Fatigue and rapid hamstring/quadriceps force capacity in professional soccer players Camila C. Greco, Wendell L. da Silva, Se ´rgio R. A. Camarda and Benedito S. Denadai* Human Performance Laboratory, Sa ˜o Paulo State University, UNESP, Rio Claro, SP, Brazil Summary Correspondence Benedito S. Denadai, Human Performance Labora- tory, UNESP, Av. 24A, 1515, Bela Vista, CEP – 13506-900, Rio Claro, SP, Brazil E-mail: bdenadai@rc.unesp.br Accepted for publication Received 14 March 2012; accepted 29 June 2012 Key words injury; isokinetic; power; strength; torque The aim of this study was to investigate the effect of fatigue induced by an exhaus- tive laboratory-based soccer-specific exercise on different hamstrings/quadriceps (H:Q) ratios of soccer players. Twenty-two male professional soccer players (231 ± 34 year) performed maximal eccentric (ecc) and concentric (con) con- tractions for knee extensors (KE) and flexors (KF) at 60° s 1 and 180° s 1 to assess conventional (H con :Q con ) and functional (H ecc :Q con ) ratios. Additionally, they performed maximal voluntary isometric contraction for KE and KF, from which the maximal muscle strength, rate of force development (RFD) and RFD H: Q strength ratio (RFDH:Q) were extracted. Thereafter, subjects were performed an exhaustive laboratory-based soccer-specific exercise and a posttest similar to the pretest. There was significant reduction in H con :Q con (060 ± 006 versus 058 ± 006, P<005) and in H ecc :Q con (129 ± 02 versus 116 ± 02, P<001) after the soccer-specific exercise. However, no significant difference between Pre and Post exercise conditions was found for RFDH:Q at 0–50 (053 ± 023 versus 057 ± 024, P>005) and 0–100 ms (053 ± 017 versus 055 ± 017, P>005). In conclusion, H:Q strength ratios based on peak force values are more affected by fatigue than RFDH:Q obtained during early contraction phase. Thus, fatigue induced by soccer-specific intermittent protocol seems not reduce the potential for knee joint stabilization during the initial phase of voluntary muscle contraction. Introduction During a soccer match, the total distance covered (9–12 km) (Mohr et al., 2003; Stølen et al., 2005), number of actions (~1300) including about 220 runs at high speed and the fre- quent change of activity or direction (Mohr et al., 2003) may induce to significant decreases in the performance during and immediately after the game. Indeed, studies have shown that the total distance covered and high-intensity activities in the second half of the game, particularly in the last 15-min period (Mohr et al., 2003; Rampinini et al., 2007), were impaired. Moreover, significant reductions in jump ability (Magalha ˜es et al., 2010), sprint (Magalha ˜es et al., 2010) and repeated sprint performance (Krustrup et al., 2010) have been found after the game. These impairments of soccer performance have been attributed, at least in part, to the reduction in maximal strength or power (i.e. fatigue) (Mohr et al., 2005). Muscle fatigue can defined as a reduction in the maximal force exerted by a muscle or a muscle group because of central and/or peripheral mechanisms (Enoka & Stuart, 1992). In fact, Rampinini et al. (2011) have recently found in high-level professional soccer players that match-related fatigue was determined by a combination of central and peripheral factors. The constant, dynamic nature of a soccer match explains, to some extent, the relatively high rates of injury among profes- sional soccer players compared with participants in other sports and occupations (Hawkins et al., 1999). Epidemiological studies have demonstrated that muscular strains are a primary injury type in professional soccer (Woods et al., 2004). Even admitting the multifactorial origin of hamstring injury, epide- miological evidence highlights that poor eccentric muscular strength and muscular strength imbalance play a central role in targeted acute muscle injuries (Croisier et al., 2008). More- over, some studies have found an increased incidence of hamstrings muscle strains in the second half, especially during the last quarter of the match (Hawkins et al., 2001). Thus, some authors have hypothesized that muscular fatigue might increase the susceptibility of a player to injury, particularly as incidents intensify towards the end of the match (Hawkins et al., 2001; Rahnama et al., 2003; Woods et al., 2004). Imbalance between the knee flexors (KF) and extensors (KE) has been traditionally assessed by conventional concen- tric hamstrings/quadriceps ratio (H con :Q con ) and functional Clin Physiol Funct Imaging (2013) 33, pp18–23 doi: 10.1111/j.1475-097X.2012.01160.x 18 © 2012 The Authors Clinical Physiology and Functional Imaging © 2012 Scandinavian Society of Clinical Physiology and Nuclear Medicine 33, 1, 18–23