A Review of Countermovement and Squat Jump Testing Methods in the Context of Public Health Examination in Adolescence: Reliability and Feasibility of Current Testing Procedures

Background In the context of a public health physical fitness (PF) examination in adolescence, a countermovement jump (CMJ) and a squat jump (SJ) are two vertical jump (VJ) tests widely used to evaluate lower limb muscle strength and power, respectively. The main criticism of both the CMJ and SJ test is the lack of test standardization. Therefore, the objectives of this review are: (a) to gather information about both jumps; (b) to investigate whether it is possible to identify common procedures referred to in the CMJ and SJ technical execution, and (c) to design standard operating procedures (SOPs) to promote CMJ and SJ standardization in an adolescent population aged 12–18 years. Methods The review partially adopted the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Statement (PRISMA). Due to growing attention in monitoring physical health through field tests in recent years, articles were collected using the PubMed, Web of Science, and Scopus databases from January 2009 to July 2019. Original articles in which CMJ or SJ were used to assess the muscular strength in adolescents were eligible for further analysis. Articles written in English was imposed as a limit. Results A total of 117 studies met the inclusion criteria. The description of the CMJ and SJ test procedures was different within the literature, with discrepancies in the jump technique, number of jumps, and measurement devices used. Conclusions A lack of method standardization for both the CMJ and the SJ test was identified. Based on the literature, SOPs for both VJs were proposed. These are useful in the context of public health PF examination in adolescents, as they facilitate an unbiased comparison of jump performance data between published studies.


INTRODUCTION
Muscular strength and power, cardiorespiratory endurance, body composition, and flexibility are health-related attributes of physical fitness (PF) (Caspersen et al., 1985) and consequently are considered key health status markers in humans (Catley and Tomkinson, 2013). To prevent pathologies and diseases that occur during adulthood, monitoring PF during adolescence is, therefore, important (Ortega et al., 2008b). In this context, the identification of children who are not developing healthy fitness habits using valid, reliable but also feasible measurement tools is essential (Davis et al., 2008;Faigenbaum et al., 2011;Garber et al., 2011). In the last decade, several research groups have focused their work on assessing the health status of children and adolescents, developing test batteries such as the ALPHA health-related fitness test battery , the ASSO project (Bianco et al., 2015), and the PREFIT battery (Ortega et al., 2015).
Physical fitness can be objectively and accurately measured through laboratory and field tests (Cooper, 1968;Astrand, 1976;Boone et al., 1978;Leger and Lambert, 1982;Inbar et al., 1996;Mayorga-Vega et al., 2014). Laboratory tests are generally more reliable, while field tests are commonly less expensive and more comfortable to administer (Heyward, 1991), and importantly they are characterized by a high level of ecological validity. Therefore, field tests are more suitable in population-based studies, especially in a school or college setting . Some field tests are subject to a standardized testing procedure, for example, the Cooper test (Cooper, 1968); the 20-m Shuttle Run Test (Leger and Lambert, 1982), or the sitand-reach test (Wells and Evelyn, 1952). Conversely, to date the vertical jump (VJ) test does not consist of such standardized testing procedures.
Vertical jumping is a multi-joint movement that requires complex motor coordination, and it has been identified as one of the fundamental movement skills (Gallahue, 2002). VJ tests are widely used to evaluate simple and complex tasks (Suchomel et al., 2016), such as sprint acceleration, sprint deceleration, throwing (Manno, 2008;Comfort et al., 2012a,b;Seitz et al., 2014), and change of direction (Nimphius et al., 2010;Spiteri et al., 2014;Suchomel et al., 2016). Furthermore, to evaluate lower limb muscular strength and due to the simplicity and richness in outcome information, VJ tests are widely used by strength and conditioning professionals, coaches, and health care professionals (Liebermann and Katz, 2003;Duthie, 2006).
The countermovement jump and squat jump (CMJ and SJ, respectively) are two examples of VJs which are both derived from the Sargent jump (Sargent, 1921(Sargent, , 1924. Both CMJ and SJ are considered reliable and valid (Markovic et al., 2004) in children (Fernandez-Santos et al., 2015). The CMJ is characterized by an initial countermovement (CM) before the toe-off phase (Bobbert et al., 1996), and the CMJ provides information about the reactive strength of the lower limbs (Young, 1995). In contrast, the SJ starts from a stationary, semi-squatting position and it provides information about leg power performance (Anderson and Pandy, 1993;Young, 1995). In sedentary individuals, as well as in elite athletes, the resultant jump height is correlated with explosive muscle strength (Sargent, 1921(Sargent, , 1924Bosco andKomi, 1979, 1980;Bosco and Viitasalo, 1982) and with performance components, such as speed (Wisloff et al., 2004), agility (Barnes et al., 2007), and power (Liebermann and Katz, 2003;Markovic et al., 2004;Patterson and Peterson, 2004;Tricoli et al., 2005). The SJ performance is also considered a measure of coordinated activities (Tomioka et al., 2001;Eloranta, 2003;Myer et al., 2005). In this context, Van Hooren and Zolotarjova (2017) in a recent review highlighted the differences between CMJ and SJ performances, emphasizing the need for future research to investigate the exact interaction of the mechanisms that explain the difference between the two jumps.
The lack of robust and consistent testing methods for CMJ and SJ evident in the literature compromises the quality of the research in this area (Eagles et al., 2015). Eagles et al. (2015) in their meta-analysis on VJ tests stated a lack of standardization in jump phase identification (i.e., starting, push-off, toe-off, and apex of the jump phase) which results in notable differences in the duration of the jump phases, the time to reach peak force, and in the rate of force development. Fitzgerald et al. (2018) suggested the use of the SJ to bypass the problem of the identification of the related phases, as this jump comprises of less variables than other kinds of jumps. Van Praagh and Dore (2002) stated that there is a distinct need to create a standardized jump protocol. Furthermore, some researchers advocate the use of standard operating procedures (SOPs) as being superior to the teaching of "laboratory manuals, " in that SOPs provide a step-by-step guide to the details related to a process which allows for the exact replication of all steps involved when repeating the process (Angiuoli et al., 2008;Tuck et al., 2009). It is important to note that SOPs are widely adopted in many other areas (Angiuoli et al., 2008), such as biology (Roseti et al., 2015) or medicine, for example in stroke prevention and treatment (Ntaios et al., 2015), critical illness (Sherren et al., 2014), or pre-hospital critical care interventions (Rognas et al., 2013).
To the best of our knowledge, no research to-date has stipulated clear guidelines for the CMJ and SJ tests. In fact, published works used different testing procedures, without specifying some essential parts for the replicability of the work.

Objectives
The first objective of this review was to gather information about testing methods used in research related to the assessment of PF, specifically muscular strength, using CMJ and SJ performance tests in adolescence. The second objective was to investigate if there are standard aspects between these CMJ and SJ testing methods and to identify the most common ones used. If these were not evident, the third objective was to develop SOPs considering: the jump phases; the devices used, and the number of jumps performed. As described by Bobbert et al. (1996), for a better understanding of the differences between the CMJ and the SJ, it is useful to divide both VJs into phases (e.g., starting position, the start of the push-off, the toe-off, and the apex of the jump phase).

Research Question
With a particular view of the adolescent population, the research question addressed whether there are clearly defined SOPs for the CMJ and the SJ test evident in the literature and then, in parallel, to gather information about both VJs.

Study Design
This review of literature partially adopted the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (Moher et al., 2009). The following key points were not used: protocol and registration (5), data items (11), risk of bias in individual studies (12), summary measures (13), synthesis of results (14), risk of bias across studies (15,22), additional analyses (16, 23), risk of bias within studies (19), results of individual studies (20), and synthesis of results (21).

Participants, Interventions, Comparators
Population, Intervention, Comparison, Outcomes, Study design criteria (PICO-S criteria) described in PRISMA (Moher et al., 2009) were considered, to include and exclude research publications. The population under examination were adolescents. According to Radnor et al. (2018), adolescent females include an age range between 12 and 18 years, and for males this age range is between 14 and 18 years. A population between 12 and 18 years old, of both genders with no particular conditions (i.e., mental disease or physical problems), was considered in the present work, to avoid misunderstandings between the different gender age ranges. Children, adults, the elderly, and elite athletes (due to the possible adaptation of the VJs for the sport characteristic) were excluded. As the authors wanted to examine the jump testing method used, interventions, comparisons, and outcomes of the studies were not considered as inclusion or exclusion criteria. Regarding the study design, original articles were eligible for further analysis in which CMJ or SJ performance tests were used to assess lower limb muscular strength as part of PF evaluations in adolescents and not as training interventions. Due to the risk of involving other populations that were not adolescents, longitudinal studies were excluded.
No restriction criteria were applied for the country of origin, but only works written in English were considered. Reviews, meta-analyses, abstracts, citations from scientific conferences, statements, opinion pieces, commentaries, editorials, letters, book reviews, books, and non-peer reviewed journal articles were excluded.

Search Strategy
The databases consulted for relevant original articles were PubMed (NLM), Web of Science (TS), and Scopus. The search strategy included the use of the terms in the search field "title" and "topic" of each database. The terms used were divided into three groups. Group A used the following keywords: "countermovement jump * , " "squat jump * , " and "vertical jump * ." Group B used the following keywords: "maximal dynamic strength, " "field-based physical fitness test * , " "fitness-test battery * , " "field test * , " "physical fitness, " "muscle strength, " "strength, " "resistance training, " "physical education, " "reliability, " and "validity." Finally, group C used the keywords: "youth, " "preadolescence * , " "adolescent * , " "public health, " and "health promotion." For each database, term by term of Group A was matched with each term of Group B and Group C using a Boolean operator (AND).

Data Sources, Studies Sections, and Data Extraction
Due to the growing interest in monitoring PF in young people in the last 10 years Bianco et al., 2015;Ortega et al., 2015), databases were searched for studies published between the 1 January 2009 up to the 8 July 2019. All original articles selected from databases were transferred to the EndNote X8 software to check the presence of duplicates. In a second screening phase, two investigators, working independently, selected the articles against the inclusion and exclusion criteria described in the section "Participants, Interventions, Comparators, " with a three steps process: (i) selection based on the titles; (ii) selection based on the abstract; and (iii) selection based on the full text. If there was disagreement between the two investigators, a third investigator took the final decision. A flow diagram that summarizes the selection process is reported in Figure 1.
The following information was extracted: first author, year of publication, sample size, participants' age (range, mean, and standard deviation), gender, aim of the study, active or sedentary, physical or sport activities practiced, jump method used, device employed, and main results. The information extracted from any section of the manuscript was consequently recorded in table format and descriptively summarized.
Following this stage, a descriptive analysis of the CMJ and SJ test technique was performed, and all common aspects between the VJs were considered.

Study Selection and Characteristics
After duplicate removal, title, abstract, and full-text comparison against inclusion and exclusion criteria, the number of publications was reduced from 18,903 to 1,860 with a final total of 119 articles being included. More details are provided in the diagram flow presented in Figure 1.
The 119 original studies were divided into subgroups as follows: Sixty-nine studies solely using the CMJ test to evaluate muscular strength in adolescents ( Table 1); Five studies solely using the SJ test to evaluate muscular strength in adolescents ( Table 2); Forty-five studies where both the CMJ and the SJ test were employed to evaluate muscular strength in adolescents ( Table 3).
A total of 9,940 individuals were considered. Of this 34% (3,373) were females, 57% (5,630) males, and for the remaining 9% (937) gender was not specified. The mean age was 15.2 years.

Study Characteristics for CMJ
There was no consistency in the description of the CMJ test method in the literature (Tables 1, 3). The protocol developed by  was the one most commonly adopted (Coelho et al., 2010;Boccolini et al., 2013;Takai et al., 2013;Alberti et al., 2014;Padulo et al., 2016a;Gallo-Salazar et al., 2017;Perroni et al., 2017;Sanchez-Urena et al., 2017;Borges et al., 2018). According to the instructions of this protocol, participants have to stay in an upright position before the execution of the VJ, which starts with a CM until the legs are bent down to 90 • . A more precise description, with information of the knee   angle during the standing position, the landing (180 • ), and the CM (reach approximately 90 • ) phases, is given by Fernandez- , cited one time (Nikolaidis and Knechtle, 2016). Yanci et al. (2016) and Ramirez-Campillo et al. (2018) cited the protocol of Maulder and Cronin (2005), giving general information on the take-off and the landing phases, which both had to be executed with extended knees and ankle joints.
Regarding the CM phase, the protocols of Cormack et al. (2008) allowed participants to self-select the CM depth. Information regarding the speed of the CM phases are given by Ortega et al. (2008a) and Chaouachi et al. (2014). In the protocol proposed by Maulder and Cronin (2005) participants were asked to "sink as quickly as possible" reaching a knee angle of approximately 120 • , which was similar to Ortega et al. (2008a) (Moraes et al., 2013;Tishukaj et al., 2017). Because of the use of the arms, it is different from the protocols previously described, as participants had to touch and mark a wall with their fingertips at a highest possible point. Finally, the protocol by Aragon-Vargas (2000) was the only report requesting that the CMJ be executed barefoot.

Study Characteristics for SJ
Likewise, no standardized jump method was detected for the SJ (Tables 2, 3). The protocol of  was used five times and required participants to perform the SJ from a half squat position with knees bent at 90 • , torso straight, and both hands on their waist (Coelho et al., 2010;Alberti et al., 2014;Padulo et al., 2015cPadulo et al., , 2016aBorges et al., 2018). Additionally, three studies Janeira, 2011, 2012;Hespanhol et al., 2013) used the 1983 protocol of Bosco (1994), but cited his work of 1994. The protocol of Lloyd et al. (2009) required the participants to take-off and land on the same spot. Furthermore, before the SJ test, some protocols instructed the participants to wait 4 (Maulder and Cronin, 2005), 3 (McGuigan et al., 2006), or 2 s (Lloyd et al., 2009) before executing the jump in order to control the assumed position. More information regarding the landing can be retrieved in the protocol of Fernandez- . According to these researchers, the knees had to be kept extended at an angle of 180 • and the ground contact during the landing had to be initiated with the toes. Straight legs in both take-off and landing was also used in the protocol of Maulder and Cronin (2005). Furthermore, according to Lloyd et al. (2009), the landing phase had to be performed with both legs fully extended while looking forward and, to maintain balance, to gaze at a specific point. Arms crossed against the chest was an instruction given by Secomb et al. (2015) citing the protocol of Hasson et al. (2004), while McGuigan et al. (2006) asked participants to hold a light weight (1.0 kg) over their shoulders.

Arm and Feet Information
Most of the CMJ and SJ tests were performed either with both hands positioned on the hips (number of articles = 41) or the waist (number of articles = 2). The arms placed in an akimbo position was also used in some works (number of articles = 4). Eight articles did not describe the hands/arms position and only stated that swinging of the arms was not permitted. However, an arm swinging movement was permitted in some CMJ-related research (number of articles = 16). A summary of information regarding the position of the upper limb for the CMJ and SJ is provided in Table 4.

Number of Jumps Performance Trials and Result Analysis
The number of trials proposed (Table 4) and the results taken for statistical analysis were either two (number of

Jump Phases Identification
The present review based the jump description and analysis according to the jump phases definition proposed by Bobbert et al. (1996): the starting position; the start of push-off; the toe-off; and the apex of the jump. Additionally, we identified the landing phase. For the CMJ, the majority of studies (number of articles = 30) agreed in defining the starting position as a standing posture. Some authors provided more details about the position of the lower limbs, suggesting that participants maintain straight legs (Lago-Penas et al., 2014) or position their feet shoulder-width apart (Holden et al., 2015).
The SJ starting position was described by 17 authors as a squat position with knees flexed at 90 • and by four authors as a semi-squatting position with knees bent at 90 • using a ruler as measurement (Maciejewski et al., 2018). Some researchers required participants to remain in the squat position for either 3 (Comfort et al., 2014;Dowse et al., 2017;Pino-Ortega et al., 2018) or 2 s (Maciejewski et al., 2018) before executing the second phase, on command, i.e., the jump phase.
The CMJ push-off is described as a downward movement without an indication of the depth (number of articles = 26). Some authors indicated that the knee angle had to reach 90 • (number of articles = 18) before starting the jump. A limited number of authors provided information regarding the speed of the downward movement, i.e., that it had to perform with a rapid descend (Alvarez-San Emeterio and Gonzalez-Badillo, 2010;Alvarez-San Emeterio et al., 2011;Negra et al., 2016Negra et al., , 2017Dowse et al., 2017). Regarding the push-off phase for the SJ, most studies performed the jump without a CM (number of articles = 14).
The toe-off phase was described as a maximal effort, i.e., as high as possible (29 works related to the CMJ and 11 works related to the SJ).
For the apex of the jump phase, a requirement of both the CMJ and the SJ was that the participants maintain extended legs (number of articles = 11).
Likewise, to provide reliable results during the execution of the jump landing, standardization is required (Borras et al., 2011). Descriptions of the landing phase were similar for the CMJ and the SJ, with most works reporting a fully extended knee landing (number of articles = 8). Similarly, six works required participants to land without legs flexed. Landing with the toes on the same spot as the take-off (number of articles = 4) and in an upright position (number of articles = 1) were other variants of the instructions given to the participants.
To emphasize the use of the leg extensors, participants were asked to maintain the torso in an upright position (Moliner-Urdiales et al., 2010;Cortis et al., 2011).
A summary of information regarding each CMJ and SJ phase is provided in Table 5.

DISCUSSION
The main finding of this review is that results and recommendations for both the CMJ and the SJ published in the literature are derived using a vast variety of testing methods and devices to evaluate lower body muscular strength in adolescents. It is questionable, therefore, whether results and, where applicable, normative jump height values, recommended for adolescents and used to assess PF are comparable. Therefore, it is necessary to create SOPs for CMJ and SJ tests that can be used in the context of health promotion and health investigations.
With this in mind, we recommend participants start the CMJ from an erect standing position with a straight torso, knees fully extended, with hands-on-hips and feet shoulderwidth apart. We also recommend maintaining this position for at least 2 s before the descending phase. The CMJ push-off phase should be characterized by a downward movement until the knee angle reaches 90 • and this should be visually inspected by the examiner and where possible, the use of accelerometer that emits audio feedback when the angle is reached (Cular et al., 2018). Instructions for the toe-off phase should explicitly state that it has to be performed with a maximal effort. Furthermore, during the apex of the jump, participants have to keep their legs fully extended. Finally, the landing phase has to occur with both feet together and with knees fully extended.
Regarding the measurement device, a photoelectric system is the most commonly employed technology, is less costly, and is very user-friendly. We, therefore, recommend it a part of the SOPs. Furthermore, the equation H = g * t 2 /8 [H: VJ height (m); t: flight time (s); g is 9.81 m/s 2 ] presents high coefficients of determination in the prediction of the VJ height (Attia et al., 2017), and is consequently suggested. Five jumps should be performed during the testing session for both CMJ and SJ, with a 1-min passive rest between jumps to ensure muscular recovery. Due to possible learning effects and consequently higher jump performances, only the best jump should be used for further analysis. Furthermore, we recommend starting the CMJ, and SJ testing session with a standardized warm-up as this can influence jump performance (i.e., a short warm-up can improve the jump height, while a high-intensity plyometric protocol deteriorates the performance) (Romero-Franco and Jimenez-Reyes, 2017). Stretching also seems to potentially cause injury rather than prevent it (Shrier, 1999). Our advice is to perform the same standardized warm-up protocol before any VJ. An example of a suitable warm-up protocol has been suggested by Pinfold et al. (2018). This comprises of two sets of the following exercise: (a) standing on one leg and nod head gently for 30 s; (b) single leg airplane squat with hip thrust (20 repetitions); (c) single leg airplane squat with trunk rotation (20 repetitions); (d) single leg airplane squat with a black theraband resistance applied to the knee that includes trunk rotation with a dumbbell held in hand (10 repetitions); (e) monster walk with a black theraband resistance positioned around the forefoot, forward, and backward (3 m each way); (f) monster walk with a black theraband positioned around the forefoot, side-to-side, i.e., left and right (3 m each way). A summary of the first part of the section "Discussion" can be seen in Table 6.
Concerning the proposed SOPs testing method, the upright position of the torso during the starting position phase for both jumps emphasizes the use of the leg extensors (Moliner-Urdiales et al., 2010;Tounsi et al., 2015). Importantly, this upright position prevents the inclination of the torso segment, a common mistake during the jump performance. In the case of a reduction of forwarding torso inclination by 50%, this can result in a 13% increase of the maximal power (Vanrenterghem et al., 2008). In contrast, hip extensors, upper body, and thigh muscles reduce their contribution on the jump performance when the torso is in a vertical position during the push-off phase, and the plantar flexors contribute mainly to the positive work while knee and hip joint muscles cannot contribute to this positive work (Kopper et al., 2012). In a simulation model, Blache and Monteil (2014) demonstrated that a non-consideration of the erector spinae muscle contribution resulted in a ∼15% reduced SJ height and, if a torso inclination was restricted, the anticipated movement and higher knee joint torque development was possible which resulted in a higher maximal power (Vanrenterghem et al., 2008). A standardized position of the knee angle in the SOPs for the CMJ and the SJ is required during the push-off phase, due to the impact it can have on either increasing or decreasing jump height (Krahenbuhl and Pangrazi, 1983;Gheller et al., 2015) caused by the hip and ankle working differently (Hara et al., 2006(Hara et al., , 2008. The execution of the VJ with a lower knee angle compromises the jump performance as a deeper squat starting position results in a higher jump, maximum force, and power output (Gheller et al., 2015). For this precise reason, the SJ has to be carefully monitored. In addition, participants tend to perform a small-amplitude CM (Bobbert et al., 1996;Hasson et al., 2004) and jumps with a CM should be discarded. As the SJ is a purely explosive VJ, some researchers (Hasson et al., 2004;Fitzgerald et al., 2018) argued that the SJ could not be influenced, and consequently performing the SJ with a standardized knee angle or a self-selected jump, apparently present no meaningful difference (Fitzgerald et al., 2018). Based on the discussion above, we strongly advocate the need for the standardized starting position of a 90 o knee flexion, feet shoulder-width apart, hands-on-hips, and with a straight torso. Furthermore, this standardized arm position avoids the contribution of the upper limbs as well as coordinative issues as a confounding variable which, as a result of the shoulder, elbow, hip, and ankle muscles working together, can impact on jump performances between 8 and 11% (Harman et al., 1990;Lees et al., 2004;Hara et al., 2006Hara et al., , 2008. Also, the instruction to maintain both legs fully extended starting from the toe-off to the landing phase (i.e., for the entire duration of the apex of the jump phase) is of crucial importance as this can affect the accuracy of the flight time (Borras et al., 2011). Likewise, the landing phase also has to be standardized to obtain equal results during the execution of the jumps (Borras et al., 2011). According to Bui et al. (2015), there are different factors such as the landing with the feet nearly flat or with the legs bent that can alter the flight time, altering the calculated jump height.
Therefore, it is important to land with straight legs, on the forefoot (Bui et al., 2015) and at the same time to amortize the movement because a stiffer technique increases the risk of injuries (Aerts et al., 2013).
Regarding the devices used, a video analysis technique that measures the displacement of the center of body mass from the standing position to the highest vertical point has been proposed as a gold standard (Aragon-Vargas, 2000). However, this specialized equipment is costly, difficult to calibrate, and transport but also requires a complex procedure to obtain data (Aragon-Vargas, 2000). Less expensive, easy to use devices are contact mats which detect the flight time. These have been reported to produce highly reliable and valid results (Markovic et al., 2004). Force plates derive jump height from the flight time, and they can measure the velocity at takeoff (Mcgown et al., 1990;Kibele, 1998;Lara et al., 2006). Furthermore, force plates accurately assess ground reaction forces, and can thus provide a preferred solution in pediatric populations (Fricke et al., 2006). Compared to force plates, photoelectric cell systems present a similar level of validity and they provide excellent test-retest reliability for the estimation of the jump height (Glatthorn et al., 2011). Vertec devices (Sports Imports, Hilliard, OH, United States) are also valid (Leard et al., 2007) and reliable (Young et al., 1997) even though not recommended to use for different reasons. Firstly, the measurement device consists of a metal stand and a height scale composed of color-coded vanes that are displaced by the participant when jumping (Klarova, 2000) which requires the use of the arms. Secondly, and in contrast to force plates, the Vertec device (Sports Imports, Hilliard, OH, United States) does not demonstrate an accurate representation of jump height (Buckthorpe et al., 2012). Claudino et al. (2017) analyzed the average of five CMJs, but, as opposed to the researchers that use jumps to investigate fatigue or super-compensation effects, we recommend to perform the same number of jumps and to use only the best performance.
The proposal of creating SOPs is supported in the literature by researchers who attempted to develop normative gender or country-related VJ test data (Taylor et al., 2010;Holden et al., 2015;Ramirez-Velez et al., 2017). However, considering the different factors that influence maximal jump height performance (e.g., different test methods or assessment criteria), it is unclear whether results are generally comparable with other populations. Furthermore, Claudino et al. (2017) proposed the CMJ to monitor the neuromuscular status using the average of five jump heights but different jumping methods thus compromising the possibility of comparing the VJ. The above further highlights the need for the development of SOPs, which offers researchers a more rigorous and robust test approach. Indeed, it has to be considered that the developed CMJ test protocol by  was used in only 5 out of the reviewed 102 original works ( Table 4). A similar situation applies to the SJ test whereby the two developed protocols by  and Bosco (1994) were fully replicated in only 3 of the considered 46 works ( Table 5).

Strengths and Limitations
The strength of this review is the stipulation of SOPs for both the CMJ and the SJ test to facilitate the evaluation of the lower limb muscular strength, in a public health context, for adolescents. If these are followed, future communications, sharing of data, result comparisons, and the development of normative data could be made easier and, importantly, these procedures should be more effective in assessing adolescents' PF. With such a vast variety of testing methods and measurement devices used, results, in fact, are not comparable which present a significant limitation of this review as it was not possible to perform a meta-analysis. The argument above is the rationale for the choice of a quantitative analysis approach of this review. Future works are recommended to review normative data using the stipulated SOPs. A second limitation of the present work is that, due to the mixed samples within the studies analyzed, gender was not considered. Future works should therefore extend their investigation to males, females, and other age groups.

CONCLUSION
The present review considered the variety of CMJ and SJ testing method procedures published in the literature, making it impossible to identify standard procedures. Consequently, SOPs for both CMJ and SJ tests have been provided in Table 6 and these are strongly recommended to researchers and health practitioners alike. It is, however, always preferred to study the context first before proposing one protocol over another, especially in the context of sporting performance.

AUTHOR CONTRIBUTIONS
LP, APal, and AB developed the research concept and study design. LP, BK, and GM performed the literature review, and data analysis and interpretation. LP and APal performed the data collection. LP, BK, GM, APao, and GD wrote the manuscript. All authors contributed to the revision and approved the submitted version of the manuscript.

ACKNOWLEDGMENTS
We would like to thank Dr. Ewan Thomas and Ph.D. students Ambra Gentile, Kaltrina Feka, and Nemanja Lakicevic of the Ph.D. Program in "Health Promotion and Cognitive Sciences" of the University of Palermo for their help in the manuscript revision. The present work has been presented in the "X Congresso della Società Italiana delle Scienze Motorie e Sportive (SISMES)" in Messina, Italy.