FIT FOR FOOTBALL?
Physiological demands of football
Football is a famous team sport played worldwide. A match lasts 90 minutes in duration, played over two 45-minute halves (Di Falco, 2015; Stolen, Chamari, Castagna, & Wisloff, 2005). The sport involves many intermittent periods of high-intensity exercise (Di Falco, 2015; Stolen et al., 2005; Zouhal et al., 2013) which will be termed “explosive football actions” throughout this article. Explosive football actions are highly reliant on anaerobic metabolism and include interactions both with and without the ball (Di Falco, 2015; Stolen et al., 2005). Those without the ball involve sprinting, jumping and rapid changes of direction in response to the flow of the game, dictated by opposing players and where the ball is (Stolen et al., 2005). These actions are interspersed with periods of jogging and walking (Stolen et al., 2005). While these explosive football actions are critical components of the sport and crucial in determining the outcome of a match, they only account for 8-12% of the total distance covered in a match (Di Falco, 2015).
At the elite level, football players cover around 10-12 kilometers (km) in a game at intensities close to the anaerobic threshold (Di Falco, 2015; Han, Can, & Sey, 2012; Impellizzeri et al., 2006; Stolen et al., 2005; Zouhal et al., 2013). Games last 90 minutes (or in some instances, 120 minutes). Subsequently, aerobic performance capabilities underpin a footballer’s ability to perform for that length of time. Aerobic metabolism dominates football with an energy contribution of approx. 90 % (Zouhal et al., 2013). Distance covered is therefore heavily associated with the aerobic capacity of the footballer, and an individual with higher maximal oxygen uptake (VO2 max) has a greater capacity to perform repetitive explosive football actions (Zouhal et al., 2013). Late in a game, the amount of distance covered reduces by 5-10% because of the onset of fatigue and, consequently, the number of explosive football actions decreases (Di Falco, 2015; Reilly, 1997; Stolen et al., 2005).
While the physiological demands do not change, the magnitude of the demands varies by playing position. Midfielders reportedly display higher VO2 max values and run greater distances in a game than other positions (Bloomfield, Polman, & O’Donoghue, 2007; Di Falco, 2015; Han et al., 2012; Stolen et al., 2005). Sprint distances also vary by position with central midfielders, wide midfielders, and strikers all sprinting 1.7 times the distance of a centre half. Elite fullbacks sprint 2.5 times more than centre-halves, the most of any position (Stolen et al., 2005). Aerobic capacity improves with training. Recovery between bouts of explosive football actions, as well as between games, will also improve as a consequence of improved aerobic performance (Stone & Kilding, 2009).
Training strategies for aerobic performance enhancement
Maximal aerobic capacity shares a correlation with performance parameters in football, e.g. distance covered and the number of sprints per game; those who are aerobically fit have more involvement in possession of the football (Turner & Stewart, 2014). Football’s intensity is close to the Lactate Threshold (LT) – 80-90% HR max – and therefore, training programmes should also focus on improving the LT. Helgerud, Engen, Wisloff, and Hoff (2001) reported that an athlete with a higher LT would be able to perform at higher intensities for the same VO2 max value. A footballer with the same VO2 max value as an opponent, but with a higher LT, would be the superior athlete (Helgerud et al., 2001). The game of football is intermittent by design, and continuous training protocols are not recommended. Instead, it is generally accepted that high-intensity interval training (HIIT) should be prescribed (Di Falco, 2015; Stolen et al., 2005).
The literature demonstrates that HIIT is the optimal method for improving VO2 max and LT in footballers. Helgerud et al. (2001) observed a 10.8% improvement in VO2 max after eight weeks of HIIT involving running at intensities above the LT. The improvement leads to a substantial increase in the amount of distance covered, and the number of involvements with the ball in a match (Helgerud et al., 2001). Small-sided games (SSG) as a form of HIIT, are considered a more specific way to improve aerobic performance than running-based HIIT. Impellizzeri et al. (2006) compared the effects of both training modes over 12 weeks, using the same training programme parameters; that is, four bouts of 4 minutes of work at high intensities (90-95% HR max). The researchers observed no statistically significant differences between the two training modes, however observed marked improvements in both groups. Impellizzeri et al. (2006) concluded that the adopted training mode could be determined based on practicality, with SSG’s potentially the method to use when training time is scarce, due to the technical and tactical benefits they offer.
Hoff, Wisloff, Engen, Kemi, and Helgerud (2002) introduced the Hoff track, a dribbling track that involves sprinting, hurdling and rapid changes of direction with a ball (Zouhal et al., 2013). They compared the Hoff track to SSG’s in terms of training intensity; higher training intensities are possible with the Hoff track (Hoff et al., 2002). One critique of this study is that the SSG format used was five aside and it is possible that a format of four or three aside could incite higher training intensities, due to the increased demand placed on players (Turner & Stewart, 2014). Nevertheless, the researchers concluded that their findings suggest a ceiling effect on training intensities can be realised during SSG for those with higher VO2 max values (Hoff et al., 2002). These individuals should train on the Hoff track due to the higher training intensities demonstrated in this study (Hoff et al., 2002).
Researchers compared the Hoff track to HIIT involving 15-second sprints followed by 15 seconds of jogging. Each training method lasted a total of 25 minutes at training intensities higher than LT (Zouhal et al., 2013). What Zouhal et al. (2013) discovered was that the Hoff track incited higher VO2 max values and blood lactate accumulation than its counterpart, adding further weight to the potential training benefits of this methodology. While there is benefit in general HIIT, the more specific methodologies discussed (Hoff track, SSG) involve the ball and allow for a range of explosive football actions to be performed during the work intervals. The interval exercise parameters discussed above are in keeping with recommendations made by Hoff and Helgerud (2004). They report that intervals in a range of 3-8 minutes at exercise intensities of 90-95% HR max are valuable. VO2 max improvements in the range of 10-30% can be expected over a time course of 8-10 weeks (Hoff & Helgerud, 2004). Rest periods of 3 minutes should be interspersed with the work intervals at 70% of HR max, as this is considered optimal for removal of waste product from anaerobic metabolism, e.g. lactate and hydrogen ions (Helgerud et al., 2001; Hoff & Helgerud, 2004; Hoff et al., 2002; Turner & Stewart, 2014).
Physiological Adaptations to HIIT
Training intensity is the primary determining factor in whether the initial physiological training response occurs in the central or peripheral component (Stone & Kilding, 2009). With training intensities below the LT, the first adaptation occurs centrally with improvements in the heart’s ability to pump and distribute blood throughout the body (Stone & Kilding, 2009). Increases in cardiac output (CO) occur because of increases in the end-diastolic volume, and left ventricular mass, off-setting improvements in stroke volume (SV) (Stone & Kilding, 2009). As discussed earlier, HIIT formats incorporated into footballer’s training programmes should be performed at intensities surpassing LT, e.g. 90-95% HR max (Di Falco, 2015; Hoff & Helgerud, 2004; Stolen et al., 2005). Under these conditions the training response occurs peripherally. Performance improvements such as VO2 max and rightward shifts in the LT result in increased muscle capillarisation, increased oxidative enzyme activity, increases in the size and density of mitochondria, and changes in myoglobin content (Stone & Kilding, 2009).
An increased capillary density in the muscle is one of the most critical training adaptations for improvements in VO2 max (Wilmore, Costill, & Kenney, 2008). The diffusion of oxygen from the capillary to the mitochondria improves with aerobic HIIT. It results in an environment conducive to oxidative energy production and an increased number of explosive football actions (Stone & Kilding, 2009; Wilmore et al., 2008). Myoglobin content increases with training, as mentioned above. When oxygen enters the muscle fibre it binds itself to the myoglobin protein, which acts to transport oxygen to the mitochondria from the cell membrane (Wilmore et al., 2008). Myoglobin also stores oxygen for use in situations where oxygen delivery is scarce, and therefore having higher myoglobin content will enhance oxygen storage capacity (Wilmore et al., 2008). The increases in mitochondrial size and density with aerobic HIIT and changes in oxidative enzyme activity will result in glycogen sparing; that is, a decline in the rate at which muscle glycogen is broken down and utilised by the body for energy production. A greater reliance on fats as an energy source will be obtained (Wilmore et al., 2008).
Football involves many anaerobic actions, yet is underpinned by a necessity to be aerobically capable. The physiological adaptations discussed will lead to improvements in VO2 max and a rightward shift in the LT. HIIT is considered the optimal training strategy for footballers, with the Hoff track the HIIT exercise of choice for advanced clientele. On-field performance improvements in distance covered, interactions with the ball, and number of explosive football actions completed, are desirable – all are attainable through HIIT. Improved aerobic fitness will also enable faster recovery between bouts of explosive football actions as well as between games due to an increased ability to buffer lactate and hydrogen.
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