Explosive Push-ups From Popular Simple Exercises to Valid Tests for Upper-Body Power.

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EXPLOSIVE PUSH-UPS: FROM POPULAR SIMPLE EXERCISES TO VALID TESTS FOR UPPER-BODY POWER DALENDA ZALLEG,1 ANIS BEN DHAHBI,2,3 WISSEM DHAHBI,4,5,6 MAHA SELLAMI,7 JOHNNY PADULO,7,8 ˇ MAROUEN SOUAIFI,5 TEA BESLIJA ,7 AND KARIM CHAMARI9 1

Research Unit, Sportive Performance and Physical Rehabilitation, High Institute of Sports and Physical Education of Kef, University of Jendouba, Kef, Tunisia; 2Faculty of Sciences, Department of Physics, University Tunis El Manar, Tunis, Tunisia; 3 Physics Department, College of Science and Arts at ArRass, Qassim University, ArRass, Saudi Arabia; 4Tunisian Research Laboratory “Sport Performance Optimisation,” National Center of Medicine and Science in Sports (CNMSS), Tunis, Tunisia; 5 Training Department, Qatar Police College, Doha, Qatar; 6Tunisian National Guard Commandos School, Oued Zarga, Tunisia; 7Faculty of Kinesiology, University of Split, Split, Croatia; 8University eCampus, Novedrate, Italy; and 9Athlete Health and Performance Research Center, ASPETAR, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar ABSTRACT Zalleg, D, Ben Dhahbi, A, Dhahbi, W, Sellami, M, Padulo, J, ˇ Souaifi, M, Beslija, T, and Chamari, K. Explosive push-ups: From popular simple exercises to valid tests for upper-body power. J Strength Cond Res 34(10): 2877–2885, 2020— The purpose of this study was to assess the logical and ecological validity of 5 explosive push-up variations as a means of upper-body power assessment, using the factorial characterization of ground reaction force-based (GRF-based) parameter outputs. Thirty-seven highly active commando soldiers (age: 23.3 6 1.5 years; body mass: 78.7 6 9.7 kg; body height: 179.7 6 4.3 cm) performed 3 trials of 5 variations of the explosive push-up in a randomized-counterbalanced order: (a) standard countermovement push-up, (b) standard squat push-up, (c) kneeling countermovement push-up, (d) kneeling squat push-up, and (e) drop-fall push-up. Vertical GRF was measured during these exercises using a portable force plate. The initial force-supported, peak-GRF and rate of force development during takeoff, flight time, impact force, and rate of force development impact on landing were measured. A significant relationship between initial force-supported and peak-GRF takeoff was observed for the countermovement push-up (CMP) exercises (standard countermovement push-up, kneeling countermovement push-up, and drop-fall push-up) and squat push-up (SP) exercises (standard squat push-up and kneeling squat push-up) (r = 0.58 and r = 0.80, respectively; p , 0.01). Furthermore, initial force supported was also negatively correlated to a significant degree with flight time for both CMP and SP (r = 20.74 and r = 20.80; p , 0.01, respecAddress correspondence to Wissem Dhahbi, wissem.dhahbi@gmail. com. 34(10)/2877–2885 Journal of Strength and Conditioning Research Ó 2018 National Strength and Conditioning Association

tively). Principal component analysis (PCA) showed that the abovementioned 6 GRF-based variables resulted in the extraction of 3 significant components, which explained 88.9% of the total variance for CMP, and 2 significant components, which explained 71.0% of the total variance for SP exercises. In summary, the PCA model demonstrated a great predictive power in accounting for GRF-based parameters of explosive push-up exercises, allowing for stronger logical and ecological validity as tests of upper-body power. Furthermore, it is possible to adjust the intensity level of the push-up exercise by altering the starting position (i.e., standard vs. kneeling).

KEY WORDS assessment, force platform, muscle strength, stretch-shortening cycle, weight-bearing exercise

INTRODUCTION

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he “push-up” is a weight-bearing exercise, in which the practitioner uses his or her body weight (BW) as a load to stimulate upper limb and torso musculature (13). Push-up exercises represent a very popular method for upper-extremity training, rehabilitation, and muscular endurance assessment (14,23,28). Their popularity can be attributed to the fact that they involve many variations in terms of technique and require little or no equipment, whereas the necessary skill is easy to learn and exercise intensity can be easily changed (6). Recent studies have shown that push-up training increases maximum dynamic force (4), joint compressive forces and joint congruency (stability) (3), and power (32), with a reduction in shear forces. In addition, it improves proprioception (34), dynamic stability, and muscle recruitment (31,35) while ensuring good neuromuscular coordination (23). Push-up training has also been shown to increase upper-body strength and power (4,15,17) when performed in a ballistic manner while using kneeling or standard position variations. VOLUME 34 | NUMBER 10 | OCTOBER 2020 |

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Face Validity of Explosive Push-ups Tests Several studies have been conducted with a focus on quantifying how modifications to body postures and the incorporation of explosive components can affect force production during push-ups (9,12,15,16,20). Some studies (9,15,20,27) have conducted kinetic analyses of explosive push-ups to quantify their intensities and include them in training programs as power exercises for the upper body. On the other hand, in only 3 recent studies (16,33,36) were explosive push-ups used as “tests of upper-body power.” These studies (16,33,36) did not only analyze the kinetic indices’ outputs of explosive push-ups but also attempted to calculate the mechanical power outputs of the upper limbs. However, their results (16,33,36) should be interpreted with caution, as the calculation method used to find the mechanical power outputs has been the subject of criticism (8,36). With the exception of Wang et al. (36), no study has assessed the validity of explosive push-ups as upper-body power tests using ground reaction force (GRF)based parameter outputs as the goal standard. In addition, previous methods have been criticized in the scientific literature (8) because their respective studies (33,36) typically used one force platform to quantify the GRF applied to the hands. Furthermore, when the GRF applied to the feet is unknown, the velocity of the body cannot be quantified to calculate power. As a consequence, the ways in which different push-up exercises may affect power production are unclear (8,16). The “face validity” (i.e., the assessment of whether a test measures what it intends to measure (7)) of the power output calculation methods of explosive push-ups has been questioned and widely criticized (8,9,36). However, other studies (9,12,15,20) have based their work on kinetic indices other than the mechanical power output calculation (e.g., initial force supported, peak-GRF takeoff, rate of force development during takeoff, flight time, impact force, and rate of force development impact) to determine the intensity and specificity of these type of exercises. This method (e.g., the one based on kinetic indices) seems more solid in terms of face validity, but its disadvantage is that it is based on many GRF-based parameters, which complicate the interpretation of the results for professionals in the field. As such, this method may fall into the redundant use of kinetic indices. Among the best solutions to reduce these disadvantages is principal component analysis (PCA), a statistical approach used to model and highlight selected data. The main advantage of PCA is that it facilitates the illustration of good models and a reduction in the variables and the number of dimensions, with a minimum loss of information (25). Therefore, the PCA method was selected for the characterization of the 5 explosive push-ups used in this investigation. The 5 “original” exercises that were analyzed in this study were inspired by the lower-limb explosive exercises (e.g., squat jump, countermovement jump, and drop jump). Analysis was performed on 2 classes of explosive push-ups (i.e., countermovement push-up [CMP] and squat push-up [SP] exercises). Hence, the objective of this study was to (a) investigate the logical and ecological validity of these 5 variations in explosive push-ups as tests of upper-body power

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by understanding the interaction between GRF-based parameter outputs, (b) extract the principal components of kinetic variables associated with the takeoff, airborne, and landing states, and finally (c) recommend the best context for the use of each exercise/test.

METHODS Experimental Approach to the Problem

This study included the same group of participants, design, and procedures described in the previous investigation undertaken by Dhahbi et al. (9). This study was a cohort-based, counterbalanced, repeated measures study that described the intensity of the following 5 push-up exercises: standard countermovement push-up (SCPu), standard squat push-up (SSPu), kneeling countermovement push-up (KCPu), kneeling squat push-up (KSPu), and drop-fall push-up (DFPu). These movements are described in detail below and depicted in Figures 1 and 2. These 5 exercises were grouped into 2 categories: CMP exercises, consisting of SCPu, KCPu, and DFPu; and SP exercises, consisting of SSPu and KSPu. All data were collected during a single session. Subjects

Thirty-seven antiterrorism soldiers from the Tunisian National Commandos School voluntarily participated in this study (age: 23.27 6 1.52 years [range 21 to 29 years]; body mass: 78.68 6 9.67 kg; body height: 179.68 6 4.27 cm; body mass index: 24.36 6 2.74 kg$m22; body fat percentage: 18.38 6 3.46%; experience in the commandos unit: 2.17 6 1.56 years; maximum repetitions of 60minute standard push-up test: 53.13 6 6.02 repetitions; and one repetition maximum bench press: 80.68 6 9.49 kg, all measurements mean 6 SD). An inclusion criterion was that participants had successfully completed the 14to 15-week training cycle of the second-level commando training courses at the Commandos National Guard School. All participants were highly active throughout their regular training in preparation for the demands of their job. Participants trained for approximately 32 h$wk21 (divided into ;14 h$wk21 for fitness training, including exercises similar to the bench press, push-ups or medicine ball push-ups, and ;18 h$wk21 dedicated to technical and tactical training). They belonged to the same environment and adhered to the same diet regimen throughout the experimental period. Participants were free from any injury or pain that would have prevented maximal effort from being exerted during testing. They all gave their written informed consent to participate in the study after receiving a thorough explanation of the study’s protocol. The protocol conformed to internationally accepted policy statements regarding the use of human participants in accordance with the Declaration of Helsinki and was approved by the Ethics Committee for Human Research (ECHR) of the National Guard Commandos School Committee of Tunisia.

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Figure 1. Vertical ground reaction force time recorded during the execution of countermovement push-up exercises (e.g., SCPu, KCPu, and DFPu). SCPu = standard countermovement push-up; KCPu = kneeling countermovement push-up; DFPu = drop-fall push-up; F0 = initial force supported; PGRF-T = peak-GRF takeoff; RMFD = rate of force development during takeoff; FT = flight time; if = impact force; RIFD = rate of force development impact.

Procedures

Participants performed no strength training ;48 hours before data collection. No nutritional or hydration control was implemented because of the repeated measures design of the study. Data were collected at approximately the same time of day (between 9.00 and 11.00 AM ) and indoors under the same environmental conditions (light: ;500 lux, temperature: 13–168 C, and humidity: 52–76%). All measurements were strictly controlled by the same expert. A familiarization session for all participants was performed 1 week before baseline testing. During this session, participants were informed of the protocol for each of the exercises examined in the study (9,15). Before testing, participants performed a 15minute warm-up (,60% V_ O 2 max), which included circumduction and flexion/extension of the upper limbs with self-selected intensity and dynamic stretching (pectorals, trapezius, elbow flexors and extensors, and wrist flexors and extensors) (10). Participants were also allocated 10 minutes so that they could refamiliarize themselves with the exercises, performing 3 trials for each

exercise during this phase. Furthermore, qualitative observation by a second expert confirmed the proper technique. After 2 minutes of rest, participants performed 3 repetitions of each push-up variation in a randomized order with 2 minutes of rest between each exercise. Standard Countermovement Push-up. From the “up” standard position, participants descended by flexing their elbows until the elbows flexed at approximately 908 (9). They immediately extended their arms to propel the upper body as high as possible. They were requested not to flex their elbows during the flight phase. Standard Squat Push-up. From the “up” standard position, participants descended by flexing their elbows until the elbows were at an angle of approximately 908 and then maintained this position for z1 second (9). When their body was completely stabilized in this motionless position, they propelled their upper body as high as possible without performing a countermovement. VOLUME 34 | NUMBER 10 | OCTOBER 2020 |

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Face Validity of Explosive Push-ups Tests

Figure 2. Vertical ground reaction force time recorded during the execution of squat push-up exercises (e.g., SSPu and KSPu). SSPu = standard squat pushup; KSPu = kneeling squat push-up; F0 = initial force supported; PGRF-T = peak-GRF takeoff; RMFD = rate of force development during takeoff; FT = flight time; if = impact force; RIFD = rate of force development impact.

(under the effect of gravity). They lowered their bodies until the elbows flexed at approximately 908. They immediately extended their arms in the same manner as the KCPu. For all exercises, on landing, the participants were requested to reduce the impact as much as possible and Kneeling Squat Push-up. This was performed in the same resume the starting position. To help achieve maximum manner as the SSPu except with support from the knees reproducibility, marks were placed on the force plate on instead of the feet (9). which participants were required to position their hands. The distance between the marks was equivalent to the Drop-Fall Push-up. Participants knelt with their shoulders in participants’ shoulder width. Participants always carried out antepulsion (908), with elbows extended and palms facing forward (9). They then allowed themselves to fall forward the tasks by resting their hands on the force plate, whereas the remaining supports (feet/ knees) were positioned on another rigid surface placed at the same level as the force TABLE 1. Descriptive force time variables for all subjects (N = 37).*† plate. Participants were Countermovement push-up encouraged to carry out all atVariables exercises Squat push-up exercises tempts with maximum intensity while trying to soften the 0.53 6 0.04 (0.52–0.61) 0.58 6 0.05 (0.56–0.59) F0 (BW) impact to their hands (15). The PGRF-T (BW) 1.12 6 0.13 (1.08–1.17) 0.93 6 0.10 (0.90–0.97) 2 best attempts from the highRMFD 2.28 6 0.98 (1.93–2.81) 0.45 6 0.38 (0.32–0.57) (BW$s21) est values of flight time were FT (s) 0.48 6 0.13 (0.44–0.53) 0.36 6 0.09 (0.33–0.39) averaged for the subsequent IF (BW) 1.55 6 0.59 (1.35–1.75) 1.65 6 0.56 (1.47–1.84) analysis. Data were also nor21 RIFD (BW$s ) 42.15 6 32.53 (31.81–54.10) 48.41 6 49.53 (31.90–64.93) malized to BW (20). Kneeling Countermovement Push-up. This was performed in the same manner as the SCPu except with support from the knees instead of the feet (i.e., the kneeling position) (9).

*F0 = initial force supported; BW = body weight; PGRF-T = peak-GRF takeoff; RMFD = rate of force development during takeoff; FT = flight time; IF = impact force; RIFD = rate of force development impact. †Values are given as mean 6 SD (CI 95%).

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Instrumentation and Data Reduction. The force applied by the hands during each push-up exercise was measured using

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Journal of Strength and Conditioning Research a portable force plate (9290AD; Kistler, Winterthur, Switzerland) operating at 500 Hz. The force platform was calibrated using known loads before each testing session. Data were recorded, with only the vertical (z-axis) data stored (QJ software, version 1.0.9.2) for later analysis. Data reduction was conducted using ROOT-based scripts (an object-oriented program and library developed by the European Organization for Nuclear Research, or “CERN,” for data analysis and plotting, written in C++) to compute kinetic variables. Specific GRF-related parameters included initial forcesupported, peak-GRF takeoff, rate of force development during takeoff, flight time, impact force, and rate of force development impact. These variables were defined and calculated in accordance with previous research and anecdotal recommendations (9,11,20) (Figures 1 and 2). Statistical Analyses

Data analyses were performed using SPSS version 23.0 for Windows (SPSS, Inc., Chicago, IL, USA). Means and SDs were calculated after verifying the normality of distributions using the Kolmogorov–Smirnov procedure. Pearson’s correlation coefficient estimations were calculated to assess the strength of relationships between the kinetic parameters for each category of exercises. Moreover, PCA was performed to identify the principal component summarizing the 6 variables using the procedure described by Kollias et al. (21). The number of principal components in the pattern matrix

TABLE 2. Principal component analysis for the countermovement push-up exercises (e.g., SCPu, KCPu, and DFPu) including factor loadings, commonalities, and eigenvalue for each variable and percentage of variance for each rotated component.*† Factor loadings Variables

1

2

3

F0 0.92 FT 20.91 RIFD 0.96 IF 0.94 RMFD 0.92 PGRF-T 0.76 Eigenvalue 2.33 1.68 1.33 Percentage of 38.75 28.03 22.11 variance

Commonalities 0.90 0.84 0.93 0.93 0.88 0.86

*SCPu = standard countermovement push-up; KCPu = kneeling countermovement push-up; DFPu = drop-fall push-up; F0 = initial force supported; FT = flight time; RIFD = rate of force development impact; IF = impact force; RMFD = rate of force development during takeoff; PGRF-T = peak-GRF takeoff. †Factor loadings lower than 0.6 were not included in the table.

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TABLE 3. Principal component analysis for the squat push-up exercises (e.g., SSPu and KSPu) including factor loadings, commonalities, eigenvalue for each variable, and percentage of variance for each rotated component.*† Factor loadings Variables

1

F0 FT PGRF-T RMFD RIFD IF Eigenvalue Percentage of variance

2

Commonalities

0.90 20.86 0.81 0.91 0.85 2.88 1.38 48.01 23.01

0.90 0.74 0.74 0.27 0.83 0.78

*SSPu = standard squat push-up; KSPu = kneeling squat push-up; F0 = initial force supported; FT = flight time; PGRF-T = peak-GRF takeoff; RMFD = rate of force development during takeoff; RIFD = rate of force development impact; IF = impact force. †Factor loadings lower than 0.6 were not included in the table.

extracted by the PCA was chosen with an eigenvalue greater than 1 (Kaiser criterion). The original matrix was rotated to extract the appropriate variables using a normalized VARIMAX rotation. Significance for all the statistical tests was accepted at p # 0.05.

RESULTS Data were normally distributed (Kolmogorov–Smirnov p = 0.17–0.78). The descriptive GRF-based parameters collected from 5 types of explosive push-ups are shown in Table 1. For the Countermovement Push-up Exercises

Initial force supported was significantly correlated with peak-GRF takeoff, flight time, and impact force (r = 0.58, r = 20.74, and r = 0.33; p , 0.001, respectively). Peak-GRF takeoff was also weakly correlated to a significant extent with rate of force development during takeoff (r = 0.48; p , 0.001) and flight time (r = 0.31; p = 0.001). Impact force was highly correlated with rate of force development impact (r = 0.83; p , 0.001). For the Squat Push-up Exercises

Initial force supported was significantly correlated with peak-GRF takeoff (r = 0.80; p , 0.001), rate of force development during takeoff (r = 0.25; p = 0.033), impact force (r = 0.43; p , 0.001), and rate of force development impact (r = 0.23; p = 0.046). However, it was negatively correlated to a significant degree with flight time (r = 20.80; p , 0.001). VOLUME 34 | NUMBER 10 | OCTOBER 2020 |

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Face Validity of Explosive Push-ups Tests Peak-GRF takeoff was correlated significantly with flight time (r = 20.57; p , 0.001) and impact force (r = 0.39; p = 0.001). The latter was also positively correlated with rate of force development impact (r = 0.62; p , 0.001). For the CMP exercises, the PCA of the 6 variables resulted in the extraction of 3 significant components. The first rotated component explained 38.8% of the total variance in the 6 variables, whereas the second rotated component explained 28.0% and the third rotated component explained 22.1% (Table 2). All of the principal components explained 88.9% of the total variance in the 6 variables. For the SP exercises, the analysis revealed the existence of 2 factors or principal components (Table 3). The first rotated principal component accounted for 48.0% of the total variance in the force data. The second rotated principal component accounted for 23.0% of the total variance. All principal components explained 71.0% of the total variance in the 6 variables.

DISCUSSION The explosive force generated by the upper extremities is an indispensable physical quality for various sports and professions, especially for commandos. Various studies have quantified the vertical GRF-based parameters that are generated when carrying out different explosive exercises involving the lower limbs (11,24). The aim of the present investigation was to examine the scientific legitimacy of using the 5 variations in explosive push-ups as tests of upper-body power. This is indeed a field where little is known (9,12,15,20).The main findings confirmed a significant relationship between flight time and initial force supported for the CMP and SP exercises. The peak-GRF takeoff during the CMP exercises was significantly (a) positively correlated with flight time, but (b) negatively correlated during the SP exercises. In addition, PCA revealed that the aforementioned 6 GRF-based variables resulted in the extraction of 3 significant components for the CMP and 2 significant components for the SP exercises. “A test has face validity or logical validity when it obviously measures the desired skill or ability” (7). For tests of basic athletic abilities, face validity is desirable, based on the assumption that anyone taking a test of physical ability wants to do well and is thus motivated by a test that appears to measure a relevant capability (5). The construct to be measured in this study is power, or, more precisely, a select expression of upper-body power as determined by explosive push-ups. The stretch-shortening cycle (SSC) is a muscletendon complex mechanism, which allows for muscle power and energy economy enhancements compared with an isolated concentric contraction (29). During the first contact phase of a jump (i.e., countermovement and push-off ), the center of mass comes from a zero vertical velocity at the end of the eccentric phase (i.e., countermovement), reaching a maximum vertical velocity at the moment when hands leave the ground (i.e., takeoff ) (2,9). The premise is to

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develop significant power during the concentric phase. The performance of SP exercises refers to jumping ability and the explosive (maximal) force production of the upper limb as a whole, i.e., the power of shortening, motor unit recruitment, and coordination. The CMP starts from the extended elbow position to achieve negative work before the phase work. However, the absence of initial velocity makes these exercises somewhat different than push motions performed in sport. The DFPu, which involves a drop from a height before making a maximum jump, can produce a maximum jump of the SSC type with an initial velocity (6). The performance of this exercise is linked to the structure of muscle fibers, the number of motor units recruited, and the coordination between muscles, proprioception, and the stretch reflex (9,22,29). It also depends on the passive structures of the upper limbs, which, by means of their mechanical properties, allow for the return of elastic energy gained during the eccentric phase by their deformation. For this reason, the explosive push-up exercises (CMP and SP), as described, would be considered as logically valid tests of upper-body power. The analyzed exercises in this study are derived from explosive exercises (e.g., squat jump, countermovement jump, and drop jump) used to develop and assess lowerlimb explosiveness. However, with these exercises, the weight supported by the arms can vary. A significant relationship between initial force supported and peak-GRF takeoff was observed for the CMP and SP (r = 0.58 and r = 0.80, respectively). Furthermore, as initial force supported gives an indication of the mass being moved during the push-up, the results support this explanation of mass affecting flight time, as the initial force supported is significantly and negatively related to flight time for both the CMP and SP (r = 20.74 and r = 20.80, respectively). By analogy with studies examining power measurements in the lower limbs, flight time is a representative index of upper-limb power output during the vertical jump (11,19,24,26). Thus, these results have called into question the optimization of the supported load in reaching the maximum flight time (9), although peak-GRF takeoff could be used to estimate flight time or jump height. In this investigation, this was confirmed by the correlation coefficients (0.31 and 20.57 for CMP and SP, respectively) between flight time and peak-GRF takeoff, which indicated that only 10 and 32% of the variance in peak-GRF takeoff could be explained by the change in flight time for CMP and SP, respectively. The muscles of the upper limbs rapidly contract in the propulsive phase to generate force. This rapid contraction of the involved muscles stretches the series elastic elements, especially the tendons. Before takeoff, the muscles contract quasi-isometrically, while the tendon recoils rapidly, increasing the power output of the related muscles (1,30). However, the impulse (force and time) generation should also affect flight time. As the velocity of the movement decreases in line with the greater body mass supported, the explosive push-ups performed in

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Journal of Strength and Conditioning Research the kneeling position should have a greater takeoff velocity and flight time. Impulse is also influenced by the mass and the time of application. However, to produce maximal power output (i.e., maximal velocity 3 optimal load (18,19)), the load can be optimized by simple changes in the posture adopted (hands, lower limbs, or both). To the best of our knowledge, only 4 studies (9,15,17,20) have investigated the rate of force parameters in the kinetic analysis of push-ups. The rate of force development indices provides the power that each exercise can generate, as well as the muscular stress during the coupling of concentric/ eccentric contractions (loading/propulsion). In addition, peak-GRF takeoff and rate of force development during takeoff provide practical information about the explosive muscular force production required in each explosive exercise (11). For both CMP and SP, a significant relationship was found between impact force and rate of force development impact (r = 0.83 and r = 0.62, respectively). This information seems to indicate that falling on the hands is a way to train muscle power for the upper limbs. Principal component analysis can be a valuable tool for selectively limiting the highly interrelated force variables used to assess explosive intensity to a smaller number of factors to explain the same amount of data variance. Consequently, PCA can group together highly interrelated predictor variables at no risk of losing important information, eliminating the burden of dealing with too many variables (21). For CMP exercises, the PCA model consisting of 3 principal components accounted for 88.9% of the total variance in the 6 GRF-based parameters selected as critical factors for assessing kinetic performance. Variables such as initial force-supported loaded highly on the first factor, with factor loading of 0.92 (Table 2). Flight time also showed a high but negative factor loading (20.91) on the first component, indicating a high yet significant relationship to initial force supported. A negative correlation of flight time with initial force supported indicates that a long flight phase resulted in a low mass supported. The second rotated principal component was identified with the impact force characteristics of the explosive push-ups. Rate of force development impact and impact force loaded highly on the second component (factor loadings of 0.96 and 0.94, respectively). The third rotated component highly linked rate of force development during takeoff (r = 0.92) and peak-GRF takeoff with moderate loading (r = 0.76). This was identified with the propulsive force characteristics of the explosive push-ups. In sum, 100% of force variables were approximated by the principal components model, as indicated by the high communality scores, which ranged between 0.84 for flight time and 0.93 for impact force. For SP exercises, the analysis revealed the existence of 2 factors or principal components, which accounted for 71.0% of the total. Variables such as initial force supported and peak-GRF takeoff loaded highly on the first factor, with loadings of 0.90 and 0.81, respectively (Table 3). Flight time also showed a high but negative factor

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loading (20.86) on the first component, indicating a high yet significant relationship with initial force supported, as was the case for CMP exercises. The second rotated principal component was identified with the impact force characteristics of the explosive push-ups. Rate of force development impact and impact force loaded highly on the second component (factor loadings of 0.96 and 0.94, respectively). Five of the 6 GRF-based parameters were approximated by the principal components model, as indicated by the high communality scores, which ranged between 0.74 and 0.90 for flight time. On the other hand, communality for rate of force development during takeoff was close to 0.27, suggesting a low prediction of this variable by the 2 principal components. For this reason, rate of force development during takeoff during propulsion was not considered in the subsequent interpretation of the present findings. Although the PCA model presented here demonstrated a great predictive power in accounting for the GRF-based parameters of explosive push-ups, it should also be noted that it has been used to examine an overly restrictive type of explosive pushup. There is no doubt that the same model would need to be validated for other push-up variations. Providing a range of 5 upper-body power exercises or tests with different kinetic characteristics (e.g., initial force supported, peak-GRF takeoff, rate of force development during takeoff, flight time, and impact force) (9) allows a coach to choose the exercise that is the most adaptable and specific to the training target. Based on the findings of Dhahbi et al. (9), using kneeling push-up exercises (e.g., KCPu, KSPu, and DFPu) is beneficial in reducing the initial load that trainees have to move, as well as a means of changing the technique used in the standard push-up (SCPu and SSPu) when an increase in load is required. In addition, SCPu and SSPu resulted in forms of the push-up with higher impact force, whereas training using these could increase eccentric strength. For subjects who aim to develop maximal power through a plyometric method without undergoing large impact force (i.e., to reduce injury risk), DFPu is recommended. Our findings have been limited to a group of young commando soldiers. Further data are needed to confirm that these explosive exercises are appropriate for assessing the dynamic performance of those engaged in sports and different levels of training, particularly female participants. Furthermore, a developed design method and instrumentation are needed to directly quantify the mechanical power output of these explosive push-up exercises (8). The effects of training adaptation on these exercises are also an important area for future enquiry. In summary, the present data indicate that there is a variety of differences in kinetic characteristics between the explosive exercises. The intensity of the mass supported by the arms and impact force can be adjusted from the starting position (i.e., the standard or kneeling position). In addition, the higher the mass supported by the arms, the shorter the flight VOLUME 34 | NUMBER 10 | OCTOBER 2020 |

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Face Validity of Explosive Push-ups Tests time achieved by the participants during the flight phase. In addition, the PCA model demonstrated a great predictive power in accounting for GRF-based parameters of explosive push-up exercises, which indicates a strong logical and ecological validity as a test of upper-body power.

PRACTICAL APPLICATIONS Explosive push-ups were originally designed to increase the power of the upper body. Considering the excellent logical validity and the presence of strong ecological validity, the explosive push-up exercises may serve as valid popular field tests of upper-body power, which in turn can be used to monitor training programs, especially those intended to improve upper-body power fitness. It is also possible to adjust the intensity level of the push-up exercise by altering the starting position (i.e., the standard vs. kneeling position).

ACKNOWLEDGMENTS This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors would like to thank all the experts, doctors, and officers of the Tunisian National Guard Commandos School for their participation in this experimental work.

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