Soccer is well-known as one of the most practiced and watched sports around the world, with an estimated 5 billion have engaged in the last edition of the FIFA (Fédération Internationale de Football Association) World Cup Qatar 2022 [1]. A main action in soccer is the kicking. Kicking is defined as a form of striking in which the foot is used to deliver force against an object [2]. In the case of soccer, the ball is the desired object for kicking (eg, for passing or shooting). Despite the evident importance of ball kicking to soccer performance, the quantity and quality of investigations are not proportional to its relevance to the game. The lack of sufficient evidence to assist practice is evidenced, for example, by the limited number of investigations available in the form of scientific studies assessing practical interventions and their effects on ball-kicking movement [3]. While adopting field paradigms is desired when measuring or testing kicking skills in soccer [4], the difficulty of capturing player movement kinematics under field conditions may justify in part the scarcity of literature (eg, large time-frame from data collection to results report) [5-7]. In this context, the emergence of technologies that can overcome such limitations is therefore necessary.

Wearables represent a range of devices that can be worn or attached to the body to record data and comprise, in general, 2 categories: independent running devices (primary) or devices that require offline transference to a primary wearable device [8]. Wearables have been extensively used to measure load demands (internal and/or external). At the same time, this can also be used in additional sport-specific movements, such as evaluating technical aspects of gameplay [9], including but not restricted to ball kicking. Examples of wearables used to monitor soccer kicking data (eg event detection and release velocity) include inertial measurement unit [10], accelerometer [11], GPS-embedded accelerometer [12], or LPS (local positioning system) [13]. One advantage of wearable devices, which provide kinematic outputs, as compared with traditional video-based tracking techniques, refers to their generally reduced cost and time effectiveness [6].

Defining some further concepts in advance is necessary when proposing a protocol for a systematic review of the measurement properties (eg, validity and reliability) of wearables used to measure ball kicking. The first one refers to the validity term, which can be assumed as whether one given instrument measures the characteristic it was designed to quantify. Concerning kinematics variables, there is a traditional acceptance that high-speed optical systems (eg VICON; Oxford Metrics Ltd) represent the “gold standard” [14]. Reliability (eg test-retest) represents more the variation of the tool, protocol, or human system [15]. Accuracy can be defined as the closeness between a measured value and the actual value [16,17]. In the case of wearables applied to capture soccer kicking features, this can be exemplified by the number of kick occurrences registered by the device or system compared with known values derived from offline observational analysis of video recordings (ie human operator labeling). Finally, while there is a knowledge base [18-20], to the extent of our knowledge, no previous systematic reviews addressed solely the measurement properties, such as validity aspects of wearable technologies specifically applied to compute ball kicking indices.

Therefore, this study aims to provide a protocol for a systematic review focused on analyzing the measurement properties (validity, reliability, and accuracy) of wearable technology systems used explicitly to measure ball-kicking features in soccer. The future review using the protocol described here will assist in answering the following question: Are the available wearables accurate enough to quantify soccer kick mechanics, especially under field conditions?

This systematic review project is ongoing. From an initial search (February 2024) conducted to find previous analyses on wearables in the context of sport (data found from inception to 2023; Table 2), there were no existing reviews, amongst those identified, which have aimed at examine solely the application of wearables to investigate ball kicking in soccer [7,9,14,18-20,39-44] (Table 3). Given the importance of the subject, a dedicated analysis is therefore justified. At the moment of submission, database searches were completed (ie, identification step), and the present authors are beginning the screening step. If there is a sufficient number of studies (ie, 3 or more on a given dependent variable) and no substantial variations are detected concerning methods used across studies, results can also be presented through a meta-analysis (quantitative synthesis). Subgroup analysis, if pertinent, will consider, for example, gender, age, and playing level. Finally, independent of whether the final manuscript will qualify for quantitative synthesis or not, qualitative synthesis will be performed using the best evidence synthesis method. In addition, a table in attachments containing the main study characteristics will be included (eg, aim, results, and findings). Results are predicted to be completed by July 2024, and the final systematic review manuscript submission by November 2024.

In this study, we set out to provide a detailed protocol for a future systematic review of the measurement properties of wearables adopted to investigate ball kicking. Ensuring data quality through the integration of new technological devices is crucial for minimizing bias in research publications and the decisions derived from using such instruments in daily practice. Therefore, scientific publishing must prioritize valid metrics, as measured in concurrent-validity studies, and reliability [45]. Based on the exploratory analysis conducted in our initial search, the majority of publications indicate that wearables help quantify soccer kick mechanics. This finding would instill confidence in their use in real-world scenarios.

Recognizing the increasing demand for ecologically focused soccer drills [46] and acknowledging the opportunities presented by new technologies for incorporating measurement systems into training, the use of wearables in ecological scenarios emerges as a high-value proposition. Indeed, this approach offers a viable solution to overcome the constraints associated with gold-standard motion-based devices, which are typically limited to specific locations, predominantly within laboratories.

With the progress in microsensors, we anticipate observing satisfactory levels of concurrent validity in wearable inertial measurement units, as supported by previous original research [47,48]. The majority of kinematic measures exhibiting acceptable concurrent validity and reliability are expected to be associated with ball release velocity [48], representing an outcome of the kicking process. Meanwhile, other outcomes will be linked to the inherent quality of movement, seeking to characterize the kinematic profile of players [12,47] by measuring linear, angular, or joint velocities. It is important to note that the judgment on whether individual wearable types/brands present sufficient research evidence to be used in practice will be based on both the quality and quantity of published studies available in the form of scientific articles, as per the rigorous synthesis method adopted.

The reliability levels observed in the wearable microsensors, as mentioned above, may enable ecological-based research and practical scenarios. Despite their accuracy and precision being comparable to gold-standard vision-based instruments, the latter are, in general, significantly limited in their ability to track kinematics in confined spaces, thereby failing to provide context and real-world applicability to practice.

As a limitation, this study focuses solely on the kicking process, thereby excluding the examination of other wearable technologies that are currently being observed for upper limb movements and throws. However, it is conceivable that these technologies may become applicable to kicking in the near future. Furthermore, the limited number of articles identified in this research field, coupled with its niche nature, may constrain the generalization of findings. Nonetheless, our study addresses a specific research and practice issue within one of the most widely practiced sports globally. Consequently, it has the potential to assist provide insights into the quality of data derived from these devices for numerous coaches.

The outcomes of this systematic review will offer researchers and practitioners a means to comprehensively summarize the evidence regarding the quality of data extracted from wearable devices for measuring kicking performance. This will enable the identification of the most accurate and reliable devices, pinpoint gaps in current research, and identify new research directions and developments that need attention in the coming years. From a practical standpoint, coaches and sports scientists can confidently use wearable technology to analyze the kinematics of kicking. Such analyses can inform the design of tasks aimed at enhancing kicking performance while also providing valuable feedback to soccer athletes to improve this skill.