Marco Toigo

New fundamental resistance exercise determinants of molecular and cellular muscle adaptations.

Physical activity relies on muscular force. In adult skeletal muscle, force results from the contraction of postmitotic, multinucleated myofibres of different contractile and metabolic properties. Myofibres can adapt to (patho-)physiological conditions of altered functional demand by radial growth, longitudinal growth, and regulation of fibre type functional gene modules. The adaptation’s specificity depends on the distinct molecular and cellular events triggered by unique combinations of conditional cues. In order to derive effective and tailored exercise prescriptions, it must be determined (1) which mechano-biological condition leads to what molecular/cellular response, and (2) how this molecular/cellular response relates to the structural, contractile, and metabolic adaptation. It follows that a thorough mechano-biological description of the loading condition is imperative. Unfortunately, the definition of (resistance) exercise conditions in the past and present literature is insufficient. It is classically limited to load magnitude, number of repetitions and sets, rest in-between sets, number of interventions/week, and training period. In this review, we show why the current description is insufficient, and identify new determinants of quantitative and/or qualitative effects on skeletal muscle with respect to resistance exercise in healthy, adult humans. These new mandatory determinants comprise the fractional and temporal distribution of the contraction modes per repetition, duration of one repetition, rest in-between repetitions, time under tension, muscular failure, range of motion, recovery time, and anatomical definition. We strongly recommend to standardise the design and description of all future resistance exercise investigations by using the herein proposed set of 13 mechano-biological determinants (classical and new ones).

Improvements in exercise performance with high-intensity interval training coincide with an increase in skeletal muscle mitochondrial content and function

Six sessions of high-intensity interval training (HIT) are sufficient to improve exercise capacity. The mechanisms explaining such improvements are unclear. Accordingly, the aim of this study was to perform a comprehensive evaluation of physiologically relevant adaptations occurring after six sessions of HIT to determine the mechanisms explaining improvements in exercise performance. Sixteen untrained (43 ± 6 ml·kg(-1)·min(-1)) subjects completed six sessions of repeated (8-12) 60 s intervals of high-intensity cycling (100% peak power output elicited during incremental maximal exercise test) intermixed with 75 s of recovery cycling at a low intensity (30 W) over a 2-wk period. Potential training-induced alterations in skeletal muscle respiratory capacity, mitochondrial content, skeletal muscle oxygenation, cardiac capacity, blood volumes, and peripheral fatigue resistance were all assessed prior to and again following training. Maximal measures of oxygen uptake (Vo2peak; ∼8%; P = 0.026) and cycling time to complete a set amount of work (∼5%; P = 0.008) improved. Skeletal muscle respiratory capacities increased, most likely as a result of an expansion of skeletal muscle mitochondria (∼20%, P = 0.026), as assessed by cytochrome c oxidase activity. Skeletal muscle deoxygenation also increased while maximal cardiac output, total hemoglobin, plasma volume, total blood volume, and relative measures of peripheral fatigue resistance were all unaltered with training. These results suggest that increases in mitochondrial content following six HIT sessions may facilitate improvements in respiratory capacity and oxygen extraction, and ultimately are responsible for the improvements in maximal whole body exercise capacity and endurance performance in previously untrained individuals.

Twenty-eight days at 3454-m altitude diminishes respiratory capacity but enhances efficiency in human skeletal muscle mitochondria

Modifications of skeletal muscle mitochondria following exposure to high altitude (HA) are generally studied by morphological examinations and biochemical analysis of expression. The aim of this study was to examine tangible measures of mitochondrial function following a prolonged exposure to HA. For this purpose, skeletal muscle biopsies were obtained from 8 lowland natives at sea level (SL) prior to exposure and again after 28 d of exposure to HA at 3454 m. High‐resolution respirometry was performed on the muscle samples comparing respiratory capacity and efficiency. Exercise capacity was assessed at SL and HA. Respirometric analysis revealed that mitochondrial respiratory capacity diminished in complex I‐ and complex II‐specific respiration in addition to a loss of maximal state‐3 oxidative phosphorylation capacity from SL to HA, all independent from alterations in mitochondrial content. Leak control coupling, respiratory control ratio, and oligomycin‐induced leak respiration, all measures of mitochondrial efficiency, improved in response to HA exposure. SL respiratory capacities correlated with measures of exercise capacity near SL, whereas mitochondrial efficiency correlated best with exercise capacity following HA. This data demonstrate that 1 mo of exposure to HA reduces respiratory capacity in human skeletal muscle; however, the efficiency of electron transport improves.—Jacobs, R. A., Siebenmann, C., Hug, M., Toigo, M., Meinild, A.‐K., Lundby, C. Twenty‐eight days at 3454‐m altitude diminishes respiratory capacity but enhances efficiency in human skeletal muscle mitochondria. FASEB J. 26, 5192–5200 (2012). www.fasebj.org

Electric pulse stimulation of cultured murine muscle cells reproduces gene expression changes of trained mouse muscle.

Adequate levels of physical activity are at the center of a healthy lifestyle. However, the molecular mechanisms that mediate the beneficial effects of exercise remain enigmatic. This gap in knowledge is caused by the lack of an amenable experimental model system. Therefore, we optimized electric pulse stimulation of muscle cells to closely recapitulate the plastic changes in gene expression observed in a trained skeletal muscle. The exact experimental conditions were established using the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) as a marker for an endurance-trained muscle fiber. We subsequently compared the changes in the relative expression of metabolic and myofibrillar genes in the muscle cell system with those observed in mouse muscle in vivo following either an acute or repeated bouts of treadmill exercise. Importantly, in electrically stimulated C2C12 mouse muscle cells, the qualitative transcriptional adaptations were almost identical to those in trained muscle, but differ from the acute effects of exercise on muscle gene expression. In addition, significant alterations in the expression of myofibrillar proteins indicate that this stimulation could be used to modulate the fiber-type of muscle cells in culture. Our data thus describe an experimental cell culture model for the study of at least some of the transcriptional aspects of skeletal muscle adaptation to physical activity. This system will be useful for the study of the molecular mechanisms that regulate exercise adaptation in muscle.

Maximum ground reaction force in relation to tibial bone mass in children and adults.

PURPOSE The purpose of the study was to assess maximum voluntary forefoot ground reaction force during multiple one-legged hopping (F m1LH) and to determine the correlation between tibial volumetric bone mineral content (vBMC, a valid surrogate of bone strength) and F m1LH. METHODS One hundred eighty-five females (8-82 yr old) and 138 males (8-71 yr old) performed multiple one-legged hopping to measure F m1LH acting on the forefoot during landing. Peripheral quantitative computed tomography scans were obtained to assess vBMC at 4%, 14%, 38%, and 66% of tibia length and calf muscle cross-sectional area at the 66% site. RESULTS In all 323 participants, F m1LH corresponded to 3-3.5 times body weight, and F m1LH predicted vBMC 14% by 84.0% (P < 0.001). vBMC 14% was better correlated with F m1LH than with the calf muscle cross-sectional area in both males (R2 = 0.841 vs R2 = 0.724) and females (R2 = 0.765 vs R2 = 0.597). F m1LH and vBMC14% both increased during growth and afterward remained constant or decreased with age but never increased above the values reached at the end of puberty. F m1LH decreased by 23.6% between 21-30 and 61-82 yr in females and by 14.0% between 31-40 and 51-71 yr in males. vBMC 14% decreased by 13.7% in females between 21-30 and 61-82 yr but remained unchanged in adult males. CONCLUSIONS Multiple one-legged hopping yields the highest (i.e., maximum) ground reaction force relative to other jumping maneuvers. Because bone strength is strongly governed by maximum muscle force, the concurrent assessment of peripheral quantitative computed tomography-derived bone strength and F m1LH might represent a new approach for the operational evaluation of musculoskeletal health.

Hypoxic training: effect on mitochondrial function and aerobic performance in hypoxia.

PURPOSE The effects of hypoxic training on exercise performance remain controversial. Here, we tested the hypotheses that i) hypoxic training possesses ergogenic effects at sea level and altitude and ii) the benefits are primarily mediated by improved mitochondrial function of the skeletal muscle. METHODS We determined aerobic performance (incremental test to exhaustion and time trial for a set amount of work) in moderately trained subjects undergoing 6 wk of endurance training (3-4 times per week, 60 min per session) in normoxia (placebo, n = 8) or normobaric hypoxia (FIO2 = 0.15, n = 9) using a double-blind and randomized design. Exercise tests were performed in normoxia and acute hypoxia (FIO2 = 0.15). Skeletal muscle mitochondrial respiratory capacities and electron coupling efficiencies were measured via high-resolution respirometry. Total hemoglobin mass was assessed by carbon monoxide rebreathing. RESULTS Skeletal muscle respiratory capacity was not altered by training or hypoxia; however, electron coupling control respective to fat oxidation slightly diminished with hypoxic training. Hypoxic training did increase total hemoglobin mass more than the placebo (8.4% vs 3.3%, P = 0.02). In normoxia, hypoxic training had no additive effect on maximal measures of oxygen uptake or time trial performance. In acute hypoxia, hypoxic training conferred no advantage on maximal oxygen uptake but tended to enhance time trial performance more than normoxic training (52% vs 32%, P = 0.09). CONCLUSIONS Our data suggest that, in moderately trained subjects, 6 wk of hypoxic training possesses no ergogenic effect at sea level. It is not excluded that hypoxic training might facilitate endurance capacity at moderate altitude; however, this issue is still open and needs to be further examined.

Mitochondrial capacity is affected by glycemic status in young untrained women with type 1 diabetes but is not impaired relative to healthy untrained women

In this study, we examined whether glycemic status influences aerobic function in women with type 1 diabetes and whether aerobic function is reduced relative to healthy women. To this end, we compared several factors determining aerobic function of 29 young sedentary asymptomatic women (CON) with 9 women of similar age and activity level with type 1 diabetes [DIA, HbA1c range = 6.9-8.2%]. Calf muscle mitochondrial capacity was estimated by (31)P-magnetic resonance spectroscopy. Capillarization and muscle fiber oxidative enzyme activity were assessed from vastus lateralis and soleus muscle biopsies. Oxygen uptake and cardiac output were evaluated by ergospirometry and N(2)O/SF(6) rebreathing. Calf muscle mitochondrial capacity was not different between CON and DIA, as indicated by the identical calculated maximal rates of oxidative ATP synthesis [0.0307 (0.0070) vs. 0.0309 (0.0058) s(-1), P = 0.930]. Notably, HbA1c was negatively correlated with mitochondrial capacity in DIA (R(2) = 0.475, P = 0.040). Although HbA1c was negatively correlated with cardiac output (R(2) = 0.742, P = 0.013) in DIA, there was no difference between CON and DIA in maximal oxygen consumption [2.17 (0.34) vs. 2.21 (0.32) l/min, P = 0.764], cardiac output [12.1 (1.9) vs. 12.3 (1.8) l/min, P = 0.783], and endurance capacity [532 (212) vs. 471 (119) s, P = 0.475]. There was also no difference between the two groups either in the oxidative enzyme activity or capillary-to-fiber ratio. We conclude that mitochondrial capacity depends on HbA1c in untrained women with type 1 diabetes but is not reduced relative to untrained healthy women.

Long-Term Endurance Exercise in Humans Stimulates Cell Fusion of Myoblasts along with Fusogenic Endogenous Retroviral Genes In Vivo.

Myogenesis is defined as growth, differentiation and repair of muscles where cell fusion of myoblasts to multinucleated myofibers is one major characteristic. Other cell fusion events in humans are found with bone resorbing osteoclasts and placental syncytiotrophoblasts. No unifying gene regulation for natural cell fusions has been found. We analyzed skeletal muscle biopsies of competitive cyclists for muscle-specific attributes and expression of human endogenous retrovirus (ERV) envelope genes due to their involvement in cell fusion of osteoclasts and syncytiotrophoblasts. Comparing muscle biopsies from post- with the pre-competitive seasons a significant 2.25-fold increase of myonuclei/mm fiber, a 2.38-fold decrease of fiber area/nucleus and a 3.1-fold decrease of satellite cells (SCs) occurred. We propose that during the pre-competitive season SC proliferation occurred following with increased cell fusion during the competitive season. Expression of twenty-two envelope genes of muscle biopsies demonstrated a significant increase of putative muscle-cell fusogenic genes Syncytin-1 and Syncytin-3, but also for the non-fusogenic erv3. Immunohistochemistry analyses showed that Syncytin-1 mainly localized to the sarcolemma of myofibers positive for myosin heavy-chain isotypes. Cellular receptors SLC1A4 and SLC1A5 of Syncytin-1 showed significant decrease of expression in post-competitive muscles compared with the pre-competitive season, but only SLC1A4 protein expression localized throughout the myofiber. Erv3 protein was strongly expressed throughout the myofiber, whereas envK1-7 localized to SC nuclei and myonuclei. Syncytin-1 transcription factors, PPARγ and RXRα, showed no protein expression in the myofiber, whereas the pCREB-Ser133 activator of Syncytin-1 was enriched to SC nuclei and myonuclei. Syncytin-1, Syncytin-3, SLC1A4 and PAX7 gene regulations along with MyoD1 and myogenin were verified during proliferating or actively-fusing human primary myoblast cell cultures, resembling muscle biopsies of cyclists. Myoblast treatment with anti-Synycytin-1 abrogated cell fusion in vitro. Our findings support functional roles for ERV envelope proteins, especially Syncytin-1, contributing to cell fusion of myotubes.

Reduced Bone Strength and Muscle Force in Women 27 Years After Anorexia Nervosa.

CONTEXT A substantial body of research findings indicate that muscle mass and bone mass are reduced in populations of anorexic females, even in such populations whose anorexia nervosa had been in remission for longer periods. OBJECTIVE This study aimed to investigate whether the bone of an anorexia nervosa recovery cohort is adapted to maximal muscle forces and whether there are alterations in the structure of the tibia in this population, as compared with a control group. DESIGN, SETTING, AND PARTICIPANTS This was a cross-sectional study of 22 women in Switzerland who have remained in stable recovery from anorexia nervosa for an average of 27 years. The measurements were compared with those of an age- and gender-matched control group (n = 73). INTERVENTIONS There were no interventions. MAIN OUTCOME MEASURES Bone characteristics of the tibia and maximal voluntary ground reaction force (Fm1LH) were measured. RESULTS The variability in volumetric bone mineral content (vBMC) at the 14% site was explained by 54.7% on the grounds of Fm1LH (P < .001). Formerly anorexic women had an 11.6% lower Fm1LH (P = .001), a significantly lower vBMC at 4% and 14% of tibia length, and an 11.9% (P = .001) lower body mass than the age- and gender-matched control population. Present body mass of the anorexia group correlated positively with vBMC at the 14% site (P < .001). CONCLUSIONS Despite the fact that findings reflected an adaptation of bone to the acting forces, most results indicated that the test cohort generally suffered from a secondary bone defect. In addition, maximal muscle force was also impaired in the formerly anorexic women.

Side-to-Side Differences in the Lower Leg Muscle-Bone Unit in Male Soccer Players.

PURPOSE To characterize side-to-side differences in the lower leg muscle-bone unit between the nondominant leg (NL) and the dominant leg (DL) using maximum voluntary forefoot ground reaction force (Fm1LH) during multiple one-legged hopping (m1LH) and tibial bone mass and geometry measured by peripheral quantitative computed tomography (pQCT). METHODS Sixty-six male high-level soccer players (age range = 12-18 yr) performed m1LH to determine Fm1LH acting on the forefoot during landing for the NL and DL separately. pQCT scans were obtained to assess bone structural variables at 4%, 14%, 38%, and 66% tibia length and calf muscle cross-sectional area at the 66% site. RESULTS First, participants displayed significant (P < 0.05) side-to-side differences in bone mass and geometry at 4%, 14%, and 38% (but not at the 66% site) of tibia length, with higher values in NL relative to DL (+0.7% to +5.6%), most evident at the 14% site. Second, no asymmetries were found for Fm1LH between the two legs (P = 0.442). Third, the relationship between Fm1LH and vBMC 14% was strong for both NL and DL (R2 = 0.48 and R = 0.54, respectively), but side-to-side differences in Fm1LH (ΔFm1LH) and side-to-side differences in vBMC 14% (ΔvBMC 14%) were not related (R2 = 0.04). CONCLUSIONS Contrary to expectations from the mechanostat theory, ΔFm1LH and ΔvBMC 14% did not differ in proportion to each other. It seems that playing soccer is a well-balanced activity with respect to Fm1LH. However, the NL contributes to the supporting of the action of the DL, meaning that the loading experienced by the tibia might be more pronounced for the NL relative to the DL, leading to the observed higher bone strength values for the NL.