Football (soccer) is a sport that requires a diversity of physical attributes, such as jumping, walking, running, sprinting and change of direction. An elite aerobic and anaerobic capacity is necessary for a football athlete to maximise performance for the duration of a match, but sometimes strength and power attributes can be overlooked. The purpose of this review is aimed to provide strength and conditioning coaches evidenced based information, to assist in the periodisation of strength and power maintenance, and maintenance during a football season. The speed, change of direction (COD), acceleration, deceleration, muscular endurance, repeated sprint ability (RSA), maximal strength and power of an athlete, are all biomotor abilities that require strength as foundation for optimisation. In recent years, strength and conditioning coaches have been incorporating rehabilitation or injury prevention programs, that include unstable exercises (UST), for healthy athletes during the entire season, rather than designing evidenced based pre-structured programs that aim to maintain and optimize strength and power. Research has shown that strength and power can be maintained with only one strength training session implemented per week for football athletes. This can be performed even if two matches are played the same week. To conclude, it is important for strength and conditioning coaches to understand, the necessity of maintaining strength and power during a football season, and how its implementation helps to maximize performance and reduce the risk of injuries.
Keywords: strength training, injury prevention, periodisation, athletic performance
Football (soccer) is a sport that demands variety of physical attributes, such as jumping, running, sprinting, and change of direction (7, 11, 14, 16, 19, 27). Additionally, aerobic capacity is important as a football match has a duration of 1.5 hours, where some of the players can reach distances up to 14km between walking, jogging, and running (7, 11, 14, 27, 34). Maximal speed and repeat sprint ability (RSA) are crucial for athlete, as they can be required to reach velocities above 30km/h for around 5% of the match (14,19, 25, 27). An athlete is also required to perform about 25 jumping actions, change direction five-hundred times, accelerate and decelerate 10 to 15 times at 20km/h and 10km/h, respectively (7, 11, 14,19, 25, 27). The literature suggests that the development of strength and power are significantly correlated to the quickness of an athlete, as well as showing that, faster athletes possess greater markers in performances (2-4, 9, 13, 17, 22, 28-33, 35). These markers include counter movement jump (CMJ), squat jump (SJ), maximum speed, maximum velocity, change of direction (COD) relative and absolute strength (squats) than slower athletes (2, 3, 9, 13, 17, 22, 29). Additionally, strength training contributes to a decreased fatigue index, as it has been found that stronger and powerful athletes had a decreased time when performing RSA tasks (5).
It has been common in recent years to implement periodised programs focusing on proprioceptive activities that are designed for rehabilitation, injury prevention and prehabilitation in football, instead of maintaining and optimizing strength and power during the competitive period (8, 10). There is limited literature that illustrates how proprioceptive exercises are beneficial for healthy athletes, therefore, it is more beneficial to follow traditional strength training on stable surfaces to improve performance outcomes (10). In addition, many studies have supported that the development of strength reduces the risks of injuries (1, 12, 34). Football players suffer from overuse and traumatic lower limb injuries (1, 12, 21). Knees, in particular the anterior cruciate ligament (ACL), and ankle sprains are the most common lower limb regions affected, as they are mostly caused by contact or by running/jumping/landing technique (1, 12, 21). Hamstring strain injuries are also common and are usually caused by fatigue and lack of strength in the lower extremities (1, 21, 32), however, the implementation of exercises specifically designed to increase hamstring strength reduces posterior thigh injuries (1).
As previously mentioned, an integrated, scientific based, and well-structured periodization structure, should always aim to develop and maintain strength and power during a competitive season (26). Moreover, a study by Ronnestad et al (26) has suggested that implementing one-strength training sessions per week for football players’ aids in maintaining strength markers developed in a pre-season period.
The purpose of this review is to provide strength and conditioning coaches with evidence-based information to assist them in periodization and programming in-season strength and power training in football athletes. It aims to provide information on the maintenance of strength and power during a competitive season for football players, rather than the implementation of prehabilitation programs, that incorporates unstable exercises (UTS), and therefore, risks the decrease of strength developed during pre-season.
The databases from Pubmed and Google Scholar were explored, utilizing the internet according to the key words. Journals published in the last 30 years were selected as they met the required criteria for the review. Research that examined and reviewed only human individuals were selected. Journals such as; The Journal of Strength and Conditioning Research and The Journal of Strength and Conditioning were also explored, amongst other individual well-respected physiology and sport science text books.
Performance Attributes for Football
Football has been considered one of the most popular sports performed worldwide (11, 14, 15, 17, 27, 34). The game is considered to have diverse components that involves multiple motor skills, such as running, jumping, kicking, tackling, dribbling, accelerating, decelerating, agility and change direction tasks (11, 14, 27). While the tactical and skill component of an athlete with the ball is essential in order to be successful, the individual’s physical and physiological condition should also reach the highest capacity, in order to be at maximum performance each match (11, 14, 15, 27). The aerobic component for a football player is important as a game of football has a duration of 90 minutes, where each player covers between 8km to 14km (depending on the position), (7, 11) at about 70%VO2Max (14, 15, 34). Each player performs a turning movement every 2 to 4 seconds for the total duration of the match (34), having an average of five hundred turning movements for the total time of the match (11). Therefore, agility and change of direction attributes are essential for a footballer (11, 14, 34). Maximal acceleration covers between 5 to 11% of the total match duration, and 90% of the efforts last less than 6 seconds (7, 11, 14). During the game athletes start sprinting actions randomly (from walking or jogging position) and usually with a recovery time of less than 60 seconds (5, 11, 15, 34). Therefore, the ability to recover faster between efforts is a significant attribute for a football athlete. In elite football, athletes have an average of 25 explosive jumps (5, 14) during a full match, in addition to anywhere between 90 and 140 interactions with the ball. These can include passing or shooting the ball and while the literature is limited to the amount of tackling performed during a game, the development of strength and power is crucial for football players, as strength contributes to the major biomotor abilities that football requires (5, 14, 27).
Strength and Power Training Effect
The term strength is defined as the integrated result of dynamic or isometric muscle contraction, while performing single or several voluntary efforts (16). It can also be defined simply as “the ability of the neuromuscular system to produce a force against an external resistance” (9, 13, 29, 30, 35). Additionally, a study by Haff and Nimphius (13) defined strength as the foundation required to develop power, establishing that a stronger athlete will perform higher power outputs. As previously mentioned, strength is also the main component for biomotor abilities such as power, maximal strength, endurance, agility and speed (9, 13, 29, 30). Power is the product between force and velocity and mostly relies on the ability of the neuromuscular system to produce the greatest impulse in a period of time (16, 20, 34). Force is a product of strength, and velocity is the product of motor unit coding, synchronization, recruitment and neuromuscular inhibition, therefore, the development of maximal strength is required to effectively produce power (30).
A study conducted by Storen et al (31) established that maximal strength training has positive effect in running economy (RE) as well as maintaining maximal oxygen uptake (VO2max). The results of the study have shown that well trained athletes significantly improved by 33.2% in one repetition maximum (1RM), 26.0% in rate of force development (RFD), 5.0% RE and 21.3% in time to exhaustion, subsequently to eight weeks. Seventeen well-trained athletes were divided into two groups, the intervention group (4 males and 4 females, 28.6 ± 10.1) performed their normal running training session as well as the 8-weeks resistance training program. The control group (5 males and 4 females, 29.7 ± 7.0) performed the running training sessions only. The 8-weeks training consisted in performing four sets of four repetitions at 4RM with 3 minutes rest in between sets, 3 times per week in half squats. Additionally, the eight weeks resistance training was designed to emphasis maximal strength training rather than muscle hypertrophy as high load and few repetitions were performed. A similar study by Hoff et al (16) concluded that a maximal strength training intervention performed by football players resulted in significant improvements in RE, 1RM and RFD (4.7%, 33%, 52.3% respectively) without changes in VO2max and body weight (BW) of the individuals. The same study also suggested that neural adaptations and changes in recruitment patterns are the main factors responsible for training improvements and responses.
As previously mentioned maximal acceleration has an important role for footballers as efforts can be classified into two categories. The first category is reflected acceleration, involving a short distance sprint with a maximum of 10m long of maximum effort. The second category is known as long sprints considered after the 30 meters long (17). In football, most of the sprints last less than 6 seconds and for more than 90% of the time maximal acceleration is covered in less than 30 meters. Additionally, in footballers, a correlation exists between strength, power and acceleration that players should possess when accelerating, sprinting, and jumping during the game (7,11, 16, 17, 26, 33). A study conducted by Wisloff et al. (33), analysed maximal strength in half squat, sprint efforts (0-10m and 0-30m sprint), 10m shuttle run and vertical jump of 17 international football players 25.8 ± 2.9 years, height 177.3 ± 4.1 cm, weight 76.5 ± 7.6 kg, and VO2max of 65.5 ± 4.3 ml/kg/min. The study concluded that there is a significant correlation between maximal strength in half squat and sprint performance, shuttle run and jumping height. The validity and reliability of the results have shown that reflect acceleration (10 m sprint) has the most significant correlation (r = 0.94, p<0.001) to 1RM half squat, then the vertical jump (r=0.78, p<0.02), 30m sprint (r=0.71, p<0.01) and lastly, the 10m shuttle run (r=0.68, p<0.02). In addition, a more specific study by Nuzzo et al () analysed countermovement jump performance variables (CMJ) (CMJ peak force, peak power (PP), peak velocity (PV) and height) compared multi-joint dynamic strength exercises, and multi-joint isometric (ISO) strength exercises. The study tested twelve (n = 12) division I-AA male football athletes (age, 19.83 ± 1.40 years; height, 179.10 ± 4.56 cm; mass, 90.08 ± 14.81 kg; % body fat, 11.85 ± 5.47%; back squat 1RM, 170.38 ± 21.72 kg; power clean 1RM, 112.50 ± 13.15 kg; back squat 1RM to body mass ratio, 1.91 ± 0.22; power clean 1RM, 1.28 ± 011) in two different testing days where the individuals performed 1RM back squat, 1RM power clean, ISO mid-thigh pull and ISO squat. The results of the study concluded that 1RM back squat and 1RM power clean have a stronger correlation (r= 0.836 and 0.856, respectively) to CMJ performance variables compared to ISO squat and ISO mid-thigh pull (r= 0.706 and 0.750, respectively). However, the implementation of back squat and power clean exercises in a training program contributes to the optimization of strength and power lower body development.
The ability to constantly change direction becomes a crucial as the player is required to rapidly change direction, velocity, decelerate, accelerate, and perform turning movements in response to reach the ball or face an opponent (11, 20). A study conducted by Spiteri et al (28) analysed the correlation between diverse lower body strength and power components (concentric, eccentric, maximal dynamic strength, isometric and power), COD (505 and TTests), and agility performances in twelve (n =12) female basketball athletes (age: 24.25 ± 2.55 years; height: 177.69 ± 7.25 cm; body mass: 75.56 ±14.55 kg) for the West Coast Waves Women’s National Basketball League (WNBL). The results of the study concluded that maximal dynamic strength, eccentric, concentric, and isometric strength components have a negative significant correlation to both COD tests (T-test: r=-0.79 to -0.89, p=0.001; 505 test: r= -0.79 to – 0.87, p= 0.001). Results also showed that eccentric squats have a negative significant correlation to both COD tests (505 test: 0.892 and T-test: 0.878). Furthermore, the research concluded that lower body strength and power attributes do not correlate with agility performance (r=-0.19 to -0.46), although strength and power play an important role in agility movements as they still required rapid and reactive actions when executed.
An important fitness attribute for any athlete is repeated sprint ability (RSA), where the literature defines it as the ability to recover rapidly from an effort, therefore, producing the same or similar performance in subsequence efforts (25). Fatigue and decrement of peak power or work are two characteristics that are present when athletes are required to perform maximal velocity spirits repetitively with short recovery rest (19). In addition, fatigue becomes present when there is a decrease in overall performance and decrease in peak power, which has a significant correlation to anaerobic power (r=0.87, p<0.05) (17, 19). A recent study by Baldi et al (5) examined the relationship between RSA (6 x 40m (20m + 20m) with 20-s resting time) and neurological and aerobic and anaerobic variables in male football players. Twenty-six players (age: 22.5 ± 3.6 years) were tested in CMJ, squat jump (SJ), and standing long jump (LJ) for VO2max, velocity at onset of blood accumulation (vOBLA), maximal aerobic speed and peak blood lactated accumulation (paLA). The individuals were also tested on a different day in best and mean time on RSA. The study concluded that RSA best (r=-0.73 and r= -0.54, p<0.01) and mean times (r=-0.69 and 0.62, p<0.01) were negatively strongly correlated to CMJ and LJ performances. As previously reviewed in this paper, CMJ is significantly correlated to power (23, 33), therefore strength and power training for lower extremities should also be enhanced in order to improve RSA performances.
Unstable Exercises vs. Injury Prevention
Injuries of the lower extremities are the most prevalent in football players (8, 21, 32). Football injuries reported as traumatic are classified as sprains, and non-traumatic injuries are classified as strains or overuse injuries (8, 21). Ankle and knee sprains have been reported to have the highest index of occurrence in football players as these types of injuries are caused during tackling actions and contacts between players, usually affecting the anterior cruciate ligament (ACL), as well as causing minor traumas in the ankle region (21, 32). Although these are contact injuries, a study by Griffin et al (12) suggested that lower limb strength reduces the risk of a non-contact ACL injury. There is evidence to suggest that another major injury reported in football players, are hamstring strains. The main causes are due to both a lack of strength in the hamstring region and fatigue (8, 21, 32). A study by Askling et al (1) concluded that specific strength training for hamstrings, that included eccentric overloading, would benefit football athletes by both maximising performance and reduce risk of injury. The study analysed thirty players from the two main Sweden divisions where subjects were divided into two groups; a training group (n=15) (age: 24.00 ± 2.6 years; height: 1.82 ± 0.06 cm; body mass: 78.00 ± 5.00 kg; body mass index: 23.5 ± 0.06 kg/m2) and a control group (n=15) (age: 26.00 ± 3.6 years; height: 1.81 ± 0.07 cm; body mass: 77.00 ± 6.00 kg; body mass index: 23.3 ± 0.09 kg/m2). The training group performed a total of 16 specific hamstring strength training sessions, one to two times per week, with 4 sets of 8 repetitions in the lying hamstring curl machine for a for a total of 10 weeks. The control group did not do this. The results of the study showed that the training significantly increased both eccentric and concentric knee flexor strength (19 and 15%, respectively, p<0.05), the same group significantly increased sprinting time in 30 meters (2.4%, p<0.05). The most significant (p<0.05) result was seen after a monitoring period of 10 months (one season) where the training group individuals reported a (p<0.05) lower number of hamstring injuries (3/15) compared to the control group (10/15). Controversially, designing programs using UST exercises has helped athletes with existing injuries to return to sport as UST are used in rehabilitation programs (8, 10, 18). UST have also been misinterpreted and programmed by strength and conditioning coaches for healthy athletes to improve performance (10, 18). In fact, a study conducted by Cressye et al (10) concluded that proprioceptive activity, including UST type of exercises, do not reflect an improvement in performance or the maintenance for healthy athletes. Nineteen (n=19) Division 1 male soccer players were divided into two groups. The experimental group (US) (n=10) performed lower-body exercises on an unstable disc as well as their normal conditioning training. The control group (ST) (n=9) performed the same exercises and training regime but on a stable surface. Lower-body exercises consisted in sets of 2 to 5 and repetitions 5 to 15, for a total of 27 sessions over 10 weeks’ period, including unilateral and bilateral exercises for both groups (unstable and stable surfaces). After post-testing at week 11, the results of the study have shown that both groups significantly improved in 40 and 10 meters, and T-test (ST: -3.9%, -7.6% and -4.4%, respectively and US: -1.8%,-4.0% and -4.4%, respectively), however Bounce Drop Jump and CMJ significantly (p<0.05) improved on athletes that trained on stable surfaces (3.2%, 2.4%, respectively).
The literature has defined periodisation as a strategic pre-structure plan in order to vigorously monitor athlete’s loading, with the ultimate goal to maximise athlete’s performance, while minimising the risk of injury (2, 9, 30, 35). Additionally, the literature has shown that several types of periodisation structures have positive outcomes in maximising strength and power, when athletes perform one or more sessions per week (2, 3, 4, 13, 26, 29, 30, 35). A study by Haff and Nimphius (13) suggested that targeting outcomes greater than 2.0 x body weight should indicate the minimum strength level for both male and female athletes. In order to maintain strength levels, a study conducted by Ronnestad et al (26) concluded that football athletes can maintain strength and sprint performance during the seasonal period, by only performing one strength session per week. The study examined fourteen (n=14) professional football players from the Norway first Division League. One group performed 1 strength training session per week during the competition season (group 2 + 1; n = 7, age 22 ± 2 years, body mass 76 ± 1 kg, height 184 ± 3 cm), whereas the other group performed 1 strength training session every second week (group 2+0.5; n=7, age 26 ± 2 years, body mass 83 ± 3kg, height 186 ± 2 cm). The strength session consisted of 3 sets of 4 repetitions at 90% 1RM half squat, for both groups. The results of the study have shown that the strength level developed in pre-season was maintained for a 12 weeks’ period for the 2 + 1 group, therefore the individuals of this group were able to half squat from 139 ± 7kg to 163 ± 8kg during the seasonal period, and the same group resulted to maintain 40-m sprint performance from 5.39 ± 0.07 seconds to 5.29 ± 0.05 seconds. Contrary, the 2 + 0.5 group results showed significant reduction in both strength and sprint performance for the trimester period (10 ± 4%, p< 0.05; 1.1 ± 0.3%, p< 0.05, respectively). Table 1 shows the organization of the weekly training schedule according to the number of matches with two possible scenarios, A and B. Previous match played on Saturdays (26). Table 2 demonstrates an Example Mixed Method Approach for Developing Power (13).
CONCLUSION AND PRACTICAL APPLICATIONS
Football is a team sport that requires diverse types of physical attributes. Strength and power are the main physical attributes for a footballer, as well as aerobic and anaerobic abilities. This paper aimed to review the literature according to all football attributes and identify how the maintenance of strength and power during a seasonal period becomes important, not only to maximise performance, but also to prevent the risk of injury. The review also aimed to provide strength and conditioning coaches evidenced based information to assist in the periodisation for strength and power maintenance and optimisation during a football season, rather than incorporating only injury prevention programs. Speed, COD, acceleration, deceleration, muscular endurance, RSA, maximal strength and power are all biomotor abilities that require strength as foundation for optimisation. The literature has also shown that the implementation of heavy resistance training during the competition period not only improves the biomotor abilities previously mentioned, but it also does not affect aerobic performance capacity (VO2max). Lastly, when a sequenced, integrated, and scientific based macrocycle plan is implemented for football players, the risk of injury is reduced. In conclusion, strength and conditioning coaches for football should understand the importance of maintaining strength and power, therefore keep implementing strength and power training sessions at least once per week during the entire season, rather than replace strength and power sessions with proprioceptive types of training.
Table 1: Organization of the weekly training schedule according to the number of matches with two possible scenarios, A and B. Previous match played on Saturdays (26).
A: One – Match
B: Two – matches
Table 2: Example Mixed Method Approach for Developing Power (13).
Type of Exercise
High Force – High Velocity
High Force – Low Velocity
Low Force – High Velocity
High Force – High Velocity
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