Standard push-up from the toes versus a modified push-up from the knees

Perform the following movements, noting differences in the effort required; explain the reasons for the differences: a) standard push-up from the toes versus a modified push-up from the knees b) sit-up with hands on the thighs versus sit-up with arms outstretched behind the head c) holding a weight in the hand with arm outstretched versus holding the same weight with the elbow at