Mobility, motion, and exercise

Abstract

Sir Isaac Newton's 1686 “Philosophiae Naturalis Principia Mathematica”¹ has repeatedly, and notably by biomedical scientists, been cited as the most influential single piece of scientific writing ever produced²: Movement, the laws pertaining to which are laid down in this work, is a fundamental characteristic of life, and as such, essential for various biological functions. Thus, life scientists across disciplines study processes that involve changes in location, from a molecular level to that of groups of complex organisms.

Most complex organisms move from one place to another—in search for nutrition, new habitats, mates or to escape predators. Importantly, humans can convey complex information through speech, while animals must often also move their bodies to communicate. This is highly relevant for animal models with respect to translational physiology and has inspired numerous creative solutions by bioscientists to enable the study of, for example, the brain during movement.³ Translational biomedical research—still so, and across disciplines—relies on animal models.^{4, 5} When cognitive processes are studied, free movement is, despite the additional challenge of controlling or monitoring sensory input in a mobile subject, a prerequisite, as, for example, crucial behavioral patterns can only be observed and studied during free movement.³ Nevertheless, telemetry-based studies in freely moving animals are extremely valuable for many more areas of application in physiology, for example in cardiovascular research,⁶ studies of vegetative function and cardiovascular reflex responses⁷ or renal function.⁸ This is exemplified in recent studies: Wu et al. show the relevance of VIP+ miRNAs in sensory processing, olfactory neural activity, and “successful” olfactory function in rodents.⁹ Toledo et al¹⁰ did not primarily observe behavioral changes; however, the modulating effects of RVLM-C1 neurons on cardiorespiratory function at rest had never before studied in conscious, adult animals able to move freely, which adds great relevance to their results. As Pilowsky remarks, one crucial advantage of this study in awake animals, with reflexes intact, is the possibility to study changes in the sleep–wake cycle and normal breathing patterns, while, however, the effects of reflexes on the phenomena observed confounds the results and need to be taken into account critically.¹¹ Baseline heart rate recordings at rest in freely moving animals¹² are of particular value from a translational perspective, as they more closely resemble the natural situation.

At the cellular level, movement occurs during cell division, intracellular transport, and the functioning of immune cells. Movement is also integral to growth, such as phototropism and gravitropism, whereby plants grow toward light and/or against gravity to optimize their conditions for photosynthesis and stability. Both growing and mature organisms respond to environmental stimuli: Tropisms in plants and taxis in microorganisms are examples of how movement helps adapt to environmental conditions. Organisms do not only move for homeostasis and survival, but also for reproductive processes: Sperms move toward the egg for fertilization in many animals, and pollen movement via wind, water, or pollinators is crucial for plant reproduction. Movement can thus be seen as a defining characteristic of living organisms that supports essential functions such as feeding, growth, reproduction, and adaptation to the environment.

When we study movement, there is a multitude of technical terms being used. Most of them share the Latin root “movēre,” which highlights their common origin related to the concept of moving or being in motion. These different terms warrant a closer look to avoid unwanted ambiguity, especially since database search engines, which rely on automatically generated thesauri for efficiency and speed, unfortunately automatically and often incorrectly synonymize these terms.

Momentum is a fundamental concept in physics: It quantitatively describes the motion an object possesses. It is thus a vector quantity with both a magnitude and direction. In a closed system with no external forces, the total momentum remains constant, which is crucial in analyzing collisions and interactions between objects. In a simplistic view, momentum is a measure of how much motion an object has and how difficult it would be to stop or change its motion. Momentum is also fundamental concept for applied human physiology: It aids, for example, the study of movement mechanics, performance enhancement, and is needed during the design process of, for example, rehabilitation and ergonomic solutions. Studying the momentum of limbs during walking or running helps in understanding the efficiency and mechanics of gait, balance, and stability. In exercise or sports physiology, momentum is vital for optimizing performance and preventing injuries when, for example, throwing a ball—properly managing and transferring momentum can lead to more effective and powerful movements. Also, understanding momentum is important in designing rehabilitation protocols and prosthetic devices to replicate the momentum of natural limbs, to help achieve smooth and efficient movement for the user. In ergonomics, momentum helps in designing tools and workspaces that minimize the risk of injury, reduce strain, and improve efficiency.

The distinction between movement and motion is essential in physiological studies of non-sedentary life. While motion is the mere change in position of an organism or object, dictated by physical laws, movement involves an intentional and coordinated action, usually driven by an organism's nervous and muscular systems. The terms motion, movement, mobility, and motility, while related, have distinct meanings. While motion refers to the change in position of both animate or inanimate objects,¹³ movement implies purposeful and coordinated behavior, intention and control, typically involving a musculoskeletal system.¹⁴ Mobility and motility, on the other hand, refer to capacities:

Mobility describes the ability of an organism or a part of an organism to either move or be moved freely and easily, thus, an overall capacity for movement—the range of motion and flexibility of joints, limbs, or the whole organism. Mobility can include passive movements (e.g., being moved by external forces) and refer to the overall ease of movement.

Motility usually refers to the ability of an organism or cell to move spontaneously and actively, consuming energy in the process—sperm cells swimming toward an egg, the movement of white blood cells toward a site of engagement,¹⁵ or the contractions of the gastrointestinal tract.¹⁶ This term is more focused on self-propelled movement, often (but not only: Delbono et al. use the term “motility” to refer to skeletal muscle innervation,¹⁷ which Fan et. al refer to as movement¹⁸) at the cellular or microbial level for active, energy-consuming processes.^19-21

When we look at the movement of individuals in groups or “swarms,” this adds another layer of complexity, when organisms adapt and synchronize their movements to form cohesive groups. Swarming and coordinated movement require communication and interaction of individual entities. Visual, auditory, chemical, or tactile signals help coordinate movements and maintain group cohesion. “Social” interactions in swarms or moving groups, such as aligning with neighbors or following a leader, can fine-tune more sophisticated group behaviors. Common general mechanisms of coordination include adjustment in direction to align with neighbors, attraction toward the center of the group to maintain cohesion, and repulsion to help individuals maintain a certain distance to, for example, avoid collisions. Benefits of swarming include predator avoidance or confusion and safe and efficient foraging and long-distance migration, seen in insects, birds, fish, and mammals alike. Understanding swarming and coordinated movement in groups does not only provide insights into the underlying principles of collective behavior, but may also have novel applications in fields ranging from robotics and artificial intelligence to crowd management and environmental conservation.

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