The musculus rectus abdominis probably is one of the most popular muscles of the human body. Not only is this muscle responsible for the very desirable six pack shape of the stomach. It also plays an important role in protecting our organs and stabilizing the pelvis and spine.
Side trunk stretches increase the mobility of the lumbar spine. Especially, the deep abdominal muscles with the quadratus lumborum will be stretched.
Iliopsoas stretch: Sportsmen and -women often stretch their iliopsoas in a suboptimal way. The iliopsoas is a pair of muscles (psoas major and iliacus) that represents the strongest hip flexor what should be stretched properly.