Effects of eccentric exercise on optimum length of the knee flexors and extensors during the preseason in professional soccer players

Abstract

Objective

To assess the effects of eccentric exercise on optimum lengths of the knee flexors and extensors during the preseason in professional soccer.

Design

Twenty-eight athletes from a professional Spanish soccer team (Division II) were randomly assigned to an eccentric exercise intervention group (EG) or a control group (CG). Over the four-week period two athletes from the control group suffered RF injuries and two athletes were contracted by other clubs. After these exclusions, both groups (EG, n = 13; and CG, n = 11) performed regular soccer training during the four-week preseason period.

Results

After the four weeks, the optimum lengths of the knee flexors were significantly (P < 0.05) increased by 2.3° in the CG and by 4.0° in the EG. The change in the EG was significantly (P < 0.05) greater than that of the CG. The optimum lengths of the knee extensors were significantly increased only in the EG by 6.5°. Peak torque levels and ratios of quadriceps to hamstring (Q/H ratios) were not significantly altered throughout the study for either group.

Conclusion

Eccentric exercise can increase the optimum lengths of both the knee extensors and knee extensors flexors during the preseason in professional soccer.

Introduction

Muscle strains are the most common lower body injuries in professional soccer (Hawkins et al., 2001, Woods et al., 2004). Particularly, the biceps femoris and rectus femoris (RF) muscles are thought to be at great risk during fast movements (i.e. sprinting, accelerating, change of direction, kicking, landing, etc.) due to their bi-articulate design (Brooks et al., 2006, Hoskins and Pollard, 2005a, Hoskins and Pollard, 2005b). Previous literature indicates that hamstring injuries alone account for between 12 and 17% of all injuries reported in professional soccer (Hawkins et al., 2001, Woods et al., 2004). In addition to the prevalence of hamstring injuries there are prolonged symptoms, a poor healing response, and a high risk of re-injury (i.e. 12–31% re-injury rate) (Woods et al., 2004). Although several prospective and retrospective studies exist on the risk factors for hamstring injuries, there is no consensus on how to treat or prevent hamstring injuries (Hoskins and Pollard, 2005a, Hoskins and Pollard, 2005b).

It has been suggested that eccentric exercise may reduce hamstring injury rates as the muscles are trained to shift the peak of the torque–angle curve in the direction of longer muscle lengths (Brockett et al., 2004, Brooks et al., 2006, Woods et al., 2004). The shift in “optimum length” is typically measured from a change in the torque–angle relationship in humans, so it should be noted that when we refer to optimum length we use it with these conventions in mind. The shift in optimum length has been reported in 11 human studies (up to 18° shift) using a variety of eccentric exercise interventions (for a recent review see Brughelli & Cronin, 2007). However, the majority of these studies have utilized an acute design (i.e. 9 of the 11 studies), where the majority of the shift was thought to be due to muscle damage (Brughelli & Cronin, 2007). There are very few studies that have reported a shift in optimum length after a period of training (Clark et al., 2005, Kilgallon et al., 2007). One of these training studies was a pilot study (Clark et al., 2005) and the other study did not use an athletic population (Kilgallon et al., 2007). Thus there have been no previous randomized controlled training studies that have reported a shift in optimum length in an athletic population. Furthermore, there are no previous training studies that have reported a shift in optimum length of two opposing muscle groups (i.e. knee flexors and knee extensors) after eccentric exercise. This shift in optimum length has been argued to be a protective adaptation for future muscle strain injuries in sport (Brockett et al., 2004, Proske et al., 2004). In support of this theory, several studies have reported reductions in hamstring injury rates after eccentric exercise in elite and sub-elite athletes (Arnason et al., 2008, Askling et al., 2003, Brooks et al., 2006, Gabbe et al., 2006). It should be noted that only the training induced shift in optimum length, and not the acute muscle damage-induced shift in optimum length, has been proposed as a protective adaptation for muscle strain injuries.

Similar to the literature presented above on the knee flexors, training studies have reported shifts in optimum length of the knee extensors after eccentric exercise (up to 15.4°) (Bowers et al., 2004, Yeung and Yeung, 2008). Thus it seems possible that shifting the optimum length of the knee extensors to longer muscle lengths could protect this muscle group from future strain injury as well. RF injuries are very common in professional soccer, especially in the preseason. For example, Woods et al. (2004) reported that RF strain injuries occurred more often (29%) than hamstrings (11%) and groin strains (12%) during the preseason among 91 professional English soccer teams. However, due to a paucity of literature on RF injuries little is known about their risk factors, time lost from injury, prevention, treatment, or re-injury rates. The only study that has investigated risk factors for quadriceps injuries (n = 163) in sport has reported the following risk factors for Australian Rules football players: previous injury, player height (shorter athletes more likely to be injured), and hard ground surfaces (Orchard, 2001).

Given that the time course of the shift in optimum length has not been investigated in many longitudinal training studies, practical training methodologies for prevention or rehabilitation of hamstrings and/or RF strain injuries are yet to be developed. Accordingly, the primary objective of this blinded, randomized/controlled training study was to determine the effects of eccentric exercise on the optimum length of both the knee flexors and extensors over the preseason (i.e. 4 weeks) in professional soccer.