What is the primary movement involved in the 'knee raises' exercise?

The primary movement involved in the 'knee raises' exercise is the action of lifting the knees toward the chest. This exercise focuses on engaging the core muscles, hip flexors, and lower abdominal region, which are crucial for stability and strength in various physical activities. By raising the knees, an individual develops coordination and enhances cardiovascular fitness, making it an effective movement for improving overall athletic performance.

In contrast, the other options describe different movements that do not align with the mechanics of knee raises. Jumping while raising arms involves vertical movements and upper body engagement, crouching while touching toes focuses on flexibility and lower body stretching, and spinning while maintaining balance incorporates rotational movements. None of these alternatives target the same muscle groups or purpose as knee raises do.