A squat is actually a closed chain kinetic exercise. Let me explain why.
When we talk about closed chain exercises, it means that the distal segment of the body, in this case, the foot, is in a fixed position and pressing against a solid surface, such as the floor. In a squat, your feet are firmly planted on the ground, and as you lower your body down and then push back up, your foot remains in contact with the floor throughout the entire movement. This is what creates a closed chain.
In contrast, an open chain exercise, like the leg curl machine you mentioned, allows the distal segment to move freely. In this case, the lower leg swings freely as you perform the exercise. This means that the foot is not fixed to any surface and can move independently.
So, to summarize, a squat is a closed chain exercise because the foot remains in contact with the floor, providing a stable base of support as you raise and lower your body. On the other hand, using a leg curl machine is an example of an open chain exercise because the lower leg swings freely without any fixed surface to push against.
I hope this clarifies the difference between closed chain and open chain exercises.