Jumping jacks are full body workouts that improve cardiovascular health and fitness. These not only target major muscle groups, but also strengthen the bones. However, jumping jacks involve a lot of jumping, which in turn put a lot of force on the lower body joints. This is why jumping jacks are associated with a risk of injury, especially to the joints in the knee and ankle.
In order to avoid such risks, it is important to first focus on strength and conditioning. Strength and conditioning involve exercises dedicated to building muscles and endurance. It includes squats, lunges, bridges, push ups, dead bugs and deadlifts.
However, if you want to still indulge in jumping jacks, start without the jumps. It is called a modified jack.