Technically speaking it's the abs. The agonist muscle in a situp is actually the hip flexors.
brachialis
In a sit-up, the primary agonist muscle is the rectus abdominis. This muscle contracts to flex the spine and bring the torso towards the thighs. Other muscles, such as the hip flexors, also assist in the movement but the rectus abdominis is the main muscle responsible for the action.
Agonist muscles are the primary muscles responsible for producing movement. However, synergist muscles are helper muscles that assist the agonist in executing a particular movement. Synergists help stabilize the movement and control the direction in which the force is exerted.
When the brachialis acts as the agonist, the biceps brachii often serves as a synergist, especially during flexion of the elbow. While the brachialis is primarily responsible for flexing the elbow joint, the biceps brachii enhances this movement, particularly when the forearm is supinated. Other muscles, like the brachioradialis, may also assist in this action, depending on the position of the arm.
The gastrocnemius is a synergist of the soleus.
A synergist muscle helps a prime mover muscle in creating a specific movement. It assists the prime mover by reducing unnecessary movements and helping stabilize the joint. Synergist muscles work together to produce coordinated movement.
A synergist muscle is a muscle that aids another by promoting the same movement.
The muscle that aids the jaw in closing is the temporalis muscle. The masseter is the muscle that primarily moves the jaw.
The synergist to the smiling muscle (zygomaticus major) is the orbicularis oculi muscle, which is responsible for the contraction around the eyes and contributes to the overall expression of a smile.
Piriformis
The rectus femoris muscle is a synergist of the iliopsoas. It works together with the iliopsoas to flex the hip joint.
infraspinatus muscle
agonist, antagonist, synergist
Gastrocnemius
Latissimus dorsi is the synergist of the teres major.Specifically, a synergist is a muscle which acts with another. The latissimus dorsi helps adduct and rotate the shoulder to elbow bone called the humerus. The teres major is a shoulder-related muscle.
brachialis
Teres major