Why is it more difficult to do a sit-up with your hands behind your head than when your arms are stretched out in front of you? A diagram may help you to answer this.
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The situation with hands behind the head is shown below:
It is evident from the diagram that the line of action of force is directed towards the axis of rotation. The force acts along the direction shown such that the lever arm has approximately zero value and henceforth, the torque value also becomes nearly zero. That’s why it is difficult to sit-up in this position.
When the hands are stretched in front, it’s easier to sit up