The mechanisms by which strength training improves performance, reduces post-match fatigue, and prevents injury are basically the same. A powerful and highly coordinated player is better able to handle the forces on the pitch. More efficient movement results in enhanced performance and reduced fatigue and is the major injury prevention factor.
1.1 Enhanced performance
Speed, power, quickness, and strength are highly interrelated abilities (Delecluse 1997; WislĂžff et al. 2004). Several studies show that there is a strong association between sprint ability and levels of strength and power in soccer players (LĂłpez-Segovia et al. 2011; WislĂžff et al. 2004). The correlations between strength and the ability to accelerate are greater than those between strength and maximal sprinting speed (WislĂžff et al. 2004).
In soccer the vast majority of sprints are performed over shorter distances (<20 m) (Haugen et al. 2014). Contact times during the acceleration phase of sprinting, changing direction, and jumping are relatively higher, which means force can be exerted over a relatively longer time period (Dintiman and Ward 2003; Mero and Komi 1986). The longer the ground contact time, the stronger the association between strength and performance and the greater the impact of strength and power training will be on performance. Enhanced lower body strength has therefore excellent transference to agility performance, vertical jump height, and the ability to accelerate (Spiteri et al. 2014). Strength training in addition to soccer training is an effective training strategy to improve the playerâs sprint, agility, vertical jump, and kicking performance (Brito et al. 2014; Buchheit et al. 2010; Chelly et al. 2009; Garcia-Pinillos et al. 2014; Keiner et al. 2014; Maio Alves et al. 2010; Manolopoulos et al. 2013; Mujika et al. 2009). Although several authors state that strength training plays a key role to improve sprint performance, not all training routines are able to augment the playerâs maximal sprinting speed (Jullien et al. 2008; Shalfawi et al. 2013). The low level of exercise and velocity specificity might explain however the ineffectiveness of some of the programs used to enhance sprinting speed. Improving the maximal sprinting speed of players with a higher training status requires velocity-specific strength training. Explosive-type strength training is needed to optimally develop athletic performance.
It is also recommended to include explosive strength training to improve the ability to recover between high-intensity actions (Bishop et al. 2011). Repeated-sprint ability is positively correlated with lower body strength and power, agility performance, acceleration ability, and the initial sprint performance (Brocherie et al. 2014; Girard et al. 2011; Ingebrigtsen and Jeffreys 2012; LĂłpez-Segovia et al. 2014; Mendez-Villanueva et al. 2008; Spencer et al. 2011). Stronger and more powerful soccer players are able to sprint faster and sprint more times without a decrement in speed (Brocherie et al. 2014; LĂłpez-Segovia et al. 2014).
The consensus is that the ability to maintain performance and attenuate fatigue for a prolonged period of time is usually related to the individualâs endurance capability. Fatigue is affected however by multiple factors and repeated-sprint ability has been related to neuromuscular and metabolic factors (LĂłpez-Segovia et al. 2014; Rampinini et al. 2009; Silva et al. 2013). Training that enhances neuromuscular and/or metabolic function can therefore improve repeated-sprint ability. Fatigue and the performance decline during subsequent sprints are manifested by a decreased sprint speed or power output (Bishop and Girard 2011; Girard et al. 2011). The decline in power output during repeated sprints has also been attributed to a reduced neural drive and motor unit recruitment (Bishop and Girard 2011; Girard et al. 2011). The potential to activate more motor units as a result of strength training can therefore attenuate fatigue and reduce the loss of power output when repeating high-intensity actions (Silva et al. 2013).
Stronger players have also a greater ability to maintain a high level of force production and power output toward the final stages of the game due to the positive relation between strength and muscular endurance (Silva et al. 2013; Zatsiorsky 1995).
Repeated-sprint ability and long-term endurance performance have also been related to musculotendinous stiffness (Bishop and Girard 2011; Girard et al. 2011). Strength and plyometric training significantly increase the stiffness of the muscleâtendon unit, which enables the muscles and tendons to store and release more elastic energy and reduce the amount of wasted energy (Saunders et al. 2004). The reduced energy demand results in less oxygen consumption, which explains the strong association between running economy and endurance performance (Saunders et al. 2004). An improved running economy and running performance following strength or plyometric training have been reported frequently (Jung 2003; Saunders et al. 2006; Spurrs et al. 2003; Turner et al. 2003).
The improvements in motor unit activation and synchronization, musculotendinous stiffness, and stretch-shortening efficiency as a result of strength training can have a beneficial impact on both sprint performance and repeated-sprint ability (Aagaard and Andersen 2010; Bishop and Girard 2011; Buchheit et al. 2010; Girard et al. 2011; Nummela et al. 2006; Silva et al. 2013).
1.2 Injury prevention
A large meta-analysis including over 26000 participants revealed that strength training reduces acute injuries by more than two-thirds and almost halves the number of overuse injuries (Lauersen et al. 2014). Several other studies that examined the effectiveness of strength training on reducing injury incidence and recurrence in soccer had similar results (Askling et al. 2003; HĂ€gglund et al. 2009, 2013; Petersen et al. 2011). Various reasons can help explain the effectiveness of strength training to prevent injuries.
Most acute sports injuries occur in the early phase of ground contact (Krosshaug et al. 2007). Fast movement reaction times and the potential to quickly produce force will benefit stability and balance during this initial phase of ground contact (Krosshaug et al. 2007). Resistance and plyometric training enhance the ability to rapidly produce force, through a reduced mechanical delay and an increased rate of force development (Aagaard et al. 2002; Wu et al. 2010). The higher musculotendinous stiffness and neuromuscular drive following strength training will shorten the time between muscle activation and contraction (mechanical delay) and increase the speed at which force is produced after the onset of contraction (rate of force development) (Waugh et al. 2014). The ability to faster produce force will not only benefit movement reaction times, but can also reduce the injury incidence through an enhanced co-activation potential during the initial phase of ground contact (Waugh et al. 2014).
Strength training can also increase the size and strength of the ligaments and tendons. As increases in muscular strength occur with strength training, ligaments and tendons also need to adapt in order to support and efficiently transmit these greater forces to the bones (Fleck and Kraemer 1987; Waugh et al. 2014).
Strength and plyometric training also improve the neuromuscular control of the lower extremities. The altered muscle activation patterns that have been reported following plyometric training include a higher pre-activation and a more symmetric co-activation between the quadriceps and hamstrings and also between the hip abductor and adductor muscles (Chimera et al. 2004; Hewett et al. 1996). These altered motor control patterns result in a better lower limb alignment and more stable knee position upon landing (Chimera et al. 2004; Cuoco and Tyler 2012). The enhanced pre-activation and stiffness of the muscle-tendon complex will increase the load absorbed by the muscles and tendons and diminish the load transmitted through the joints and ligaments (Chimera et al. 2004; Fouré et al. 2010, 2011).
A stronger musculoskeletal system and enhanced pre-activation will also improve the reactive stability and restraint against sudden movement perturbations, such as being pushed, contact with the opponent, or unforeseen game situations (Blazevich et al. 2007; Blickhan et al. 2007; Bosch 2012).
Players with higher strength levels are also less prone to injury due to a higher muscular endurance and resistance to f...