Reflections on Momentum

The arm swing results from muscular activity originating in locomotor circuits in the Central Nervous System (CNS), largely mechanically passive. There is also active contribution required from muscles other than stretch reflexes and a possible generating role of the Central Pattern Generator (CPG) in normal gait cycles (Meyns et al., 2013). The Liu and Ballard report on whole body movement (2021) states arbitrary whole-body movements have been less studied.

The pendulum of swing of the arms and elastic quality in the joints of the body are key to well-coordinated, fluid and relaxed movement when performing our coordination patterns. They can be better understood through biomechanical analysis merged with imagery, via the image of Newton’s Cradle. If you are new to physics, this discussion serves as a simple introduction and explanation as to why and how this approach can enhance your movement quality. Newtons Cradle demonstrates principles of conservation of momentum and energy. Unlike the swinging spheres, we do not see an actual collision of the hands at the top of the arm swing (as that would really hurt!). Also, real humans are messy! Unlike perfect systems in physics equations, individuals exhibit diverse behaviours, resistances, and deviations. But we can still grasp complex concepts like momentum and kinetic energy conservation through imagery. In human movement, energy transfers and transforms, but the total energy remains constant even as it gets "messy."

We see a momentary suspension of the hands in space before the return swing. Think back to written accounts of watching the great Nijinsky perform, he appeared to pause in the air mid flight before returning to the ground . We can imagine the top of our arm swing (arm abducted on the transverse plane) as the sphere ready to swing. From the open arm centre position, the arm adducts into the swing and crosses the midline of the body to meet the opposite arm, gaining kinetic energy. In Newtons cradle, energy is transferred from one sphere to the next in a chain reaction. In our bodies, using this image can help us in considering the chain reaction through our anatomical structure to move with the appearance of effortlessness in fluid and efficient motion.

The rhythmic swinging of the spheres reflects the cyclical nature of many human movements like walking and running, making this image a good visual aid for understanding patterns of movement and well coordinated muscle activation for minimal energy expenditure. The periodic motion inherent in pendulum swings serves as a valuable tool across domains to demonstrate how momentum in movement can be performed using minimal effort, providing important considerations when bringing dance movement into rehabilitation domains. Research shows the arm swing is significantly reduced in Parkinson’s disease (Lopez et al., 2022).

Caveat: It is important we recognise the complexities of human biomechanics and therefore the limitations of this metaphor. Real-world collisions involve energy losses due to imperfections and external factors. This means that kinetic energy conservation is an ideal that's rarely, if ever, perfectly achieved in practice. Human movement involves more complex interactions than only linear transfers of momentum with multiple muscle groups working, generating and absorbing forces in various directions, making our systems much more intricate than a simple pendulum. We also need to move in three dimensions (sagittal, frontal and transverse planes) while the pendulum in confined to a single plane. Extending the analogy to 3D movement is possible but would require additional considerations. Minimising effort requires significant training and practice to optimise coordination and minimise energy expenditure so this could be an image a little too far out of reach for absolute beginners.

References

Liu, L., & Ballard, D. (2021). Humans use minimum cost movements in a whole-body task. Scientific Reports, 11(1), 1–15. https://doi.org/10.1038/s41598-021-99423-5

Lopez, L. D., Reschke, P. J., Knothe, J. M., & Walle, E. A. (2017). Postural Communication of Emotion: Perception of Distinct Poses of Five Discrete Emotions. Frontiers in Psychology, 8. https://www.frontiersin.org/articles/10.3389/fpsyg.2017.00710

Meyns, P., Bruijn, S. M., & Duysens, J. (2013). The how and why of arm swing during human walking. Gait & Posture, 38(4), 555–562. https://doi.org/10.1016/j.gaitpost.2013.02.006