Many people have lots of time and need to stay fit. Some are walking every day. But they also need to do things to maintain their strength and flexibility. Pushups and situps are ideal because they don’t take much space or time. How effective they are depends on how many repetitions they do and how they do them. Pushups are simpler and the main technique consideration is how wide apart their hands are. How many one should do depends on their age and gender.  Situps use different muscles and there is a wider range of positions of one’s arms. How difficult it depends on where you put your arms. It’s basic engineering. Your body is like a bunch of chunks of meat: head, arms, torso, abdomen, legs. If you extend your arms over your head your situps will be a lot more difficult than if you keep them along your legs. Some people are fans of “crunches”, which are shorter and harder motions. Your choice is like choosing a religion.