INTRODUCTION: STRENGTH OR POWER?
Researchers involved in the training methodology to improve jump ability proposed a variety of techniques and exercise modalities (22,15,4). Nevertheless, the question on what percentage of training should be dedicated to strength and what to power seems unresolved. First of all, to better understand the matter, we have to stress the difference between strength and power. Generally speaking, in athletics, the rate of force development (RFD) is much more important than strength alone; the average RFD (mRFD) can be estimated as the ratio Fmax/Tmax, where Tmax is the time to reach the peak force, whereas Fmax is the maximum force. This ratio is also named explosive strength, commonly computed, for applicative purposes per 1 kilogram of body weight (26). Explosive strength is a fundamental conditional capacity for a soccer player, and more specifically, vertical jump height is primal to successfully impact the ball. Explosive strength is fundamental for a soccer player to obtain the jump height necessary to head the ball or, in other words, the maximal ability of a muscle to exert force or torque at a specific velocity (16). This kind of neuromuscular quality is often assessed by the 1 repetition maximum (RM) test, but 1RM strength maybe useful only for a few athletic events like weightlifting because during the 1RM test, low acceleration values are usually obtained. Referring to concentric movements, the difference among strength and power generation is due to the velocity of the muscle contraction. In fact, power is given by the product of force and velocity: the higher the force, the lower the velocity of concentric muscle action (14). This is explained in Figure 1.
SOCCER JUMPING HEADER: TECHNIQUE AND TRAINING CONSIDERATIONS
This skill (Figure 2) begins with the loading of the leg (or both legs) in preparation for the jump, then the arms move from bottom to top, and the take off starts. At the beginning of the flight, the trunk and the legs extend backward; the further back the trunk extends, the greater the maximum forward velocity of the upper trunk and head. Force is produced by a strong contraction of the trunk flexors, hip flexors, and knee extensors before impact (20). After impact, the jumping header finishes the movement with the landing phase.
It is clear how take off is a key point to reach a consistent jump height and needs specific training. This phase is similar to a counter movement jump (CMJ) (with both legs or with a single leg depending on the game situation), where the muscles involved are first stretched and then shortened to accelerate the limb. As a consequence, storage of elastic energy in both muscle and tendon occurs, with a direct reutilization in the subsequent concentric phase, contributing to increase the jump performance (5). Another contribution to strengthen the concentric action is because of the increase of muscle neural stimulation elicited by the mechanical stretch stimulus (10,11). This action of the lower limb muscles, known as stretch-shortening cycle (SSC) (17), involves some interesting neural and mechanical processes extensively studied in the scientific literature (5,8,10,12,23). A second training point is the strengthening of trunk and hip flexors together with the knee extensors to obtain great trunk and head acceleration for a powerful ball heading. In addition, the complexity of heading the ball requests great degree of coordination. As a matter of fact, explosive strength depends on the level of intermuscular coordination considering agonist, antagonist, and synergist muscle activities. In fast movements, low levels of resistance should be recorded, and the relaxation of the antagonist muscles should be simultaneous and well coordinated with the agonist ones. In this direction, specific training may reduce agonist-antagonist cocontractions, strengthening the coordination pattern and, as a consequence, the role of agonist and synergist muscles (23).
Literature investigations on soccer heading concentrated mainly on biomechanical aspects. Kristensen et al. (20) investigated the segmental characteristics in jumping headers with particular attention to the head's velocity relative to the torso at impact along with the contributions of the upper and lower extremities (19). Results showed that the head accelerated relative to the torso throughout the impact phase as a nonrestricted free segment and that the arms had little role in creating high ball speed after impact. The authors concluded that movement of the legs was the single most important factor in the skill. Because of the body jackknife movement around the pelvis, the authors suggest developing muscle strength in the stomach, back, and pelvis and to put no restrictions on head and arm movement to optimize the jumping header (20). Marcolin and Petrone (21) proposed a functional evaluation method considering the height of the jump, the ball velocity, and its angle after impact. In particular, they identified a maximum efficient heading elevation of the header related to the ball speed increments and to the correct ball angles useful to evaluate training effects during the season. Furthermore, they introduced an elevation index defined as the percent ratio between the jump elevation and the anthropometric height of each subject and correlated it with the ball velocity to classify a player's ability to head high balls with high impact energy. Shewchenko et al. (24) developed biomechanical methods and a numerical model to investigate head impact response and the influence of heading technique. If the aim is to reduce impact severity, their mathematical modeling suggested an increased risk of neck loads performing alternative techniques.
TRAINING CONSIDERATIONS TO IMPROVE JUMPING HEADER PERFORMANCE
Bosco and Komi (5) studied the relationship between jump performance and stretch load application showing an increase of the jump height with an increase in the stretch load. This explains, for example, why in the drop jump, the performance improves with the increase of the drop height. On the other hand, a too high stimulus with excessive stretch loads lead to an inhibition of the muscle contraction because of the Golgi tendon organ reflex (10,23). It is noteworthy that nonathletes or athletes unaccustomed to intense SSC tasks may show better performance during a CMJ than during a drop jump, and often, their drop jump height could be lower than the squat jump (SJ) (23). It also appears logical that vertical jump performance improves more markedly after strength training in subjects with an initial low value (1) than in previously strength-trained athletes (12). In addition, combined strength-power training seems to be more effective than power or strength training alone in recreationally trained subjects (7). According to these results, it is clear that the ability to rapidly generate force is the major contributor to vertical jump height both in presence of SSC (CMJ) and in the absence of it (SJ) (6). For soccer players, both skills are important to be trained, allowing development of great levels of strength in short periods (Figure 3).
Many scientists (11,3) demonstrated the importance of mRFD for explosive performance and that squat training with heavy loads (70–120% of 1RM) improves maximal isometric strength but not mRFD (13). Therefore, in soccer, intermediate loads should be preferred to the heavy ones to train lower limb force and power. To support this, recent data (2) showed that weightlifting and plyometric exercise have different effects on muscle activation, and knee and hip angle of soccer players, suggesting that weightlifting training might be more appropriate in the precompetition period improving vertical jump height via changes in power and technique (9), whereas the plyometric exercises should be preferred in the competition period. As reported in the description of the jumping header technique, trunk and hip flexors and also abdominals have an important role in giving speed to the impacted ball. The trapezius muscle group stabilizes the head and trunk in preparation for ball impact (25). Therefore, weightlifting has an important role not only for the quadriceps but also in reinforcing the role of the trapezius muscle groups in heading the ball. For these reasons, in the preseason, training including crunches and jackknives should be taken into account (18). During the transition phase from precompetition to the competition period, plyometric exercises should be mixed with series of CMJs (also with single leg) with the arms free to move in such away to simulate a double or a single-leg soccer heading take off. The use of the ball is recommended to improve the coordination of the jumping heading and should be emphasized above all in the competition period, whereas in the precompetition, the main focus of the training should be to increase the strength and muscle power. An example of a periodized program for ball heading in advanced soccer players is illustrated in the Table and should be integrated with the other athletic and technical training sessions.
The importance of jumping heading is well documented in the literature; a study across different soccer leagues indicated that players impact the ball more than 6 times per game (20). This skill can also determine the outcome of a match, considering that in the Japan and Korea Fédération Internationale de Football Association world cup, more than 20% of the goals were scored by headers (20).
Vertical jump height, along with the ability to head the ball with power, is the key point for an efficient jumping heading. Vertical jump performance depends not only on lower limb level of strength but also on the rate at which they are able to generate force, on the contraction velocity, on the ability of SSC utilization, and on the degree of intermuscular and intramuscular coordination. The increase of strength obtained with traditional weight training appears to be effective to increase jump height only in athletes with low level of initial strength. With experienced well-trained athletes, programs should aim to improve RFD and muscle power output. Because of the multifaceted nature of the vertical jump, a multivariate training approach seems to be more effective with respect to a single approach because it provides various stimuli for the development of different athletic qualities. For this purpose, particularly interesting seems to be the combined training of traditional weight training with sprints and Olympic-style exercises performed in different periods of the competitive season. The proposal of plyometric exercises mixed with a series of CMJs with the arms free to move simulating a double or a single-leg soccer heading take off is helpful to link exercises focused on increased jump height with training to improve trunk/lower limbs coordination. Finally, a simplified video analysis method for a functional evaluation of the jumping headers considering some of the parameters presented in the literature (21) such as jump height, ball velocity, and the initial angle described by the ball trajectory would be useful in comparing the players classifying their ability in heading high balls with high impact energy and in evaluating training effects during the season.
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