Why Are We So Sore From Balancing Postures?

The muscles of the body most often use concentric contraction to create movement. This means that a muscle shortens as it contracts, pulling the skeleton into position. For example, in backward bends the erectors of the back contract and shorten, and the spine bends backward. In forward bends, the rectus abdominis (6-pack) shortens and contracts to bend the spine forward. This type of contraction is also how we walk, run, stand up and do many other basic functions.
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So why are our legs so sore after poses like Standing Bow, Standing Separate Arms Balancing Stick (pictured) or Standing Splits? 
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These poses require eccentric contraction, meaning that a muscle lengthens as it contracts. The hamstrings of the standing leg eccentrically contract in these balancing postures, bearing the weight of the upper body as we tilt forward on the hip. The hamstrings must contract to keep the body from collapsing forward. It is a common misunderstanding that the standing leg hamstrings should relax in Standing Bow or the others. This couldn’t be farther from the truth; the hamstrings actually do most of the work.

This complexity—asking the hamstrings to lengthen even as they hold the weight (eccentric contraction)—makes the muscles a lot more sore than concentric contraction.