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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).

| 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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... In contrast to endurance running, sprint kayaking imposes high demands on the upper-body endurance capacity of the athlete, as the athlete propels the boatbody-system against water resistance involving his/her relatively small upper-body muscles [10]. Differences in muscle mass, muscle fibre spectrum, oxygen extraction [11] as well as glucose and lipid oxidative capacity [11][12][13] between upper and lower body require different qualitative and quantitative training stimuli for distinct adaptation. From this perspective, the TID in kayak sprinting when compared to other leg-dominated sports should be different. ...
... In contrast, sprint kayaking imposes high demands on the endurance capacity of the athletes' upper body, as the athlete propels the boat-body-system against water resistance involving relatively small upper-body muscles [10]. Differences in muscle mass, muscle fibre spectrum, oxygen extraction [11] as well as glucose and lipid oxidative capacity [11][12][13] between upper and lower body require different qualitative and quantitative training stimuli for distinct adaptation. Recent studies in elite cross-country skiers investigated upper and lower-body muscles and found upper-body muscles to be less capable to oxidize fat and to rely more on carbohydrate oxidation than lower-body muscles [11,12]. ...
... Differences in muscle mass, muscle fibre spectrum, oxygen extraction [11] as well as glucose and lipid oxidative capacity [11][12][13] between upper and lower body require different qualitative and quantitative training stimuli for distinct adaptation. Recent studies in elite cross-country skiers investigated upper and lower-body muscles and found upper-body muscles to be less capable to oxidize fat and to rely more on carbohydrate oxidation than lower-body muscles [11,12]. Thus, it may seem plausible to assume that kayakers may not be able to perform as much volume in the higher intensity zones (Z2, Z3), as arm-glucose storages depleted earlier and therefore kayakers (have to) implement more time in Z1 compared to whole-body and/or lower-body sports. ...
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Background Research results on the training intensity distribution (TID) in endurance athletes are equivocal. This non-uniformity appears to be partially founded in the different quantification methods that are implemented. So far, TID research has solely focused on sports involving the lower-body muscles as prime movers (e.g. running). Sprint kayaking imposes high demands on the upper-body endurance capacity of the athlete. As there are structural and physiological differences between upper- and lower-body musculature, TID in kayaking should be different to lower-body dominant sports. Therefore, we aimed to compare the training intensity distribution during an 8-wk macrocycle in a group of highly trained sprint kayakers employing three different methods of training intensity quantification. Methods Heart rate (HR) and velocity during on-water training of nine highly trained German sprint kayakers were recorded during the final 8 weeks of a competition period leading to the national championships. The fractional analysis of TID was based on three zones (Z) derived from either HR (TIDBla-HR) or velocity (TIDBla-V) based on blood lactate (Bla) concentrations (Z1 ≤ 2.5 mmol L⁻¹ Bla, Z2 = 2.5–4.0 mmol L⁻¹ Bla, Z3 ≥ 4.0 mmol L⁻¹ Bla) of an incremental test or the 1000-m race pace (TIDRace): Z1 ≤ 85% of race pace, Z2 = 86–95% and Z3 ≥ 95%. Results TIDBla-V (Z1: 68%, Z2: 14%, Z3: 18%) differed from TIDBla-HR (Z1: 91%, Z2: 6%, Z3: 3%) in each zone (all p < 0.01). TIDRace (Z1: 73%, Z2: 20%, Z3: 7%) differed to Z3 in TIDBla-V (p < 0.01) and all three TIDBla-HR zones (all p < 0.01). Individual analysis revealed ranges of Z1, Z2, Z3 fractions for TIDBla-HR of 85–98%, 2–11% and 0.1–6%. For TIDBla-V, the individual ranges were 41–82% (Z1), 6–30% (Z2) and 8–30% (Z3) and for TIDRace 64–81% (Z1), 14–29% (Z2) and 4–10% (Z3). Conclusion The results show that the method of training intensity quantification substantially affects the fraction of TID in well-trained sprint kayakers. TIDRace determination shows low interindividual variation compared to the physiologically based TIDBla-HR and TIDBla-V. Depending on the aim of the analysis TIDRace, TIDBla-HR and TIDBla-V have advantages as well as drawbacks and may be implemented in conjunction to maximize adaptation.
... This may be an expression of different muscle energetics in the lower limbs muscles that during daily activities have a more varied high-intensity energy turnover, compared to the upper body muscles that may be less exposed to high-intensity activation and are more prone to repetitive movements and low-intensity contractions. This finding may also be supported by the ~ 60% higher maximal activity in baseline citrate synthase in the lower body muscle, which confirms previous studies in different populations (Helge et al. 2008;Nordsborg et al. 2015;Ørtenblad et al. 2018), despite similar MHC-isoform expression in the two muscles, indicating the superior capacity for oxidative phosphorylation in lower versus upper body muscles. ...
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Purpose Studies have indicated upper body involvement during football, provoking long-term muscular adaptations. This study aimed at examining the acute metabolic response in upper and lower body skeletal muscle to football training organized as small-sided games (SSG). Methods Ten healthy male recreational football players [age 24 ± 1 (± SD) yrs; height 183 ± 4 cm; body mass 83.1 ± 9.7 kg; body fat 15.5 ± 5.4%] completed 1-h 5v5 SSG (4 × 12 min interspersed with 4-min recovery periods). Muscle biopsies were obtained from m. vastus lateralis (VL) and m. deltoideus (DE) pre- and post-SSG for muscle glycogen and metabolite analyses. Blood lactate samples were obtained at rest, middle and end of the SSG. Results Muscle glycogen in VL decreased (P < 0.01) by 21% and tended (P = 0.08) to decrease in DE by 13%. Muscle lactate increased in VL (117%; P < 0.001) and DE (81%; P < 0.001) during the game, while blood lactate rose threefold. Muscle ATP and PCr were unaltered, but intermuscular differences were detected for ATP at both time points (P < 0.001) and for PCr at pre-SSG (P < 0.05) with VL demonstrating higher values than DE, while muscle creatine rose in VL (P < 0.001) by 41% and by 22% in DE (P = 0.02). Baseline citrate synthase maximal activity was higher (P < 0.05) in VL compared to DE, whereas baseline muscle lactate concentration was higher (P < 0.05) in DE than VL. Conclusion The upper body may be extensively involved during football play, but besides a rise in muscle lactate in the deltoideus muscle similar to the leg muscles, the present study did not demonstrate acute metabolic changes of an order that may explain the previously reported training effect of football play in the upper extremities.
... According to Calbet et al. [15], in adult athletes, the neuromotor performance of the upper limbs seems to exhibit a higher efficiency of glycolytic capacity compared to the lower limbs. However, Ørtenblad et al. [47] point out that there are no differences in relation to the distribution of oxidative and glycolytic fibers of the brachial triceps and the vastus lateralis of adult athletes, the authors add that oxidative and glycolytic muscle fibers exhibited similar aerobic capacity, regardless of whether they were located in a muscle of the upper or lower limbs. In this way, the present study identified that apparently during the late maturational stage and synchronized the differences in glycolytic efficiency between the upper and lower limbs are significant. ...
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We have previously demonstrated the relation between the biological maturation (BM) and the power of both upper (ULP) and lower limbs (LLP) in young athletes. We have also observed that the BM has a higher influence on the ULP, and the LLP is more prone to changes in hormonal and morphological factors. However, we have not yet compared the power of upper limbs and lower limbs in relation to the BM stage to verify whether the BM exerts more relation on the muscle power of a segment of the body. To attend this question, in this transverse study we have analyzed the relative muscle power from 241 young athletes (male: 57%, female: 43%, 12.3 ± 1.7 years) in relation to the BM stages of—delayed, synchronized and advanced. The ULP was verified by the medicine ball test and the LLP was verified by jump test in strength platform. Measures were standardized in Watts. The ULP was superior than LLP when individuals were at the late (male: p < 0.0001, female: p < 0.0001), or synchronized (male: p < 0.000, female: p = 0.0003) stages of BM, whereas no differences were found when the individuals were at the advanced stage of BM (male: p = 0.3, female: p = 0.3). Athletes at the late or synchronized stages of BM display of a higher power of superior compared inferior limbs. It is concluded that biological maturation has a greater relationship with the power of the upper limbs when compared to the lower limbs. Graphical abstract
... The fact that XC skiers can reach more than 90% of their RUN-VO 2max during DP further demonstrates that DP involves whole-body work (Van Hall et al., 2003;Holmberg et al., 2006;Hegge et al., 2016;Danielsen et al., 2018). The difficulty of reaching VO 2max in DP is likely to be related to longer diffusional distances, shorter mean transit times, and lower oxidative capacity in the upper than the lower body (Van Hall et al., 2003;Calbet et al., 2005;Ortenblad et al., 2018). Therefore, upper-body muscles are reported to extract ∼10% lower O 2 than leg muscles (Calbet et al., 2005) and contribute, together with a lower vascular conductance (Calbet et al., 2004), to lower VO 2peak values in DP compared to running . ...
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Purpose The objective of this study was to compare physiological and kinematic responses to double poling (DP) between long-distance (LDS) and all-round (ARS) cross-country skiers. Methods A number of five world-class LDS (28.8 ± 5.1 years, maximal oxygen uptake (VO 2max ): 70.4 ± 2.9 ml·kg ⁻¹ ·min ⁻¹ ) and seven ARS (22.3 ± 2.8 years, VO 2max : 69.1 ± 4.2 ml·kg ⁻¹ ·min ⁻¹ ) athletes having similar training volumes and VO 2max performed three identical tests; (1) submaximal and incremental tests to exhaustion while treadmill DP to determine gross efficiency (GE), peak oxygen uptake (DP-VO 2peak ), and peak speed; (2) submaximal and incremental running tests to exhaustion to determine GE, VO 2max (RUN-VO 2max ), and peak speed; and (3) an upper-body pull-down exercise to determine one repetition maximum (1RM) and peak power. Physiological responses were determined during both DP and running, together with the assessments of kinematic responses and electromyography (EMG) of selected muscles during DP. Results Compared to ARS, LDS reached higher peak speed (22.1 ± 1.0 vs. 20.7 ± 0.9 km·h ⁻¹ , p = 0.030), DP-VO 2peak (68.3 ± 2.1 vs. 65.1 ± 2.7 ml·kg ⁻¹ ·min ⁻¹ , p = 0.050), and DP-VO 2peak /RUN-VO 2max ratio (97 vs. 94%, p = 0.075) during incremental DP to exhaustion, as well as higher GE (17.2 vs. 15.9%, p = 0.029) during submaximal DP. There were no significant differences in cycle length or cycle rate between the groups during submaximal DP, although LDS displayed longer relative poling times (~2.4% points) at most speeds compared to ARS ( p = 0.015). However, group × speed interaction effects ( p < 0.05) were found for pole angle and vertical fluctuation of body center of mass, with LDS maintaining a more upright body position and more vertical pole angles at touchdown and lift-off at faster speeds. ARS displayed slightly higher normalized EMG amplitude than LDS in the muscles rectus abdominis ( p = 0.074) and biceps femoris ( p = 0.027). LDS performed slightly better on 1RM upper-body strength (122 vs. 114 kg, p = 0.198), with no group differences in power in the pull-down exercise. Conclusions The combination of better DP-specific aerobic energy delivery capacity, efficiency, and technical solutions seems to contribute to the superior DP performance found among specialized LDS in comparison with ARS.
... 22 Myosin heavy chain (MHC) composition was analyzed using gel electrophoresis. 33 Briefly, muscle homogenate (80 ml) was mixed with 200 ml sample buffer (10% glycerol, 5% 2-mercaptoethanol, 2.3% SDS, 62.5 mmol/L Tris-base, and 0.2% bromophenolblue at pH 6.8), boiled in a water bath at 100°C for 3 min, and loaded with three different amounts of protein (10-40 ml) on an SDS-PAGE gel [6% polyacrylamide (100:1, acrylamide:bis-acrylamide), 30% glycerol, 67.5 mmol/L Tris-base, 0.4% SDS, and 0.1 mmol/L glycine]. Gels were run at 80 V for at least 42 h at 4°C and MHC bands made visible by staining with Coomassie and three separate bands could be detected and characterized as MHC-1, MHC-2A, and MHC-2X. ...
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The present study examined skeletal muscle metabolism and changes in repeated sprint performance during match play for n = 20 competitive elite women outfield players. We obtained musculus vastus lateralis biopsies and blood samples before, after, and following intense periods in each half of a friendly match, along with 5 × 30‐meter sprint tests and movement pattern analyses (10‐Hz S5 Global Positioning System [GPS]). Muscle glycogen decreased by 39% and 42% after an intense period of the second half and after the match, respectively, compared to baseline (p < 0.05). Post‐match, 80% type I fibers and 69% type II fibers were almost empty or completely empty of glycogen. Muscle lactate was higher (p < 0.05) after the intense period of the first half and post‐match compared to baseline (14.3 ± 4.6 (±SEM) and 12.9 ± 5.7 vs. 6.4 ± 3.7 mmol/kg d.w.). Muscle phosphocreatine was reduced (p < 0.05) by 16% and 12%, respectively, after an intense period in the first and second half compared to baseline. Blood lactate and glucose increased during the match and peaked at 8.4 ± 2.0 and 7.9 ± 1.2 mmol/L, respectively. Mean 5 × 30 m sprint time declined by 3.2 ± 1.7 and 7.0 ± 2.1% after the first and second half, respectively, and 4.7 ± 1.6% (p < 0.05) after an intense period in the first half compared to baseline. In conclusion, match play in elite female football players resulted in marked glycogen depletion in both fiber types, which may explain fatigue at the end of a match. Repeated sprint ability was impaired after intense periods in the first half and after both halves, which may be associated with the observed muscle metabolite perturbations.
... Due to their adaptive nature, myofibers can change in response to exercise (Zierath and Hawley, 2004; advancements in next generation sequencing (NGS) and the development of the OMNI ATAC-Seq protocol (Corces et al., 2017), analysis of chromatin accessibility of samples with an input of as low as 500 cells is now possible (Corces et al., 2017). However, myofibers present additional challenges with their rigid membrane and high levels of mitochondria (Janssen et al., 2000;Ortenblad et al., 2018;Mishra et al., 2015). Here, we report a robust protocol for the successful application of ATAC-Seq on a single myofiber isolated from the EDL muscle. ...
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Myofibers are the main components of skeletal muscle, which is the largest tissue in the body. Myofibers are highly adaptive and can be altered under different biological and disease conditions. Therefore, transcriptional and epigenetic studies on myofibers are crucial to discover how chromatin alterations occur in the skeletal muscle under different conditions. However, due to the heterogenous nature of skeletal muscle, studying myofibers in isolation proves to be a challenging task. Single cell sequencing has permitted the study of the epigenome of isolated myonuclei. While this provides sequencing with high dimensionality, the sequencing depth is lacking, which makes comparisons between different biological conditions difficult. Here we report the first implementation of single myofiber ATAC-Seq, which allows for the sequencing of an individual myofiber at a depth sufficient for peak calling and for comparative analysis of chromatin accessibility under various physiological and disease conditions. Application of this technique revealed significant differences in chromatin accessibility between resting and regenerating myofibers, as well as between myofibers from a mouse model of Duchenne Muscular Dystrophy (mdx) and wild type (WT) counterparts. This technique can lead to a wide application in the identification of chromatin regulatory elements and epigenetic mechanisms in muscle fibers during development and in muscle-wasting diseases.
... Sprint kayakers propel both the mass of their body and of the boat against the resistance of the water, which places extensive demands on the endurance of the relatively small upper-body muscles (Ualí et al., 2012). Upper-and lower-body muscles differ substantially with respect to mass, fiber composition, extraction of oxygen (Calbet et al., 2005), and the contractile properties of muscle fibers (Gejl et al., 2021), as well as in the oxidation of glucose and lipid (van Hall et al., 2003;Calbet et al., 2005;Helge, 2010;Zinner et al., 2016;Ørtenblad et al., 2018). This indicates that the training required to achieve optimal adaptations in the upper and lower bodies differs both qualitatively and quantitatively. ...
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Purpose: To evaluate retrospectively the training intensity distribution (TID) among highly trained canoe sprinters during a single season and to relate TID to changes in performance. Methods: The heart rates during on-water training by 11 German sprint kayakers (7 women, 4 men) and one male canoeist were monitored during preparation periods (PP) 1 and 2, as well as during the period of competition (CP) (total monitoring period: 37 weeks). The zones of training intensity (Z) were defined as Z1 [<80% of peak oxygen consumption (VO 2peak )], Z2 (81–87% VO 2peak ) and Z3 (>87% VO 2peak ), as determined by 4 × 1,500-m incremental testing on-water. Prior to and after each period, the time required to complete the last 1,500-m stage (all-out) of the incremental test (1,500-m time-trial), velocities associated with 2 and 4 mmol·L ⁻¹ blood lactate (v2 [BLa] , v4 [BLa] ) and VO 2peak were determined. Results: During each period, the mean TID for the entire group was pyramidal (PP1: 84/12/4%, PP2: 80/12/8% and CP: 91/5/4% for Z1, Z2, Z3) and total training time on-water increased from 5.0 ± 0.9 h (PP1) to 6.1 ± 0.9 h (PP2) and 6.5 ± 1.0 h (CP). The individual ranges for Z1, Z2 and Z3 were 61–96, 2–26 and 0–19%. During PP2 VO 2peak (25.5 ± 11.4%) markedly increased compared to PP1 and CP and during PP1 v2 [bla] (3.6 ± 3.4%) showed greater improvement compared to PP2, but not to CP. All variables related to performance improved as the season progressed, but no other effects were observed. With respect to time-trial performance, the time spent in Z1 ( r = 0.66, p = 0.01) and total time in all three zones ( r = 0.66, p = 0.01) showed positive correlations, while the time spent in Z2 ( r = −0.57, p = 0.04) was negatively correlated. Conclusions: This seasonal analysis of the effects of training revealed extensive inter-individual variability. Overall, TID was pyramidal during the entire period of observation, with a tendency toward improvement in VO 2peak , v2 [bla] , v4 [bla] and time-trial performance. During PP2, when the COVID-19 lockdown was in place, the proportion of time spent in Z3 doubled, while that spent in Z1 was lowered; the total time spent training on water increased; these changes may have accentuated the improvement in performance during this period. A further increase in total on-water training time during CP was made possible by reductions in the proportions of time spent in Z2 and Z3, so that more fractions of time was spent in Z1.
... Furthermore, a cross-sectional study of the association of 25(OH)D status with muscle strength (n = 419; healthy men and women; 20-76 y) has also reported a stronger association between 25(OH)D and muscle strength in the arms compared to the legs (39). One potential explanation for this discrepancy is the reported greater distribution of VDR in type 2 muscle fibres (40) which make up a greater proportion of upper body skeletal muscle (41)(42)(43)(44). Vitamin D affects both the diameter and the number of type 2 muscle fibres, which are important for not only young athletes but also the elderly, due to their capacity to reduce the risk of falls, for example (45,46). ...
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Purpose Low serum vitamin D status has been associated with reduced muscle mass in observational studies although the relationship is controversial and a causal association cannot be determined from such observations. Two-sample Mendelian randomization (MR) was applied to assess the association between serum vitamin D (25(OH)D) and total, trunk, arm and leg fat-free mass (FFM). Methods MR was implemented using summary-level data from the largest genome-wide association studies (GWAS) on vitamin D (n=73,699) and total, trunk, arm and leg FFM. Inverse variance weighted method (IVW) was used to estimate the causal estimates. Weighted median (WM)-based method, and MR-Egger, leave-one-out were applied as sensitivity analysis. Results Genetically higher serum 25(OH)D levels had a positive effect on total (IVW = Beta: 0.042, p = 0.038), trunk (IVW = Beta: 0.045, p = 0.023) and arm (right arm IVW = Beta: 0.044, p = 0.002; left arm IVW = Beta: 0.05, p = 0.005) FFM. However, the association with leg FFM was not significant (right leg IVW = Beta: 0.03, p = 0.238; left leg IVW = Beta: 0.039, p = 0.100). The likelihood of heterogeneity and pleiotropy was determined to be low (statistically non-significant), and the observed associations were not driven by single SNPs. Furthermore, MR pleiotropy residual sum and outlier test did not highlight any outliers. Conclusions Our results illustrate the potentially causal, positive effect of serum 25(OH)D concentration on total, trunk and upper body appendicular fat-free mass.
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Across different cell types and within single cells, mitochondria are heterogeneous in form and function. In skeletal muscle cells, morphologically and functionally distinct subpopulations of mitochondria have been identified, but the mechanisms by which the subcellular specialization of mitochondria contributes to energy homeostasis in working muscles remains unclear. Here, we discuss the current data regarding mitochondrial heterogeneity in skeletal muscle cells and highlight potential new lines of inquiry that have emerged due to advancements in cellular imaging technologies.