Disturbances in skeletal muscle lipid oxidation might induce ectopic fat deposition and lipotoxicity. Nevertheless, the cellular mechanisms that regulate skeletal muscle lipid oxidation have not been fully determined. We aimed to determine whether there was an association between relative whole body lipid oxidation and mitochondrial size or mitochondria-sarcoplasmic reticulum interactions in the skeletal muscle. Twelve healthy men were included [mean (standard deviation), 24.7 (1.5) yr old, 24.4 (2.6) kg/m2]. The respiratory quotient (RQ) was used to estimate relative lipid oxidation at rest and during exercise (50% maximal oxygen consumption, 600 kcal expended). A skeletal muscle biopsy was obtained from the vastus lateralis at rest. Transmission electron microscopy was used to determine mitochondrial size and mitochondria-sarcoplasmic reticulum interactions (≤50 nm of distance between organelles). Protein levels of fusion/fission regulators were measured in skeletal muscle by Western blot. Resting RQ and exercise RQ associated inversely with intermyofibrillar mitochondrial size ( r = −0.66 and r = −0.60, respectively, P < 0.05). Resting RQ also associated inversely with the percentage of intermyofibrillar mitochondria-sarcoplasmic reticulum interactions ( r = −0.62, P = 0.03). Finally, intermyofibrillar mitochondrial size associated inversely with lipid droplet density ( r = −0.66, P = 0.01) but directly with mitochondria fusion-to-fission ratio ( r = 0.61, P = 0.03). Our results show that whole body lipid oxidation is associated with skeletal muscle intermyofibrillar mitochondrial size, fusion phenotype, and mitochondria-sarcoplasmic-reticulum interactions in nondiabetic humans.

Purpose The present study compared the effects of eccentric cycling (ECC) and conventional concentric cycling (CONC) training on muscle function, body composition, functional performance, and quality of life (QOL) of patients with moderate chronic obstructive pulmonary disease (COPD). Methods Twenty patients (age: 69.6 ± 10.1 years, forced expiratory volume in 1-s: 73.2 ± 11.4% of predicted) were randomly allocated to ECC (n = 10) or CONC (n = 10) group. They performed 12 weeks of ECC or CONC training at similar perceived exertion. The workload, heart rate (HR), blood oxygen saturation (SpO 2 ), and dyspnea were monitored during cycling. Outcomes measures included maximal voluntary isometric contraction (MVC) strength of the knee extensors, rate of force development (RFD), lower limb fat-free (LLFFM) and fat (LLFM) mass, 6-min walking test (6MWT), timed upand-go test (TUG), stairs ascending (SAWT) and descending walking time (SDWT), and QOL assessed by the Saint George's respiratory questionnaire. Results ECC produced on average threefold greater (P < 0.001) workload (211.8 ± 106.0 kJ) than CONC (78.1 ± 62.6 kJ) over 34 training sessions. ECC showed 1.5 ± 2.1% greater SpO 2 , 24.7 ± 4.1% lower HR, and 64.4 ± 29.6% lower dyspnea in average than CONC (P < 0.001). ECC increased LLFFM (4.5 ± 6.2%; P = 0.03), while CONC decreased LLFM (3.3 ± 6.4%; P = 0.04) after training. Both ECC and CONC reduced (P < 0.05) SAWT (− 16.1 ± 9.3% vs − 10.1 ± 14.4%) and SDWT (− 12.2 ± 12.6% vs − 14.4 ± 14.7%), and improved (P < 0.05) QOL (33.4 ± 38.8 vs 26.1 ± 36.6%) similarly, but only ECC improved (P < 0.05) RFD (69-199%), TUG (13.6 ± 13.6%), and 6MWT (25.3 ± 27.7%). Conclusion These results suggest that ECC training with less cardio-pulmonary demands was more effective in increasing functional performance and muscle mass for COPD patients than CONC training. KeywordsChronic obstructive pulmonary disease • Muscle strength • Rate of force development • Functional performance • Body composition • Quality of life Abbreviations 6MWT Six-minute walking test ANOVA Analysis of variance ATP Adenosine triphosphate CONC Concentric cycling COPD Chronic obstructive pulmonary disease DEXA Dual-energy X-ray absorptiometry DOMS Delayed onset muscle soreness ECC Eccentric cycling ES Effect size Communicated by William J. Kraemer.

An interaction between mitochondrial dynamics, physical activity levels, and COVID‐19 severity has been previously hypothesized. However, this has not been tested. We aimed to compare mitochondrial morphology and cristae density of PBMCs between subjects with non‐severe COVID‐19, subjects with severe COVID‐19, and healthy controls. Additionally, we compared the level of moderate‐vigorous physical activity (MVPA) and sitting time between groups. Blood samples were taken to obtain PBMCs. Mitochondrial dynamics were assessed by electron microscopy images and western blot of protein that regulate mitochondrial dynamics. The International Physical Activity Questionnaire (IPAQ; short version) was used to estimate the level of MVPA and the sitting time The patients who develop severe COVID‐19 (COVID‐19++) not present alterations of mitochondrial size neither mitochondrial density in comparison to non‐severe patients COVID‐19 (COVID‐19) and control subjects (CTRL). However, compared to CTRL, COVID‐19 and COVID‐19++ groups have lower mitochondrial cristae length, a higher proportion of abnormal mitochondrial cristae. The COVID‐19++ group has lower number (trend) and length of mitochondrial cristae in comparison to COVID‐19 group. COVID‐19, but not COVID‐19++ group had lower Opa 1, Mfn 2 and SDHB (Complex II) proteins than CTRL group. Besides, COVID‐19++ group has a higher time sitting. Our results show that low mitochondrial cristae density, potentially due to physical inactivity, is associated with COVID‐19 severity.