Complex systems in nature are defined as having many individual components that are free to vary and interact with each other, exemplified by a sand pile, a weather system, and social collectives such as animal colonies and sports teams. An athlete can also be studied in this way. In the complexity sciences, the term degrees of freedom typically refers to the independent components of a system that can be reorganized in many different ways as surrounding constraints change. When considering the human body as a complex system, an important challenge is to understand how coordination emerges among the large number of motor system degrees of freedom (e.g., the muscles, joints, limb segments). In motor learning, this challenge is known as Nikolai Bernstein’s degrees of freedom problem: How can humans organize the large number of motor system degrees of freedom to consistently produce functional actions such as catching a ball with one hand? Even a simple movement of reaching and grasping an object with the hand and arm could require a catcher to regulate 7 degrees of freedom (dfs) of the arm, involving flexion–extension, medial–lateral movement, and rotation of joints (3 at the shoulder, 1 at the elbow, and 3 at the wrist). Of course, more degrees of freedom need to be regulated in coordinating more complex actions such as performing a triple somersault in gymnastics.
Bernstein proposed that performers initially cope with the large number of motor system degrees of freedom by rigidly fixing or freezing a small number into a basic motor pattern to achieve a task goal. This strategy leads to the characteristic stiffness that many individuals portray early in learning. The freezing of motor system degrees of freedom is a completely understandable coping mechanism when anyone is placed in an unfamiliar performance context and shows how an individual’s intentions, perception, and action interact to constrain the movement pattern that emerges. For example, when novices learn to swim their main intention is to remain afloat and maintain stability in the water in order to breathe and not sink. This intention contrasts with those of an Olympic-level swimmer seeking to move rapidly and efficiently through the water to reach a race endpoint in the shortest time possible. An initial coordination mode in the breaststroke corresponds to an iso-contraction of the nonhomologous limbs: the in-phase muscle contraction of arms and legs together. System stability is enhanced by synchronizing the flexion and extension of both arms and legs together, rather like the directional movements of an accordion. The accordion mode of coordination corresponds to a juxtapositioning of two contradictory actions: leg propulsion during arm recovery and arm propulsion during leg recovery. It is not mechanically effective and does not provide high swim speed because each propulsive action is thwarted by a recovery action. However, this freezing coordination strategy is functional for novice swimmers because it is the most stable and easiest to perform early in learning.
As learners become more familiar with a task, their intentions change quickly and they can abandon the coping strategy of freezing degrees of freedom by reorganizing them into specific functional muscle–joint linkages or synergies. Bernstein advocated that these more functional groupings help learners compress the numerous physical components of the movement system to make the relevant dfs for an action become mutually dependent. Synergies between motor system components help make the body more manageable for learners when they discover and assemble strongly coupled limb relations to cope with the huge number of movement system degrees of freedom.
Synergies are functional, being designed for a specific purpose or activity, such as when groups of muscles are temporarily assembled into coherent units to achieve specific task goals, like throwing a ball or performing a triple salchow in ice skating. Good quality perceptual information is necessary in assembling coordinative structures because the details of their specific form or organization are not completely predetermined and emerge under the constraints of each performance situation. Assembling a synergy is a dynamical process dependent on relevant sources of perceptual information related to key properties of the performer (e.g., haptic information from muscles and joints) and the environment (e.g., vision of a target or surface). Synergies emerge from the rigidly fixed configurations that learners use early on to manage the multitude of motor system dfs and become dynamic and flexible as learners use information to tune their functional organization.
Bernstein’s ideas were a precursor to recognition of the human body as a complex system and were instrumental for movement scientists seeking to understand how coordination can emerge in human movement systems with their huge number of degrees of freedom, such as muscles, joints, and limb segments. It has been suggested that the degrees-of-freedom problem can be resolved in a human movement system if the human movement system is conceptualized as a complex, dynamical system in which cooperation between subsystem components can lead to a reduction in system dimensionality through the emergence of synergies or more compact movement patterns. Some research on how skilled and unskilled individuals kick a football has supported these ideas. D. I. Anderson and Ben Sidaway’s detailed analysis of kicking confirmed the different ways that motor system degrees of freedom are reorganized during learning. They demonstrated that novice kickers did not display the same coordination patterns as skilled individuals. The rigidity of novice movement patterns and the flexible nature of skilled kicking patterns were clearly depicted in their work. Before practice, the joint range of motion (ROM) for knee flexion and extension during kicking by unskilled participants was smaller in magnitude than the values observed in skilled kickers. Smaller ranges of joint ROM tend to signify greater rigidity of movement patterns. After practicing for 10 weeks at 15 minutes per week, the novice group’s coordination pattern began to lose its rigidly fixed characteristic and tended to resemble the more flexible pattern of skilled kickers.
- Anderson, D. I., & Sidaway, B. (1994). Coordination changes associated with practice of a soccer kick. Research Quarterly for Exercise and Sport, 65, 93–99.
- Bernstein, N. A. (1967). The coordination and regulation of movement. London: Pergamon Press.
- Seifert, L., & Davids, K. (2012). Intentions, perceptions and actions constrain functional intra and interindividual variability in the acquisition of expertise in individual sports. The Open Sports Sciences Journal, 5, 68–75.