Plyometric training (PT) consists of dynamic and rapid stretching of muscles (eccentric action) immediately followed by a concentric of shortening action of the same muscles and connective tissues (1). This training focuses on learning to move from a muscle extension to a contraction in a rapid or ‘explosive’ manner, such as in specialized repeated jumping. Exercises are of high-intensity, explosive muscular contractions combining strength and speed for acquisitions of benefits in power. PT involves hops and jumps used to capitalize on the stretch-shortening cycle of the muscle (1). The stored elastic energy within the muscle is used to produce more force than can be provided by a concentric action alone. It is distinguished by a rapid deceleration of mass followed immediately by its rapid acceleration in the opposite vertical direction. For the lower limbs, PT entails exercise such as hopping, bounding or drop-jumping (depth jumping) from a raised box or platform and immediately jumping vertically after an ‘amortization’ period of ground contact (2). The PT programme typically includes sport-specific exercises including exercises for shoulder and muscles of arms (3,4) and has traditionally been used for sprinting, jumping, and sports with rapid changes in direction.

The adaptability of PT has increased in popularity, thereby proven valuable for application to a range of sports as an effective tool in increasing lower body power (5), as measured in several studies using adult subjects (6). However, it has been reported that PT may also have negative aspects, particularly in its early phase. High intensity and repetition of eccentric contraction lead to a delayed onset of muscle soreness (7). In addition, only athletes who have already achieved a high level of strength through standard resistance training should engage in plyometric drills (8). It has been also shown that gains in jump performance following PT were due to change in mechanical properties of muscle-tendon complex more than to muscle activation (9). Taken altogether these data suggest that PT may constitute a high risk of injury of the youth growth plate (6,10) and may contraindicate them. Additionally, vigorous movements that incorporate sprinting, jumping and throwing actions (11,12) in PT may be damaging for youth subjects or adult females, even if improvements in balance, coordination and agility and injury prevention after use throughout pre-season training are likely, although the suggestion is less well supported by experimental evidence. Nevertheless, data demonstrating the use of PT for developing skill performance in soccer players (13), particularly in youth and female players, are available, although there is currenlty little scientific information available regarding whether PT actually enhances skill performance in soccer players. The purpose of this review was to examine the effects of PT on soccer player in young and adult athletes of both genders.

Primarily used by martial artists, sprinters and high jumpers to improve the performances of athletes, PT has emerged in two forms that have evolved since 1980. An original version defined as the ‘shock method’ was created by the Russian scientist Yuri Verkhoshansky, the second version is widely used in the United States (?). In the first form, the athlete is required to drop down from a height and experience a ‘shock’ upon landing. This in turn led to a forced eccentric contraction, which was then immediately switched to a concentric contraction as the athlete jumped upward. The landing and take-off were executed in an extremely short period of time (0.1–0.2 sec). The shock method is the most effective method used by athletes to improve their speed, quickness, and power after development of a strong strength base. The second version of pyometrics widely used in the United States, relates to doing any form of jump regardless of execution time. This involves jumps that are lower in intensity and execution, while the time required for transitioning from eccentric to the concentric contraction is much greater.

Currently, typical PT comprises three phases (Fig. 1). The first phase is a rapid muscle lengthening movement known as the eccentric phase. The second involves a short resting period known as the amortization phase and, in the third phase, the athlete engages in an explosive muscle shortening movement, termed the concentric phase. The athlete repeats this three-part cycle as rapidly as possible, with the goal being to decrease the amount of time in between the eccentric and concentric movements. Reductions of time in between eccentric and concentric movement induce the athlete to become faster and more powerful (14) as this primarily improves muscular, tendon and nerve functions (14). The increase in physical power make athletes run faster, jump higher and hit harder and develop specific skills such as injury protections relative to specific practiced sport (15). In addition, the forces involved and the quickness of execution induced the involvement of the central nervous system (16).

Soccer is an intermittent, highly-intensive and complex sport. A successful performance is dependent on basic abilities, in particular, repeated explosive burst, strength, power, kicking, tackling, and their derivatives such as jumping, turning, sprinting, and changing pace (43), all making important contributions to the performance of the soccer player. Nevertheless, soccer relies primarily on aerobic metabolic for energy, and it has been suggested that up to 98% of the total energy expenditure during 90 min of the game play is derived from aerobic metabolism (44). Aerobic endurance performance in soccer is governed by three interrelated mechanisms including, VO₂ max, lactate threshold and RE (45). The average intensity is high, with a range of 75–80% of VO₂ max, despite periods of recovery (46,47). Consequently, maximal oxygen consumption corresponds to the most important component of aerobic endurance performance in soccer (44), and there is evidence that indicates maximal aerobic power correlates with soccer success (46,48). Accordingly, a plyometric agility training program may increase the percentage of VO₂ peak in female soccer players (47), increase several muscle powers and endurance measured after a 6-week PT program in young soccer players (49) and improve and maintain the soccer kick for ball speed (50), thereby confirming the place of PT in skill performance (51) and decisive determining of neuromechanical training responses in high-level soccer players (52).

In line with the abovementioned data, strength prior to PT intervention is also an important variable (53) to consider as men tend to be stronger than women (54). Additionally, in non-trained individuals, men gain in vertical jump performance compared to women after PT (54) while similar sprint (55) and endurance (56) performance adaptation are reported in men and women after PT. This difference suggests that PT-induced adaptation occurred in men and women independently of the initial performance prior to training (57) or basal differences in important performance-related hormonal markers (58). However, in young (~21 years) players with a similar training load and competitive background assigned to training women and men, and control men and women groups, no differences in performance improvement was observed between the PT groups (59). The two PT groups improved more in all the performance tests compard to the controls group, suggesting that adaptation to PT does not differ between men and women. In addition, short-term (6 weeks) PT intervention induced higher maximal-intensity exercise and endurance performance improvements compared to soccer training alone, and the improvements induced by PT were not affected by gender (60). Thus, male and female soccer players with similar competitive background and training load can undergo similar PT programs (60). Accordingly, PT induced significantly greater kicking distance in a group of adolescent females (~13 years) after 14 weeks of training while no significant difference was seen in vertical jump height between the groups at pretest (61). PT improved depth vertical jump performance, agility and isometric knee extensor strength (62) in young soccer players.

Due to the multifaceted nature of physical requirements in soccer, including strength, endurance, power and agility, soccer training must be able to fulfill the needs of improvement. Taken altogether, the data demonstrated a strong ability of PT to transfer and improve specific cardiovascular and neuromuscular fitness. PT induces an increase in VO₂ max, maximal strength, sprinting speed, solid kick, endurance, agility, particular soccer player skills and vertical jump ability (63) in male and female individuals at any age, whether in recreational or professional athletes. In addition, improvements include muscular and tendon strengthening, resulting in the abililty to avoid injuries (15).

Thus, PT must be a part of soccer player training programs as is the case in many types of sports. Safety consideration must be taken into account, including, evaluation of the athlete, ensuring facilities and equipment are safe, establishing sport-specific goals, determining program design variables, and teaching the athlete proper technique and properly promoting the program.