Staying fit can feel impossible when you have goals that don't revolve around being fit. Maybe you want to conquer the world with your computer, but guess what, your computer doesn't look great in a weight room, it looks like a cautionary tale. Maybe you're a full-time mom and feel like kids are a workout enough. Maybe that new Stranger Things marathon looks like more fun to you than an actual marathon. Me too honey.
But fitness is about a lot more than body image, it's about full-body health, and especially the health of the heart and mind. When you don't do cardio regularly the strength of your heart and lungs decreases, and when paired with a poor diet plaque builds up and your chances of cardiovascular problems increase exponentially. It also means that your body isn't producing as many happy endorphins, sometimes producing what's known as the "runner's high" (in my mind it's the only reason to run a marathon in the first place.) When you don't have the regular production of these endorphins and hormones you feel lethargic, depressed, irritable. Not only does no one want to feel these things, but it also makes motivating yourself to get out and exercise all the more difficult. But, then again, it makes everything more difficult.
That's why, even if you do nothing else, you should do 100 jumping jacks every day. Now, I prefer to do mine all at once as soon as I wake up because it's how I wake myself up. This serves as a warm-up to any other exercise I plan to do that day. I went months only doing my 100 jumping jacks every day and intermittently exercising when I would be invited to go on a hike or attend a similar activity with a friend, which is not a great exercise plan. But, it's far better than how I felt when school and work took over my life and I let my 100 jumping jacks fall to the wayside. I thought that I was prioritizing my time to what I actually needed to work on, but 100 jumping jacks takes less than 10 minutes total, requires no equipment, and can be done nearly anywhere so I really wasn't saving any time by ignoring my body's basic needs. And, as I said, not exercising takes a huge toll on your body, so everything became harder, I was always stressed, and I suffered a bout of intense anxiety over the course of that semester.
If you can't do 100 jumping jacks all at once, that's entirely fine, I couldn't either when I started, it's about doing them throughout the whole day. Push yourself to the point of a bit of breathlessness, you want your heart rate to go up so that it's working, but don't push yourself to the point of pain or feeling like you can't breathe at all.
There are also simple adaptations you can make to jumping jacks to make them more effective or to make them easier on the knees. To make them more effective, swing your arms so that they make a circle above and below your shoulders, this increases strength and flexibility in your shoulders while your core and legs do the brunt in the actual jumping jack. You can also add a squat into the jumping jacks while your feet are apart (a sumo squat) and when they are together (chair squat) to add additional strength training. If you feel any pain in your knees, or anticipate feeling pain in your knees, then don't jump, step out and back in, alternating which foot you step out with. In order to get the same cardiac benefits, you need to do any of these variations at an upbeat tempo, though before you begin you should make sure your form is perfect, and if it falls out of being perfect stop and reset. If done correctly jumping jacks and squats improve your joint and muscle strength in tandem and so your joints should hurt less not more.