It is also important to consider the relationship between training status and asymmetry. Stronger individuals may be anticipated to demonstrate less asymmetry (3,4) and would also be expected to perform better in jumping tasks (63). For example, Bazyler et al. (4) median split recreationally trained male subjects into strong and weak groups based on isometric squat peak force. The reported symmetry index of the strong group (1.9 ± 1.1%) was significantly lower than that of the weak group (3.9 ± 1.8%; p = 0.007). Bailey et al. (3) observed similar findings when analyzing peak IMTP force in collegiate athletes. The symmetry index of the strongest decile (4.7 ± 0.1%) was also significantly lower than that of the weakest decile (9.4 ± 0.1%; p = 0.03; d = 0.82). Importantly, Bazyler et al. (4) noted that the strength-asymmetry relationship can be modulated by training. After a 7-week squat training intervention, both strong and weak groups improved 1-repetition maximum (RM) squat performance (strong: 5.0%, weak: 6.6%; both p < 0.05), although only the weak group reduced isometric squat asymmetry (from 3.9 ± 1.8 to 1.9 ± 1.5%; p < 0.05). Such findings suggest that the presentation of asymmetry can be greatly affected by an athlete's training status. Where seeking to examine relationships between asymmetry and performance, training status and exercise familiarity must be considered when selecting appropriate tests to identify asymmetry.
Asymmetries within sprinting kinetics and kinematics have been examined in 3 investigations (20,28,48). All 3 studies have reported significant interlimb asymmetries; however, none have reported an association between asymmetry and overall sprint performance (20,28,48). Although the findings of Meyers et al. (48) were observed in a cohort of 11- to 16-year-old boys, Exell et al. (20) and Haugen et al. (28) sampled from populations of well-trained sprint athletes. Haugen et al. (28) also evaluated the influence of asymmetry on an intraindividual level. Between sprinters best and worst sprint trials, there were no differences in the magnitude of asymmetries recorded. On consideration of the current evidence base, it would therefore appear that asymmetries should not be considered detrimental to sprinting performance.
As change-of-direction (COD) performance is a group 1 unilateral task, it may be reasonable to propose that asymmetries would be less likely to influence performance than type 3 or 4 bilateral tasks. Performances in COD tasks rely on the extension forces generated by single limb, as opposed to an interlimb “trade-off” that is evident during bilateral tasks. Indeed, it appears likely that asymmetries evident during bilateral tasks are a function of neural (i.e., regulation of interlimb effort) and not mechanical (i.e., maximal force production capacity) factors (6,42,61).
The presence of kinetic asymmetries within cycling has reported for several decades (13,15,19,29). Investigations had previously reported that an increase in cadence workload is associated with a reduction in the presentation of asymmetry (12,55,62). However, examination of the relationship between asymmetry and performance has only recently been examined.
Given the apparently symmetrical action of cycling, a type 3 task, it is perhaps unexpected that Bini and Hume (7) observed a significant positive relationship between asymmetries in effective pedaling force and 4-km time trial performance (i.e., larger asymmetries were associated with faster times). However, Bini and Hume (7) reported no relationship for total pedaling force, a finding subsequently replicated (8). The findings of García-López et al. (23) also question the existence of an association between asymmetry and performance. Kinetic or kinematic asymmetries determined during cycling did not differ between elite and subelite cyclists.
Two studies have examined asymmetries during front crawl swimming and their association with swimming performance, yielding conflicting results. dos Santos et al. (18) suggested a deleterious effect of kinetic asymmetry on performance over a 2-minute duration, whereas Morouço et al. (50) did not report such an effect over a 30-second duration. Whether this discrepancy could be related to the duration of the performance or the ages of the cohorts evaluated (age: ∼22 vs. ∼16 years) could be further explored.
Similar disparate findings have been reported for kicking performance. Hart et al. (26) reported a negative effect of isometric strength asymmetry on kicking accuracy in Australian Rules football. When evaluating isokinetic torque asymmetry and kicking accuracy in futsal, Vieira et al. (71) did not observe an association. Once more, the lack of consistency between asymmetry measures and cohorts evaluate makes it hard to draw any conclusions.
Although clear inferences cannot be drawn given the current body of literature, there is some evidence to suggest that athletes demonstrating preintervention asymmetry may experience concomitant increases in performance and reduction in asymmetry. At this stage, it is not possible to determine if the effect of reducing asymmetry on performance is independent of the general improvements in neuromuscular capacity.
A bilateral sporting asymmetry indicates the existence of a unilateral deficiency. For example, the left leg can achieve a jump height of 0.30 m during a single-leg CMJ, but the right leg can only achieve 0.24 m. This scenario suggests that the weaker limb is underdeveloped (Figure 2). In training, the principle of diminishing returns describes the notion that the potential for positive adaptations to occur will decrease as the system becomes more positively adapted (52). Thus, the overall stress applied to the system will need to increase for adaptation to continue to occur. This review has previously established that, in sport, the force dominant side will experience more stress than the nondominant side. Over a period of months, years, and decades, this will lead to the accumulation of a greater overall “training volume” for the dominant side and, in line with the principle of diminishing returns, a gradual narrowing of the potential for subsequent adaptation to occur (Figure 2). Therefore, in relative terms, the potential for adaptation should be wider in the nondominant side and could be identified as a window for development. With this approach in mind, the training program would merely be seeking to pick the low-hanging fruit rather than be directly “targeting” the reduction of asymmetry. Such a viewpoint could explain the findings observed within some of the training interventions, which have been discussed (4,9,24).
A clear link between asymmetry and athletic performance cannot be currently determined given the lack of consistency between investigations. Fluctuating asymmetries such as nostril width or ear size have been used as surrogate markers of developmental stability. There is evidence associating FAs with impaired performance; however, these types of asymmetry have not been evaluated with the same prolificacy as “sporting” asymmetries. Sporting asymmetries, such as force output or jump height, are likely to be a function of limb dominance and magnified by an athletes' long-standing participation within his or her sport. Sporting asymmetries do not seem to carry a clear influence on athletic performance measures given the current balance of the available literature. Research to date has not has only marginally addressed how specific sporting backgrounds may modulate any relationships between sporting asymmetry and performance. For example, the potential for the influence of sporting asymmetry on performance to differ between bilateral- and unilateral-dominant athletes has not been explored. Recent investigations have demonstrated that training interventions can reduce sporting asymmetries and improve performance. However, these studies have not sought to evaluate the extent to which the reduction in asymmetry may be associated with the improvement in performance. There is a clear need for randomized controlled trials that seek to differentiate between training-induced improvements in outcome variables (i.e., max force, jump height, etc) and the direct reduction in sporting asymmetries.
Asymmetry has been widely asserted as detrimental to athletic performance, but this proposition is not strongly supported by scientific evidence and the type of asymmetry is often not defined. Fluctuating asymmetries are thought to indicate the developmental stability of an organism and may be negatively associated with performance. Nonetheless, the application of FA testing within strength and conditioning would be unlikely to influence practice. Many “sporting” asymmetries are a consequence of limb dominance and are magnified by sporting participation. Reported findings to date have failed to demonstrate a clear influence of sporting asymmetries on performance. Accordingly, testing for the existence of asymmetries, fluctuating or sporting, for potential intervention planning may not be beneficial. Training interventions can reduce sporting asymmetries and improve performance, although it is yet to be determined if such changes are related or independent of one another. It is perhaps best for the practitioner to view sporting asymmetries in the light of “development windows” that can be targeted for attaining more general neuromuscular improvements.
No benefits in any form have been or will be received from any commercial party or grant body related directly or indirectly in relation to this manuscript. The results of the present study do not constitute the endorsement of any product by the authors or by the National Strength and Conditioning Association.
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