Los problemas de la seguridad

The differences in the magnitude of force loss reported in the literature may be due to the different protocols used to bring about damage. It is not unreasonable to suggest that all damaging protocols are not equal in their ability to bring about damage to a group of target muscles. For example, it is possible that the completion of 300 eccentric contractions of the quadriceps on an isokinetic dynamometer (Barnes et al. 2010; McIntyre et al. 1996) or 100 barbell squats with a load of 80 % of subjects concentric one repetition maximum, as employed by Byrne and Eston (2002b), is more damaging than 150 unweighted walking lunge steps (Malm et al. 1999), 40 minutes of downhill walking (Nottle and Nosaka 2007) or 100 vertical jumps (Twist and Eston 2005; 2007). Indeed, research has revealed that a number of variables contribute to the severity of EIMD related force loss.

The high force and low motor unit recruitment that occurs during eccentric contraction is likely to place large mechanical strain on the sarcomeres and connective tissue responsible for generating force (Enoka 1996). Whether strain or force is principally responsible for EIMD, and associated losses in force, is unclear with findings confirming that each factor plays a role in the damaging process. The magnitude of strain a muscle is exposed to as it lengthens, rather than the high force being produced, has been suggested as the main contributing factor (Lieber and Fridén 1993), conversely others have shown that force loss, and therefore EIMD, is closely associated with the force generated during eccentric exercise (Deschenes et al. 2000; McCulley and Faulkner 1985; Warren et al. 1993a).

The length of the muscle during contraction also influences the magnitude of damage with muscles exercised at long lengths displaying greater changes in strength, and other characteristics associated with EIMD, than those exercised at shorter lengths (Child et al. 1998; McHugh and Pasiakos 2004; Newham et al. 1988; Nosaka and Sakamoto 2001; Panchangam et al. 2008; Paschalis et al. 2005a). Additionally, when time under tension is controlled for, the rate at which the muscle lengthens during eccentric work has also been shown to influence the magnitude of EIMD (Chapman et al. 2006). Finally, the number of eccentric contractions (Baker et al. 2007; Chapman et al. 2008; Hesselink et al. 1996; Nosaka et al. 2002) and intensity (Paschalis et al. 2005c) during the exercise bout also impacts the severity of damage seen in the days after exercise; more contractions and higher intensities result in greater levels of damage. Together these factors suggest that significant EIMD is likely to occur when a muscle repeatedly and rapidly lengthens under high force at long lengths, as occurs when the hamstring contracts eccentrically during sprinting (Yu et al. 2008), for example.

2.2.3.2.4 Implications of EIMD on functional performance

As discussed above, the effect of EIMD on voluntary force production is well established however the performance of an individual muscle group under isokinetic or isometric conditions may not be representative of muscle function during dynamic, multi-joint movement, as occurs during exercise and sport. This lack of application is behind the relatively recent increase in research investigating changes in a variety of functional performance measures after strenuous eccentric exercise.

2.2.3.2.4.1 Power output

While the generation of maximal forces during exercise is important, optimal power development, the rate at which force is generated, is a critical component of many sports (Duchateau and Hainaut 2003). The rate of force development and peak power output (PPO) have been suggested as key variables to sporting success, particularly in sports that rely heavily on strength and power (Stone et al. 2002) such as throwing and jumping sports and the contact football codes which include American football, rugby union, rugby league, Gaelic football and Australian Rules football. Stone et al. (2002) suggest that PPO is likely to be the most important attribute in determining the outcome of sporting performance. Given the relationship between power output and

force production (Power = force x distance / time) it is likely that any condition that limits the generation of force, such as EIMD, would directly impact PPO.

Research investigating the effect of EIMD on PPO during maximal anaerobic cycle tests, such as the 30 s Wingate anaerobic test, has been contradictory with a lack of change reported by Malm et al. (1999) and Nottle and Nosaka (2007). Conversely, a decrease in PPO in the days following damaging exercise has been reported by Sergeant and Dolan (1987), Byrne and Eston (2002b) and Twist and Eston (2005; 2007). Additionally, the latter study by Twist and Eston (2007) found that time to PPO was impaired 48 h after the completion of a repeated vertical jump protocol. The inability to produce peak power, particularly at the start of exercise, may be due to an inability to produce maximal forces as a result of a combination of peripheral and central mechanisms including the preferential damaging of type II fibres, decreased voluntary activation and EC coupling failure (Byrne and Eston 2002b; Twist and Eston 2007), as discussed above.

These incongruous results are difficult to explain given the corresponding changes in other measures commonly associated with EIMD, in particular decreases in voluntary force production and elevations in CK, reported by Nottle and Nosaka (2007), Byrne and Eston (2002b) and Twist and Eston (2005; 2007). It is worth noting that, although the subjects in Malm et al. (1999) reported significant levels of DOMS two days post exercise, no significant change in performance or CK was evident at any time post - exercise. Therefore it is unclear whether the protocol used was sufficient to bring about significant levels of EIMD.

More conclusive are the decrements in vertical jump height observed up to three days post-exercise. Vertical jump performance is closely related to sprinting speed (Cronin and Hansen 2005) and provides an accurate measure of lower body power output when combined with body mass and the appropriate regression equation (Harman et al. 1988; Sayers et al. 1999). While decreases in jump performance have been seen with squat, counter movement (CMJ) and drop jumps the magnitude and mechanism of change may differ between the three jump types (Byrne and Eston 2002a). Decrements in squat jump height, which relies more on force production than the other two jump types, appears to be most affected. This is possibly due to the lack of

contribution from the muscles elastic elements during this concentric only jump (Byrne and Eston 2002a; Marginson et al. 2005). On the other hand, decreases in drop jump performance (Byrne and Eston 2002a; Horita et al. 1999; Horita et al. 2003; Twist and Eston 2007), which relies on the stretch – shortening cycle, and accompanying increases in ground contact time (Twist and Eston 2007) have been observed up to 72 h post – exercise. Along with decrements in force production, changes in drop jump height may be impaired by the muscles decreased tolerance to impact and stretch. During the drop jump, for example, ground contact time may increase as a result of subjects attempting to dampen impact with the ground thus reducing the stretch reflex (Byrne and Eston 2002a; Twist and Eston 2007).

Reductions in power output provide some insight into the functional effects of EIMD; however few sports involve single bouts of explosive movement. Perhaps more relevant to most sports is how repeated power output is altered in the days following a bout of damaging exercise. Surprisingly, repeated (10 x 10 s) sprint cycle performance was found to improve in the days following a bout of damaging exercise (Malm et al. 1999). However, as previously stated, whether the exercise model used by Malm et al. (1999) was appropriate to bring about EIMD is somewhat questionable. Others have shown repeated sprint ability, both on a cycle ergometer (10 x 6 s) and on foot (10 x 10 m), to be negatively impacted up to 48 h after damaging exercise (Twist and Eston 2005). Similarly, speed over 10 m and agility, which is reliant on PPO (Stone et al. 2002), was also reduced in the presence of EIMD (Highton et al. 2009). Not surprisingly, the same mechanisms as those responsible for changes in PPO have been implicated in these reductions in repeated sprint and agility performance (Twist and Eston 2005; Highton et al. 2009). Irrespective of the mechanism it is clear that EIMD results in significant alterations in both one off and repeated power output; outcomes which are likely to have serious consequences for sports performance.

2.2.3.2.4.2 Running economy

As with force and power production alterations in running economy (RE), as measured by steady state oxygen consumption during fixed, sub-maximal running speeds, may negatively impact sports performance (Saunders et al. 2004). Once again the results of studies investigating this topic have proven contradictory with a number

of studies (Hamill et al. 1991; Marcora and Bosio 2007; Paschalis et al. 2005b; Vassilis et al. 2008) failing to see any change in RE after damaging exercise. Although Marcora and Bosio (2007) reported a decrease in the distance covered during a treadmill time trial this was unrelated to changes in RE, but instead appeared to be the result on an altered perception of effort. That is, subjects reported higher ratings of perceived exertion (RPE) at lower speeds when running after damaging exercise compared to pre-exercise values. This result is similar to the changes in force matching and sense of effort (Carson et al. 2002; Proske et al. 2004; Proske et al. 2003; Weerakkody et al. 2003) with EIMD reported earlier. Paschalis et al. (Paschalis et al. 2005b) and Vassilis et al. (2008) suggest that the lack of change in RE in their study may have been intensity related with sub-maximal running economy likely to be influenced by type I motor units whereas type II motor units may be preferentially recruited and therefore damaged during eccentric exercise. This implies that only at maximal intensities, such as during time trial performance, as observed by Marcora and Bosio (2005), would the impact of EIMD truly be seen. This may be a valid explanation however this does not fit with the findings of other studies, discussed below, that tested RE at a similar range of intensities. A more likely reason, as Paschalis et al. (2005) and Vassilis et al. (2008) also propose, is that the exercise used to bring about damage only targeted the quadriceps and therefore, because running involves many more muscles than just the quadriceps, this exercise protocol may not have been appropriate for use in investigating RE. This highlights the specific nature in which EIMD affects certain physical parameters and suggests that both the exercise model and performance measures used should be considered carefully when investigating the effects of EIMD on physical performance.

Contrary to the findings discussed above, Braun and Dutto (2003) and Chen et al. (2007b) found that RE may be reduced by between 3 and 7 % for up to three days post-exercise when downhill running is used to bring about EIMD. This affect was correlated with alterations in running form, in particular a decrease in stride length, stride frequency and joint range of motion, and occurred on a different time line to changes in muscle function. Chen et al. (2007b) speculate that reductions in RE may be due in part to impairment in the stretch–shortening cycle and increased recruitment of muscle fibres to compensate for those damaged during exercise. It is also possible that the reduced stride length and increased stride frequency reduces impact during

running, in a similar way that increasing the contact time during jumping does, so that the sensations of pain that are typically associated with EIMD are reduced.

2.2.3.2.4.3 Cycling performance

Endurance cycling performance is affected by EIMD. As with short duration sprint cycling (Bryne and Eston 2005; Twist and Eston 2005; 2007), PPO and average power output, and therefore distance covered, during five (Twist and Eston 2009) and 15 (Burt and Twist 2011) minute cycling time trials is reduced 48 h after damaging exercise. Similarly, cycling time to exhaustion is also reduced at the 48 h point (Davies et al. 2009). At sub-maximal intensities the VO2 response to cycling is not

altered by EIMD (Davies et al. 2009; Gleeson et al. 1995; Schneider et al. 2007; Twist and Eston 2009) suggesting that maximal, but not sub - maximal cycling is affected in the days after eccentric exercise. As with changes in running time trial performance (Marcora and Bosio 2007), reduced cycling performance appears to be closely related to altered perceptions of effort, at both sub-maximal (Twist and Eston 2009) and maximal intensities (Burt and Twist 2011; Davies et al. 2009; Twist and Eston 2009) such that subjects believe that they are performing at a higher intensity than they actually are. Together these findings indicate that EIMD may influence pacing ability during endurance cycling and running, an outcome that has direct implications for competition and training (Twist and Eston 2009).

2.2.4 Rugby Union

The sport of Rugby Union, or simply rugby as it will be referred to from here on, has a close and historic relationship with alcohol (Collins and Vamplew 2002). As previously discussed, rugby players are amongst the group of sports people who regularly consume hazardous amounts of alcohol through binge drinking behaviour (O'Brien and Lyons 2000; Quarrie et al. 1996). While the physical nature of rugby means that players are susceptible to muscle injury, through what is often violent contact with other players and/or the ground and as a result of eccentric muscular contraction, it is currently not known how the combination of alcohol and the physical stress induced by a rugby match impacts recovery in the days following the match.