Metabolic Adaptations to Sprint Training in Adolescent Male Athletes
The ability of a muscle to produce energy is associated with three different metabolic adaptations (Ross & Leveritt, 2001). These involve an increase in enzymatic activity or energy stores in the muscles and the muscle’s ability to deal with and eliminate waste associated with fatigue from energy production (Burgomaster, Hughes, Heigenhauser, Bradwell, & Gibala, 2005). Sprint training (ST) is a form of anaerobic training characterised by its short durations of activity lasting less than 30 seconds at maximal exercise intensities (Harmer et al., 2000). ST can vary in form, for instance it can be performed by running or cycling and differ in distance and recovery periods (Gibala & McGee, 2008). ST has been shown to increase anaerobic power due to metabolic adaptations involving enzymes such as myokinase (MK) and creatine phosphokinase (CPK) as well as changes in the breakdown rate of adenosine triphosphate (ATP) and phosphocreatine (PCr) (Spencer, Bishop, Dawson, & Goodman, 2005; Ross & Leveritt, 2001). In addition, there are changes in enzymatic activities in anaerobic glycolysis involving lactate dehydrogenase (LDH), glycogen phosphorylase (PHOS) and phosphofructokinase (PFK) and contribution from aerobic pathways as well (Ross & Leveritt, 2001). As many sports require the ability to generate energy rapidly for activities such as cycling, field sports and track and field (Creer, Ricard, Conlee, Hoyt, & Parcell, 2004; Spencer et al., 2005) it is very important for strength and conditioning (S&C) coach to understand the metabolic adaptations of ST and its consequence on athletic development and sports selection.
Understanding the metabolic adaptations to ST in a specific population is important for the S&C coach when devising a training plan, as it will influence training modality to better suit the athlete’s background and sports demand (Lätt et al., 2010; Farpour-Lambert, Carlson, Bradney, & Van Praagh, 2000). For instance, adolescence is a very important stage in athletic development that is marked by many physiological and musculoskeletal changes (Farpour-Lambert et al., 2000). By the time a male athlete reaches adolescence they are usually starting to compete in their chosen sport (Robertson & Way, 2005). Manipulation of training methods such as frequency and volume, duration and exercise intensity as well as recovery periods can lead to different metabolic adaptations (Rodas, Ventura, Cadefau, Cussó, & Parra, 2000; Parra, Cadefau, Rodas, Amigo, & Cusso, 2000). Provided the S&C coach understands the metabolic adaptations to certain forms and variations of training, such as of ST, they could potentially enhance the training of adolescent athletes, benefiting their long-term performance. As noted before, regulation of energy production is affected by enzyme activity (Ross & Leveritt, 2001). During sprint activities of less than 10 seconds there is a great demand on PCr breakdown and ATP resynthesis and when exercise duration increases different energy pathways are utilised such as glycolysis and aerobic pathways (Spencer et al., 2005; Hirvonen, Rehunen, Rusko, & Härkönen, 1987). ATP provides muscles with the required immediate source of energy in highly demanding physical activities (Spencer et al., 2005). ATP is resynthesised from adenosine diphosphate (ADP) by the enzyme MK which has been shown to increase in both short sprints lasting less than 10 seconds and longer sprints of 15 seconds or more (Ross & Leveritt, 2001; Thorstensson, Sjödin, & Karlsson, 1975). Dawson et al. (1998) investigated the effects of ST lasting less than 10 seconds in 9 trained adult male subjects and reported non-significant increases in MK activity after 6 weeks of training. However, in an earlier study Thorstensson et al. (1975) found significant increases in MK activity in adolescent male athletes post ST of 30 seconds long as well as an increase in ATP and PCr stores. This study was over a longer period lasting 8 weeks with more frequent sessions and shorter recovery periods. Despite existing evidence suggesting ST increases MK activity, the connection between enhanced performance and increases in ATP resynthesis remains somewhat unclear in adults or adolescent athletes (Ross & Leveritt, 2001).
During sprints of up to 30 seconds, ATP stores usually deplete to half of their resting state or less, with this value being lower the shorter the sprints (Spencer et al., 2005). On the other hand studies have shown significant decreases of PCr stores in the muscle during high intensity exercise with duration not having a major impact as stores deplete rapidly (Spencer et al., 2005). The breakdown of PCr is catalysed by the enzyme CPK, which has been shown to increase as a result of ST (Parra et al., 2000), however there is also research that shows increases in PCr breakdown with no changes in CPK in adolescent athletes (Cadefau et al., 1990). This can be because of the different testing methodologies used by researchers or the initially high levels of CPK (Cadefau et al., 1990). Energy production from ATP resynthesis and PCr breakdown have been shown to contribute to less than 40% of energy production in sprint activities lasting less than 30 seconds (Spencer et al., 2005). It is also now clear that anaerobic glycolysis starts to contribute instantly to metabolic energy from the commencement of physical activity and this contribution increases with the length of the activity overtaking the initial contribution of PCr breakdown (Spencer et al., 2005; Ross & Leveritt, 2001). In activities lasting 15 to 30 seconds or activities that require short repeated sprints over a longer duration, there is a greater contribution from anaerobic glycolysis (Burgomaster et al., 2005). Cadefau et al. (1990) found that ST leads to a significant increase in muscle glycogen stores in adolescent male athletes and, according to Burgomaster et al. (2005), increased glycogen stores as a result of ST can improve endurance capacity by delaying the onset of muscle fatigue. In a study by Rodas et al. (2000) looking at anaerobic metabolism, significant increases were shown in PCr and glycogen post ST involving 15 seconds all out bouts on cycle ergometer followed by 45 seconds rest and also in 30 seconds all out exercise followed by 12 minutes rest. The study shows significant increases in CPK, PFK and LDH but did not find any performance gains post 30 seconds sprint. Interestingly, it did show that using a progressive method improved maximum oxygen consumption. Moreover, increases have been shown in the enzyme LDH that catalyses pyruvate into lactate and the enzyme PHOS that mobilises muscle glycogen in ST of short and long duration (MacDougall et al., 1998). Linossier, Denis, Dormois, Geyssant, & Lacour (1993) suggest that anaerobic glycolysis can be linked to increases in energy production as lactate production was shown to increase post training along with a 20% increase in PFK and LDH. However Cadefau et al. (1990) found no changes in LDH levels only significant increases in PFK post ST and neither enzyme can be directly linked to enhanced sprint performance (Ross & Leveritt, 2001).
During short high intensity activities the aerobic energy pathway also has a contribution to energy metabolism that increases the longer the activity lasts or if rest times are kept short, with citrate synthase (CS) and succinate dehydrogenase (SDH) showing changes in sprints of longer duration (Rodas et al., 2000). Dawson et al. (1998) showed significant improvement in VO2max performance as well as an increase in PHOS activity but a decrease in CS activity using ST of less than 10 seconds. Gibala & McGee (2008) also reported improvements in endurance performance following intermittent ST protocol of 15 minutes of highly demanding exercise over two weeks with increases in VO2peak. Cadefau et al. (1990) found a significant increase in SDH post ST in adolescent athletes after 8 months. Although ST has been shown to cause improvements across the different energy pathways including the aerobic system (Dawson et al., 1998), the role of CS and SDH are still not clear as research into the effects of ST on endurance performance is still limited (Ross & Leveritt, 2001). Linossier et al. (1993) and Cadefau et al. (1990) both state that ST can lead to major adaptations to the athlete metabolic performance that continue even if training is ceased for a lengthy period of time before they return to baseline data.
ST training not only has consequences on metabolic adaptations but also leads to morphological adaptations, the knowledge of which is very important in early athletic development (Ross & Leveritt, 2001; Fournier et al., 1982; Cadefau et al., 1990). Research by Dawson et al. (1998) using short sprints of less than 10 seconds demonstrated an increase of muscle fibers of type II. It is interesting to note that this study utilised short rest periods of 24 seconds whereas in another study examining muscle fiber composition, Jansson, Esbjörnsson, Holm, & Jacobs (1990) also found an increase in type IIa muscle fibers using 30 seconds of high intensity sprints on Wingate bicycle followed by 15-20 minutes of rest. This shows that ST induces muscle fiber adaptations possibly caused by increased fiber activation regardless of actual rest times (Jansson et al., 1990). Cadefau et al. (1990) reported a significant increase in type I muscle fibers post ST in adolescent male athletes accompanied by increases in only type IIa fibres. There was also an increase in glycogen muscle stores and hypertrophy was evident in both type I and II fibers. In another research examining muscle metabolites Karatzaferi, de Haan, van Mechelen, & Sargeant (2001) found that ATP breakdown was greatest in IIa fibers whereas PCr breakdown occurred across all muscle fiber types. Therefore it can be concluded that ST causes morphological adaptations that lead to further metabolic adaptations via enzymatic activity changes and energy storage, which can be advantageous for many sports. Bangsbo, Mohr, & Krustrup (2006) state that during a football game for example, a player can perform up to 250 bouts of intense movement that creates high demand on the rates of PCr breakdown and utilisation of glycogen via glycolysis. Hence, it is extremely important for the athlete to be able to breakdown energy quickly and recover efficiently. In another study looking at the effects of ST, Markovic, Jukic, Milanovic, & Metikos (2007) determined that ST is as or more effective in producing performance gains in muscle function and athletic performance when compared to plyometric training. The authors found significant improvements across a range of tests including jump performance, shuttle run tests and sprints over 20 meters and recommend ST as an important tool in developing explosive performance. In a study by Sharp, Costill, Fink, & King (2008) looking at muscle buffer capacity, results concluded that ST can also increase muscle buffer capacity due to changes in lactate concentration not seen in endurance training (ET). On the other hand, Fournier et al. (1982) investigated the metabolic and morphological effects of sprint and ET in adolescent boys and found that ET resulted in significant increases in VO2max and slow twitch muscle fiber surface area as well as SDH activity whereas ST only resulted in increased PFK activity. Measures in both ST and ED returned to baseline data after a period of detraining. It is clear that the different methodologies used by researchers can results in contrasting results however the overall consensus shows ST has many benefits that a young athlete can benefit from.
In summary, using ST as tool not only improves anaerobic energy metabolism but can also enhance aerobic fitness. Although more research is required to back this information in adolescent male athletes, improvements in glycogen metabolism are likely to help young athletes train for longer by resisting fatigue. The athlete can also benefit from the morphological adaptations resulting in the development of type II muscle fiber type if required by their sport. An S&C coach can manipulate sprint time, rest duration, frequency and volume to induce different adaptations that can be retained for an extensive period of time after detraining (Ross & Leveritt, 2001).
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