Kicking is one of the most frequently used skills in soccer, and the most fundamental for soccer performance (5,8,25,32,44,48). Soccer performance and other athletic kicking usually depend on kicking ball velocity and kicking accuracy (4,8,42). This speed could be particularly important while kicking towards the goal, because the chances of scoring increase with an increased ball speed (assuming that the kick is accurate) because the goalkeeper has less time to react (19,42).
Kicking can be described as a summation of forces (58). The motion pattern is generally accepted as a proximal-to-distal sequence in which the distal segments are allowed to lag behind the proximal segments as they move forward (20), in which the foot is the last segment to intervene and the fastest segment in the open kinetic chain (65). The timing of muscle activation can be described with activation of hip flexors such as the iliopsoas followed by the rectus femoris, which is a hip flexor and knee extensor, and finally by activation of the knee extensors such as the vastus lateralis. Therefore, hip flexor and knee extensor muscles are important for developing a high foot velocity (18). In addition, the knee flexors (hamstrings) and hip extensors (gluteal muscles), which function as antagonists to decelerate the swinging-leg after ball impact, are also significantly active in a maximum soccer kick (18,57). In general, these antagonists require fundamentally eccentric strength, whereas agonists require concentric strength (18).
The ball velocity after a soccer kick is more strongly affected by the foot velocity at the initial instant of the impact phase than any other factors (26). Furthermore, significant correlation was found between these 2 variables (7,17,37) and the coefficient of restitution (9), which is influenced by skill factors, such as the part of the foot that makes contact with the ball and the stiffness of the foot at impact (65). The mass of the shank–foot segment does not influence the velocity of the ball significantly (3).
The role of the supporting leg in generation of foot speed is not clear, but it could be speculated that the strength of the support leg is important for providing a stable platform to quickly swing the kicking leg (9). Significant correlations between single-leg balance and kicking accuracy, but not velocity, were found (14).
It is clear that the strength of the lower limbs' muscles could be related to the ball velocity. Therefore, several studies have investigated the relationship between the ball velocity after a maximal soccer kick and the strength and power values of the lower limbs (10,13,15,28,30,38,44–47,51,60,61) as well as the relationship between ball velocity and the velocity values of linear sprint runs (15) and nonlinear sprint runs (13). Other studies have explored the influence of different strength training programs on ball velocity values after a soccer kick (1,10,12,27,39–41,50,55,62,63). Finally, another group of studies have validated new kick test protocols to assess the biomechanics and physiological parameters in soccer kicking (2,8,22,36,42,59,63).
Therefore, the purpose of this article is to systematically review the existing data about strength and ball velocity, discuss potential limitations of the literature, and suggest directions for future research in kicking velocity performance. Additionally, practical applications of the results will be presented for the strength & conditioning practitioners, because they may have limited knowledge of this topic or are unaware of up to date research.
The following databases were used in this review: Medline, Sport Discus, Dialnet, Google scholar, and Scopus. The keywords used were combinations of “soccer,” “strength,” “training,” “kick,” “kicking,” “maximal,” “ball,” “fatigue,” and “velocity.” Studies in English, Spanish, and Portuguese were included. Studies from 1979 to 2014 were included in this review with a focus on the maximal kicking velocity and one or more of the following factors: soccer, kicking, velocity ball, strength, accuracy, muscle fatigue, and training. We analyzed more than 300 papers, of which 210 were studies about ball velocity and 96 of them were included in this review. The results of this review are classified in 2 categories, studies that describe the relation between strength and kicking velocity and studies that explore the effect of strength training on the kicking velocity.
THE RELATIONSHIP BETWEEN STRENGTH AND BALL VELOCITY
Isokinetic strength in soccer kicking performance
Differences in strength have been found among players from different divisions (11,49). However, the relationship between muscle isokinetic strength and kicking performance remains a subject of controversy in the soccer research field (Table 1). Cabri et al. (11) found a high correlation between kick distances and the isokinetic strength exerted by knee flexor and extensor muscles, but moderate correlation with isokinetic strength of hip flexor and extensor muscles. In addition, in young soccer players, a significant relationship between ball kicking velocity and the maximal isokinetic forces of the thigh and shank was found (38). In elite soccer players, ball velocity values after a soccer kick correlated significantly with isokinetic muscle torques reached at different angular velocities (51). These authors also reported that the intensity of the relationship tends to decrease with the increase in angular velocities. Significant correlation between knee extensors isokinetic peak torque and kick velocity score was also found in trained and untrained soccer players. (5). In addition, Masuda et al. (44) examined relationships between muscular concentric isokinetic strength and kicking ball velocity achieved with both legs during 3 different approach angles. The mean ball velocities correlated significantly with isokinetic strength of several muscles on the kicking and the supporting legs (Table 2). They concluded that different approach angles could alter the requirement on muscle strength potential of both legs during kicking. Especially, an angled approach toward the kick direction could require greater hip extension and abduction strength from the supporting leg to increase its stabilization capacity.
Maximal strength, explosive strength, and kicking performance
Elite young soccer players can be distinguished from subelite and recreational young soccer players by their higher performance in explosive and strength tests such as maximal isometric force, vertical jump height, pedaling rate, or 10 m sprint time (24). However, maximal strength in a full squat exercise did not correlate with maximal ball kicking velocity after a soccer kick in university soccer players (31). There was no correlation between ball velocity and the performance in several jump tests (squat jump; countermovement jump (CMJ)) with young soccer players (29), elite young soccer players (30,61), and elite inside soccer players (28). Furthermore, no relationship was found between the performance in a 10 meter sprint test and ball kicking velocity values. However, 2 studies found a weak relationship between explosive strength and maximal strength with the maximal ball velocity after a soccer kick. (13,23) (Tables 2 and 3).
Induced fatigue effects on ball kicking velocity
The soccer kick has been mainly studied under nonfatigued conditions. Few researches have examined the effects of fatigue on soccer and maximal ball kicking velocities (6,21,33,34,35) (Table 4), and there is only one study about fatigue effects on short-passing ability (56).
In summary, those studies have shown that there is a significant decrease in ball velocity which has been reported after: (a) a 6 minute step exercise protocol (35); (b) 90 minutes of intermittent exercise protocol (33); (c) knee extension and flexion motions on a weight-training machine until exhaustion (6); (d) a soccer specific circuit (with jumps, skipping, multiple changes of direction, dribbling the ball, passing, bursts of sprinting and jogging) (21), and (e) consecutive soccer instep kicks (34).
The significantly slower ball velocity observed in the fatigue condition may result from a reduced lower leg swing speed and poorer ball contact (6). Moreover, fatigue obscured the eccentric action of the knee flexors immediately before ball impact, which might increase the susceptibility to injury (6). This could be attributed to alterations in the function of the neuromuscular system and force generation capacity (33), and a poorer intersegmental coordination (6,35).
Finally, one study (34) showed that during repetitive kicks there was a significant reduction in the ball velocity between the first and the fifth and subsequent kicks. Therefore, it seems that no more than 4 consecutive kicks would be recommended to evaluate the kicking performance in the absence of fatigue (34).
Effects of strength training on maximum ball velocity
Several studies have explored the effect of strength training programs on maximal ball kicking velocity. Table 5 summarizes the results and strength protocols used in those studies. Most of these studies have been conducted in nonprofessional soccer players. The studies have reported a significant increase in ball velocity values after the application of different strength training programs in young amateur (27,50,53,54), young elite (16,43,52,55,62), adult amateur (39–41), and elite females soccer players (12). The strength training program has always been implemented as an extra session to regular soccer training. The strength training programs involved maximal strength training with a loaded kicking simulating exercise (62), combined strength and kick coordination training (39–41), an explosive and/or plyometric strength training program without kicking specific exercise (12,16,43,50,53–55), core strength training performed on stable surfaces (52), and electrostimulation on both quadriceps (10).
The studies conducted by Manolopoulos et al. (39–41) deserve special attention because they record muscle activity by surface electromyography. In those studies, the strength training program led to a significant decrease in joint angular velocities with an increase in biceps femoris electromyography of the kicking leg during the backswing phase (40,62). In addition, an increase in segmental and joint velocities and muscle activation of the same leg during the forward swing phase were found (40,63). They also reported a significantly higher vertical ground reaction force on the supported leg together with an increase in the rectus femoris and gastrocnemius activations (40,63). Therefore, the authors reported that the increase in ball velocity values after a soccer kick was accompanied by changes in kinetic and kinematic indices of the kicking performance (10,40,41) mainly due to an altered soccer kick movement pattern, characterized by a more explosive backward–forward swinging movement and higher muscle activation during the final kicking phase (39).
In other studies, the enhancement in maximum kicking performance was accompanied by increases in the performance of muscular strength tests like sprint (41,50,52–55,64), CMJ (12,39–41,53–55), and other strength (Isometric) measurements (10,39–41,62).
It should be noted that a few studies have been conducted in professional soccer players. Trolle et al. (63) and Aagaard (1) did not find a significant improvement in maximal kicking velocity in professional soccer players after strength training with several intensities with (1) and without (63) regular soccer training. Although, both studies reported improvements in knee extension strength, those results were not accompanied by an increase in the maximal kicking velocity.
In this article, we review the existing data about strength and ball kicking velocity in soccer. We focus on the relationship between both parameters and also the effects of strength training on maximum ball velocity. The main outcomes of this review indicate that the relationship between strength and kicking velocity in soccer is inconsistent. In addition, strength training programs seem to enhance maximal ball kicking velocity although its effectiveness is questionable in elite soccer players.
STRENGTH IN SOCCER KICKING PERFORMANCE
In general, soccer players are stronger than nonsoccer players and differences in strength can be found among players from different divisions (11,49). However, it is not possible to establish a cause-and-effect relationship between the maximal strength and kicking performance (65) according to studies using maximal isokinetic or full squat tests (5,11,15,19,38,44,46–47,51,60). Therefore, neither isokinetic torque nor maximal full squat seems to be a good predictor of ball velocity. It is likely that those tests do not reproduce the muscular demands involved in the soccer kick.
It is reasonable to assume that tests involving explosive muscular actions could be more suitable when exploring the relationship between strength training and ball kicking velocity, than the above-mentioned tests. Vertical jumps or sprints have been the most common tests used to explore the relationship between explosive strength and ball kicking velocity. However, the small number of studies and their contradictory findings (11,13,15,23,28–31,61) make it difficult to reach a definitive conclusion about this relationship. It is important to note that the performance of vertical jumps required familiarization sessions to obtain reliable data. Most of those studies did not report whether or not such familiarization sessions were conducted, thus, we cannot assess an association between vertical jumps and ball kicking velocities.
We suggest that soccer-specific strength tests, like a maximal kicking test, should be included in the selection of young soccer players and in the elite adult soccer player's training process to guide and control their progress, because this test closely reflects the role of muscles used in this action.
INDUCED FATIGUE EFFECTS ON BALL KICKING VELOCITY
The effects of fatigue on soccer kick performance have only been studied by a few researchers. There is an agreement that after implementation of different fatigue protocols, there is a significant decrease in ball kicking velocity (6,33,34). This could be attributed to alterations of the force generation capacity and a poorer intersegmental coordination. However, no researchers have examined the effects of actual match fatigue in maximum kicking velocity. Therefore, we do not have available information about how muscle fatigue could be related to impairment in ball kicking velocity during a real match. This is because, in part, to explore that relationship during an actual match would be a challenge. The simulation of a real game with small pauses to perform explosive strength tests (i.e., vertical jumps and maximal kicking test) could be an interesting approach.
Previously, we have suggested that less than 5 trials seems to be adequate to avoid the fatigue effect and maintain high kinematics and kinetic responses, when measuring the maximum kicking velocity (34). Although this is interesting for evaluation purposes, it does not represent a real situation, where it is unlikely that a player can perform more than 4 maximal kicks in a 2 minute period as in Khorasani's study (34).
EFFECTS OF STRENGTH TRAINING ON MAXIMUM BALL VELOCITY
Based on the review of literature, it is difficult to reach a final conclusion about the efficacy of strength training programs in improving the ball kicking velocity. Several factors can account for the different results across studies such as: (a) the heterogeneous profile of the samples (from amateurs to elite soccer players); (b) a disparity of the features of the strength training programs (from isokinetic to plyometric exercises), and (c) the inclusion of regular soccer training sessions during the strength programs. Nevertheless, we can speculate in the following paragraphs the role of those factors in the efficacy of the strength training programs.
There is evidence in the literature that the application of different strength training programs can increase ball velocity values in soccer players, depending on gender and playing level (12). In amateur adult, young elite, and young soccer players, strength training programs in combination with technical training lead to significant increases in maximum ball velocity values (39–41,62). However, the scarce information on adult elite soccer players (Table 5), may be due to the difficulty in accessing this type of sample. Only a small pool of studies has been conducted in that population (1,12,63). One of them found a significant improvement with female soccer players (12), while no studies found any effect of a strength training program in male soccer players (1,63). Therefore, it seems that the years of experience and expertise can affect the potential enhanced effect of strength training programs. This is reasonable because the margin for improvement becomes narrower with higher levels of performance. However, this does not necessarily mean that there is still no opportunity for improvement, but it is clear that the development of strength training programs for elite soccer players is more complex.
From the diversity of training programs reviewed in the current study, it seems that the most effective approaches to improve ball kicking velocity of the programs reviewed are those that combine explosive strength training with regular soccer training. The lack of improvement in training programs that include isokinetic strength exercises (1,63) could be expected to be according to the poor specificity between isokinetic and kicking performances (see previous discussion about this issue). However, the use of plyometric exercises in the form of vertical or horizontal jumps could lead to improvements in ball kicking velocity (23,53,54) even in combination with isometric strength exercises (23). In those studies, the increases in maximum kicking velocity were accompanied by an increase in the performance of countermovement jumps. Another important factor is whether or not the strength training program is combined with regular soccer training or technical kicking training. From all the studies that lead to improvements in ball kicking velocity, 94.1% of them (1,4,10,12,16,23,27,39–41,50,52–55,62) combined strength and regular soccer training. Therefore, this supports Aagaard's suggestion (1) that strength training should be integrated with other types of training, involving the actual movement pattern to increase the performance within more complex movement patterns.
Studies about the effect of strength training programs on ball kicking velocity are focused on the dominant leg, whereas studies evaluating the nondominant leg are clearly limited. Only 3 studies have explored or compared the improvements between legs (12,16,23). Interestingly, their results showed higher gains in the kicking ball velocities for the nondominant in comparison with the dominant legs. It is likely that diminished baseline performance kicking with the nondominant leg could explain this result. Nevertheless, more studies must be conducted to explore the effects of strength training programs in kicking ball velocity with the nondominant leg because the ability to kick with both legs leads to an advantage for the soccer player.
The current review explores the existing data about the relationship between strength tests and ball kicking velocity and the efficacy of the strength training programs. The results of this review suggest that tests such as isokinetic, 1RM, or vertical jumps are not strongly related to maximal kicking velocity. Thus, their role in understanding the force requirements during soccer ball kicking is questionable. In addition, the development of a strength training program with the goal of improving the ball kicking velocity must include explosive strength exercises in combination with regular soccer training. We also recommend evaluation in the efficacy of those programs by a direct measurement of ball kicking movement (i.e., by radar) rather than other strength measurements.
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