Which of the following activities is an example of internal kinetic force?

Internal kinetic force refers to the forces produced within the body as it moves. This type of force involves the muscles contracting to facilitate movement of the limbs and joints.

Lifting one's leg directly involves the contraction of the hip flexor muscles and other surrounding muscle groups as they generate the necessary force to elevate the leg against gravity. This internal action of the muscles, working to create movement at the joints, exemplifies internal kinetic force.

On the other hand, running on a treadmill, cycling on a stationary bike, and swimming in a pool are activities that, while they may involve muscular effort, primarily focus on external kinetic forces acting on the body due to the interaction with the environment – whether it’s the treadmill belt, the pedals of the bike, or the water resistance. While internal forces are certainly at play in these activities, they do not serve as the most direct examples of internal kinetic forces like the act of lifting a leg does.