Generally, the main trunk flexors are Rectus Abdominus and the Obliques (Internal and External). Quadratus Lumborum, Trapezius and Latissimus Dorsi also play a small role in flexion.
scalene muscles
The muscle of the Erector Spinae group that is located closest to the spine is the spinalis muscle. It runs alongside the vertebral column and is the most medial component of the Erector Spinae group, which also includes the longissimus and iliocostalis muscles. The spinalis primarily functions to extend and laterally flex the spine.
The large thick muscle located on either side of the lumbar vertebrae is called the erector spinae. Its main function is to extend and laterally flex the spine.
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.
When you flex a muscle, it puts pressure on the muscle fibers, causing microscopic damage. This damage triggers an inflammatory response, leading to pain and soreness in the muscle.
When you flex your bicep, the muscle contracts and pulls on the tendons that attach it to your bones. If there is inflammation or strain in the muscle or tendon, it can cause pain when you flex your bicep.
supraspinatus muscle
The triceps brachii muscle does not flex the elbow; it is responsible for extending the elbow joint when it contracts.
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A muscular contraction that causes a muscle to shorten is called a concentric contraction. A concentric contraction is a type of isotonic contraction.
Spine. Bone, not muscle.