Enhancing Repeated Sprint Ability In Football Players
Introduction
Football is a fast paced sport that requires players to sprint for short and long distances covering approximately 10 to 12km during a game (Stølen, Chamari, Castagna, & Wisløff, 2005). The term repeated sprint ability (RSA) is given to sports such as football as they require the athlete to consistently produce force over extended periods of time with intermittent periods of rest (Girard, Mendez-Villanueva, & Bishop, 2011). The ability to minimise fatigue and enhance the athlete’s RSA is key to success on the football field (Spencer, Bishop, Dawson, & Goodman, 2005). A good understanding of the mechanisms that contribute to fatigue and the energy pathways during exercise can help the strength and conditioning (S&C) coach enhance performance (Stølen et al., 2005).
Defining Fatigue
Muscular fatigue can be defined as the reduction in the ability of the muscle to continually produce or sustain force or power as a result of exercise or any barrier that may interfere with muscle contraction (Green, 1997). The nervous system consists of two parts, the central nervous system (CNS) and the peripheral nervous system (PNS) (McArdle, Katch, & Katch, 2010). The brain and spinal cord are part of the CNS and connect it to the rest of the body (Kirkendall, 1990). The PNS consists of motor neurons that carry signals from the CNS to the muscle in order to initiate voluntary movement (Kirkendall, 1990). The development of muscle fatigue is due to central and peripheral factors associated to each of these parts of the nervous system.
Mechanisms of Fatigue
Neuromuscular Mechanisms
Central fatigue factors involving the CNS can be influenced by motivation and psychology of the athlete (Davis, 1995). Central factors have been attributed to an alteration in CNS recruitment, neural transmission or reduced stimulus (Kirkendall, 1990), however the causes of central fatigue remains an area for future research (Davis, 1995). Peripheral fatigue factors, on the other hand, involve a reduction in muscle excitation coupling activity possibly due to a poor transmission through the neuromuscular junction and a decrease in energy supplies and force generation (Kirkendall, 1990; Sahlin, 1992).
Metabolic Mechanisms
In order to understand the metabolic mechanisms of fatigue it is important to examine the energy pathways utilised during RSA. Skeletal muscle movement requires energy, which comes in the form of an unstable molecule called adenosine triphosphate (ATP). The resynthesis of ATP utilises different metabolic pathways involving phosphocreatine (PCr) resynthesis, anaerobic glycolysis and aerobic metabolism (McArdle et al., 2010). It is important to note that in order to maintain high intensity exercise, ATP resynthesis must be at the same rate that it is utilised, fact that highlights the importance of the ATP-PCr system and anaerobic glycolysis.
The ATP-PCr system
During high intensity exercise where ATP utilisation is rapid, PCr plays an important role in the resynthesis of ATP (Girard et al., 2011). ATP holds energy that is released when its phosphate bonds are broken to form adenosine diphosphate (ADP) and inorganic phosphate (Pi) (Glaister, 2005). The enzyme creatine kinase is then used to break down the PCr into creatine and phosphate allowing the formation of ATP again through the addition of the phosphate to ADP (Glaister, 2005). It has been suggested that RSA performance is determined by the ability to resynthesise PCr (Bishop, Girard, & Mendez-Villanueva, 2011). As PCr stores are limited and can only provide energy for short period of time lasting no more than 10 seconds, the levels deplete rapidly during intense exercise forcing the body to utilise different energy sources (Bishop et al., 2011).
Anaerobic glycolysis
Anaerobic glycolysis is stimulated with the rapid drop of PCr and increase in Pi only a few seconds into a sprint (Bishop et al., 2011). Anaerobic glycolysis requires the breakdown of glycogen and glucose via a complex series of reactions involving various enzymes in order to produce ATP (Glaister, 2005). As anaerobic glycolysis takes place without oxygen, it produces by-products (Spencer et al., 2005). Changes in blood lactate concentrations as a result of hydrogen ions (H+) and lactic acid accumulation have been suggested to inhibit the enzymatic reaction processes eventually leading to muscle fatigue (Spencer et al., 2005). According to Bangsbo, Mohr & Krustrup (2006) muscle glycogen stores play the most important role in energy production with fatigue setting at the latter stages of the match due to depletion of glycogen levels. The author found that consuming a high carbohydrate diet increases glycogen availability during exercise and can enhance performance in field sports such as football.
Hypotheses of Fatigue
The causes of metabolic fatigue have been explained using the exhaustion and accumulation hypothesis (Sahlin, 1992). The exhaustion hypothesis states that due to the depletion of energy substrates such as ATP, PCr and glycogen and the inability of the body to resynthesis ATP at the rate it is used, this ultimately leads to muscle fatigue. The accumulation hypothesis relates to the increases in metabolic by-products such as H+ and lactic acid, when the production of these by-products exceeds the removal rate leading to fatigue (Sahlin, 1992). Muscular fatigue is likely to occur as a result of both factors and is influenced by the conditions of RSA.
Session Plan To Enhance RSA
In order to induce the required metabolic adaptations for optimum performance in RSA, it is important to consider the mechanisms of fatigue during the specific sporting activity. Using the relevant exercise intensities and recovery periods will allow the coach to mimic the conditions and train the relevant energy pathways and design an efficient and specific plan that will enhance performance.
Sample Plan
As a football game is played over two 45 minutes halves and requires intermittent periods of sprinting, it is important that a training session stresses both the anaerobic and aerobic energy systems (Stølen et al., 2005). The following RSA sample session aims to develop aerobic capacity whilst still enhancing anaerobic pathways (Dawson et al., 1998; Meckel, Machnai, & Eliakim, 2009).
Rationale Behind the Plan
In order to develop anaerobic capacity it is important to incorporate short burst of explosive activity lasting no longer than 10 seconds (Hirvonen, Rehunen, Rusko, & Härkönen, 1987). Spencer et al. (2005) suggests the use of sprints lasting less than 5 seconds because they represent similar durations to field sports activity. The number of sprint repetitions also plays an important role since data shows that 4-7 bouts of intense exercise may best represent field sport activity (Spencer et al., 2005). The rest time utilised aims to help with PCr stores recovery and to replicate the work to rest ratios of approximately 1:4-6 respectively observed during competitive match play with 3 passive rest minutes used at the end of each set to aid with metabolic fatigue (Billat, 2001).
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