2.0 Muscular Strength vs. Single Sprint Performance
Muscular strength is a physiological characteristic thought to facilitate sprint performance, mainly due to the increased ability of the muscle to generate muscular contraction during short-term high intensity activity (Baechle and Earle, 2008). Experts have defined muscular strength as the ability of a muscle or muscle group to exert maximal force against a resistance once, through the full range of motion (Newman et al., 2004). Specifically, literature has deemed the force production capabilities of the legs to be the decisive factor in improving ones ability to sprint (Thorland et al., 1987; Alexander, 1989; Dowson, 1998). For instance, an initial study by Alexander (1989) examined the correlation between leg strength and 100m sprint performance in 23 elite sprinters. For all participants, a significant relationship was found between the two variables. In support, a similar study conducted by Dowson et al. (1998) examined the relationship between leg strength and sprint performance in 18 elite male athletes. Again, leg strength was deemed to be a major contributor to ones ability to sprint over shorter distances of 15 and 35m. Although ample studies have investigated the relationship between muscular strength and sprint performance, with several findings recognising strength as a significant facilitator (Thorland et al., 1987; Alexander, 1989; Dowson et al., 1998), a similar investigation by Baker and Nance (1999) deduced that no measure of strength was correlated to either 10m or 40m sprint performance in Rugby League players. Moreover, Cronin and Hansen (2005) examined the relationship between muscular strength and measures of first-step quickness (5-m time), acceleration (10-m time), and maximal speed (30-m time), with all 3 measures of speed proving to be unaffected by muscular strength.
It appears that studies examining the relationship between leg strength and sprint performance have produced contradictory conclusions. However more importantly, the investigations discussed above only implemented single sprint trials. Current research has identified that perhaps repeated-sprint efforts reflect a more accurate representation of the intermittent physiological demands of multiple sprint sports, rather than a single sprint (Dawson et al., 1991; Duthie et al., 2006; Oliver et al., 2007). For instance, a study carried out by Dawson et al. (1991) noted that for high intensity efforts of less than 5 seconds such as a 1RM squat or single 40m sprint test, the largest contribution to the energy demands was made by the phosphagen (ATP-CP) energy system. Moreover, Dawson et al. (1991) recognised that the contribution of the glycolytic system in ATP re-synthesis significantly increased when sprint efforts were repeated, which more accurately reflects the physiological demands of intermittent sports. This notion has since received support from other studies examining repeated sprint ability (Newman et al., 2004; Bishop and Edge, 2006). Therefore, although studies have formed contrasting opinions on the effectiveness of leg muscle strength in predicting a single sprint performance (Thorland et al., 1987; Alexander, 1989; Dowson et al., 1998; Baker and Nance, 1999; Cronin and Hansen, 2005), the validity of these investigations appears limited when considering the imprecision of using a single-sprint to assess multiple-sprint sport performers.
2.1 Isokinetic Strength vs. Multi Joint Strength Measures
As discussed, it is widely recognised within the literature that RSA is more ecologically valid than a single sprint when assessing team-sport athletes (Dawson et al., 1991; Newman et al., 2004; Duthie et al., 2006; Oliver et al., 2007). Despite this knowledge, the association between muscular leg strength and repeated sprint ability has received scant consideration. However, one investigation by Newman et al. (2004) did specifically examine the relationship between isokinetic knee strength and repeated sprint ability in soccer and rugby players. Newman and colleagues reported that no measure of strength was related to RSA, thus concurring with other similar investigations (Baker and Nance, 1999; Cronin and Hansen, 2005) who albeit implemented single sprint trials. Despite Newman et al. (2004) notions, the methods employed to assess leg strength may have influenced their findings and reduced their validity. For instance, Newman et al. (2004) highlighted that the isokinetic knee extension test they implemented was not specific to the conditions and movement patterns performed in the sporting environment. To elaborate, Flint-Wagner et al., 2009 highlighted that isokinetic strength tests generally require participants to be placed in a position that isolates the tested muscle. Specialized apparatus then gives resistance so that no matter how much force is exerted by the participant, movement takes place at a constant speed. Although this method of testing has been established as beneficial during the injury rehabilitation process for testing a specific area or joint movement (Cardone et al., 2004), the validity of the procedure has been has been questioned in able team-sport athletes. For example, research by Duthie et al. (2006) illustrated that during sports performance, seldom does one body part work in isolation at a constant speed. Instead, Newman et al. (2004) and later Duthie et al. (2006) recognised that muscles are required to work synergistically in an integrated and co-ordinated fashion. This indicates that perhaps using a multi-joint strength measure would give a more valid reflection of muscular strength in rugby players, as muscles are required to interdependently. Even so, studies evaluating the effect of leg strength on the ability to sprint have been inclined to implement laboratory based isokinetic strength measures (Adams et al., 1992; Blazevich and Jenkins, 1998; Newman et al., 2004; Kin-Isler et al., 2008). Therefore, a study examining the relationship between a multi-joint strength measure and repeated sprint ability warrants investigation.
2.2 Brief vs. Prolonged Repeated Sprint Ability
In addition to the strength measure used, recent studies on RSA have begun to examine the structure of the brief RSA protocols previously implemented (Oliver et al., 2007; Rampinini et al., 2007). To elucidate, a study by Oliver et al. (2007) examined the physiological relationship between brief and prolonged repeated sprint ability protocols. They identified that the participant's maximal speed significantly decreased in the latter sets of the brief RSA test, but not in the prolonged test. This suggests that the brief RSA protocol previously employed by Newman et al. (2004) will have influenced the strength/RSA correlation witnessed. For example, Newman and colleagues implemented 20 second recovery periods in the RSA test, less than the half-life of CP re-synthesis (Gaitanos et al., 1993). According to Bishop and Edge (2006) who examined the determinants of RSA in female athletes, a short recovery period between sprints induces the accumulation of H+ ions, causing a reduction in repeated sprint ability. Therefore, if Newman et al. (2004) had implemented prolonged periods of recovery between sprints (i.e. more than the half-life of CP re-synthesis), the relative contribution of the glycolytic and phosphagen systems will have changed, which would have affected the relationship between muscular leg strength and RSA. This idea was demonstrated by Oliver et al. (2007) who indicated that a relationship between muscular strength and prolonged repeated sprint recovery could be plausible, provided the recovery was long enough for near phosphagen repletion. Furthermore, Oliver et al. (2007) theorised that although brief RSA protocols are accurate for reflecting short intense periods of play, longer periods of recovery between sprints perhaps reflect a more accurate portrayal of the recovery patterns experienced throughout the course of a whole match. This view was endorsed by Bishop and Edge (2006) who stated that a large majority of sprints experienced during intermittent sports are separated by rest periods long enough (> 1 min) to allow complete or near complete recovery, over double the amount of rest implemented in the majority of RSA studies (Dawson et al., 1991; Newman et al., 2004; Edge et al., 2006; McGawley and Bishop, 2006; Spencer et al., 2008).
Consequently, it appears that the correlation between leg strength and prolonged RSA has yet to be adequately understood, and thus, a study investigating this relationship requires examination.
2.3 Repeated Sprint Ability Protocols
In addition to the length of recovery performed, recovery mode has been shown to affect ones performance during repeated sprint exercise, making it a key component of any RSA protocol. Recently, studies have produced contrasting findings in determining whether passive or active recovery is the most beneficial in resisting fatigue. Some studies have proposed that active recovery is superior as blood muscle flow is maintained, which enhances the buffering and removal of hydrogen ions (Bogdanis et al., 1996; Toubekis et al., 2008). Conversely, other studies have indicated that active recovery negatively affects the subsequent sprint performance, by inducing fatigue and slowing the rate of PC resynthesis (Dupont et al., 2003; Spencer et al., 2006). Despite this conflict, time motion analysis has illustrated that phases of recovery in team sports typically involve some sort of active work (i.e. jogging or shuffling into defensive position) (Spencer et al., 2004; Jougla et al. 2009). Therefore, active recoveries appear to represent the most valid form of recovery when testing repeated sprint performance in rugby union players. Despite this knowledge, numerous repeated-sprint studies on team sport athletes have implemented passive recovery in their protocols (Bishop et al., 2004; Edge et al., 2006).
Another factor that can affect repeated sprint performance is the length of each executed sprint. Previous studies on repeated sprint ability have typically used sprint distances requiring 6 seconds of work to assess their participants (Gaitanos et al., 1993; Dawson et al., 1997; Bishop et al., 2004; Edge et al., 2005). However, Spencer et al. (2004) suggested that shorter sprint durations of 4 seconds provide a more accurate portrayal of the sprint distances typically experienced during team sports. Spencer and colleagues based this notion on their time-motion analysis of RSA patterns in elite field hockey, which is deemed by Spencer et al. (2004) to elicit similar physiological demands to rugby union. In support, other time motion analysis studies by Duthie et al. (2006) and later Deutsch et al. (2007) reported that for all rugby positions, the mean duration of sprints was 2-4 seconds. This suggests that although 6 second sprints have been widely used within the literature, they may not be optimal for testing RSA in team sport athletes. This may explain why recent studies on team sport athletes have begun to implement shorter distances of 3-5s in their RSA protocols (Spencer et al., 2006; Oliver et al., 2007; Spencer et al., 2008), rather than the 6 second distances previously employed.
Additionally, RSA studies on team sport athletes have typically included 5-10 sprints in their protocols, as this is thought to represent the most accurate depiction of a brief intense period of play (Gaitanos et al., 1993; Dawson et al., 1997; Bishop and Spencer., 2004; Bishop et al., 2004; Edge et al., 2005; Spencer et al., 2006; Oliver et al., 2007; Spencer et al., 2008), shown in table 1. This idea was forwarded by Spencer et al. (2004) who examined the number of repeated sprints executed during a brief intense period of play in elite hockey. A brief intense period of play was defined as a minimum of three high intensity sprints, with a mean recovery duration of less than 21 s. Spencer et al. (2006) findings revealed that the majority of intense phases of play were comprised of 3-7 sprints. Therefore, based on Spencer et al. (2004) findings and the majority of previous RSA studies, 5-10 sprints appear to most accurately represent a short intense period of play in team sports.
A further variable to consider when designing RSA protocols is the mode of exercise performed. Although non-motorised treadmills and over-ground sprints provide the most accurate mode of assessing RSA in team sport athletes, they have been sparsely administered within the literature (Spencer et al., 2006; Oliver et al., 2007). In contrast, Table 1 shows that the majority of RSA investigations have implemented cycle ergometers to analyse their participants (Gaitanos et al., 1993; Dawson et al., 1997; Bishop et al., 2004; Bishop and Spencer, 2004; Edge et al., 2005). Recently, experts have questioned the validity of employing cycle ergometry to assess team sport athletes, who primarily execute over-ground sprints in game situations (Fitzimmons et al., 1993; Bishop et al., 2001; Oliver et al., 2007). Therefore, it appears that although cycle ergometers may provide a convenient means for recording muscle biopsies, analysing gas samples and eliminating environmental conditions. For most field sports, cycle ergometers provide a poor reflection of the physiological movements typically experienced during a match. Hence, when assessing RSA in team sport athletes such as soccer, hockey and rugby players, the most valid method of assessment appears to be over-ground sprints, as used by Spencer et al. (2008) which is shown in Table 1.
2.4 Explosive Power
Although the relationship between muscular leg strength and sprint performance has produced ambiguity within the literature (Thorland et al., 1987; Alexander, 1989; Dowson et al., 1998; Baker and Nance, 1999; Newman et al., 2004; Cronin and Hansen, 2005; Kin-Isler et al., 2008), recent studies have suggested that perhaps explosive power is more of an accurate determinant of sprint performance (Dowson et al., 1998; Hennessy and Kilty, 2001). Explosive power is defined as the maximal force that a muscle or muscle group can generate at high speeds, or (work/time) (Baechle and Earle, 2008). Investigations by Dowson et al. (1998) and Hennessy and Kilty (2001) have tested the effectiveness of explosive power in predicting sprint performance by implementing field based tests such as vertical countermovement jumps. Interestingly, Dowson et al. (1998) identified that the magnitude of force generated during a countermovement jump significantly correlated with the amount of speed an athlete produced during a single-sprint performance, indicating that explosive power could potentially be a direct predictor of sprint performance. In support, Hennessy and Kilty (2001) reported a similarly significant relationship between countermovement jumps and sprint performance in female athletes. They attributed this relationship to the stretch-shortening cycle (SSC) witnessed during a countermovement jump. To elaborate, the stretch-shortening cycle was found to mimic the eccentric-concentric contractions of the leg extensor muscles experienced during sprinting, which directly facilitated sprint performance. After reviewing the literature, it appears that studies ascertaining the relationship between explosive power and a single sprint performance have produced consistent results, as well as being valuable in terms of ecological validity (Dowson et al., 1998; Hennessy and Kilty, 2001) compared to such laboratory based strength investigations (Adams et al., 1992; Blazevich and Jenkins, 1998; Newman et al., 2004; Kin-Isler et al., 2008). However, research has continued to solely focus on the relationship between explosive power and a single sprint, neglecting the opportunity to examine the association between explosive power and repeated-sprint ability, despite research highlighting the specificity of RSA to the patterns of play witnessed during multi-sprint sports (Dawson et al., 1991; Newman et al., 2004; Bishop and Edge, 2006; Duthie et al., 2006; Oliver et al., 2007). Subsequently, an investigation examining the effect of explosive power on brief and prolonged repeated sprint ability appears warranted in order to fully establish explosive power as the most accurate physiological predictor of RSA.
Accordingly, the primary aim of current study was to examine the effect of muscular strength on brief and prolonged repeated sprint ability using a multi-joint strength measure. A secondary purpose was to ascertain if explosive power was a more valid determinant of brief and prolonged repeated sprint ability than muscular strength.