What is the primary muscle group functioning concentrically in the frontal plane during a lateral step-up?

In the context of a lateral step-up movement, the primary muscle group functioning concentrically in the frontal plane is indeed focused on stabilizing and controlling the body during the exercise. When performing a lateral step-up, the movement involves lifting the body sideways onto a step, which requires significant activation of the hip adductors.

The hip adductors play a crucial role as they are responsible for bringing the leg toward the midline of the body, especially when stabilizing during lateral movements. As one leg steps up and bears the body's weight, the hip adductors contract to maintain balance and control while elevating the body upward onto the step. This concentric contraction is essential for executing the lateral step-up effectively and safely.

In this exercise, other muscle groups like the quadriceps, gluteus maximus, and hamstrings certainly contribute to movement as well, but they primarily assist in different capacities or planes. The quadriceps and hamstrings are more focused on movements in the sagittal plane (forward and backward), while the gluteus maximus mainly works to extend the hip. Thus, the hip adductors are the key players specifically for the lateral movement being performed in the frontal plane.