Magnitude, development and interrelationship of morphological and functional asymmetry in tennis players

Almost every individual tends to have a preferred upper and/or lower extremity when performing voluntary movements due to the process of lateralisation. The preferred use of an upper or lower extremity eventually results in morphological (e.g., in terms of bone and muscle tissue) and functional (e.g., in terms of strength and power) adaptations to the extremity most often used, given that it is subjected to a higher mechanical loading and neuromotor triggering. The corresponding upper and lower extremity morphological and functional asymmetry magnitudes (i.e., which are expressed as a percentage) can be further accentuated by (intensively) practicing a unilateral sport such as tennis. Both morphological and functional asymmetry, however, are related to an increased injury risk as well as a decreased sports performance. Therefore, enhancing our knowledge regarding morphological and functional asymmetries could be beneficial.
Despite the reported asymmetrical loading on the lower extremity when playing tennis, no study has combined the examination of both upper and lower extremity morphological and functional asymmetry in female as well as youth male and female tennis players. Also, research regarding the relationship between morphological asymmetry and functional asymmetry for both the upper and lower extremities is currently lacking. Specifically, it is unknown whether high(er) magnitudes of morphological asymmetry is related to high(er) magnitudes of functional asymmetry, which is to be considered both at upper and lower extremity level. Finally, research is needed regarding the development of morphological as well as functional asymmetry in youth tennis players according to their maturity offset. Therefore, this doctoral research project aims to increase our current knowledge and understanding regarding morphological and functional asymmetry by examining the magnitude, development and the interrelationship of both types of asymmetries in tennis players.
Results The findings of the original research chapters of this doctoral research project are displayed in PART 2 of this dissertation.
In Chapter 1, a meta-analysis reported that upper extremity BMC was significantly higher in the dominant upper extremity (i.e., in terms of handedness) compared to the non-dominant upper extremity. Additionally, males, adult tennis players and early starters demonstrated higher upper extremity BMC asymmetry magnitudes compared to females, youth players, senior players and late starters, respectively. The crosssectional study presented in Chapter 2 examined upper as well as lower extremity morphological asymmetry magnitudes in 19 elite female adult tennis players using anthropometric assessment and DXA. With the exception of FM, all anthropometric and DXA related outcome measures of the upper extremity were significantly greater in the dominant upper extremity (i.e., in terms of handedness) when compared to the non-dominant upper extremity. Statistically significant morphological asymmetry magnitudes between both lower extremities were also apparent for BMC, LM and FM. The 22 elite female adult tennis players included in Chapter 3 performed a physical performance test battery to examine upper and lower extremity functional asymmetry using a cross-sectional study design. Subsequently, statistically significant functional asymmetry magnitudes were apparent for all upper and lower extremity physical performance tests. The direction-specificity of functional asymmetry across tests was also examined. Perfect levels of agreement were found for the upper extremity meaning that the dominant upper extremity consistently displayed the dominant performance value across the physical performance tests. In contrast, the lower extremity only displayed slight to poor levels of agreement across physical performance tests highlighting the direction-specificity across tests of lower extremity functional asymmetries.
Using a cross-sectional design, the study presented in Chapter 4 examined the relationship between LM (i.e., examined using DXA) and functional asymmetry (i.e., examined using a functional (field-based) test battery) in terms of magnitudes and asymmetry direction in 22 elite female adult tennis players. Statistically significant LM and functional asymmetry magnitudes were apparent in both the upper and lower extremity. Nevertheless, both asymmetry types were not related in terms of their magnitudes and direction. In Chapter 5, upper and lower extremity morphological and functional asymmetry magnitudes were examined in 41 male and female elite youth tennis players and 41 sex- and age-matched controls using anthropometric assessment, BIA and a physical performance test battery respectively. Statistically significant morphological and functional asymmetry magnitudes at the upper and lower extremity level were apparent in both the youth tennis players and the controls. In both groups, there was also no relationship apparent between both upper and lower extremity LM and functional asymmetry magnitudes in terms of magnitudes and direction.
Using a two-year longitudinal study, being presented in Chapter 6, the development of upper extremity morphological asymmetry values (i.e., BMD, BMC and LM) as well as their asymmetry magnitudes were examined in 49 male and 31 female elite youth tennis players according to their maturity offset. All included outcome measures were consistently higher in the dominant upper extremity (i.e., in terms of handedness) as compared to the non-dominant upper extremity. As opposed to upper extremity LM asymmetry magnitudes, both BMD and BMC asymmetry magnitudes increased significantly according to players’ maturity offset. In the 53 male and 37 female elite youth tennis players under study in Chapter 7, the development of upper and lower extremity functional asymmetries according to maturity offset was examined using a three-year longitudinal design. Additionally, the consistency in directionality across time as to which extremity performed dominantly across the yearly test occasions was examined. Statistically significant functional asymmetry magnitudes were apparent for every upper and lower extremity physical performance test in both the male and female elite youth tennis players. Functional asymmetry magnitudes did not change significantly according to players’ maturity offset for the physical performance tests. When examining the consistency in directionality across time as to which extremity performed dominantly across the yearly test occasions, perfect levels of agreement were found for the upper extremity tests whilst only poor to fair levels of agreement were found for the consistency in direction in terms of the lower extremity performing dominantly across time.
Conclusion This doctoral research project aimed to increase the current knowledge and understanding regarding morphological and functional asymmetry in tennis players. As such, statistically significant morphological and functional asymmetries were found in both female adult as well as youth (male and female) tennis players. There was, however, no relationship apparent between morphological and functional asymmetry in terms of magnitudes and direction. Only upper extremity BMD and BMC asymmetry magnitudes increased significantly according to maturity offset in youth tennis players, which was opposed to upper extremity LM as well as upper and lower extremity functional asymmetry magnitudes showing no significant change. Future longitudinal research is warranted to prospectively examine if and to what extent (i.e., perhaps after surpassing a certain threshold) the magnitudes of morphological and functional asymmetry influence the increase in injury risk and/or the decrease in sports performance.