Im not having a go but you led me to belive you were having trouble maintaining position through weak hams, however the truth is you can hold the weight fine but dont want to or something, i mean OF COURSE its gonna hurt your hams they are engaged continuously....
You're right - I think it may be just me being overly cautious with a new excercise and also not being fully aware of every muscle that this excercise brings into play - the caution comes from not being aware that the hams would get stressed rather than the stress itself.
Thanks for the tips CW!