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| Mitochondria content and subcellular localization in distinct fiber types and at whole-muscle level of leg and arm muscles. There was a tendency (P = 0.095) toward a higher mitochondrial content in the intermyofibrillar (IMF) and subsarcolemmal (SS) regions, of arm muscle (open bars) compared with leg muscle (filled bars) (A). This tendency is also apparent when calculating total mitochondrial content (IMF + SS) (B). (C) Weighted mitochondrial volumes in the arm and leg muscle, estimated from a fiber type distribution of 57 and 37% MHC-I for the leg and arm (n = 9), respectively. These MHC weighted values of whole-muscle mitochondrial content in arm and leg muscles are similar. Values are means ± SE (n = 29-30 fibers from 10 subjects).
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As one of the most physically demanding sports in the Olympic Games, cross-country skiing poses considerable challenges with respect to both force generation and endurance during the combined upper- and lower-body effort of varying intensity and duration. The isoforms of myosin in skeletal muscle have long been considered not only to define the con...
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... line with this, Essén et al. (1975) reported an equally high SDH activity in the type 2 and type 1 muscle fibers in top endurance runners [with a maximal oxygen uptake ( ˙ VO 2max ) > 72 ml·kg −1 ·min −1 ], with untrained having a clear fiber type difference with only half the SDH activity in their type 2 muscle fibers. Also, the mitochondrial volume density is generally considered to be strongly fiber type-dependent. In untrained humans, the mitochondrial volume varies from 6% in type I fibers to 4.5% in type 2a and 2.3% in type 2x fibers (Howald et al., 1985), with a more pronounced difference in animal studies of oxidative and glycolytic muscle, i.e., 2.7 times higher in rabbits and 4.5 times higher in rats ( Saltin and Gollnick, 1983;Jackman and Willis, 1996). In the current study, we compared equally trained arm and leg muscle based on the same CS activity (Table 2), the same average capillarization (Table 3), and no difference in the mitochondrial content at whole-muscle level. Based on this, we state that arm and leg muscle are equally trained. In these endurance-trained humans, there is a twofold higher mitochondrial volume density between type 1 and 2 fibers (Figure 3). Furthermore, the volume density of the type 2 fibers from trained is equal to (Howald et al., 1985) or higher (Nielsen et al., 2010a) than in type 1 fibers from untrained individuals. Thus, fiber type mitochondrial content is extremely malleable with muscle activity and inactivity (Hoppeler, 1986;Nielsen et al., 2010b). These changes in fiber metabolic characteristics are clearly not fiber-type-dependent, and a considerable variation exists within each fiber type with a clear overlay between fiber types. In line with this, a recent study indicated that type 2a fibers can possess equally high or even higher mitochondrial respiration as type 1 fibers ( Boushel et al., 2014). The equal volume density of mitochondria and CS activity in different types of fibers suggest that the intrinsic characteristics of mitochondria are variable and not determined solely by fiber ...
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... the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content ( Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms (Figure 3C). Thus, differences in leg and arm whole-muscle metabolic characteristics may not solely be explained by the dissimilar fiber type distribution in the limbs. The high mitochondrial content in type 2 fibers in arm could either be a consequence of the high metabolic demand in the upper body of these trained subjects or, possibly, due to a high demand for glycolytic flux in type 2 fibers. Thus, there is a clear necessity for being able to convert lactate to pyruvate within the mitochondrial intermembrane space with pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized ( Brooks et al., 1999;Hashimoto et al., 2006;Jacobs et al., 2013). Furthermore, peak arm blood flow and O 2 delivery per unit muscle mass during arm exercise is higher than that to leg muscle during leg cycling reflecting the proportional matching of oxygen delivery to oxidative capacity (Boushel et al., ...
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... the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content ( Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms (Figure 3C). Thus, differences in leg and arm whole-muscle metabolic characteristics may not solely be explained by the dissimilar fiber type distribution in the limbs. The high mitochondrial content in type 2 fibers in arm could either be a consequence of the high metabolic demand in the upper body of these trained subjects or, possibly, due to a high demand for glycolytic flux in type 2 fibers. Thus, there is a clear necessity for being able to convert lactate to pyruvate within the mitochondrial intermembrane space with pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized ( Brooks et al., 1999;Hashimoto et al., 2006;Jacobs et al., 2013). Furthermore, peak arm blood flow and O 2 delivery per unit muscle mass during arm exercise is higher than that to leg muscle during leg cycling reflecting the proportional matching of oxygen delivery to oxidative capacity (Boushel et al., ...
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... the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content ( Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms (Figure 3C). Thus, differences in leg and arm whole-muscle metabolic characteristics may not solely be explained by the dissimilar fiber type distribution in the limbs. The high mitochondrial content in type 2 fibers in arm could either be a consequence of the high metabolic demand in the upper body of these trained subjects or, possibly, due to a high demand for glycolytic flux in type 2 fibers. Thus, there is a clear necessity for being able to convert lactate to pyruvate within the mitochondrial intermembrane space with pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized ( Brooks et al., 1999;Hashimoto et al., 2006;Jacobs et al., 2013). Furthermore, peak arm blood flow and O 2 delivery per unit muscle mass during arm exercise is higher than that to leg muscle during leg cycling reflecting the proportional matching of oxygen delivery to oxidative capacity (Boushel et al., ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... line with this, Essén et al. (1975) reported an equally high SDH activity in the type 2 and type 1 muscle fibers in top endurance runners [with a maximal oxygen uptake ( ˙ VO 2max ) > 72 ml·kg −1 ·min −1 ], with untrained having a clear fiber type difference with only half the SDH activity in their type 2 muscle fibers. Also, the mitochondrial volume density is generally considered to be strongly fiber type-dependent. In untrained humans, the mitochondrial volume varies from 6% in type I fibers to 4.5% in type 2a and 2.3% in type 2x fibers (Howald et al., 1985), with a more pronounced difference in animal studies of oxidative and glycolytic muscle, i.e., 2.7 times higher in rabbits and 4.5 times higher in rats ( Saltin and Gollnick, 1983;Jackman and Willis, 1996). In the current study, we compared equally trained arm and leg muscle based on the same CS activity (Table 2), the same average capillarization (Table 3), and no difference in the mitochondrial content at whole-muscle level. Based on this, we state that arm and leg muscle are equally trained. In these endurance-trained humans, there is a twofold higher mitochondrial volume density between type 1 and 2 fibers (Figure 3). Furthermore, the volume density of the type 2 fibers from trained is equal to (Howald et al., 1985) or higher (Nielsen et al., 2010a) than in type 1 fibers from untrained individuals. Thus, fiber type mitochondrial content is extremely malleable with muscle activity and inactivity (Hoppeler, 1986;Nielsen et al., 2010b). These changes in fiber metabolic characteristics are clearly not fiber-type-dependent, and a considerable variation exists within each fiber type with a clear overlay between fiber types. In line with this, a recent study indicated that type 2a fibers can possess equally high or even higher mitochondrial respiration as type 1 fibers ( Boushel et al., 2014). The equal volume density of mitochondria and CS activity in different types of fibers suggest that the intrinsic characteristics of mitochondria are variable and not determined solely by fiber ...
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... the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content ( Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms (Figure 3C). Thus, differences in leg and arm whole-muscle metabolic characteristics may not solely be explained by the dissimilar fiber type distribution in the limbs. The high mitochondrial content in type 2 fibers in arm could either be a consequence of the high metabolic demand in the upper body of these trained subjects or, possibly, due to a high demand for glycolytic flux in type 2 fibers. Thus, there is a clear necessity for being able to convert lactate to pyruvate within the mitochondrial intermembrane space with pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized ( Brooks et al., 1999;Hashimoto et al., 2006;Jacobs et al., 2013). Furthermore, peak arm blood flow and O 2 delivery per unit muscle mass during arm exercise is higher than that to leg muscle during leg cycling reflecting the proportional matching of oxygen delivery to oxidative capacity (Boushel et al., ...
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... the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content ( Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms (Figure 3C). Thus, differences in leg and arm whole-muscle metabolic characteristics may not solely be explained by the dissimilar fiber type distribution in the limbs. The high mitochondrial content in type 2 fibers in arm could either be a consequence of the high metabolic demand in the upper body of these trained subjects or, possibly, due to a high demand for glycolytic flux in type 2 fibers. Thus, there is a clear necessity for being able to convert lactate to pyruvate within the mitochondrial intermembrane space with pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized ( Brooks et al., 1999;Hashimoto et al., 2006;Jacobs et al., 2013). Furthermore, peak arm blood flow and O 2 delivery per unit muscle mass during arm exercise is higher than that to leg muscle during leg cycling reflecting the proportional matching of oxygen delivery to oxidative capacity (Boushel et al., ...
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... the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content ( Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms (Figure 3C). Thus, differences in leg and arm whole-muscle metabolic characteristics may not solely be explained by the dissimilar fiber type distribution in the limbs. The high mitochondrial content in type 2 fibers in arm could either be a consequence of the high metabolic demand in the upper body of these trained subjects or, possibly, due to a high demand for glycolytic flux in type 2 fibers. Thus, there is a clear necessity for being able to convert lactate to pyruvate within the mitochondrial intermembrane space with pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized ( Brooks et al., 1999;Hashimoto et al., 2006;Jacobs et al., 2013). Furthermore, peak arm blood flow and O 2 delivery per unit muscle mass during arm exercise is higher than that to leg muscle during leg cycling reflecting the proportional matching of oxygen delivery to oxidative capacity (Boushel et al., ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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... electron microscopy images showing the subcellular localization of skeletal muscle mitochondria in the highly trained cross-country skiers are shown in Figure 1, clearly demonstrating a very high mitochondrial volume in these trained muscles. The SS mitochondria were unevenly distributed below the sarcolemma, with a higher volume located near the capillaries and around the nuclei. The IMF mitochondria are wrapped around the myofibrils, mainly located on each side of the z-line. These mitochondria in the I-band are often connected to an adjacent mitochondrion in the same sarcomere through the A-band. Individual values for the total volume of mitochondria per volume of myofiber are given in Table 5. The total volume of mitochondria is a volume-weighted average of the superficial region and the central region of the myofiber as well as the SS space. The individual values are based on 8-12 myofibers from two different biopsies. The total mitochondrial volume averaged 8.6 ± 1.6 and 9.0 ± 2.0 µm 3 ·µm −3 , for the arm and leg, respectively. The relative distribution of the mitochondrial subcellular regions was estimated in a total of 29 or 30 fibers from the 10 participants. In these highly endurance-trained athletes, the skeletal muscle mitochondria had similar relative distribution between IMF and SS localizations in both leg and arm muscles and in type 1 and 2 fibers. Thus, 83-86% of the mitochondria are localized in the IMF region and 11-14% in the SS region. The mitochondrial content and subcellular localization in distinct fiber types and at the whole-muscle level of leg and arm muscles is shown in Figure 3. Intriguingly, there was a tendency toward (10-20%) a lower mitochondrial content in the IMF and SS regions of leg muscle fibers compared with arm muscle fibers ( Figure 3A, P = 0.095). This is also apparent when calculating a total (IMF + SS) mitochondrial content ( Figure 3B). By taking the different MHC composition of leg and arm muscles into account, the average fiber type-mitochondrial volume can be estimated, given a fiber type distribution of 57 and 37% MHC-1 in leg and arm, respectively. Weighting the fiber type distribution, the whole-muscle mitochondrial volume in leg and arm muscle was similar ( Figure 3C). Thus, at the whole-muscle level, the non-significantly higher mitochondrial content in the arms mediated, despite a relatively higher number of MHC-2 TABLE 2 | The profile of myosin heavy chains and enzyme activities in the arm (triceps brachii) and leg (vastus lateralis) muscles of elite cross-country skiers (n = 10). The maximal activities of 3-hydroxy-acyl-CoA-dehydrogenase (HAD) and citrate synthase (CS) are given in µmol/g dw/min. * Significantly different from the leg muscle. Capillary density was assessed immunohistochemically. Number of capillaries is given in: total number of capillaries per total number of fibers (#cap/fiber); total number of capillaries per muscle area (cap/mm 2 ), and number of capillaries around each fiber for each fiber type and average for all fibers. * Significantly different from the corresponding value for leg muscle; # significantly different from the corresponding values for the other fiber types. fibers, an equal whole-muscle mitochondrial content in the legs and arms ( Figure 3C). There was a significant correlation (P = 0.02) between the total mitochondrial content in arm muscle and whole body VO 2 max (L·min −1 ), which was not apparent in leg ...
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Clenbuterol induces a slow-to-fast fiber type transition in skeletal muscle. This muscle fiber transition decreased mitochondrial oxidative capacity and respiratory function. We hypothesized that the clenbuterol-mediated reduction in oxidative capacity is associated with the alteration in mitochondrial morphology. To verify this hypothesis, we exam...
Citations
... Enhanced vagal tone due to water immersion can lead to lower HRpeak values during swimming than during on-land exercises [55]. Another possible reason for differences in the HRpeak values between swimming and on-land exercises may be related to increased external pressure and heat conduction in water compared with those in on-land exercises [56]. ...
... The lowest VȮ 2 peak and V̇Epeak values usually achieved during arm cranking compared with other modes of exercise, such as cycling, could be explained by the smaller muscle mass involved, as well as the muscle fibre ratio, because arm muscles consist of a higher percentage of type II fibres [16]. Moreover, arm muscles consume less oxygen and have a lower oxidative capacity than leg muscles [56]. The differences in VȮ 2 peak and V̇Epeak values between swimming and cycling could be due to different recruitment and activation levels of muscles. ...
Background
This study aimed to compare the aerobic capacity in swimming, cycling and arm cranking in swimmers aged 11–13 years.
Methods
Eleven swimmers (mean age, 12.1 ± 1.0 years) performed three incremental exercise tests. One of the tests was performed under specific conditions (front crawl swimming), and the other two were under non–specific conditions (cycling and arm cranking). Data on the pulmonary gas exchange were recorded using the portable analyser MetaMax 3B (Cortex, Leipzig, Germany). One-way analysis of variance for repeated measures was employed to test the null hypothesis and determine statistically significant differences between the indicators obtained under specific and non–specific testing conditions. Pearson’s correlation coefficient was calculated to assess the relationships between the indicators of the pulmonary gas exchange.
Results
The relative peak oxygen uptake (V̇O2peak) value during swimming was 49.3 ± 6.2 mL/kg/min, which was higher than that during arm cranking (39.6 ± 7.3 mL/kg/min; P < 0.01) but lower than that during cycling (54.3 ± 7.8 mL/kg/min; P < 0.01). The peak minute ventilation (V̇Epeak) value during swimming (84.9 ± 12.6 L/min) was higher than that during arm cranking (69.4 ± 18.2 L/min; P < 0.01) but lower than that during cycling (98.4 ± 15.4 L/min; P < 0.01). Strong positive correlations were observed in the absolute and relative V̇O2peak values between swimming and cycling (r = 0.857, P < 0.01; r = 0.657, P < 0.05) and between swimming and arm cranking (r = 0.899, P < 0.01; r = 0.863, P < 0.05). A strong positive correlation was also observed in V̇Epeak values between swimming and arm cranking (r = 0.626, P < 0.05).
Conclusion
Swimmers aged 11–13 years showed V̇O2peak and V̇Epeak values during the specific swimming test greater than those during arm cranking but lower than those during cycling. However, aerobic capacity parameters measured during specific swimming conditions correlated with those measured during non–specific arm cranking and cycling conditions.
... Mitochondrial biogenesis function in skeletal muscle is closely linked to the fiber type composition [45]. Fiber types I and 2A, which are known to affect muscular endurance, typically exhibit high mitochondrial density [46]. Our study found that the combination of mistletoe extract and apple peel extract increases the number of mitochondria and the expression of genes related to mitochondrial biogenesis (Figure 4a,b,e). ...
Muscular strength and endurance are vital for physical fitness. While mistletoe extract has shown efficacy in significantly increasing muscle strength and endurance, its accessibility is limited. This study explores combining mistletoe and apple peel extracts as an effective muscle health supplement. Analyses of histology, RNA, and protein in the combined extract-treated mouse group demonstrated significant enhancements in muscle strength and endurance, evidenced by larger muscle fibers, improved mitochondrial function, and a higher ratio of type I and IIa muscle fibers. Combining half doses of each extract resulted in greater improvements than using each extract separately, indicating a synergistic effect. Pathway analysis suggests that the observed synergy arises from complementary mechanisms, with a mistletoe extract-induced decrease in myostatin (MSTN) and an apple peel extract-induced increase in IGF1, leading to a sharp rise in AKT, S6K, and MuRF1, which promote myogenesis, along with a significant increase in PGC-1α, TFAM, and MEF2C, which are critical for mitochondrial biogenesis. This research provides practical insights into developing cost-effective, natural supplements to enhance muscle performance and endurance, with potential applications in athletic performance, improving muscle growth and endurance in children, and addressing age-related muscle decline.
... it has been shown that mitochondrial characteristics, such as volume density, enzyme activity, respiratory function, and morphology, vary in different skeletal muscle fiber types and that these distinct characteristics may reflect differences in mitochondrial proteins between fiber types (Figures 1(b) and 2). a. Volume density: in human fibers, the average mitochondrial volume (as determined by transmission electron Microscopy-teM) varies from 6% in type i fibers to 4.5% in type iia fibers and 2.3% in type iix fibers (Kiessling et al. 1971;Howald et al. 1985;Ørtenblad et al. 2018). b. ...
... citrate synthase (cS), malate dehydrogenase (MDH), and succinate dehydrogenase (SDH); Jackman and willis 1996; Howlett and willis 1998) and the β-oxidation pathway (e.g. carnitine palmitoyltransferase i (cPt1), 3-hydroxy-acyl-coA dehydrogenase (β-HAD); Kim et al. 2002;Ørtenblad et al. 2018) are greater in type i compared with type ii fibers. Similar findings have been reported for SDH in humans (Gollnick et al. 1972). in contrast, the activities of isocitrate dehydrogenase (iDH) and mitochondrial glycerol-3-phosphate dehydrogenase (GPD-a marker of glycolytic metabolism) are higher in type ii than type i fibers (Peter et al. 1972;Jackman and willis 1996). ...
... They are categorized based on their contractile and metabolic characteristics, resulting in three major types of muscle fibers: slow-twitch oxidative fibers (Type I), fasttwitch oxidative fibers (Type IIA), and fast-twitch glycolytic fibers (Type IIB or IIX). Generally, each fiber type possesses a distinctive composition of contractile proteins influenced by several factors, including 1) myosin heavy chain (MHC) isoform, 2) contractile characteristics, and 3) calcium handling properties [4,5]. On the other hand, the metabolic capacity of the muscle fiber depends on 1) capillary density, 2) mitochondrial content, and 3) insulin sensitivity [4]. ...
... Generally, each fiber type possesses a distinctive composition of contractile proteins influenced by several factors, including 1) myosin heavy chain (MHC) isoform, 2) contractile characteristics, and 3) calcium handling properties [4,5]. On the other hand, the metabolic capacity of the muscle fiber depends on 1) capillary density, 2) mitochondrial content, and 3) insulin sensitivity [4]. ...
Purpose of Review
The global obesity epidemic has become a major public health concern, necessitating comprehensive research into its adverse effects on various tissues within the human body. Among these tissues, skeletal muscle has gained attention due to its susceptibility to obesity-related alterations. Mitochondria are primary source of energy production in the skeletal muscle. Healthy skeletal muscle maintains constant mitochondrial content through continuous cycle of synthesis and degradation. However, obesity has been shown to disrupt this intricate balance. This review summarizes recent findings on the impact of obesity on skeletal muscle mitochondria structure and function. In addition, we summarize the molecular mechanism of mitochondrial quality control systems and how obesity impacts these systems.
Recent Findings
Recent findings show various interventions aimed at mitigating mitochondrial dysfunction in obese model, encompassing strategies including caloric restriction and various dietary compounds.
Summary
Obesity has deleterious effect on skeletal muscle mitochondria by disrupting mitochondrial biogenesis and dynamics. Caloric restriction, omega-3 fatty acids, resveratrol, and other dietary compounds enhance mitochondrial function and present promising therapeutic opportunities.
... Given the risk of overestimating the differences in mitochondrial content between type I and type II fibers, the intrinsic difference in fiber typespecific respiration should be interpreted with caution. Thus, to fully confirm the findings presented here, additional studies are required in which intrinsic fiber type-specific respiration is normalized using additional methods of assessing mitochondrial content such as proteomics [53], transmission- [54], or focused ion beam scanning electron microscopy [5,16]. ...
Objective
Human skeletal muscle consists of a mixture of slow- and fast-twitch fibers with distinct capacities for contraction mechanics, fermentation, and oxidative phosphorylation. While the divergence in mitochondrial volume favoring slow-twitch fibers is well established, data on the fiber type-specific intrinsic mitochondrial function and morphology are highly limited with existing data mainly being generated in animal models. This highlights the need for more human data on the topic.
Methods
Here, we utilized THRIFTY, a rapid fiber type identification protocol to detect, sort, and pool fast- and slow-twitch fibers within 6 h of muscle biopsy sampling. Respiration of permeabilized fast- and slow-twitch fiber pools was then analyzed with high-resolution respirometry. Using standardized western blot procedures, muscle fiber pools were subsequently analyzed for control proteins and key proteins related to respiratory capacity.
Results
Maximal complex I+II respiration was 25% higher in human slow-twitch fibers compared to fast-twitch fibers. However, per mitochondrial volume, the respiratory rate of mitochondria in fast-twitch fibers was approximately 50% higher for complex I+II, which was primarily mediated through elevated complex II respiration. Furthermore, the abundance of complex II protein and proteins regulating cristae structure were disproportionally elevated in mitochondria of the fast-twitch fibers. The difference in intrinsic respiratory rate was not reflected in fatty acid–or complex I respiration.
Conclusion
Mitochondria of human fast-twitch muscle fibers compensate for their lack of volume by substantially elevating intrinsic respiratory rate through increased reliance on complex II.
... Similarly, skeletal muscle citrate synthase (CS) activity was significantly higher in EG than both other groups, a consequence of long-term endurance training [24]. Skeletal muscle type 1 fiber proportion was significantly higher in EG compared with both other groups (Table 1, Figure S1), a difference that has been observed before [25,26]. Muscle fiber cross-sectional area (CSA) was significantly Original Article lower in CG for type 1 compared to both other groups. ...
Objective
Long-term high-level exercise training leads to improvements in physical performance and multi-tissue adaptation following changes in molecular pathways. While skeletal muscle baseline differences between exercise-trained and untrained individuals have been previously investigated, it remains unclear how training history influences human multi-omics responses to acute exercise.
Methods
We recruited and extensively characterized 24 individuals categorized as endurance athletes with >15 years of training history, strength athletes or control subjects. Timeseries skeletal muscle biopsies were taken from M. vastus lateralis at three time-points after endurance or resistance exercise was performed and multi-omics molecular analysis performed.
Results
Our analyses revealed distinct activation differences of molecular processes such as fatty- and amino acid metabolism and transcription factors such as HIF1A and the MYF-family. We show that endurance athletes have an increased abundance of carnitine-derivates while strength athletes increase specific phospholipid metabolites compared to control subjects. Additionally, for the first time, we show the metabolite sorbitol to be substantially increased with acute exercise. On transcriptional level, we show that acute resistance exercise stimulates more gene expression than acute endurance exercise. This follows a specific pattern, with endurance athletes uniquely down-regulating pathways related to mitochondria, translation and ribosomes. Finally, both forms of exercise training specialize in diverging transcriptional directions, differentiating themselves from the transcriptome of the untrained control group.
Conclusions
We identify a “transcriptional specialization effect” by transcriptional narrowing and intensification, and molecular specialization effects on metabolomic level Additionally, we performed multi-omics network and cluster analysis, providing a novel resource of skeletal muscle transcriptomic and metabolomic profiling in highly trained and untrained individuals.
... Type I (slowtwitch) fibers, optimised for endurance and sustained activities, are generously endowed with mitochondria, underscoring their aerobic proficiency. In contrast, type II (fast-twitch) fibers, tailored for quick and explosive actions, have a comparatively diminished mitochondrial content (55). In chronic presentations of polymyositis and DM, attenuated muscular endurance may be attributable to an insufficiency of oxidative, type I myofibers (56). ...
Impairment in cellular bioenergetics as either the cause, consequence, or major contributor of tissue damage has drawn increasing scientific curiosity across aging and chronic health conditions, with mitochondrial dysfunction emerging as a central mechanism in the pathogenesis of a variety of inflammatory and degenerative disorders. Beyond bioenergetics, mitochondria play critical regulatory roles in programmed cell death of dysfunctional/defective cells as well as in metabolite synthesis and metabolic signalling. Further, extra-cellular exposure to fragmentation of injured mitochondria is associated with incitement of systemic and organ-based inflammation. Thus, mitochondrial function has recently drawn intense, spectral scientific interest as an integral component across maladies.In muscle, mitochondrial dysfunction is clinically associated with atrophy and diminished endurance. Direct myo-histopathological evidence characterising loss of mitochondrial integrity as a hallmark of muscle compromise was first noticed in inclusion body myositis (IBM). This was followed by the discovery of multiple deletions in mitochondrial DNA in sarcopenia, IBM, and other inflammatory myopathies, like dermatomyositis. Though fraught with bioethical considerations, the transplant technology of mitochondrial transfer is swiftly gaining prominence in cellular biology and muscle physiology to remediate mitochondrial diminution and dysfunction. Assembling seminal works and recent developments, this review ventures into the rapidly evolving landscape of mitochondrial transfer, focusing on its implications on muscle function, and offers an integrated perspective on the potential roles of mitochondrial transfer and its implications for preserving and restoring muscle health. Presented here is a consolidated viewpoint on mitochondrial transfer in idiopathic inflammatory myopathies.
... 9 The highest capillary to fiber ratios reported in high level endurance athletes are in the range of 3.0-3.5. 18,[64][65][66] In amateur cyclist, international level cyclist and team pursuit riders, capillary to fiber ratio have been reported to be 2.4 ± 0.3, 2.9 ± 0.3 and 3.2 ± 0.5, respectively. 66 Elite road cyclist, studied at the age of 25, were found to have 35% higher capillarization compared to their 4-year younger elite competitors 67 suggesting that the capillary network continues to adapt to training, even in very well-trained subjects. ...
Sufficient delivery of oxygen and metabolic substrates, together with removal of waste products, are key elements of muscle performance. Capillaries are the primary site for this exchange in skeletal muscle and the degree of muscle capillarization affects diffusion conditions by influencing mean transit time, capillary surface area and diffusion distance. Muscle capillarization may thus represent a limiting factor for performance. Exercise training increases the number of capillaries per muscle fiber by about 10%–20% within a few weeks in untrained subjects, whereas capillary growth progresses more slowly in well‐trained endurance athletes. Studies show that capillaries are tortuous, situated along and across the length of the fibers with an arrangement related to muscle fascicles. Although direct data is lacking, it is possible that years of training not only enhances capillary density but also optimizes the positioning of capillaries, to further improve the diffusion conditions. Muscle capillarization has been shown to increase oxygen extraction during exercise in humans, but direct evidence for a causal link between increased muscle capillarization and performance is scarce. This review covers current knowledge on the implications of muscle capillarization for oxygen and glucose uptake as well as performance. A brief overview of the process of capillary growth and of physical factors, inherent to exercise, which promote angiogenesis, provides the foundation for a discussion on how different training modalities may influence muscle capillary growth. Finally, we identify three areas for future research on the role of capillarization for exercise performance.
... Subsequently, Helge 36 found that performing exercises with the upper limbs has a lower fat oxidation compared to those of the lower limbs, suggesting glycolytic predominance in the upper limbs compared to the lower limbs. Ørtenblad et al. 37 analyzed the metabolic differences and predominance of muscle fiber types in the triceps brachii and vastus lateralis thigh muscles in elite adult skiing athletes. It was suggested that the lower limbs pointed to a higher oxidative capacity due to the percentage of 3-hydroxy-acyl-CoA dehydrogenase capacity being higher in the leg muscles. ...
Success in speed swimming depends on the efficiency of the anaerobic system for the production of cellular energy, especially during muscle power production. In the adolescent athletes much is unknown with regards to the relationships between relative power of upper and lower limbs with speed swimming performance. The aim the present study was to identify differences in relative muscle power of upper and lower limbs in adolescent swimmers and relate these to speed swimming performances. Sixty adolescents, of both sexes (50% female, 50% male, 30 swimmers and 30 controls), were recruited. The relative upper limb power (ULP[W/kg]) was assessed by a medicine ball test and the relative lower limbs power (LLP[W/kg]) by a jump test on a jumping platform. Lean mass of the upper and lower limbs was assessed by dual-energy X-ray absorptiometry (DXA) (g). Sport performance was assessed during national level competition (50-m swimming time [in seconds]). Biological maturation (BM) was indexed by years from attainment of peak height velocity. ULP(W/kg) was higher than LLP(W/kg) in both groups (p < 0.05). Upper and lower limb lean mass (g) correlated significantly with ULP(W/kg) and LLP(W/kg) in both groups (p < 0.05). ULP(W/kg) and LLP(W/kg) correlated with 50-m swimming performance (s), in both sexes (p < 0.05). Advanced BM was associated with ULP(W/kg) and LLP(W/kg) in both groups (p < 0.05), and with 50-m swimming performance (s) in both sexes (p < 0.05). We concluded that ULP(W/kg) is higher than LLP (W/kg) in adolescent swimmers. Upper and lower limb lean mass and BM were both positively associated with increased ULP (W/kg) and LLP (W/kg).
... [10][11][12][13] However, there was restricted data about RTS on neuromuscular performance among athletes after ARinf and most available studies also focus on the lower-body. [14][15][16] Upper-body strength and power were of particular importance to some sports, like kayaking, cross-country skiing, rowing, etc. 17,18 Furthermore, the effects of RTS during the pandemic on maintaining body composition in athletes remain mixed results. [19][20][21] Given above, it is necessary to elucidate the influences of ARinf and RTS in athletes who demand high-level neuromuscular function and optimum body composition, such as kayakers. ...
... 3,20,41 Furthermore, the effects of ARinf on the upper and lower-body neuromuscular function may differ due to the relatively small muscle mass and differences in muscle fiber type, oxygen uptake, glucose and fat oxidation ability compared to the lowerbody. 17,18 Regarding the upper-body function was crucial for many other exercises (e.g., rowing, kayaking, water-polo, cross-country skiing, etc), 18,22 it may be useful in future studies to refine the RTS strategy according to the importance of different body parts of the athletes for performance. ...
... 3,20,41 Furthermore, the effects of ARinf on the upper and lower-body neuromuscular function may differ due to the relatively small muscle mass and differences in muscle fiber type, oxygen uptake, glucose and fat oxidation ability compared to the lowerbody. 17,18 Regarding the upper-body function was crucial for many other exercises (e.g., rowing, kayaking, water-polo, cross-country skiing, etc), 18,22 it may be useful in future studies to refine the RTS strategy according to the importance of different body parts of the athletes for performance. ...
Purpose
This study aimed to examine the short-term effects of SARS-CoV-2 infection and return to sport (RTS) on neuromuscular performance, body composition, and mental health in well-trained young kayakers.
Methods
17 vaccinated kayakers (8 male, 9 female) underwent body composition assessment, peak power output bench press (BP), and 40-s maximum repetition BP tests 23.9 ± 1.6 days before and 22.5 ± 1.6 days after a SARS-CoV-2 infection. A linear transducer was used to examine the BP performance. The perception of training load and mental health were quantified with Borg's CR-10 scale and the Hooper questionnaire before and after infection. The difference and relationship of variables were used Wilcoxon test, Student t-test, Pearson's, and Spearman's r correlation coefficients.
Results
There was a significant increase in body mass, fat-free mass, and skeletal muscle mass, but no significant changes in body fat, fat mass, and all BP performance after infection (p < 0.05). There was a significant reduction in training hours per week, session rating of perceived exertion (sRPE), internal training load (sRPE-TL), fatigue, muscle soreness levels, and Hooper index, but no changes in sleep quality and stress levels after infection (p < 0.05). The training and mental health during the RTS period was significantly correlated (r = −0.85 to 0.70) with physical performance after infection.
Conclusion
A SARS-CoV-2 infection did not appear to impair the upper-body neuromuscular performance and mental health of vaccinated well-trained young kayakers after a short-term RTS period. These findings can assist coaches, and medical and club staff when guiding RTS strategies after other acute infections or similar restrictions.
