Asymmetry as a Foundational and Functional Requirement in Human Movement

The appeal of symmetry is deep. It conveys ideas of perfection, balance, and equilibrium. It makes reality seem rational and understandable. It soothes our spirits and provides a notion that the world is perfectly equitable and unbiased. And so, humans have searched for symmetry everywhere, and when nature did not abide, we imposed symmetric ideals. Mountains, rivers, and planets are irregular and asymmetric. But the human imagination has proven itself powerful and, through amazing conceptual jumps, attempted to impose symmetry onto nature. Triangles, circles, squares: all are completely imaginary figures, idealized concepts that have no connection to reality. Physics—as we will see—created several theoretical symmetries based on beautiful mathematical theories. But all these theories require the establishment of numerous symmetry-breaking events to be compatible with the known universe. So, while people search for symmetry, the key to existence is asymmetry.

Physics has established that reality relies on a series of phase transitions, resulting in symmetry-breaking events [1], and this can be extended to chemistry as well [2]. At a fundamental level, electrons and quarks, two elementary constituents of matter, have an intrinsic angular momentum—the spin—which, by convention, is said to be left-handed when rotation is clockwise and right-handed when it is anticlockwise. Only left-handed particles experience weak interaction or force, thus violating parity symmetry [3]. The electromagnetic field also emerges from a symmetry-breaking phenomenon, whereby it is no longer unified in an electroweak field [1]. In topological superconductors, initially symmetric crystalline structures produce spontaneous phase transitions that break time-reversal symmetry [4]. Asymmetry is also a fundamental geostatistical property, as it is a natural consequence of dynamic processes, such as land surface elevation and groundwater contamination [5].

There is evidence that skeletal asymmetries in the bones of the upper limbs have been intimately related to right-hand dominance since the dawn of the genus Homo; this type of asymmetry is also believed to have occurred in Neanderthals [1]. An anthropological study of 780 Holocene adult humans has shown that modern humans present bilateral asymmetry in the length and especially in the diaphyseal breath of long bones (e.g., the femur) [2]. Furthermore, the authors found a systematic right-bias in all dimensions for the upper-limb bones and a slight left-bias in diaphyseal breadth and femoral length. In another anthropological study of 509 Holocene adult humans, Auerbach and Raxter [3] found contralateral asymmetries in both the clavicle and the humerus, with greater asymmetries observed in diaphyseal breath than in length. Furthermore, the authors state that the asymmetries in diaphyseal breadth could be related to variations in the physical activities practiced by the groups.

Many daily activities are typified by automated routines, most of which we are not even aware of. From brushing our teeth to holding and eating an apple, to opening doors, to driving, the number of hours spent daily performing highly asymmetric actions is considerable. Overall, human movement does not fit with the notion of perfect symmetry [1]. Writing is perhaps the best example of how asymmetries make our actions more efficient, as humans spend countless hours of their lives writing, especially during the school years. Writing consists of more than dexterity, as it also has implications for posture. For example, in a study conducted by Flatters et al. [2], the authors explored the relationship between body stability and manual dexterity in children (n = 278) aged 3–11 years. The results showed that postural control and manual control are interdependent and that the development of both postural control and manual control has a degree of task-specific codependency.

Based on the knowledge portrayed in the previous chapters, it would be surprising if asymmetries were not a factor in athletic performance. Ask a person to get ready to sprint, and that person will most likely always (and unconsciously) put the same foot forward. Ask a person to punch or kick a boxing bag as hard as possible, and their lateral preferences will tend to be very strong. These preferences are self-evident and can be easily accessed. But even in scenarios where it would be theoretically beneficial to perform actions with equal prowess on both sides, this does not occur in practice. In sports such as judo, boxing, or jiu-jitsu, most high-level competitors have well-established lateral preferences. Having two equally skilled sides would certainly prove to be a competitive advantage, and so we should ask ourselves why such symmetry is not common at the highest levels of practice, even when coaches stimulate bilateral practice in athletes starting from young ages. In elite-level basketball, players don’t attempt three-point shots in equal percentages with both hands. Indeed, despite claims that bilateral asymmetries may be detrimental to sports performance, research does not entirely support this notion, as has been shown in the review of Maloney [1].

Many postural and therapeutic protocols rely on reestablishing idealized levels of symmetry, specifically, left–right symmetry. For example, the ninth edition of the American College of Sports Medicine’s Guidelines for Exercise Testing and Prescription states that a “training program should induce symmetrical and balanced muscular development” [1]. There is even a clinic called Symmetry Physical Therapy (https://​www.​symmetry-physicaltherapy.​com/​). Contrariwise, the rationale developed so far suggests that this perspective may be flawed. In this context, the systematic review of Bishop et al. [2] highlights the fact that we don’t actually know the practical effects of attempting to reduce existing asymmetries. That is to say that even if such a reduction would be desirable, it may not be possible to achieve in the long run. In fact, motor asymmetries may be functional for performance and not necessarily associated with a greater injury risk [3]; remember that this feature has also been reported in polo horses [4]. Indeed, because symmetry is less efficient than asymmetry, most systems will spontaneously break symmetry [5], and human bodies that are forced toward more symmetric states might simply break symmetry at the earliest opportunity. In addition, most activities performed during sports performance will likely increase certain asymmetries [6].

The goal of this work was to convey that asymmetry, far from being a prejudicial concept, is a prerequisite for our existence, as it is found even in the form of matter–antimatter asymmetry [1, 2] and as far back as the creation of time itself [3]. Across a wide variety of levels, asymmetry emerges as an efficient solution [4] and provides the foundation upon which structures develop. Asymmetry is present in the rotation of subatomic particles [5], topological superconductors [6], and geostatistical processes [7]. Naturally, these asymmetries are necessary for proper gene expression and regulation, as well as for embryological development [8, 9]. Developmental, structural, and functional asymmetries are a feature of all vertebrates and chordates [10]. Therefore, reality fundamentally depends on asymmetry.