Final answer:
The Sartorius is a muscle in the thigh that assists with the flexing, abducting, and externally rotating the hip, as well as flexing the knee. It's the longest muscle in the human body and essential in walking and sitting cross-legged.
Step-by-step explanation:
The Sartorius muscle is a long, thin, band-like muscle found in the human thigh. It is the longest muscle in the human body, spanning from the anterior superior iliac spine on the pelvis down to the medial aspect of the proximal tibia. Its primary functions include flexing, abducting, and externally rotating the hip, as well as flexing the knee. When you sit in a cross-legged position, the Sartorius is active in helping to achieve this position. It is also involved in the action of lifting the leg to take a step forward, making it important for walking and running.
In addition to describing the Sartorius, it's worth noting the function of other muscles such as the intrinsic skeletal muscles of the back and neck, which are essential for maintaining posture and facilitating movement of the head and vertebral column. The skeletal muscles of the abdominal wall and thorax, including muscles like the serratus anterior, are vital for respiration and maintaining core stability.