This is the first study to investigate the relationship between lumbar spine abnormalities in asymptomatic elite adolescent players and serve kinematics. The aim of this study was to compare the effect of lumbar spine pars abnormalities, gender, and serve type on the kinematics and temporal sequencing of the serve in adolescent tennis players. No differences in peak lumbar spine, pelvis, or ball toss kinematics were observed between those with and without abnormalities. However, some differences in peak lumbar spine, pelvis, and ball toss kinematics existed between gender and between serve types. This study was also novel in its comparison of the effect of gender and serve type on the kinematics of the adolescent serve.
The Relationship Between Lumbar Abnormalities and Serve Kinematics
Surprisingly, the lumbar spine kinematics were comparable in magnitude, independent of the presence of abnormalities. Consequently, our first hypothesis was rejected. These findings contrast with previous research that has inferred a link between serve kinematics and low back pain among adolescent male tennis players [14]. Despite being informed by previous research [14], our hypothesized reduction in lumbar and pelvis rotation in both the drive and forward-swing phases of those without abnormalities was not substantiated. Unexpectedly, lumbar left lateral flexion, lumbar and pelvis left rotation, and pelvic anterior tilt were also comparable in the forward-swing phase. While discrete kinematics are valuable in determining peak/moment-in-time differences, there are shortcomings of analyzing these values in isolation. For example, while there were no observed differences in peak lumbar kinematics, the order and timing of the kinematics did vary considerably between groups. These variations might prove instructive for coaches when identifying players at risk of lumbar abnormalities [29], whereby the temporal features of serving might be observed via high speed video or sensor-based technologies [30]. Specifically, players with lumbar spine abnormalities tended to enter peak front knee flexion and initiate anti-clockwise pelvis rotation before RHP, which is in contrast to players without spine abnormalities. These two kinematics could be detected and monitored by coaches using the abovementioned technologies. It is worth noting however that further research investigating the statistical significance of the relationship between the order and timing of key serve events with lumbar spine abnormalities is needed.
Lastly, the variation in age and skill level likely contributed to our findings, that is, the younger participants in our study displayed large amounts of variation in their kinematics, potentially indicative of still maturing technique.
The Relationship Between Gender and Serve Kinematics
As expected, there were kinematic differences between the junior male and female serve. Peak posterior pelvic tilt was ~ 11° greater in male players during the drive phase of both the flat and kick serve. Most females adopted a more upright trunk posture during the ball toss (between ~ 3 and 4° more trunk extension), a probable by-product of these female players maintaining a neutral or anteriorly tilted pelvis during the drive phase compared to males. This trunk alignment tended to coincide with more pronounced peak front knee flexion, which saw female players assume a squat-like or more vertical (up-down) serve than male players.
Males made serve impact significantly further into the court on the flat (~ 16 cm) and kick (~ 15 cm) serves, even when held relative to their standing stature. The forward impact location of the adolescent male flat serve was similar to past research [17, 27] that has found junior and adult players to impact the ball ~ 52-58 cm forward of the front toe. The adolescent female players in the current study however tossed the ball up to 20 cm closer to the baseline than previous descriptions of the adolescent female serve [28]. It is possible that this was linked to the adoption of the abovementioned upright trunk position during the drive phase, which likely contributes to a reduced shoulder-over-shoulder rotation.
Interestingly, males impacted the flat serve 25 cm and kick serve 44 cm to the left of their front toe, which is substantially higher than some elite adult players [17]. If we assume that the average standing height of male player in past research is 183 cm, then the difference in relative lateral impact position (adults 0.19; adolescents 0.26) is even more extreme. Although speculative, we expect that this leftward positioning of the ball relates to a combination of the heightened need to impart spin to the ball to clear the net as well as introduction of the kick serve at this age. Importantly, for players to position themselves in this way, there is likely to be compensation elsewhere. For example, pelvis obliquity (where the right hip was vertically higher than the left) was much higher than reported in other elite junior populations [14]. This appeared to result in players’ bodies being rotated laterally, potentially explaining why players in this study impacted the ball further across their body compared to similar previously studied populations [14]. This type of alignment of the body might be injurious if unconstrained and is worth coaches and health professionals monitoring.
The Relationship Between Serve Type and Serve Kinematics
Flat and kick serve kinematics were notably different, largely supporting our third hypothesis. The kick serve displayed increased lumbar flexion and pelvis obliquity (left down), suggesting that players adjust their sagittal plane lumbar kinematics and pelvis position to achieve laterally displaced impacts. Similar to the observed differences in impact position based on gender, serve type also significantly alters the relationship between ball and racquet at impact. As with previous research in the adult game [17], players in this study made flat serve impact significantly further forward (51 cm vs kick 34 cm) and with higher horizontal velocity.
Interestingly, in contrast to previous work in elite tennis players [31], peak vertical racquet velocity was significantly higher for the flat serve. Conversely, previous work has established that vertical racquet velocities are higher for second serves in order to impart topspin on the ball [31]. A combination of comparatively smaller player heights and inexperience, as these junior players were likely only recently introduced to the kick serve, present as the most likely explanations of this finding.
Temporal Kinematics
The order and timing of key serve events was different between the P and NP groups, upholding our final hypothesis. Specifically, peak right lumbar lateral flexion and pelvis left rotation as well as peak front knee flexion occurred prior to RHP in players with abnormalities indicating possible early initiation of leg drive. Indeed, this difference in sequencing coupled with their earlier engagement of peak right knee flexion meant that the RHP of players with abnormalities was substantially different to those without abnormalities. The importance of RHP to the serve’s rhythm has been emphasized previously [27], and the lower (8 cm) ball zenith of the P group afforded them less time to self-organize in order to impact the ball. Keeping in mind that the players in the P group were asymptomatic, it is possible that the difference in their timing of serve events might lead to different loading of the spine which may ultimately lead to LBP. Therefore, more work is needed to explore the differences in serve sequences in players with lumbar abnormalities and whether these serve sequences result in additional spinal loading.
Female players with abnormalities tended to reach peak lumbar extension and peak lumbar left lateral flexion earlier than players without abnormalities. This is likely related to their reduced lumbar extension and commencement of pelvis rotation prior to RHP. As the lumbar spine is extended during the drive phase, increasing the duration of time spent in lumbar extension may be deleterious due to the amount of stress placed on the spine in this position [8].
Sample size was a limitation in this study due to the strict criteria and limited number of elite adolescent athletes available. This in turn resulted in participants’ age varying. This study also recruited players who reported as pain free at the commencement of the study.