The body remembers and stretches this capacity to its natural limits. Youthful humans stretch more easily. As time passes, the body, unless carefully maintained, loses the ability to stretch as it one did. The memory of that same ability is not lost, but stored. Lack of flexibility begins to limit the motion that was once easily and effortlessly attained. As the atrophy of flexibility occurs, motion in many aspects is diminished. If the practices of stretching and flexing muscles are not encouraged through regular exercise, the body will slowly grow more rigid and loose the variability of posture that was once available.

    A spring has memory too. Added energy is required to pull a spring from a resting position to an extended position. When the potential energy is released, the spring returns to its resting position. A spring will continue to have this potential motion until at some point, the spring has been extended beyond the elastic limit for elastic potential and the spring can no longer return to the original resting position. Like a spring the muscles of the human body have memory. Postural muscles work in pairs, allowing for extension and contraction. The body remembers how far a muscle can be stretched and how many muscle fibers must be recruited to make the action occur. The body learns the limits to the flexion of muscles through experience and remembers. Like the spring, a muscle can be overextended leading to injury. Sometimes such an injury is irreparable.

    The poem “Birches” by Robert Frost exemplifies the understanding of body thinking at several levels. Initially, I consider the boy. He is lithe and limber and easily climbs to the top of a selected Birch tree. He swings outward allowing his mass (with some help from his kicking legs) to stretch the tree like a spring, extending its top as it supports his motion toward the ground. Robert Frost must have completed this very action. I see him looking backward with empathy for the boy he was. In these verses, one can see that he well remembers the role and plays it out in the words of the poem. In another view, Frost also remembers the body knowledge of the event, the skill required to pull himself upward as he climbed as well as the finesse of the launching out at exactly the right time. He describes the action of his legs kicking outward as he hangs by his arms while tightly clinging to the tree. The hidden muscle memory for the oft repeated event is saved in muscle memory or body knowledge. Perhaps the swinging was so repeated that some of his father’s trees were permanently bowed as if by the weight of accumulated ice. Like any other sport, the boy acquired a skill that improved with practice resulting in proprioception, a visceral and emotional feeling that remained with Frost into his old age. One senses in the poem a desire to activate or return to these emotions.

    Also it is clear that Robert Frost is considering the process of aging. Here he is sadly aware of what had been the flexibility of his younger body and the current constraints of an aging body. He might wistfully contemplate how it might be to return to a time when his aging body had more “spring.” His muscles have atrophied. He no longer has the agility, stamina or strength to accomplish the complexity of tree climbing or riding on a stretched Birch tree. Yet, due to his body thinking, he can remember clearly the sensations and excitement of the play in the woods.  

   From a Physics perspective if is interesting to consider the memory of the spring. The spring does not possess proprioception like the boy, yet it does remember where the resting place was and also the amount of elastic potential energy required to stretch to the elastic limit. Like the Birch trees, a point is reached where the spring cannot return to its initial resting position. Due to the added energy, the spring has exceeded the elastic limit, and the spring is permanently deformed. The trees in the poem act with two potentials. When energy is transferred into the stretching tissue of the tree, and the boy lets go of the tree, the tree returns to its initial upright status. When the ice stretches the tree, the mass does not let go, and the tree exceeds an elastic limit, either breaking under the load or permanently stretching, incapable, due to the  deformation, to return to the upright form.