Does jumping jacks with forward bending hurt the knees?

In general, whether squat jacks (or "squat jumps") harm the knees depends on an individual's physical condition. For people in good health, squat jacks typically do not injure the knees; however, individuals with pre-existing knee injuries or conditions such as knee arthritis or patellar chondromalacia should avoid this exercise. The detailed analysis is as follows:

For individuals who are physically healthy, squat jacks generally do not cause knee damage. Squat jacks are a full-body aerobic exercise that can improve cardiovascular and pulmonary function and strengthen lower limb muscles.

However, for individuals who already have old knee injuries or suffer from conditions like knee arthritis or patellar chondromalacia, performing high-impact exercises such as squat jacks may increase stress on the knees, exacerbate pain, and even accelerate joint wear.

To reduce the risk of injury, it is recommended to train under the guidance of a professional to ensure proper technique, and to perform adequate warm-up and stretching exercises. Additionally, training intensity and duration should be reasonably planned according to individual physical condition and fitness level to avoid overtraining.