Final answer:
The frontal lobe, located behind the forehead, is responsible for muscle movements, speech, planning, and cognitive functions such as decision-making.
Step-by-step explanation:
The portion of the cerebral cortex that is just lying behind the forehead and is involved in speaking, muscle movements, and in making plans and judgments is known as the frontal lobe. This area of the brain houses the motor cortex, which plays a significant role in planning and implementing movement and is mapped to various muscle groups. Additionally, the frontal lobe is critical for cognitive functions, including maintaining attention, producing speech through Broca's area, and decision-making processes. Studies suggest that it also contributes to personality and social behavior.