Help: Question on biomechanics

[Boris Lekht]

I know that all body postures and movements (such as standing, push-up,
pull-up, sitting etc.) are based on principles of physical levers ( 1,
2 and 3-rd classes). However, sometimes it is not easy for me to
determine what class of lever a body posture or movement is represent.
Please help me to understand the general rule (or rules) which I could
apply for resolution of my problem.
Thank you.
Boris boris7@ix.netcom.com