of knee flexion exercise in a general preparation phase-as competition moves closer we will stick with training hamstrings as hip extensors (this does not mean we dont revisit the lifts at times throughout the year), whether it be a glute ham raise, or for those without this try a nordic hamstring curl/natural glute ham raise. Search on youtube and you will find examples.