Introduction

Kicking is a fundamental movement skill and is highly prevalent and essential in many sports, including soccer, rugby, American Football and Australian Rules Football [15]. Kicking motion in all types of sports is a skill used  to score goals or points, which necessitates the generation of a high ball-releasing velocity [32]. Kicking performance is evaluated by ball release velocity [39] and accuracy [18]. Previous research has focused predominantly on the kinematics of the lower body [29, 35, 38], and the recommended instruction for coaching the upper-body motion is limited [15]. With the increasing number of recent kicking biomechanics studies, it is worth reviewing the necessity of adding an upper body rotation technique.

From a biomechanics perspective, generating high ball release velocities can be explained in terms of the kinetic link principle, where angular momentum is sequentially transferred from proximal to distal body segments. Although kicking performance includes accuracy [18] consistently, the kinetic link principle cannot account for this definition of kicking performance; instead, it explains  a coordinated sequencing of body segments to produce a high ball release velocity. From a sports coaching perspective, descriptions of the kicking skill [15] typically indicate that torque at the hip joint initiates the motion [35], followed by the peak extension of the ipsilateral knee joint [38], rather than from rotational motions of the upper body. These lower limb motions are explained by mathematical equations [2, 38] that indicate the transfer of angular velocities from the thigh to the shank (or lower leg) to generate a high foot velocity at ball impact. Foot velocity is further noted as the most significant predictor of ball release velocity [2, 9, 17]. However, the origin of the angular velocity of the hip action remains unclear. Thus, upper body segment motions should be considered (in addition to other body motions such as the action of supporting leg) to ascertain their contribution to prestrike foot velocity and, thus, ball release velocity in this review.

The kicking motion has already been defined as a ‘whip-like’ motion. The application of the kinetic link principle in biomechanics analysis is evident in studies examining other ‘whip-like’ motions, such as overarm throws [16, 19]. The angular momentum from upper torso rotation is sequentially transferred to the upper chest, upper arm, lower arm and hand to generate a maximum ball release velocity [19]. The possibility and its impact of an angular velocity transferred from the upper to the lower body segments in kicking warrants investigation.

The role of the upper body transferring angular momentum to the hip and knee joints during the kicking motion has been explored in previous studies [18, 32]. However, due to a variety of variables and differing definitions of the body segments involved, the contributions of the upper body segments remain unexplored in the current guideline [15, 21]. Thus, this systematic review aimed to synthesise findings in the extant literature from seven major sports science research databases to explore the role of upper body rotations in optimising kicking skills.

Methods

A predefined systematic review protocol was registered with the PROSPERO International Prospective Register of Systematic Reviews (registration ID: CRD42020176108). The review was conducted and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [31].

Literature Identification

An initial systematic search of existing literature was conducted using the combined keywords ‘All ball kicking sports AND Kick AND Upper body’ (Appendix 1). These keywords were generated in consultation with a specialist librarian to locate literature that explored the role of upper body rotations in kicking performance outcomes. Comprehensive searches were conducted on MEDLINE, CINAHL, SPORTDiscus, Web of Science, PUBMED, SCOPUS and EMBASE from the first record to 30th October 2023. Additional studies that met the inclusion criteria were identified from the reference lists of the included studies from the database search.

Inclusion Criteria

We included original research reporting on the relationship between upper body kinematics (segments above the hip) and ball kicking performance. We also included participants’ characteristics, including the type of sport, sex, age, health status, playing status of participant and languages. Studies reporting on the coordination patterns of the upper body during ball kicking, coaching and/or training applications, as well as adverse effects were included.

Exclusion Criteria

We excluded research reporting outcomes not related to the application of upper body rotation training on athletes such as robotic prototypes, muscle strength, or cross-sectional area, the lower extremity (segments including only the hip or below the hip). We also excluded editorials, conference papers, systematic reviews or unpublished studies without a peer-review process.

The title, abstract and full-text screening of the retrieved papers was independently conducted by three authors (AF, JL and JC) based on the inclusion and exclusion criteria. The final review of all included studies was then conducted by all authors. Any discrepancies were addressed through discussion with all authors.

Methodological Quality Assessment

The quality of each included study was assessed independently by three authors (AF, JL and JC) using an assessment checklist specifically designed for this review (Table 1 and Appendix 2). This checklist was modified based on the Standard Quality Assessment Criteria [26]. Items 1, 2, 4, 5, 6, 7, 9, 10, 12 and 13 were included from the existing list. Subsections were created within items 4, 5 and 6 and items 3, 8 and 11 were added to make this checklist specifically relevant towards biomechanical analysis in ball kicking motions. A score of 0 was given for each item when the corresponding criterion was not met, and a score of 2 when the criterion of the item was met fully. A score of 1 was given for each item when the information was reported partially and required interpretation or prior understanding. A total score of 80% or higher in a study indicated a high-quality study, as the methods used were considered reproducible to answer the intended research question. Any discrepancies in scoring were resolved through discussion by all authors.

Table 1 Methodological quality assessment checklist for included studies
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Data Extraction

Data extraction was undertaken by AF, JL and JC, with all authors checking for accuracy. Data extracted were the year of publication, the purpose of the study, mode of kicking, participant characteristics (age, sex, skill level), study design and methods, alongside the main finding (Table 2).

Table 2 Abridged summary of records screened in the systematic search. Listed by alphabetically by mode of kicking
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Data synthesis and Analysis

Due to the heterogeneity of included studies in terms of study methodology and analysis of joints and segments, a meta-analysis would be potentially misleading. Thus a narrative synthesis of the results was presented. The findings of the kicking motion in differing sports were discussed separately due to variances in ball size, mass and shape, as well as differences in kicking technique. Where reported, for each study, results were considered significant if the reported p-value was less than their stated critical p-value.

Results

Of the initial 1486 papers, 27 papers were deemed eligible for inclusion (Fig. 1) [3, 5, 7, 10, 12, 13, 20, 22,23,24,25, 27, 28, 32, 33, 36, 41,42,43,44,45,46,47,48,49, 51, 15]. This raises the question of whether the movement pattern or the energy transfer from the upper body could complement these guidelines, reducing the risk of injuries or contributing to the occurrence of these injuries. Previous studies demonstrated that when the whip-effect is greatly increased, the chance of injury may also be increased [14, 37]. DeLang et al. indicated that he probability of injury of the inertial leg is 1.6 times that of the non-inertial leg in soccer. At the same time, while among youth athletes, the rate has increased by another 1.5 times [14]. Hamstring strain injuries have already been proven to be highly prevalent in sports [30]; risk of such injury may be increased by an excessive strain during eccentric contraction in the late kicking phase. Further studies could investigate the impact of energy flow between the trunk and the extremities and their contribution to injuries.

Currently, there is no standardised protocol to quantify the strength of evidence in biomechanics studies with different study designs. However, given that 10 of the 12 included studies demonstrated a positive relationship between upper body rotation and ball release speed. This review appears to provide at least moderate evidence that increased upper body rotations promote ball release velocity following the kinetic link principle. The impact of the upper body rotation is not highlighted in current coaching manuals, which should be updated to improve the kicking skill acquisition.

Perspective

This review provides moderate evidence that increased upper body rotations play a crucial role in promoting ball release velocity, aligning with the kinetic link principle. Specifically, develo** thoracic and pelvic rotations has been found to have a positive impact on ball release velocity during the in-step kicking motion. However, an important gap emerged in the current coaching guidelines, as they do not emphasise the critical significance of upper body rotation, underscoring the urgent need for updates to enhance kicking skill acquisition. By gaining a deeper understanding of how the upper trunk and upper limb segments interact with the pelvis and lower limb segments, coaches can effectively prescribe techniques, and athletes can acquire the necessary skills for improved performance. To comprehensively optimise ball-kicking motion, thereby updating the coaching manual, further research is warranted to explore the effects of timing and the ranges of motion of all relevant upper and lower body segments on maximum ball release velocity.