Chapter 9, Problem 13Q

Why is it more difficult to do sit-ups when your knees are bent than when your legs are stretched out?

To determine

Why is it more difficult to do sit-ups when your knees are bent than when your legs are stretched out?

Answer to Problem 13Q

Solution:

It is more difficult to do sit-ups when your knees are bent than when your legs are stretched out because the lever arm of the muscles of the abdomen in the case ofknees bentis less thanthe lever arm of the muscles of the abdomen in the case of legs stretched out

Explanation of Solution

Take the pivot at waist height, for a person doing crunches with the legs stretched we have the force of gravity doing torque counterclockwise, while the muscles of the abdomen do torque clockwise, if the legs are stretched out the lever arm of the abdomen muscles is greater than lever arm of gravity, so it is easy to counteract the torque made by the force of gravity. Now in the case of a person doing crunches with the knees bent, the arm lever of abdomen muscles is much smaller than in the first case (legs stretched out). In this way it is much more difficult for the muscles of the abdomen to counteract the torque caused by gravity.