The effects of High Intensity Interval Training (HIIT) on the predicted VO2max of Youth Academy Football Players

The effects of High Intensity Interval Training (HIIT) on the predicted VO2max of Youth Academy Football Players

Adam Brooks

Abstract
The purpose of this study was to compare the effects of High Intensity Interval Training (HIIT) on the predicted VO2max of Youth Academy Football Players. It was hypothesised that by part taking in a HIIT session once a week over a 6-week can increase predicted VO2max more significantly as compared to an individual who only participates in general steady state exercise. Results showed a significant difference in the mean value of the post-test predicted VO2max of the Training Group (TG) when compared to the Control Group (CG). The mean difference between the predicted VO2max of CG and TG was 4.07 ml/kg/min. The improvement in VO2max from HIIT training was in accordance with previous studies (Gibala & McGee, 2008: Helgerud et al, 2001: Gromley et al, 2008: Gorostiaga et al, 1991).

In sport, fitness refers to the ability to perform specific physical skills or activities, to their optimal performance level. “In the sporting arena where teams and individuals are constantly seeking to better themselves at all levels of performance, fitness is one of the most influential and changeable elements of performance that a coach can effect” (Brewer, 2005). Vaeyens et al (2006) found Speed, Agility and Aerobic Power to be among the strongest predictors of talent in youth soccer players. Hawkins (2007), states that the demands placed on footballers are numerous and require players to develop every aspect of their physiological character. A study by Reilly et al (2000) found elite youth players to be leaner and to possess more aerobic power. It has been repeatedly demonstrated throughout research, that measures of aerobic fitness are closely related to soccer performance. As a result the assessment of aerobic fitness on a regular basis is vital for monitoring the effectiveness of physical training programs and the preparedness of soccer players for competition (Castanga et al, 2006). Traditional means of measuring ones aerobic fitness have focused on the ‘aerobic capacity’ or ‘VO2max’ of an individual. VO2Max is defined as the maximum volume of oxygen that can be utilised in one minute during maximal or exhaustive exercise (Hill and Lupton, 1923). VO2Max is primarily used to determine an athlete’s capacity to perform sustained exercise and is generally considered as the best indicator of cardio vascular endurance and aerobic fitness. O’Reilly & Wong (2012) state that high levels of aerobic fitness in elite soccer players is extremely important. Various investigations have shown significant decreases in high intensity activity towards the latter stages of a soccer match. For this reason, O’Reilly & Wong (2012) believe that, knowledge of a soccer player’s aerobic capacity is a valuable resource for coaches when identifying prospective talent. “A soccer match generally lasts for about 90 minutes, and recent evidence suggests that, for elite soccer players, the average oxygen uptake during a match, after allowing for factors such as dehydration, hyperthermia and mental stress, is around 70% of maximum oxygen uptake” (O’Reilly & Wong, 2012).
A recent study by Helgerud et al (2002) demonstrates a direct correlation between VO2 max and match performance, specifically, an increases in a subjects VO2Max of 11%, results in an increase of 5% in match intensity and distance covered by 1800 metres. Brewer (2005) is also in agreement and describes how an improvement in a player’s VO2Max will lead to an improvement in performance and the relative intensity at which a player can work. McMillan et al (2005) discuss how important it is, in the modern game, for elite players to possess high aerobic fitness because aerobic metabolism provides 90% of the energy cost during a match. Helgerud et al (2001) support with further evidence the correlation between enhanced performance and VO2max. They discusses their findings, namely, that the rank among the best four teams in the Hungarian top soccer division was directly correlated to the players rank among their average VO2max. This assertion is also supported by the results of demonstrating a significant difference in VO2max between the top team and a lower placed team in the Norwegian elite division (Helgerud et al, 2001). Research has determined mean values for elite footballers VO2max to be between 56 and 69 ml.kg-1.min-1. These values are similar to those reported in other team sports (Reilly et al, 2012). Jeukendrup & Martin (2001) understandably found that an initial high VO2max results in smaller increases through training, whereas an initially low VO2max at the onset of training will result in larger improvements.
Leger & Lambert, (1982) states that in order for one to assess the maximal aerobic power of a large group of subjects, one has to rely on valid, reliable, safe and inexpensive tests. The 20-m multistage fitness test (MSFT) is arguably, the most widely used test of aerobic fitness in the United Kingdom and is employed by many agencies including schools, sports clubs and the armed forces (Davies et al, 2008). A study by Ramsbottom et al (1988) suggested that a progressive shuttle run test, provides a more valid estimate of VO2max. The MSFT has been used for many years as a predictor of VO2max (Ramsbottom et al 1988 ; Leger & Lambert 1982). Chatterjee et al (2009) describes the MSFT as an excellent performance indicator of aerobic fitness, which requires many changes of direction. The 20-m shuttle run test offers the advantages of requiring no sophisticated equipment and enabling the testing of many individuals at the same time. It has been concluded that the 20-m MST is the most accurate and reliable form of field-testing to predict Vo2max, however, not in its original form (Chatterjee et al, 2009).
Training is recognised as one of the main modifiers of exercise performance (Jeukendrup & Martin, 2001). Stone & Kilding (2009) discuss numerous factors that interact to determine the overall magnitude of the performance enhancements in response to training. These factors include, exercise intensity and frequency, the duration of training, overall length of training programmes and the initial fitness level of individuals. Based on the analysis of a soccer game it is clear that, the training of elite players should focus on improving their ability to perform intense exercise and to recover rapidly from periods of high-intensity exercise. Bangsbo et al (2005) states that, performing aerobic and anaerobic training on a regular basis does this best. Recent evidence from Gibala & McGee (2008) suggests that, a number of metabolic adaptations usually associated with traditional high-volume endurance training can be induced quicker than previously thought, with a surprisingly small volume of High Intensity Interval Training (HIIT). Although there is no universal definition Gibala & McGee (2008) describe HIIT as “repeated sessions of relatively brief intermittent exercise, often performed with an ‘‘all-out’’ effort or at an intensity close to that which elicits VO2peak.” Depending on the intensity of training, a single effort may last from a few seconds to several minutes, with multiple efforts separated by up to a few minutes of rest or low-intensity physical activity. It has been proven that interval training enhances aerobic endurance in soccer players by increasing distance covered, enhancing work intensity, and increasing the number of sprints and involvements with the ball during a match (Helgerud et al, 2001). Helgerud et al (2007) found HIIT to be significantly more effective at improving ones VO2max compared to training at moderate and low intensities. However, Overend et al (1992) offer a contrasting opinion and concluded that interval training offered no advantage over continuous training. Gorostiaga et al (1991) demonstrated that the use of 30s work intervals at 100% VO2max separated by 30s rest intervals produce a greater increase in VO2max than that of continuous training at 50% of VO2max. Billat et al (2001) found that not only did interval training enhance aerobic capacity but also that interval training also stimulates the rate of lactate removal. The higher the concentration of lactate the greater the rate of removal. It was therefore found that interval training that increases blood lactate levels would also stimulate an improvement in lactate removal. Billat et al (2001) recommends active recovery during rest intervals in order to avoid blood lactate accumulation. Gibala & McGee (2008) recommend further studies to examine modified interval-based training programmes in order to identify the optimal combination of training intensity and volume necessary to induce adaptations to aerobic capacity in a practical time-efficient manner.
Having assessed current research in this field, the following study will compare the effects of a structured High Intensity Interval Training (HIIT) and generic low intensity continuous training on a group of young academy football players and their predicted Vo2Max.

Hypothesis
It is predicted that by part taking in a HIIT session once a week over a 6 week can increase predicted VO2max more significantly as compared to an individual who only participates in general steady state exercise.

Methods
Subjects
Twelve young male football players, (age 13-14) all from the same English League 1 Football Academy volunteered for the study. The experimental protocol was fully explained to all participants. The tests were demonstrated to the subjects before actual administration and the subjects that agreed signed a statement of formal consent. Having closely examined the protocol, ethical approval was given by Hartlepool College of Further Education and the League 1 Football Academy.

Experimental Design
The 20-M MSFT was undertaken to establish the pre-training predicted VO2max for all subjects prior to training. The Subjects start running back and forth on a 20-meter (m) course and must touch the 20-m line. The initial speed is 8.5 km/h, which gets progressively faster (0.5 km/h every minute), in accordance with a pace dictated by a sound signal on an audiotape. Several shuttle runs made up each stage, and subjects were instructed to keep pace with the signal for as long as possible. When the subject could no longer follow the pace, the last stage announced was used to predict maximal oxygen uptake using the table of predicted maximum oxygen uptake values (VO2 Max) for the Multistage Fitness Test. It was developed by the Department of Physical Education and Sports Science Loughborough University, 1987.
The twelve participants were randomly assignment to two separate groups.
Controlled Group (CG) – taking part in generic steady state aerobic exercise once a week for 6 weeks, along with prior specific football coaching session commitments.
Training Group (TG) – taking part in HIIT for 42 minutes once a week for 6 weeks, along with prior specific football coaching session commitments.
HIIT session consisted of a cycling interval training protocol on gym based spin bikes with variable resistance. Participants completed a 15 minute progressive warm up before beginning the maximal sprints. The HIIT component of the session was based on a 1:1 work – rest ratio of 12 x 30 second sprints interspersed with 30 seconds active recovery. All participants then completed a 15 minute cool down. Throughout the session a gym instructor gave direction and verbal motivation to all subjects. Post-test VO2max was then established for by Control Group (CG) and Training Group (TG) once again using the 20-M MSFT.

Statistical Analysis
A paired t-test was adopted for statistical analysis of the data. The statistical package for Social Sciences (SPSS) MS Windows Release V20 was used for statistical analysis.

Results
The mean value of the post-test predicted VO2max of the Training Group (TG) by 20m MSFT shows a significant difference when compared to the Control Group (CG). The mean difference between the predicted VO2max of CG and TG was 4.07 ml/kg/min. The results therefore shows that the Hypothesis can be accepted, since the t-test tail probability is (Sig.(2.tailed)) is .003. This is written as: t = 3.81;df = 10; S
This is confirmed by the P<0.01 indicating a 99% Confidence Interval of Difference.

Discussion
The HIIT training protocol used in this study has been proven to increase VO2max significantly more than that of steady state aerobic exercise. This improvement in VO2max from HIIT training was in accordance with previous studies (Gibala & McGee, 2008: Helgerud et al, 2001: Gromley et al, 2008: Gorostiaga et al, 1991). Although a significant difference was found in favour of HIIT training, over 80% of the CG group also displayed increased predicted VO2max through continuous training. This supports research from Stone & Kilding (2009), who found that both approaches promote similar increases in aerobic fitness and sport performance when training intensity and volume are constant. Stone & Kilding (2009) argues that a benefit of performing sport specific conditioning as opposed to structured HIIT is that it allows for both aerobic fitness and game skills to be developed simultaneously. In relation to the current investigation it was not designed to examine these criteria however it would be an interesting follow on study. Stone & Kilding (2009) concluded that sport-specific and traditional aerobic conditioning approaches to training elicit no significant differences in terms of developing aerobic fitness and match performance. However, one may find aerobic conditioning difficult to monitor during a football specific session. Without the use of heart rate monitors it would be difficult to establish the intensity of athlete’s performance throughout the game or at specific points. If the goal is to train at an intensity range between 90 and 95% of VO2max, this would be impractical under match conditions (Helgerud et al, 2001). However, Overend et al (1992) & Stone & Kilding (2009) have very little support in this area as Gromley et al (2008) explains the benefits of HIIT. They established that higher intensities of exercise elicit greater improvements in VO2max than lower intensities of exercise over a 4 – 6 week training period in healthy, young adults. This is complete agreement with the findings of the current study. Based on the physical demands and characteristics of team sport competition, and the potential importance of aerobic fitness, it is clear that a significant portion of conditioning programmes for team sport, players should focus on improving their ability to repeatedly perform high-intensity exercise and to recover (Stone & Kilding, 2009). Although research has shown similar benefits of both training methods Gibala & McGee (2008) state that HIIT is indeed a very time-efficient strategy to induce adaptations normally associated with endurance training again fully supported by the evidence presented by the current study.
Although much research has determined that within soccer, VO2max is one of the most influential factors in determining players exercise intensity (Chamari et al, 2004), contradictory research has questioned that VO2max is not always a good indicator of exercise performance (Jeukendrup & Martin, 2001) and therefore it is difficult to predict performance improvements from these studies. However it is likely that these training programmes resulted not only in an increased VO2max but also in a significant shift of the lactate threshold. Helgerud et al (2001) also states that an improvement in VO2max is normally followed by an improvement in Lactate Threshold (LT).
Chatterjee et al (2009) found that the application of the present form of the MSFT may be justified in the standard adult population. However, to better predict VO2max among youth players (12-16yrs) they developed a new equation based on their own research.
Sport-specific conditioning methods have been demonstrated to promote increases in VO2peak. However, little research to date has addressed the subsequent effects on game performance. The effectiveness of sport-specific conditioning appears to be influenced by the skill level of the athlete, where those with a lower skill level may not be able to maintain the skill or drill at a suitable intensity to promote the desired aerobic adaptations. Current fitness must also be considered. Players with already high levels of fitness may easily achieve the desired physical load during small-sided games and thus not achieve a training effect (Stone & Kilding, 2009).

Conclusions
To conclude the current study has demonstrated that HIIT is significantly more beneficial to enhance VO2max than steady state continuous exercise. This study contributes to a growing body of evidence concerning the beneficial effects of higher-intensity exercise for improvements in VO2max, as well as potential benefits for increasing lactate threshold.

Future recommendations
In order to establish greater validity a larger sample size should be tested. With varied methods of testing participants performance rather than relying on a single measure (Predicted VO2max). One could also incorporate, blood lactate testing, notational analysis of match performance and qualitative feedback in the form of rate of perceived excursion (RPE) from the subjects. Future research is recommended within this subject area.

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