Cardio-Metabolic Changes in Response to Exercise Training
Introduction
The human body has 3 major yet very unique energy systems that are used to provide the active skeletal muscles with the energy demands they require (Gastin, 2001). These 3 systems are the Phosphocreatine (PCr), Glycolytic and Aerobic systems. The PCr system involves “splitting of the high-energy phosphagen, phosphocreatine (PCr), which together with the stored ATP in the cell provides the immediate energy in the initial stages of intense or explosive exercise.” (Gastin, 2001, p.726). The key enzyme involved is ATPase and creatine kinase during the PCr shuttle (Maughan and Gleeson, 2010). The Glycolytic system involves the “nonaerobic breakdown of carbohydrate, mainly in the form of muscle glycogen, to pyruvic acid and then lactic acid through glycolysis.” (Gastin, 2001, p.726). The key enzymes used in this energy system are glycogen phosphorylase, hexokinase (this ensures glucose can enter the cell has a phosphate group attached and therefore cannot diffuse back out the cell) and phosphofructokinase (PFK) (Maughan and Gleeson, 2010). The aerobic system involves metabolising carbohydrates and fats with the influence of oxygen (Gastin, 2001). These reactions, catalysed by pyruvate dehydrogenase (PDH) all take place in the mitochondrion (Maughan and Gleeson, 2010).
When examining exercise intensity, there is two different scales of classification that can be used. Absolute exercise intensity is a generic quantitative intensity value that can be applied to anyone exercising with no factors taken into consideration e.g. body weight, height etc. The relative exercise intensity is also a quantitative value and is proportional to an individual’s specific maximal capability, fitness level and other factors (mentioned above) that are taken into consideration (Bird, Smith and James, 1998).
Mean ± SD
Age (years) 21 ± 3
Height (cm) 175 ± 11
Weight (kg) 71.6 ± 12.9
The aims and hypothesis of this study was to examine the effects of a 12-week training programme on 10 “unfit” male individuals using an incremental test of maximal aerobic capacity (VO2max) on a cycle ergometer in order to investigate changes in substrate oxidation (see Table 1. for subject characteristics) with the presumption that lipid oxidation will increase following training.
Results
Statistical Analysis. The VO2max and substrate oxidation rates both pre and post training were analysed using the Statistical Package for Social Sciences (SPSS, IBM Corp) software. A significant difference (*) was set at P<0.05.
VO2max. The mean absolute value post-training (2.73 L.min-1, s=0.07) was significantly higher (t9=8.85, P<0.01) than the mean absolute value pre-training (2.50 L.min-1, s=0.03) The 95% CI for the mean of the differences was from 0.17 to 0.29 L.min-1 (see figure 1.). The mean relative value post-training (39.31 ml.kg-1.min-1, s=7.24) was significantly higher (t9=7.08, P<0.01) than the mean relative value pre-training (36.04 ml.kg-1.min-1, s=6.50). The 95% CI for the mean of the differences was from 2.22 to 4.32 ml.kg-1.min-1. (see figure 2.).
Substrate Oxidation. The mean carbohydrate oxidation rate pre-training (0.35 g.min-1, s=0.06) was significantly higher (t9=2.91, P=0.017) than the mean carbohydrate oxidation rate post-training (0.29 g.min-1, s=0.08). The 95% CI for the mean of the differences was from 0.01 to 0.10 g.min-1. The mean fat oxidation rate post-training (1.13 g.min-1, s=0.07) was significantly higher (t9=7.26, P<0.01) than the mean fat oxidation pre-training (0.87 g.min-1, s=0.07). The 95% CI for the mean of the differences was from 0.18 to 0.34 g.min-1. (see figure 5.)
RPE. This showed a near linear increase from rest up until 20 minutes of exercise both pre and post training, however the was a decrease in the mean RPE post training. The mean RPE pre-training was 13 and post training was 11. (see figure 3.)
Heart Rate. Increased linearly with exercise intensity up to 10 minutes after which a plateau occurred for the rest of the duration of the exercise. The mean resting heart rate decreased from 76 bpm to 72 bpm following the training and the participants mean peak heart rate also decreased after training from 160 to 153 bpm. (see figure 4.)
Discussion
Endurance is a very versatile and widely used term used in sport but can be defined as “the capacity to sustain a given velocity or power output for the longest possible time” (Jones and Carter, 2000 p.373). Throughout this study, the overall aim was to analyse the effects (if any) that 12 weeks of steady state endurance-based training, consisting of 30-60 minutes 5 times per week at 65% REL (relative) pre-training VO2max (see table 2.), had on 10 individual’s substrate oxidation rates. The substrates being fat and carbohydrate. Heart rate and RPE were measured during each stage of exercise duration at the end of every 3-minute stage (see figures 3&4 respectively). The findings indicated that the hypothesis of endurance training results in a chronic increase in lipid oxidation following an exercise programme, the participants fat oxidation rates were significantly higher post-training and that carbohydrate oxidation rates were significantly lower post-training as opposed to the basal pre-training basal rates (see results section above and figure 5.).
Mean ± SD
Pre-training Post training
VO2max (L.min-1) 2.50 ± 0.03 2.73 ± 0.07*
VO2max (ml.kg-1.min-1) 36.0 ± 6.5 39.3 ± 7.2*
In 2001, Carter, Rennie and Tarnopolsky lead a study which resulted in findings that were similar to our study. They found that by exercising for 7 weeks at 60% VO2max for 90 minutes, fat oxidation increased at the same ABS (absolute) workload and oxidation of carbohydrate was decreased at the same ABS workload. This reduction in CHO oxidation for ATP resynthesis leads to the preservation of muscle glycogen (Kiens et al., 1993) which will inherently increase muscle glycogen stores (Greiwe et al. 1999 cited in Jones and Carter 2000 p. 380) and therefore provide a greater resistance to fatigue as exhausting glycogen stores can lead to poor performance (Costill et al. 1973 cited in Jones and Carter 2000 p. 380). Increased fat oxidation through physical activity has been shown to increase insulin sensitivity in obese individuals and help prevent diabetes (Goodpaster, Katsiaras and Kelley, 2003).
In 1993, a study by Kiens et al. using seven male subjects performing a one-legged knee extension exercise over a course of 8 weeks concluded that net uptake of FFA in the trained leg did not plateau unlike it did in the non-trained leg, indicating that fat oxidation had increased following endurance training based of RER (respiratory exchange ratio), Respiratory Quotient (RQ) values and adaptational angiogenesis of trained muscle which allows the transport of more FFA. They found that over the exercising thigh, RQ values averaged 0.81 in the trained leg vs. 0.91 in the non-trained leg. This value shows that the trained leg utilizes more fat than the non-trained leg as lower RQ indicates fat utilization (Seidenberg and Beutler, 2008). Decreases in RER and RQ have also been linked to higher fat oxidation in other studies such as Jones and Carter (2000) and Carter, Rennie and Tarnopolsky (2001) who also demonstrated that females had lower values both pre and post training than males.
However, in contrast to our study van Baak, (1999) concluded that there wasn’t a definitive answer as to whether exercise increases fat utilisation during rest, exercise or 24 hours post-exercise in obese female individuals. There is also some controversy in regards to the origination of the oxidised FFA. Some studies such as Hurley et al. (1986 cited in Carter, Rennie and Tarnopolsky 2001 p. E898) and Phillips et al. (1996 cited in Carter, Rennie and Tarnopolsky 2001 p. E898) indicate that there is an increase in IMTG oxidation whereas Bergman et al. (1999 cited in Carter, Rennie and Tarnopolsky 2001 p. E898) found that there was an increase in plasma FFA oxidation. In addition to this increase Hoppeler et al., 1985 suggested that the FFA could be better utilised if the mitochondria sites were closer to the FFA supply, hence increases in angiogenesis and mitochondrial biogenesis.
According to Egan and Zierath (2013) acute exercise bouts increase AMPK phosphorylation and activation of this enzyme (which is a key modulator in cellular metabolism) supresses levels of glycogen synthesis. (Carling and Hardie 1989, cited in Egan and Zierath 2013 p.169). Bergeron et al., (2001 cited in Egan and Zierath 2013 p.170) epitomised that chronic activation of AMPK stimulates mitochondrial biogenesis which results in a larger mitochondrial density which in turn magnifies respiratory control sensitivity. An enhanced control over the respiratory sensitivity increases lipid oxidation and decreases carbohydrate oxidation (Talanian et al., 2010 cited in Egan and Zierath 2013 p. 174). This marked decreases in CHO oxidation as shown in our study, will help preserve levels of glycogen meaning that there will be a greater resistance to fatigue during exercise. As a consequence of increased density and number of mitochondria, Dudley et al., (1987); Phillips et al., (1996 cited in Egan and Zierath 2013 p. 174) post training responses show that there are smaller decreases in ATP and PCr and increases in free ADP levels.
In summary, our results show that there was a marked increase in the rate of lipid oxidation post training and marked decrease in CHO oxidation post training which helps preserve glycogen levels and provides a resistance to fatigue when performing. Several other studies have conducted similar programmes and found similar results. However, our study only looked at a small number of male subjects whereas most other studies include both male and female subjects to obtain a greater range of results and a number of studies include data on RER which is a more direct guide to intensity and substrate utilization than RPE. It is highly beneficial to take muscle biopsies using electron microscopy to show effects to IMTG stores and adaptations to mitochondria and capillaries more precisely. These are some recommendations for future studies.
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