Symmetry has long been regarded as a synonym for health in sports training (Brown et al., 2017b), therapy (Botelho et al., 2017) and daily practice (Parrington and Ball, 2016). However, the movement and posture of human does not conform to the concept of complete symmetry (Tomkinson and Olds, 2000). Asymmetry may be widespread even in high-performance sports (Maloney, 2019). Movement symmetry is an essential technical parameter in some competitive sports, such as sprint running (Haugen et al., 2018), walking race (Tucker and Hanley, 2017) and rugby (Brown et al., 2017a). In addition, most athletes have dominant limbs for certain tasks, and these preferences may be determined by different motor tasks (Maloney, 2019). Asymmetry of athletes is one of the main causes of musculoskeletal diseases, sports injuries and poor performance (Bishop et al., 2018). Although bilateral asymmetry is widely believed to be detrimental to sports performance of athletes, previous studies does not fully support this association (Afonso et al., 2020). Therefore, the athletes and coaches would benefit from an biomechanical examination of quantitative asymmetry of the bilateral movement or posture, rather than depending exclusively on subjective determination during daily training (Maloney, 2019; Xiang et al., 2022; Xu et al., 2022; Yahya et al., 2022).
Quantification of bilateral asymmetries has been widely examined in the available studies as well as the quantification method is not uniform (Bishop et al., 2018). Asymmetrical gait may be associated with injury risk, athletic performance, and the legality of the movement (Tucker and Hanley, 2017), although contradictory findings have been reported. The literature reports that the associated symptoms may occur only when the degree of asymmetry exceeds certain thresholds (Bishop et al., 2018). Previous studies have shown a potential relationship between athletes’ limb asymmetry greater than 15% and the occurrence of sports injuries (Barber et al., 1990, Grindem et al., 2011). In addition, other researchers have set asymmetry of less than 10% as the goal for discharge and returning to the sport of athletes with unilateral limb injury (Kyritsis et al., 2016, Rohman et al., 2015). Trivers et al. (Trivers et al., 2013) reported that symmetry can be identified as one of the indicators of early talent recognition in athletes. A long-term Jamaican study observed that the athletes’ knee asymmetry at age 8 could predict their sprint performance 14 years later (Trivers et al., 2013). On the other hand, opponents argue that musculoskeletal coordination forms the basis for the symmetry of an athlete’s static and dynamic movements (Preatoni et al., 2013). In practice, interpreting motor coordination is more complex than classical biomechanical measurements (Warmenhoven et al., 2018). Therefore, athletes performing an action with bilateral asymmetry may cause a decrease in biomechanical parameters of one or both sides. For example, water rowing is widely evaluated by bilateral continuous variables of force symmetry rather than coordination (Afonso et al., 2020). These findings suggest that asymmetry is an adaptive consequence magnified with long-term physical activity participation (Maloney, 2019). One of the causes of bilateral asymmetry is an abnormality of the spine (Vincent and Vincent, 2019). The pressure generated during movement is transferred to the spine to stabilize the upper body and keep it balanced and upright. Therefore, biomechanical assessment of athletes’ bilateral symmetry is the main method to develop recovery strategies to restore normal function in clinical practice.
Limited literature suggests that greater than 10% power and force asymmetry of the bilateral lower extremity can reduce the change of direction speed times (Hoffman et al., 2007) and jumping performance (Bell et al., 2014), indicating that increased asymmetry can impair athletic performance. In addition, previous scholars have reported that legal and effective walking techniques require certain gait symmetry, so it is essential to monitor the differences between limbs during training (Preatoni et al., 2010). Tomkinson et al (Tomkinson et al., 2003) have reported that the athletes who are symmetrical can improve the sport performance. Although further research is needed into the relationship between symmetry and athletic performance, the potential applications of this research should also be considered. On the other hand, opponents argue that bilateral asymmetry may negatively affect athletic performance (Maloney, 2019). Loturco and colleagues (Loturco et al., 2019) analyzed the relationship between vertical asymmetry and basal performance in high level female soccer players and they found that bilateral countermovement jump (CMJ) performance was significantly associated with strength on sprinting and squat tests, while asymmetry of unilateral squat jump (USJ) was not associated with athletic performance. Radzak (Radzak et al., 2017), for example, has found that asymmetries in the running gait may be beneficial after neuromuscular fatigued. This is due to the higher mechanical efficiency of intra-limb asymmetry than symmetry, although limited data in previous study support this view (Afonso et al., 2020). However, the increase of excessive asymmetry may also produce adverse effects such as sports injuries.
Within the previous studies, a stronger topic surrounding patients or rehabilitated people to have been explored then the participants of athletes. Asymmetry of the bilateral body has been evidenced to be indicative of movement function (Nigg, 2007). Therefore, The symmetry of biomechanical parameters were often used in the clinical and motion capacity assessment, which was important for restoration of abnormal function through appropriate of treatment strategies (Tomkinson and Olds, 2000, Botelho et al., 2017). Increased symmetry is considered by clinicians to be a sign of successful recovery and can increase the confidence of athletes to return to sport safely and effectively (Bishop et al., 2018). The degree of asymmetry determines whether an athlete may have a potential injury risk (Parrington and Ball, 2016, Jordan et al., 2015). Asymmetry of bilateral loading can contribute to the increase of unilateral limb damage such as ACL injuries, especially in female athletes (Hewett et al., 2013, Mokhtarzadeh et al., 2017). The non-contact injury rates in soccer were 68% in non-dominant limbs for females and 74% in dominant limbs for males (Montalvo et al., 2019). Similarly, Brown and Brughelli (Brown and Brughelli, 2014) used symmetry of lower limbs as a decisive factor in assessing rugby players’ return-to-sport status. Asymmetrical gaits often cause increased work on one limb in motor techniques, which may damage one limb due to excessive load (Exell et al., 2012b). Schache et al. (Schache et al., 2009) observed a soccer player with a unilateral hamstring strain due to a 5.7° difference in peak knee extension between legs and a 7% vertical peak force during the swing. However, the methods used to assess symmetry vary greatly, so caution should be exercised when establishing a correlation between asymmetry and injury (Maloney, 2019). Previous research has shown that when asymmetry exceeds a certain threshold, it can negatively impact an athlete’s health, although it may be beneficial for specific athletic performance (Afonso et al., 2020). However, these thresholds are still an unsolved problem in current studies and may vary between individuals and individual states. Therefore, these complex explanations should be considered in future studies. By better understanding of the effects of limbs asymmetry on athletes’ physical activities can provide an important basis for coaches and athletes to design training strategies and rehabilitation testing.
Although previous studies generally believe that bilateral asymmetry in competitive sports has a negative impact on sports performance and is positively correlated with a sports injury, studies do not fully support this association. In addition, few attempts have been made in the literature to distinguish between asymmetries of different exercise types. Therefore, this review aims to promote a clearer understanding of intra-limb asymmetry in competitive sports as well as to summarize their correlation with sports performance measurement and sports injuries. It also analyzes the evaluation methods of athletes’ asymmetry in competitive sports.
Current systematic review was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines (PRISMA). A thorough computer-aided literature search of the database of PubMed (all years), Web OF Science (1960-present) and ScienceDirect (all years) were performed until 19 February 2022, to identify all relevant studies, using the keywords (‘biomechanics’ OR ‘kinetics’ OR ‘kinematics’) AND (‘symmetry’ OR ‘asymmetry’ OR ‘symmetric’) AND (‘athlete’ OR ‘player’ OR ‘competition’ OR ‘match’). The effects of symmetry assessment on different modes of competitive performance and injury are discussed by searching the articles related to the application of symmetry in competitive sports.
We have registered this study on the International Platform of Registered Systematic Review and Meta-analysis Protocols (INPLASY, Registration number: INPLASY202280023).
Studies that meet the following criteria were excluded: (1) Participants were included in recreational sports groups rather than competitive athletes; (2) The athletes had a physical injury during the test. (3) Studies that scored less than 75%. Endnote X9 (Thomson Reuters, Carlsbad, California, USA) was used to perform article collation, and duplicate articles delete functions.
The assessment scheme for the quality of these literatures was based on a established scales used in sports science. This method is commonly used to check literatures conducted in an exercise-based training environment (Brughelli et al., 2008). This quality system developed by Black et al., 2016 was used by a evaluator to assess the grading article quality (Black et al., 2016). The articles were evaluated using 9 different criteria(Score: 0–2), and total(Score: 0–18): (1) Inclusion criteria stated(Score: 0–2); (2) Subjects assigned appropriately (random/equal baseline); (3) Intervention described;(4) Dependent variables defined; (5) Assessments practical; (6) Training duration practical (acute vs. long term);(7) Statistics appropriate (variability, repeated measures); (8) Results detailed (mean, standard deviation, percent change, effect size); (9) Conclusions insightful(clean concise, future directions). Where each criterion is graded from 0(no) to 1(maybe) or 2(yes). In order to ensure the fairness of the quality assessment of the included studies, we evaluated the scores as a percentage (Range: 0–100%).
A total of 386 articles were initially returned, as shown in Figure 1. Any competitive athlete-related articles were included in the initial collection process and engender a total of 31 literatures. After identification, screening, and applying the exclusion and inclusion criteria, 22 articles were included in this study. A total 22 articles that included the ultimate investigation (The specific research methods were shown in Tables 1, 2, 3, 4), 8 of these articles focused on asymmetry in gait related sports, 7 examined asymmetry in upper limb sports, 3 of these studies attend to ball players asymmetries, and 4 related asymmetries in multifarious sports players. The specific athlete categories of sports studies include: sprinting players (6), multifarious sports players (4), soccer (2), Walking race players (1), Long-distance running players (1), Rowing players (1), archers (1), overhead sports player (1), cricket (1), Racing wheelchair player (1), paralympic powerlifting (1), pole vault player (1), rugby player (1).
|AUTHOR||SPORTS||PARTICIPANTS||ASSESSMENT METHODS||ASSESSMENT PARAMETERS||ASYMMETRY TESTS/METRICS MEASURED||FINDINGS||QUALITY SCORE|
|Tucker and Hanley (2017) (Tucker and Hanley, 2017)||Walking race||n = 35
senior-level (n = 18) and junior-level (n = 17)
|Race walked (speeds: 103% of the season’s best time for 20 km and 10 km, respectively.)||
Spatiotemporal: (SL, SF, CT, FT).
Kinetics: (Impact force; Loading force; Mid-stance force; Push off force; Impulse).
|SA||Twelve athletes were found to have asymmetrical stride sizes, which an underlying movement imbalance could cause.||100%|
|Haugen et al., 2018 (Haugen et al., 2018)||Sprint||Competitive sprinters (n = 22)||2–3 sprints over 20 m.||
Spatiotemporal: (SV, SL, SF, CT, FT).
Kinematics: (TD angle; thigh angle at TD; Liftoff angle; Thigh angle at LO; Knee angle at LO; MTF; Range of thigh motion; Knee flexion at MTE; ankle velocity.)
|Interlimb asymmetry||No significant correlation between individual asymmetry and sprint performance.||100%|
|Girard et al. (2019) (Girard et al., 2019)||Long-distance running||Well-trained, un-injured distance runners(n = 9)||Running at seven running velocities of 60s. (10, 12.5, 15, 17.5, 20, 22.5, and 25 km.h–1)||
Spatiotemporal: (SL, SF, CT, FT).
Kinetics: (Mean loading rate; Peak vertical Forces; Maximal downward vertical displacement; leg compression; vertical stiffness; leg stiffness; Braking phase duration; Peak braking force; Braking impulse; Push-off duration; Peak push-off force; Push-off impulse.)
|SA||Changes in running speed did not cause changes in bilateral limb symmetry.||100%|
|Girard O et al. (2017) (Girard et al., 2017)||Sprint running||Male sprints athletes (n = 13)||Five 5-s sprints with 25-s recovery.||
Spatiotemporal: (SL, SF, CT, FT, SV).
Kinetics: (Average vertical forces; Average horizontal forces; Average total forces; Propulsive power; Peak vertical forces; Centre of mass vertical displacement; Leg compression; Vertical stiffness; Leg stiffness).
|Interlimb asymmetry||There is the asymmetry in kinematics, kinetics and spring-mass characteristics during repeated sprints. Fatigue rates were similar in both lower limbs.||83%|
|Exell et al. (2012a) (Exell et al., 2012b)||Sprint running||Trained athletes (n = 8)||maximal velocity sprint running||
Spatiotemporal: (SL; SF; SV).
Kinetics: (Net horizontal impulse; Net vertical impulse; Maximum vertical force; Mean support movement; Network around the ankle, knee and hip joint.).
Kinematics: (Maximum hip height during contact; Maximum knee lift during contact; Minimum knee angle during swing; Maximum hip angle at end of contact; Touchdown distance;).
|SA||The occurrence of significant asymmetry was athletes specific. Variability within limbs should be considered in the asymmetric analysis.||100%|
|Exell et al. (2012a) (Exell et al., 2012a)||Sprint running||Male sprint trained athletes (n = 8)||maximal velocity sprint running||
Spatiotemporal: (SL; SF; SV).
Kinetics: (Net horizontal impulse; Net vertical impulse; Maximum vertical force; Mean support moment; Net ankle work; Net knee word; Net hip work.)
Kinematics:(Maximum hip height; Minimum knee lift; minimum knee angle; Maximum hip extension Touchdown distance;)
|SA and Composite Asymmetry scores.||The individual difference of lower limb asymmetry among different sprinters is great.||94%|
|Brown et al. (2017b) (Brown et al., 2017b)||Sprint running||Mate sprint athlete (n = 1)||submaximal sprints for ~8s at 50, 70 and 80% and a single short maximal trial for ~3s.||
Kinetics:(Hip relative Horizontal Force;
Hip relative power).
Kinematics: (Hip velocity).
|SA||Reducing the hip horizontal force asymmetry can reduce the risk of injury and increase sprint performance.||94%|
|Brown et al. (2017a) (Brown et al., 2017a)||Sprint running||Un-injured male rugby athletes (n = 30)||8-s sprints.||
|SA||The kinetic asymmetry can improve sprint performance.||94%|
|AUTHOR||SPORTS||PARTICIPANTS||ASSESSMENT METHOD||ASSESSMENT PARAMETERS||ASYMMETRY TESTS/METRICS MEASURED||FINDINGS||QUALITY SCORE|
|Warmenhoven et al. (2018) (Warmenhoven et al., 2018)||Rowing||n = 27 national- level (n = 14) and international level (n = 13)||Row for 1000 m (250m×4, stroke rates: 20, 24, 28, and 32 strokes per minute))||Kinetics:(propulsive pin forces).||SI||Asymmetry may lead to better performance in rowing.||94%|
|Sanchis-Sanchis et al. (2020) (Sanchis-Sanchis et al., 2020)||archers||males (n = 18) and females (n = 12) archers||The skin temperature of the trunk and upper limbs was measured pre, post and after 10 min of a simulated archery competition.||Anthropometry: (trunk skin temperature; upper limbs skin temperature).||The difference between the two sides||The asymmetry of skin temperature caused by Archery exercise is a major factor in maintaining muscle vitality and postural performance.||94%|
|Oyama et al. (2008) (Oyama et al., 2008)||overhead sports||Athlete (n = 43, including 15 baseball pitchers, 15 volleyball players, and 13 tennis players.)||Electromagnetic tracking device to measure both side scapular position and orientation.||
Kinematics:(Scapular upward-downward and internal-external rotation angle;
Scapular anterior-posterior tilt angle;
Scapular protraction-retraction angle;
Scapular elevation-depression angle;).
|Separate analyses of variance.||The dominant lateral shoulder blades of overhead athletes have more internally rotated and anteriorly tilted. This asymmetry may be normal.||89%|
|Gray et al. (2016) (Gray et al., 2016)||Cricket||adolescent provincial league specialist fast bowlers (n = 25, 16 with and 9 without LBP)||Supine (Static ultrasound images)||Anthropometry: (Muscle thickness of internus abdominis; obliquus externus and transversus abdominis).||ANOVA||The asymmetry of abdominal muscle thickness in fast bowlers is caused by the asymmetric biomechanical characteristics of the sport.||100%|
|Goosey et al. (1998) (Goosey and Campbell, 1998)||Racing wheelchair||Endurance-trained wheelchair racers (n = 7)||The athletes pushing their own racing wheelchairs at 6.58 ms-1||
Spatiotemporal:(Cycle time; time spent in contact whit the hand rim).
Kinematics: (Range of elbow flexion;
|Wilcoxon Matched-Pairs Signed-Ranks tasks.||The symmetry exists during wheelchair propulsion in the elbow movement pattern for trained wheelchair racers.||94%|
|Dalla Bernardina et al. (2021) (Ramos Dalla Bernardina et al., 2021)||Paralympic powerlifting||Paralympic powerlifting athletes (n = 10, 8 men and 2 women)||Kinematic of athletes performing in bench press at submaximal intensities (50% and 90% of the one-repetition maximum).||Kinematics:(Linear velocity).||FANOVA||Asymmetry exists in the higher effort of disabled weightlifting tasks.||100%|
|Panoutsakopoulos et al., 2021 (Panoutsakopoulos et al., 2021)||Pole vault||Pole vaulters (n = 24, 11 males and 13 females)||The athlete attempts to complete the pole vault as best he can, recording the last eight steps near the start of the jump.||Spatiotemporal: (SL; SF; SV).||SA||There were significant gender differences in the asymmetry of step frequency and step length.||94%|
|AUTHOR||SPORTS||PARTICIPANTS||ASSESSMENT METHOD||ASSESSMENT PARAMETERS||ASYMMETRY TESTS/METRICS MEASURED||FINDINGS||QUALITY SCORE|
|Teixeira et al (2008) (Teixeira and Teixeira, 2008)||indoor soccer||6-year-old (n = 8), 8-year-old (n = 8) and 10-year-old (n = 8) indoor soccer players||Leg preference was evaluated separately for three task categories: balance stabilization, soccer related mobilization, and general mobilization.||
Functional variables:(averaged scores for the three tasks preference).
Kinematics:(Peak ankle, knee and hip velocity and peak delay).
|Repeated measures ANOVA and Newman-Keuls procedures||Age had no effect on leg preference. There was a similar increase in leg performance from ages 6-8 to 10.||89%|
|Fousekis et al. (2010)(Fousekis et al., 2010)||Soccer||Soccer players (n = 100)||Isokinetic concentric and eccentric strength of ankle and knee muscles be performed.||
Kinetics:(Knee Extension torque;
Knee flexion torque;
Ankle Dorsal flexion torque;
Ankle Planter flexion torque;
Knee strength ratios).
|The difference between the two sides||There is the asymmetry in the strength adaptation of knee joint and ankle joint in football players. More experienced player can more easily handle the risk of injury caused by asymmetry.||94%|
|Brown and Brughelli (2014) (Brown and Brughelli, 2014)||Rugby||Professional rugby league player (n = 1)||Lower limbs isokinetic strength and sprint kinetics be performed.||
Kinetics: (Keen and Hip Extension and flexion torque).
Kinematics: (angle of peak torque).
|SA||The reduction of asymmetry can be used as an indicator of athletes’ return to sport.||83%|
|STUDY||SPORTS||PARTICIPANTS||ASSESSMENT METHOD||ASSESSMENT PARAMETERS||ASYMMETRY TESTS/METRICS MEASURED||FINDINGS||QUALITY SCORE|
|Alvarenga et al. (2019) (Alvarenga et al., 2019)||multifarious sports||n = 13(9 women, 4 men)||Static Position, Free squat, CMJ.||Kinetics: (Peak vGRF).||SI||SMT intervention changed static symmetry greatly, but did not change dynamic symmetry significantly.||78%|
|Paterno et al. (2010) (Paterno et al., 2010)||multifarious sports||Athletes after ACLR (n = 56, 35 female, 21 male) and return to sport.||Single limb dynamic postural stability test. Kinematic and kinetic analysis during DVJ maneuver.||
Kinetics:( Transverse plane hip net moment impulse; sagittal plane knee moments at initial contact).
Kinematics:(2-dimensional frontal plane knee joint range of motion).
|The difference between two limbs.||Greater asymmetry in internal knee extensor moment at initial contact during DVJ maneuver of ACLR athletes.||100%|
|Kotsifaki et al. (2022) (Kotsifaki et al., 2022)||multifarious sports||Male athletes (n = 47, 24 athletes after ACLR and 23 healthy male)||Triple-hop test.||
Kinetics:(Knee extension moment;
Hip, knee, ankle and total work).
Kinematics:(Hip, knee, ankle, trunk and anterior pelvic tilt angle).
|SI||The triple hop symmetry for distance masked important deficits in knee joint work and other biomechanical parameters in ACLR athletes.||100%|
|Morishige et al. (2019) (Morishige et al., 2019)||multifarious sports||Female collegiate (n = 23, basketball players, n = 15; soccer players, n = 8) and recreational athletes (n = 19, basketball players, n = 10; volleyball players, n = 9;).||DVJ||
Kinetics: (Peak vGRF).
Kinematics:(Knee joint angle of IC and peak value; Knee joint moment within 40 milliseconds from IC).
|Two-tailed paired t-tests||The degree of asymmetry of knee abduction Angle of leisure and college athletes is the opposite.||100%|
As shown in Tables 1, 2, 3, 4, the average quality assessment rate for selected articles in this systematic review is 94.4 ± 6.3%. Each of these literatures illustrated the inclusion criteria in their studies, respectively. The best-reported criteria were “Results detailed” and “Conclusions insightful” and the least reported criterion was “Inclusion criteria stated”. As shown in Figure 2(A), the grade of average quality assessment of “Training Duration practical”, “Statistics appropriate”, and “Results Detailed” was 2 (yes). The grade of “Inclusion criteria stated” and “Intervention described” in ball sports is the lowest (average quality = 1.33).
Furthermore, as shown in Tables 1, 2, 3, 4, we identified 8 studies related to the asymmetry in gait related sports. Moreover, 7 of 22 articles investigating the asymmetry in upper limb sports. In addition, 4 studies involved athletes in multifarious competitive sports. We identified 3 (2 soccer and 1 rugby) studies related to the asymmetry in ball sport athletes. As shown in Figure 2(B), we identified 6 studies on the variables of asymmetry on sprinter and 4 on multifarious sports. The number of analyzed the variates of kinetic and kinematics asymmetries was the highest among all the included studies, 14 and 13, respectively. 9 studies analyzed the asymmetry of spatiotemporal variables (Figure 2(C)). In addition, SA was used as an assessment tool for asymmetry in 8 studies, and 6 studies used the method of SI (3 studies) and two side differences (3 studies), respectively. 5 of 22 articles used general statistical check approaches to identify bilateral asymmetries, ANOVA (2 studies), N-K procedures (1 study), FANOVA (1 study), W M-Pairs Signed (1 study) and Separate analyses of variance (1 study), as shown in Figure 2(D).
The purpose of the current systematic review was to investigate the existing literature on the application of limbs asymmetry in competitive sports and to discuss the correlation with sports performance measurement and sports injuries. Inter-limb asymmetry appears to have a positive effect on physical performance in upper limb movement, while it may cause injury to occur and have a detrimental effect on performance in gait related sports. However, the evidence pertaining to inter-limb asymmetry in ball athletes and different athletes is less conclusive. Mixed results were also found in a specific sport, suggesting that the effects of bilateral limbs asymmetry on different athletes may be task-specific.
Since the human body is a large and complex system, consequently, gait motion can be realized in many different ways. For example, the muscle group of the normal leg can compensate for the other leg with the weak muscle group during gait movement (Levine et al., 2012; Patra et al., 2022). Therefore, Gait asymmetry can increase the workload of one limb. By analyzing the gait variability and symmetry of 35 race walkers, Tucker and Hanley reported the asymmetrical step lengths were persistent in individual athletes, which may be caused by the underlying gait imbalance (Tucker and Hanley, 2017). Further data has also linked gait asymmetries to sprint running performance. Brown et al (Brown et al., 2017a) used acceleration and maximal velocity sprinting to assess athletic performance in thirty male rugby athletes (development-level). Trivial to small correlations was proved between the Vmax and symmetry angle of vertical and horizontal force in both acceleration (R2 = 0.021 and 0.100) and maximal velocity sprint phases (R2 = 0.179 and 0.0002), while the correlations between the symmetry angle in acceleration and maximal velocity sprint phases were 0.459 for vertical force and 0.721 for horizontal force. These results suggesting that the asymmetry of vertical and horizontal force may be the crucial components for acceleration performance in sprinting. However, the relationship between athlete performance with asymmetries is not clearly stablished. Another similar case study indicated that asymmetry was negatively associated with a lower risk of injury and high sprinting performance (Brown et al., 2017b). On the other hand, opponents argue that the symmetry of kinematic parameters during the stride cycle was no relationship with sprinter sports performance and the prevalence of injury (Haugen et al., 2018).
In addition, Exell and colleagues used asymmetry composite scores to quantify the intra-limb asymmetry in eight male sprint athletes and reported that asymmetrical measures exist for inter-participant differences (Exell et al., 2012a). A similar study compared and evaluated the spatiotemporal parameters and GRF asymmetries of 18 elderly and 17 young walkers and found that although there was no overall mean asymmetry, the individual analysis found asymmetries of several athletes (SA ≥ 1.2%) (Tucker and Hanley, 2017), This is somewhat supported by Girard who highlighted the relatively large range of asymmetries between individuals should be taken into account in the analysis (Girard et al., 2019). Therefore, these findings should be interpreted with caution, significant asymmetrical variables may be athletes specific, and therefore, intra-limb variability should be included in asymmetrical analyses to avoid misleading results (Exell et al., 2012b). In addition, Previous studies have hypothesized that gait asymmetry may be due to running fatigue (Brown et al., 2017a). Girard and colleagues have examined whether inter-limb asymmetry in lower limb mechanics increases with fatigue and found that similar fatigue rates exist in bilateral lower limbs during sprinting exercise (Girard et al., 2017). Consequently, the cause of bilateral lower extremity asymmetry should be the focus of future research.
The asymmetry of human body structure will cause the asymmetry of bilateral limb function. Similarly, asymmetrical movement over a long period of time can promote structural asymmetry. This potentially vicious cycle may have a negative impact on athletes’ training efficiency, so exploring the causes of asymmetry should be the focus of future research. Previous studies have reported that dexterity is one of the important causes of upper extremity gross anatomical asymmetry (Auerbach and Raxter, 2008). Oyama et al. (Oyama et al., 2008) assessed the asymmetry of bilateral scapular position and orientation in 3 groups of healthy overhead athletes (13 tennis players,15 baseball pitchers and 15 volleyball players). More internally rotated (p < 0.01) and anteriorly tilted (p < 0.01) of the scapula was showed on the dominant side of the overhead athletes and a more protracted of the scapula position occurred on the dominant side of the tennis players (p < 0.05). These results indicate that the cause of the asymmetry may be related to the athletic attributes of the athletes, and clinicians should be cautious in evaluating the asymmetry of the upper limbs in such athletes. However, the authors did not analyze the correlation between asymmetry and exercise experience, and more future research would be required to confirm this suggestion. More definite conclusion have been discribed for the Paralympic powerlifting athletes. Dalla Bernardina and colleagues (Ramos Dalla Bernardina et al., 2021) analyzed functional asymmetries during different submaximal intensities (50% and 90% of the one-repetition maximum, 1RM) using linear velocity. Powerlifting performed symmetrically at 50% of 1RM. In comparison, significant asymmetry in favour of the dominant limb occurred at 90% of 1RM. By comparing the sensitivity of ANOVA and FANOVA to body asymmetry, the authors found that the latter is the most suitable for examining the asymmetry of the performance of paralympic weightlifters. However, Further research is needed to confirm the relationship between bilateral asymmetry and weightlifting performance. Similar disparate findings have been reported for racing wheelchair propulsion. Goosey reported that no statistical difference was found in the elbow height, elbow angular displacement and propulsion phase of the racing wheelchair athletes.
For sports requiring a high level of the unilateral upper extremity, such as archery, previous studies have shown that the nature of the movement leads to an asymmetry in skin temperature. The authors point out that the asymmetry of different temperatures can reflect the muscle activation of archers and make an important contribution to their posture. The influence of exercise experience on skin temperature needs to be further explored in combination with neuromuscular signal analysis. Asymmetry is generally thought to affect athletic performance negatively, but the scientific evidence to support this claim is insufficient. In addition, asymmetric types are usually not defined. Warmenhoven et al. (Warmenhoven et al., 2018) noted that high-level rowers are more likely to use adaptive asymmetric strategies for rowing, suggesting that asymmetries have a functional role in a rowing movement. However, more scientific evidence is needed to determine whether asymmetry boosts rowing performance. Gender differences is also widely believed to be an important reason for individual differences in asymmetric parameters. Male pole vaulters with greater explosive power have greater step length and step frequency asymmetry during competition. This gender difference could be attributed to the athletes’ physical condition and pole characteristics. Gray et al. (Gray et al., 2016) By comparing abdominal muscle thickness asymmetry in fast bowling players with and without LBP, athletes with LBP had more symmetrical abdominal muscle size. However, whether this phenomenon has clinical significance remains unclear.
Although associative studies may indicate the potential influence of asymmetries assessment on return-to-sport status, it is important to consider degree of asymmetry related to the degree of rehabilitation. The findings of Brown and Brughelli suggest that a multi-component strength and dynamics assessment strategy can be used to reflect more fully the changes in athletes’ symmetry. This finding can provide evidence for the development of return-to-sport for athletes with different types of injuries (Brown and Brughelli, 2014). However, the specific symmetry recovery threshold of return to competition should be the focus of future research. Two studies have examined the performance asymmetries and their association with different training ages in soccer players, yielding conflicting results. Teixeira et al. (Teixeira and Teixeira, 2008) was evaluated separately balance stabilization, general and soccer related mobilization of the dominant and non-dominant lower limbs in right-footed soccer players children of different training ages. The results showed that the lower score of leg preference and the greater difference between individuals occurred in the stabilization task and training age had no effect on bilateral limb asymmetry. This finding provides evidence for the amplitude of stabilization of performance of bilateral lower extremity asymmetry during childhood development. Whereas Fousekis et al. (Fousekis et al., 2010) Soccer players with longer training age can use their lower extremities more evenly to deal with musculoskeletal asymmetry and may reduce the risk of injury. Whether this discrepancy is related to the age of the assessed cohort and the assessed indicators can be further explored.
Limited data are available on the effect of motor tasks on limb asymmetry. Further studies in a broad population of athletes are needed to clearly determine whether various body asymmetries are associated with motor tasks. There were 4 studies included in this review that did not evaluate the symmetry of a specific type of athletes but recruited different types of athletes with sports experience as subjects. Liu and Jensen (Paterno et al., 2010) calculated asymmetry of kinematics and kinetics in 56 athletes who underwent ACLR and found that asymmetries in sagittal plane knee moments at initial contact during the landing phase of a DVJ are strong predictors of second ACL injury. This prediction model only applies to the prediction of secondary ACL injury in athletes who have experienced ACLR, and further research is needed for the prediction of injury risk in healthy athletes. A similar study assessed changes in lower extremity symmetry in athletes who had experienced ACLR after return to sports criteria and found that patients used hip, pelvis, and trunk compensatory strategies to address inter-limb differences in knee function (Kotsifaki et al., 2022). A further consideration for the inducement of asymmetry would be the Athletic level. Minimal literature has focused on the difference in biomechanical symmetry of lower limbs in athletes of different sports levels. Morishige et al. (2016) compared the leg asymmetry between 23 female collegiate and 19 recreational athletes during the landing phase of a DVJ, and the results showed that the asymmetry of bilateral knee abduction Angle was opposite in the two groups. However, further evidence is needed to determine whether this interesting phenomenon is related to the injury. The presence of biomechanics asymmetries within athletes has been reported for several decades. Investigations had previously reported that an increased asymmetry of bilateral lower limbs was one of the potential causes of spinal abnormalities. However, investigation of the ameliorating interventions of asymmetry has only recently been examined. Alvarenga et al. (Alvarenga et al., 2019) reported that the intervention of lumbar SMT can improve the immediate static asymmetry of athletes, more interventions related to dynamic asymmetry need to be explored in the future.
The current review provides an exhaustive summary of the application of asymmetric assessment in different competitive sports and its relationship with sports performance and injury. Although asymmetric monitoring may be implemented in low-level sports settings, some studies of asymmetric assessment of non-athletes may have been excluded. Currently, in the absence of higher consistency of the results and conclusions of the included studies, confidence in understanding the relationship between bilateral asymmetry and performance, injury and function remains limited. Although the results in this review highlight that asymmetry is detected in the gait-related sports and associated with motion injury, a particular threshold size of asymmetry may cause damage has not been identified. Asymmetry seems to positively affect athletic performance, as demonstrated in upper limb athletes, although the conclusions are not entirely consistent. The cumulative literature suggests that large individual differences exist in asymmetrical measurement between athletes’ limbs, experience, age, gender and different sports tasks have also been proved to be the potential causes of asymmetrical measurement. The results of current systematic review emphasize the complexity of bilateral asymmetries in athletes of competitive sports. Future studies should aim to investigate specific asymmetric thresholds that may cause sports damage in different competitive tasks and assess symmetry’s specific effect on performance.
This study was sponsored by the China Scholarship Council (CSC NO.202108330003).
The author has no competing interests to declare.
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