Final answer:
The rectus abdominis is the primary muscle strengthened by sit-ups, commonly known as the "sit-up" muscles, responsible for the 'six-pack abs' appearance and important for trunk movement and posture support.
Step-by-step explanation:
The muscle that is strengthened through the exercise of sit-ups is called the rectus abdominis. This muscle is a pair of long, linear muscles that extend along the length of the body's trunk. They originate at the pubic crest and symphysis and are enclosed by a sheet of connective tissue known as the rectus sheath. The rectus abdominis is commonly known as the "sit-up" muscles, and they are segmented by transverse bands of collagen fibers called the tendinous intersections, which give rise to the appearance of "six-pack abs" when these muscles are well-developed and hypertrophied through exercises like sit-ups.
Apart from the rectus abdominis, the anterior abdominal wall also consists of three layers of muscles that include the external oblique, internal oblique, and the transversus abdominis. However, the main muscle targeted during sit-ups is the rectus abdominis. These muscles play a crucial role not only in trunk movements but also in compressing the abdominal organs and maintaining posture.