During curl ups, the trunk flexors exert less force if the arms are thrown forward during the trunk movement because of

When performing curl-ups, throwing the arms forward during the trunk movement can reduce the engagement of the trunk flexors (abdominal muscles). This is because the arm movement introduces momentum and assists in lifting the upper body, shifting some of the workload away from the abdominal muscles. Several biomechanical factors contribute to this phenomenon:

1. Inertia: The rapid movement of the arms creates inertia, which can help lift the upper body. This reduces the resistance that the trunk flexors need to overcome.
2. Leverage: The arms act as levers, and when thrown forward, they create a longer lever arm. This longer lever arm decreases the force required by the abdominal muscles to lift the upper body against gravity.
3. Recruitment of Other Muscles: Throwing the arms forward may involve recruitment of other muscle groups, such as the hip flexors and the muscles of the lower back. This can shift the focus away from the abdominal muscles.

For a more effective engagement of the trunk flexors during curl-ups or abdominal exercises, it is recommended to keep the arms crossed over the chest or placed behind the head. This minimizes the contribution of arm movement and momentum, placing a greater load on the abdominal muscles and promoting better muscle activation.