Some movements involving voluntary muscle contraction happen automatically as a reflexive response to intended movements or external stimulation. Here the body is acting before you can think about it. When a muscle contracts in response to stretching within the muscle, this is called a stretch reflex. With eccentric contraction—for example, the hamstrings while folding forward into Uttanasana—it is easy to generate a stretch reflex. In folding forward we ideally relax the hamstrings, allowing them to stretch more easily. But before we know it, the hamstrings are actively engaging to control the weight of the upper body moving forward and down. It is as if the hamstrings want to pull the body back up into its natural anatomical position, fully upright and stable. Stretch reflexes limit the development of flexibility and must be circumvented through countervailing muscular actions in order to cultivate full flexibility.
When students move very quickly in and out of asanas, they are likely to trigger stretch reflexes that not only limit flexibility but also increase the risk of straining muscles or tearing ligaments. As we will explore in some detail when discussing how to “play the edge,” listening to the body’s natural feedback through the breath, heartbeat, and nervous-system messages is the key to moving with ease and stability.