Maximal oxygen uptake is proportional to muscle fiber oxidative capacity, from chronic heart failure patients to professional cyclists

Training-Induced Changes in Mitochondrial Content and Respiratory Function in Human Skeletal Muscle

A sedentary lifestyle has been linked to a number of metabolic disorders that have been associated with sub-optimal mitochondrial characteristics and an increased risk of premature death. Endurance training can induce an increase in mitochondrial content and/or mitochondrial functional qualities, which are associated with improved health and well-being and longer life expectancy. It is therefore important to better define how manipulating key parameters of an endurance training intervention can influence the content and functionality of the mitochondrial pool. This review focuses on mitochondrial changes taking place following a series of exercise sessions (training-induced mitochondrial adaptations), providing an in-depth analysis of the effects of exercise intensity and training volume on changes in mitochondrial protein synthesis, mitochondrial content and mitochondrial respiratory function. We provide evidence that manipulation of different exercise training variables promotes specific and diverse mitochondrial adaptations. Specifically, we report that training volume may be a critical factor affecting changes in mitochondrial content, whereas relative exercise intensity is an important determinant of changes in mitochondrial respiratory function. As a consequence, a dissociation between training-induced changes in mitochondrial content and mitochondrial respiratory function is often observed. We also provide evidence that exercise-induced changes are not necessarily predictive of training-induced adaptations, we propose possible explanations for the above discrepancies and suggestions for future research.

Lack of cyclin D3 induces skeletal muscle fiber-type shifting, increased endurance performance and hypermetabolism

The mitogen-induced D-type cyclins (D1, D2 and D3) are regulatory subunits of the cyclin-dependent kinases CDK4 and CDK6 that drive progression through the G1 phase of the cell cycle. In skeletal muscle, cyclin D3 plays a unique function in controlling the proliferation/differentiation balance of myogenic progenitor cells. Here, we show that cyclin D3 also performs a novel function, regulating muscle fiber type-specific gene expression. Mice lacking cyclin D3 display an increased number of myofibers with higher oxidative capacity in fast-twitch muscle groups, primarily composed of myofibers that utilize glycolytic metabolism. The remodeling of myofibers toward a slower, more oxidative phenotype is accompanied by enhanced running endurance and increased energy expenditure and fatty acid oxidation. In addition, gene expression profiling of cyclin D3-/- muscle reveals the upregulation of genes encoding proteins involved in the regulation of contractile function and metabolic markers specifically expressed in slow-twitch and fast-oxidative myofibers, many of which are targets of MEF2 and/or NFAT transcription factors. Furthermore, cyclin D3 can repress the calcineurin- or MEF2-dependent activation of a slow fiber-specific promoter in cultured muscle cells. These data suggest that cyclin D3 regulates muscle fiber type phenotype, and consequently whole body metabolism, by antagonizing the activity of MEF2 and/or NFAT.

Muscle morphology of the vastus lateralis is strongly related to ergometer performance, sprint capacity and endurance capacity in Olympic rowers

Rowers need to combine high sprint and endurance capacities. Muscle morphology largely explains muscle power generating capacity, however, little is known on how muscle morphology relates to rowing performance measures. The aim was to determine how muscle morphology of the vastus lateralis relates to rowing ergometer performance, sprint and endurance capacity of Olympic rowers. Eighteen rowers (12♂, 6♀, who competed at 2016 Olympics) performed an incremental rowing test to obtain maximal oxygen consumption, reflecting endurance capacity. Sprint capacity was assessed by Wingate cycling peak power. M. vastus lateralis morphology (volume, physiological cross-sectional area, fascicle length and pennation angle) was derived from 3-dimensional ultrasound imaging. Thirteen rowers (7♂, 6♀) completed a 2000-m rowing ergometer time trial. Muscle volume largely explained variance in 2000-m rowing performance (R² = 0.85), maximal oxygen consumption (R² = 0.65), and Wingate peak power (R² = 0.82). When normalized for differences in body size, maximal oxygen consumption and Wingate peak power were negatively related in males (r = -0.94). Fascicle length, not physiological cross-sectional area, attributed to normalized peak power. In conclusion, vastus lateralis volume largely explains variance in rowing ergometer performance, sprint and endurance capacity. For a high normalized sprint capacity, athletes may benefit from long fascicles rather than a large physiological cross-sectional area.

Adaptations in muscle oxidative capacity, fiber size, and oxygen supply capacity after repeated-sprint training in hypoxia combined with chronic hypoxic exposure

In this study, we investigate adaptations in muscle oxidative capacity, fiber size and oxygen supply capacity in team-sport athletes after six repeated-sprint sessions in normobaric hypoxia or normoxia combined with 14 days of chronic normobaric hypoxic exposure. Lowland elite field hockey players resided at simulated altitude (≥14 h/day at 2,800-3,000 m) and performed regular training plus six repeated-sprint sessions in normobaric hypoxia (3,000 m; LHTLH; n = 6) or normoxia (0 m; LHTL; n = 6) or lived at sea level with regular training only (LLTL; n = 6). Muscle biopsies were obtained from the m. vastus lateralis before (pre), immediately after (post-1), and 3 wk after the intervention (post-2). Changes over time between groups were compared, including likelihood of the effect size (ES). Succinate dehydrogenase activity in LHTLH largely increased from pre to post-1 (~35%), likely more than LHTL and LLTL (ESs = large-very large), and remained elevated in LHTLH at post-2 (~12%) vs. LHTL (ESs = moderate-large). Fiber cross-sectional area remained fairly similar in LHTLH from pre to post-1 and post-2 but was increased at post-1 and post-2 in LHTL and LLTL (ES = moderate-large). A unique observation was that LHTLH and LHTL, but not LLTL, improved their combination of fiber size and oxidative capacity. Small-to-moderate differences in oxygen supply capacity (i.e., myoglobin and capillarization) were observed between groups. In conclusion, elite team-sport athletes substantially increased their skeletal muscle oxidative capacity, while maintaining fiber size, after only 14 days of chronic hypoxic residence combined with six repeated-sprint training sessions in hypoxia. NEW & NOTEWORTHY Our novel findings show that elite team-sport athletes were able to substantially increase the skeletal muscle oxidative capacity in type I and II fibers (+37 and +32%, respectively), while maintaining fiber size after only 14 days of chronic hypoxic residence combined with six repeated-sprint sessions in hypoxia. This increase in oxidative capacity was superior to groups performing chronic hypoxic residence with repeated sprints in normoxia and residence at sea level with regular training only.

Reduced dietary intake of micronutrients with antioxidant properties negatively impacts muscle health in aged mice

BACKGROUND

Inadequate intake of micronutrients with antioxidant properties is common among older adults and has been associated with higher risk of frailty, adverse functional outcome, and impaired muscle health. However, a causal relationship is less well known. The aim was to determine in old mice the impact of reduced dietary intake of vitamins A/E/B6/B12/folate, selenium, and zinc on muscle mass, oxidative capacity, strength, and physical activity (PA) over time.

METHODS

Twenty-one-month-old male mice were fed either AIN-93-M (control) or a diet low in micronutrients with antioxidant properties (=LOWOX-B: 50% of mouse recommended daily intake of vitamins A, E, B6, and B12, folate, selenium, and zinc) for 4 months. Muscle mass, grip strength, physical activity (PA), and general oxidative status were assessed. Moreover, muscle fatigue was measured of m. extensor digitorum longus (EDL) during an ex vivo moderate exercise protocol. Effects on oxidative capacity [succinate dehydrogenase (SDH) activity], muscle fibre type, number, and fibre cross-sectional area (fCSA) were assessed on m. plantaris (PL) using histochemistry.

RESULTS

After 2 months on the diet, bodyweight of LOWOX-B mice was lower compared with control (P < 0.0001), mainly due to lower fat mass (P < 0.0001), without significant differences in food intake. After 4 months, oxidative status of LOWOX-B mice was lower, demonstrated by decreased vitamin E plasma levels (P < 0.05) and increased liver malondialdehyde levels (P = 0.018). PA was lower in LOWOX-B mice (P < 0.001 vs. control). Muscle mass was not affected, although PL-fCSA was decreased (~16%; P = 0.028 vs. control). SDH activity and muscle fibre type distribution remained unaffected. In LOWOX-B mice, EDL force production was decreased by 49.7% at lower stimulation frequencies (P = 0.038), and fatigue resistance was diminished (P = 0.023) compared with control.

CONCLUSIONS

Reduced dietary intake of vitamins A, E, B6, and B12, folate, selenium, and zinc resulted in a lower oxidative capacity and has major impact on muscle health as shown by decreased force production and PA, without effects on muscle mass. The reduced fCSA in combination with similar SDH activity per fibre might explain the reduced oxidative capacity resulting in the increased fatigue after exercise in LOWOX-B mice.

Critical determinants of combined sprint and endurance performance: an integrative analysis from muscle fiber to the human body

Optimizing physical performance is a major goal in current physiology. However, basic understanding of combining high sprint and endurance performance is currently lacking. This study identifies critical determinants of combined sprint and endurance performance using multiple regression analyses of physiologic determinants at different biologic levels. Cyclists, including 6 international sprint, 8 team pursuit, and 14 road cyclists, completed a Wingate test and 15-km time trial to obtain sprint and endurance performance results, respectively. Performance was normalized to lean body mass^2/3 to eliminate the influence of body size. Performance determinants were obtained from whole-body oxygen consumption, blood sampling, knee-extensor maximal force, muscle oxygenation, whole-muscle morphology, and muscle fiber histochemistry of musculus vastus lateralis. Normalized sprint performance was explained by percentage of fast-type fibers and muscle volume ( R² = 0.65; P < 0.001) and normalized endurance performance by performance oxygen consumption ( V̇o₂), mean corpuscular hemoglobin concentration, and muscle oxygenation ( R² = 0.92; P < 0.001). Combined sprint and endurance performance was explained by gross efficiency, performance V̇o₂, and likely by muscle volume and fascicle length ( P = 0.056; P = 0.059). High performance V̇o₂ related to a high oxidative capacity, high capillarization × myoglobin, and small physiologic cross-sectional area ( R² = 0.67; P < 0.001). Results suggest that fascicle length and capillarization are important targets for training to optimize sprint and endurance performance simultaneously.-Van der Zwaard, S., van der Laarse, W. J., Weide, G., Bloemers, F. W., Hofmijster, M. J., Levels, K., Noordhof, D. A., de Koning, J. J., de Ruiter, C. J., Jaspers, R. T. Critical determinants of combined sprint and endurance performance: an integrative analysis from muscle fiber to the human body.