MS-3409

A 68-year-old man visits his doctor accompanied by his daughter, reporting a recent onset of tremors and slower movements. During the examination, the doctor observes a shuffling gait, slower movements, and a resting pill-rolling tremor of the right hand with cogwheel rigidity. As part of the neurological examination for Parkinson’s disease, the doctor assesses the coordination of the lower limbs by instructing the patient to place his left foot on his right knee and slide it down his leg.

Which muscle is the most crucial for this movement?