Which muscles perform knee extension and spinal flexion?

The muscles that are primarily responsible for knee extension are the quadriceps. This group consists of four muscles located at the front of the thigh, and their main function is to extend the knee joint, allowing for movements such as standing up or kicking.

Simultaneously, spinal flexion is largely accomplished by the rectus abdominus. This muscle runs vertically along the front of the abdomen and is primarily engaged during movements that involve bending forward, such as sit-ups or crunches.

Together, the quadriceps and rectus abdominus perform functions critical to both lower body movement and core stability, making them the correct pair of muscles for knee extension and spinal flexion, respectively. Understanding the functions of these muscles is essential for creating effective exercise programs and understanding human movement mechanics in fitness and rehabilitation settings.