Slumping the back and collapsing the chest is a common occurrence in Sukhasana, especially in those who are new to yoga. Factors that can contribute to this posture include fatigue, defeated mental state, and tight muscle groups. Many yoga poses are designed to counteract these factors, Sukhasana being one of them.
Sitting up in Easy Cross-Legged Pose aids to bring the spinal column into alignment, so that the vertebral bodies and their discs support the torso; expanding the chest forward enhances breathing. With practice this can lead to a comfortable, easy position that is reflected on the central nervous system—an example of the mind/body connection in yoga
Here’s the Cue
Place the hands with the palms facing down on the knees in Chin Mudra. Then, while holding onto the knees, attempt to draw the hands back towards the torso. This engages the latissimus dorsi. The hands are constrained, so the force of contracting the latissimus is transmitted to its origin along the midline of the back.
The result is what is known in kinesiology as a “closed chain” movement, whereby the origin of the muscle moves (instead of the insertion). Activating the lats in this manner lifts the spine and expands the chest forward. If you tend to hyperextend the lumbar, then engage the abdominals to counteract this. Note the effect.
Slumping tilts the pelvis backwards into retroversion, so that one is sitting on the back part of the ischial tuberosities (the sitting bones). A portion of the latissimus dorsi originates from the back of the iliac crest, so that activating this muscle also tilts the pelvis forward, bringing the sitting bones more upright.
This technique is portable to other poses. In Tadasana, for example, simply fix the palms against the sides of the hips and attempt to drag them backwards. Note how the chest expands forward and the back straightens.
Here’s the Anatomy
The latissimus dorsi originates from the spinous processes of thoracic vertebrae 6—12, lumbar vertebrae 1—5 (via the thoracolumbar fascia), ribs 9—12, the supraspinous ligament, and the posterior third of the ilium. It inserts onto the intertubercular groove on the humerus and the deep fascia of the arm.
The latissimus dorsi extends, adducts, and internally rotates the shoulder (open chain movement). It extends the spine and lifts and tilts the pelvis forward (closed chain movement) and is also an accessory muscle of respiration.
Always, in your particular case, consult your health care provider before practicing yoga or any other exercise program. Always practice yoga under the direct supervision of a qualified instructor. See full disclaimer here.