Why Sports Should Embrace Bilateral Asymmetry: A Narrative Review

Abstract

(1) Background: Asymmetry is ubiquitous in nature and humans have well-established bilateral asymmetries in their structures and functions. However, there are (mostly unsubstantiated) claims that bilateral asymmetries may impair sports performance or increase injury risk. (2) Objective: To critically review the evidence of the occurrence and effects of asymmetry and sports performance. (3) Development: Asymmetry is prevalent across several sports regardless of age, gender, or competitive level, and can be verified even in apparently symmetric actions (e.g., running and rowing). Assessments of bilateral asymmetries are highly task-, metric-, individual-, and sport-specific; fluctuate significantly in time (in magnitude and, more importantly, in direction); and tend to be poorly correlated among themselves, as well as with general performance measures. Assessments of sports-specific performance is mostly lacking. Most studies assessing bilateral asymmetries do not actually assess the occurrence of injuries. While injuries tend to accentuate bilateral asymmetries, there is no evidence that pre-existing asymmetries increase injury risk. While training programs reduce certain bilateral asymmetries, there is no evidence that such reductions result in increased sport-specific performance or reduced injury risk. (4) Conclusions: Bilateral asymmetries are prevalent in sports, do not seem to impair performance, and there is no evidence that suggests that they increase injury risk.

No Effect of EVA and TPU Custom Foot Orthoses on Mechanical Asymmetries during Acute Intense Fatigue

“…This is in contrast to a study of children with flat foot disorders who wore prefabricated orthoses, in which a reduction in hip abduction moment symmetry was observed only in the dominant limb during walking [15]. However, this comparison is limited due to the highly task-, metric-, and individual-specific nature of assessing bilateral asymmetries [2]. Our study suggests that well-trained runners maintain relatively even strides with or without inserts, indicating that unilateral and bilateral mechanical assessments of (repeated) sprints may offer similar information for quantifying the effects of CFOs in injury-free individuals.…”

“…The levels of agreement for consistency in the direction of asymmetry (i.e., which limb is favored [2]) were generally less than 100%, suggesting that the limb producing the greatest values occasionally switched sides between conditions. However, levels of agreement were relatively comparable between all three comparisons for a given variable.…”

“…Human gait for a range of slow-to-fast running speeds is rarely perfectly symmetrical, even on a treadmill in laboratory-controlled conditions [1]. As such, practitioners regularly quantify bilateral leg differences or asymmetries as a screening/testing tool [2], as asymmetrical stride patterns can lead to higher metabolic energy expenditure [3] and an increased risk of re-injury [4]. Multiple techniques are available to calculate asymmetry (such as limb symmetry index, asymmetry index, or bilateral strength index), each with merits and drawbacks (for review, see [5]), exist.…”

Body Composition and Strength Symmetry of Kettlebell Sport Athletes

“…Antisymmetry depends on genetic and environmental factors and is typically influenced by the differential use of body segments [ 2 , 3 , 4 ]. It can be observed in the different sizes, body composition, strength, and functionality of contralateral limbs and is widespread in the general population and among athletes [ 5 ]. In both sexes and through the life cycle, the dominant leg and arm (more frequently the right arm) show higher values of lean mass and strength [ 2 , 6 ], while the differences in the fat mass (FM) are generally less accentuated [ 6 ].…”

“…In sports science, studies on asymmetry focused on physical capacity and strength more than on body composition and mainly aimed to analyse their relationship with performance or risk of injuries. Although asymmetry has been frequently considered to be related to adverse outcomes, recent reviews doubt the robustness of such associations and highlight the need for further research [ 5 , 14 , 15 , 16 , 17 ]. The literature results are still controversial because of the large variability of characters and phenomena under study, which include different outcomes (size, body composition, performance, injury risk), different body segments (anthropometric details, upper or lower limbs…), different kinds of relationships (causal or not), different types of asymmetry (antisymmetry or fluctuating asymmetry), and asymmetry indices.…”