Animals are constantly moving. Insects crawl, fish swim, frogs hop, penguins waddle, horses trot and gallop, while humans walk, reach, grasp, jump and much, much more. Clearly, there are many ways to coordinate the muscles, joints, ligaments, tendons, and bones that make up the skeletal-articular-muscular movement apparatus (i.e., our bodies). While the question of how we coordinate movement may seem like an intuitive and trivial problem to solve, the scientist faces many challenges in finding a satisfactory answer. This begs the question: what makes motor coordination so challenging? At the same time, we can see clear differences in the demands placed on coordination by different animals, not least because of their different bodily structures. No doubt coordinating movement has become much harder since the beginning of life. For example, single-celled organisms definitely do not face the same level of difficulty coordinating movement as humans or dogs do. In this blog, I draw heavily on Essays 2 and 3 of Nikolai Bernstein's On Dexterity and Its Development (Bernstein, 1996) to outline the challenges of motor coordination and to identify their evolutionary origins. While the essays tackle these issues separately, I aim to provide a coherent synthesis of Bernstein's insights by drawing direct links between each coordination challenge and a specific evolutionary event. Of course, this is not to say that the problems of coordination are caused by single evolutionary events. Rather, they are highlighted merely as notable events among the many which have led to such difficulties in controlling movement.
Let's begin with a preliminary definition of coordination. According to the Oxford dictionary, coordination refers to "the organisation of the different elements of a complex body or activity so as to enable them to work together effectively". A definition that is specific to movement is also provided: "ability to use different parts of the body together smoothly and efficiently". For our purposes, we'll zoom in on two main aspects of coordination. Firstly, coordination involves combining many elementary units of a wider system. Secondly, coordination is functional. With the rare exception (e.g., artistic sports like ballet, figure skating, and artistic gymnastics), movements are typically coordinated to serve an external, outcome-related goal. These aspects of coordination provide a foundation for understanding the challenges humans (and animals) face in controlling and coordinating their movements. Bernstein also provides his own definition in Essay 2, but we'll circle back to it at the end of the blog.
The problem of distributed control
That coordination involves so many moving parts hints at the first challenge of motor control identified by Bernstein. Given the reasonable assumption that the nervous system controls every muscle and joint, much like a puppet master manipulates the strings of her puppets, it seems that the brain is burdened with explicitly and separately controlling each elementary unit of the human skeletal-articular-muscular system. Yet, the well-known limits of attention make it difficult to believe that this is what's going on! In other words, there is a challenge of distributed attention: How can the nervous system allocate adequate attentional resources to each degree of freedom (i.e., separate directions of mobility like that of muscles and joints) given the incompatibility between the sheer volume of elementary movement components and the finite limits of attentional capacity? For this to work, we either have to show that our nervous systems are indeed capable of such explicit control, or find mechanisms and principles that allow for 'brain-free' control (e.g., self-organisation, tensegrity), or that reduce the number of degrees of freedom to control (e.g., formation of synergies).
From an evolutionary perspective, we find that this challenge is faced by organisms later in the evolutionary chain. To give an extreme example, single-celled organisms likely do not face the problem of distributed attention. Not only is there a lack of degrees of freedom to manage, but there is also no nervous system to distribute attention. The origin of this problem seems to involve the development of the elongated body shape. Unlike their spherical and symmetrical counterparts, animals with elongated body shapes have separate openings for consuming nutrients and expelling waste. According to Bernstein, these animals actively sought food with the mouth end of their bodies, making them the first organisms to move headfirst. Naturally, receptor sensitivity at the head increased dramatically. For instance, while earlier organisms interacted with the world through contact sensitivity (i.e., touch), elongated animals slowly evolved and became attuned to chemical, mechanical, and electromagnetic information, leading to the senses of smell, hearing, and vision, respectively. These receptors (i.e., teloreceptors) differed from those of touch in that they allowed for long-range contact with the world. Animals without telereceptors could only detect proximal stimuli, leading to movements comprising mainly of local and minor displacements in posture. On the other hand, the world of animals with teloreceptors was drastically enlarged. When a prey or predator is detected from a distance, local movements no longer suffice. Instead, animals had to engage in more global coordination of their movement systems to locomote toward or away from the stimulus source. In other words, animals were now in the business of coordinating a larger set of moving parts to survive, leading to the emergence of Bernstein's challenge of distributed control.
The problem of redundancy
The next two challenges of motor coordination that Bernstein identified belong to a broader category known as the problem of redundancy. Also known as the degrees-of-freedom problem, these challenges relate to the second aspect of our coordination definition: that movements are usually coordinated to solve a specific problem or accomplish an external goal. In this light, motor coordination is analogous to solving an external problem. This is where the problem of redundancy comes in. Given the many degrees of freedom present in the skeletal-articular-muscular system, there seems to be an infinite number of movement solutions for any given motor problem! For example, consider the relatively 'simple' action of reaching for a cup. where there seems to be an uncountable number of ways of accomplishing the task! The three joint angles of the wrist, elbow, and shoulder can organise themselves in countless ways that still allow for the fingers to touch the cup. At the same time, the muscles can produce a wide variety of force patterns that, while accelerating and decelerating the upper limb in various manners, still achieve the initial goal of getting the hand near the cup. Given the infinite number of ways movement can be coordinated for any given task, how do we land on the one that is actually implemented? If the first problem of distributed attention asks how all the degrees of freedom can be simultaneously organised to produce even a single coordination pattern, the problem of redundancy questions how we choose among the many coordination patterns afforded by the large number of movement degrees of freedom that still help us solve a given movement problem.
Types of degrees-of-freedom problems
Bernstein identified two kinds of redundancy. The kinematic (i.e., the study of motion) degrees-of-freedom problem concerns the many different directions of motion the skeletal-articular-muscular apparatus can achieve given its structure and organisation. One clear evolutionary event that exacerbated this problem is the development of extremities. One of the earliest vertebrates, fish, only had to concern themselves with the twisting and turning of their spines to propel themselves through water, with their fin appendages limited to simpler tasks like directional control. The evolution of fish to amphibians, reptiles, birds, and eventually mammals and humans was accompanied by the development of more complex extremities with more mobile joints. This increased the kinematic degrees of freedom in subsequent movement systems, thereby contributing to redundancy.
Meanwhile, the dynamic (i.e., the study of forces) degrees-of-freedom problem concerns the elastic properties of muscles and how this poses a challenge for precise and stable motor control. This elasticity introduces unwanted unpredictability and variability, where even the slightest difference in initial muscle exertion can lead to drastically different force outputs and movements. Borrowing some terminology from the science of chaos, we might say that our skeletal-articular-muscular apparatus demonstrates sensitive dependence on initial conditions, otherwise known as the butterfly effect. Crucially, while part of the dynamic degrees-of-freedom problem involves the infinite number of muscle exertion patterns one can implement to achieve a specific movement goal, Bernstein was more concerned here with the degrees of freedom associated with the uncontrolled forces that accompany muscle exertion due to the butterfly effect. An obvious evolutionary event associated with the dynamic degrees-of-freedom problem is the development of the striated muscle. Unlike the smooth muscles of earlier soft-bodied, jelly-like organisms, striated muscles gave later organisms the advantage of power and speed. Yet, their powerful, impulse-like contractions meant that striated muscles act more like forceful, ballistic rubber bands -- hard to control and highly variable in output! The subsequent vertebrate blueprint, in which the skeleton was surrounded by striated muscles, tendons, and ligaments, also led to a highly unstable movement apparatus that had to be constantly supported by passive muscle tone just to stay upright. This constant interplay between passive and active muscle contractions is likely what introduced the dynamic degrees-of-freedom problem to movement systems.
Coordination, according to Bernstein
The previous sections outlined the three coordination challenges faced by animals and how they might be linked to specific evolutionary events. We are now at a better place to introduce Bernstein's definition of coordination. But before that, a quick note: if one were to read the above for the first time, they might think poorly of the immense degrees of freedom present in our bodies. After all, every coordination difficulty seems to go back to degrees of freedom (fittingly, Bernstein is credited for introducing the degrees-of-freedom problem in the motor control literature). But this should be caveated by reminding the reader of the benefits afforded by these degrees of freedom -- they grant the organism flexibility and adaptability, especially when faced with novel and unexpected situations! The main challenge comes only when there are excessive degrees of freedom. Anecdotal evidence suggests that most healthy individuals are rather adept at coordinating movement despite this excess. This brings us to Bernstein's definition: Coordination is overcoming the excessive degrees of freedom of our movement organs, that is, turning the movement organs into controllable systems. This is a useful definition for future explorations into motor control. For instance, when we ask, How do we coordinate movements?, we're really asking, How do we overcome excessive degrees of freedom?, or, How do we reduce the dimensionality of our movement systems to make it more controllable? Admittedly, I have a limited understanding of the solutions in the ecological and dynamical literature (e.g., synergies, self-organisation). Still, they definitely seem to be coherent answers when pitted against the questions above!
Concluding remarks
At this time, it would be good to be transparent and inform the reader that this blog was written after reading only the first three essays of Bernstein's On Dexterity and Its Development. I'm excited to learn more and to connect my reflections to the subsequent essays, but just know that any misinformed claims should be attributed to me, not to Bernstein. Nonetheless, I believe there are strong (though implicit) connections between the content on the difficulties of motor control in chapter 2 and that on the origins of movement in chapter 3. Hopefully, the reader finds the explicit links drawn between the chapters sensible and illuminating. I'll see you in the next blog!
References
Bernstein NA (1996) On dexterity and its development. In: Latash ML, Turvey MT (Eds.) Dexterity and Its
Development, pp. 1–244, Erlbaum Publ.: Mahwah, NJ.
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