I actually have this problem myself quite frequently. It is due to very tight hamstring, buttocks and back muscles. A client’s abdominal core may also be weak in this instance. Personally, I find yoga and pilates to be the best at treating my tight hams, glutes and erectors while strengthening my core. I also use a foam roller to do myofascial release of the area. This is commonly found in runners.
I suggest that you perform tests for the individual muscles. I do not know whether your ACE Personal Trainer Manual contains information for it but I found a link where you can test all the muscles for flexibility http://www.exrx.net/Lists/Tests.html . Thus, you should be able to identify them. Be mindful that there is a school of thought (to which I subscribe) that says that muscles are tight to protect vulnerable areas, and stretching for its own sake may be counterproductive.
Tight hamstrings would likely be the culprit, as opposed to tight hip flexors which would help keep the person sitting in that position.
You will want to pay close attention to the client and do not let them move past the point of discomfort in case it is due to an injury. Other muscles used to help lift could also be weak.
Therefore, figure out if this is due to poor flexibility, poor strength, or both, with open communication with the client. Good luck.
Many people have trouble sitting in this manner.
If I have a client that really struggles with this I will not have them sit without support for their backs
I agree with Karin also.
Usually it’s more than just one muscle causing this inability. Try using stability balls or have them sit with their knees bent.
We really don’t sit this way much but we were very curious why both of us–at different parts of the age spectrum but both with strong cores, including lower backs–would have this problem. I’m suspecting hamstrings as a major player so we have been doing Pilates (and I do yoga) and, yes, Julie, she is a runner so that makes sense.