Let us imagine a human-sized mantis (1.7m tall, 70kg / 5'8", 154 lbs.), but with the forelegs modified to have a shape closer to human arms. So each arm would have two main segments, and end up in a multi-segmented, multi-fingered, grasping-capable hand.
In order to make this creature feasible without violating the square-cube law, let us imagine that it evolved a closed circulatory system and a pair of huge, highly vascularized and alveolated lungs, capable of expanding and contracting just like ours. Add a diaphragm, just because. The mantisman would have tracheas only in the abdomen, and those tracheas would lead into those lungs. Its metabolism and phisiology would be on par with ours.
The point where such a creature differs from us is the skeleton. This creature's skeleton is its external chitin layer. The muscles are inside and attach to their skeleton in a different way, compared to us.
I know, not a mantis, but it's the best image showing insect muscles that I could find.
What I would like to know is how this creature's musculature would compare to ours regarding strength, more specifically lifting and carrying weight. Other aspects of strength, such as arm-wrestling or doing pull ups would be appreciated, but are not needed for this question.
Edit: I know that some insects and arachnids are capable of feats of strength, such as ants and spiders being able to lift many times their own weight. What I don't know is if that would scale up with size, nor if the reason they are able to do so is directly related to their muscle arrangement.