Surya Namaskar: Chaturanga Dandasana

The fifth posture in Surya namaskar is the chaturanga dandasana or the four limb stick pose.
• From the equestrian pose, bring the right foot back next to the left foot, keeping the arms extended at elbow and assume the plank position. The distance between the feet should be about 6-12 inches and no more than hip width apart. The entire body is like a straight stick, with four limbs in contact with the floor, hence the name four limb stick pose. The feet are dorsiflexed and resting on ball of feet, the knee and hip joints are neutral, the spine is neutral with neck elongated and shoulders directly over the wrist joints, with wrists in extension and weight bearing through palmar surface of hands. The entire trunk muscles work in co-contraction and there is complete synchrony of synergists, agonists and antagonists to maintain stability. From here maintaining stable trunk, slowly lower body as unit by keeping elbows close to trunk and bending the elbows, while shoulder joints are extended in close chain movement. As the body weight lowers, and the arms contribute to weight bearing, weight shifts to upper quadrant. The heels draw slightly forward to accommodate this weight shift and the ankle joints are at near 90° angle. This is the final chaturanga dandasana posture.
 It is important to keep spine in neutral and not hyperextend at the lumbar spine (inadequate spinal stabilization), which causes compensatory flexion of the thoracic spine with shoulder protraction or alternatively, the lumbar spine may be forced to hyperextend by hip flexion (tight muscles) with anterior pelvic rotation. Sometimes tight hip flexors with weak gluteus maximus may cause hip to flex and the ischial tuberosity to be higher than it should resulting in inverted “V” posture instead of stick pose. So be vary of any variation that may occur. 
• Muscles: The toe extensors and foot dorsiflexors are engaged, as are the quadriceps and gluteus maximus. To maintain stability of legs in the transverse plane, the abductors and adductors co-contract. Plenty of muscles are activated in the trunk to maintain stability as the rectus abdominus co-activates with the gluteus maximus to prevent anterior pelvic rotation and hence hyperextension of lumbar spine. The transverse abdominus, multifidi, and erector spinae are all active and the quadratus lumborum with the obliques balances the pelvis(maintains symmetry) and prevents unilateral pelvic rotation (or dipping of the pelvis on one side). The rhomboids and middle trapezius keep the scapula or shoulder blades close to the midline/ spine and provide the ideal channel for weight transference. The serratus anterior fires making load transfer from stable shoulder blades to the arms. The rotator cuff(set of muscles- supraspinatus and infraspinatus, subscapularis and teres minor) stabilizes the shoulder girdle and keeps the head of the humerus (upper arm bone) snug in the glenoid socket of the scapula (shoulder joint). Triceps contract as do the forearm pronators and wrist extensors with wrist in 90° of extension as the hands make contact with the ground.
 That is a long list of muscles activating to maintain a stable position. A wonderful workout and hence important to make sure you are comfortable doing it. In yoga Patanjali says “sthiram sukham asanam,” which means the “stable comfortable posture,” and this posture can amply demonstrate this if one tunes into (or increases awareness of the comfort level of the body during its performance.
 It is important to experience comfort during execution of this posture. Stop at once if you notice discomfort or unease in your back, shoulders or wrists or any other body part. It may indicate the need to work on stability of isolated parts or simplifying posture by decreasing intensity through bringing knees to floor and building up missing or weak components.
• During transition from the ashwa sanchalanasana to the chaturanga dandasana, breathe out. When you reach the final posture and hold the posture, breathe in and hold breath in. Alternatively if you cannot hold for that long, inhale and exhale as long as you can, finishing up with inhalation. Breathe in slow and prolong breaths as you can. In yoga, much benefit takes place through prolonged holding of postures and breath for stability and quicker pace has more to do with skill or ease of movement. 
"Om Khagaya Namaha"
I bow to thee, one who moves in the sky