Aim We investigated architectural, functional and molecular responses of human skeletal muscle to concentric (CON) or eccentric (ECC) resistance training (RT). Methods Twelve young males performed 10 weeks of concentric (CON) or eccentric (ECC) resistance training (RT) (n = 6 CON, 6 ECC). An additional 14 males were recruited to evaluate acute muscle fascicle behaviour and molecular signalling in biopsies collected from vastus lateralis (VL) after 30 min of single bouts of CON or ECC exercise. VL volume was measured by magnetic resonance imaging. Muscle architecture (fascicle length, Lf; pennation angle, PA) was evaluated by ultrasonography. Muscle remodelling signals to CON or ECC loading [MAPK/AKT‐mammalian target of rapamycin (mTOR) signalling] and inflammatory pathway (TNFαMurf‐1‐MAFbx) were evaluated by immunoblotting. Results Despite the ~1.2‐fold greater load of the ECC group, similar increases in muscle volume (+8% CON and +6% ECC) and in maximal voluntary isometric contraction (+9% CON and +11% ECC) were found after RT. However, increases in Lf were greater after ECC than CON (+12 vs. +5%) while increases in PA were greater in CON than ECC (+30 vs. +5%). Distinct architectural adaptations were associated with preferential growth in the distal regions of VL for ECC (+ECC +8% vs. +CON +2) and mid belly for CON (ECC +7 vs. CON +11%). While MAPK activation (p38MAPK, ERK1/2, p90RSK) was specific to ECC, neither mode affected AKT‐mTOR or inflammatory signalling 30 min after exercise. Conclusion Muscle growth with CON and ECC RT occurs with different morphological adaptations reflecting distinct fibre fascicle behaviour and molecular responses.

Narici, M. V. (2013). Skeletal muscle adaptations to physical inactivity and subsequent retraining in young men. Biogerontology (Dordrecht), 14,(247)(248)(249)(250)(251)(252)(253)(254)(255)(256)(257)(258)(259) Dette er siste tekst-versjon av artikkelen, og den kan inneholde små forskjeller fra forlagets pdf-versjon. Forlagets pdf-versjon finner du på link.springer.com: http://dx.doi.org/10. 1007/s10522-013-9427-6 This is the final text version of the article, and it may contain minor differences from the journal's pdf version. The original publication is available at link.springer.com: http://dx.doi.org/10.1007/s10522-013-9427-6 AbstractSkeletal muscle structure and function are markedly affected by chronic disuse. With unloading, muscle mass is lost at rate of about 0.4%/day but little is known about the recovery of muscle mass and strength following disuse. Here we report an extensive data set describing in detail skeletal muscle adaptations in structure and function in response to both disuse and retraining. Eight young men (23±2.2 yrs) underwent 3 weeks of unilateral lower limb suspension (ULLS) followed by a 3-week resistance training recovery program. Knee extensor (KE) isometric torque, voluntary activation (VA), quadriceps femoris (QF) muscle volume (QF vol ), fascicle length (Lf) and pennation angle (θ), physiological cross-sectional area (PCSA) of all four heads of the QF muscle, were measured before, after ULLS, and post-ULLS-resistance training. Needle biopsies were taken from the vastus lateralis muscle of a subgroup (n=6) of the same subjects and cross sectional area of individual muscle s and myosin content of muscle samples were determined. Following 3 weeks of ULLS, isometric torque decreased by 26%, PCSA by 3%, QF vol by 10%. Lf and θ of all four heads of QF significantly decreased (P≤0.05). Following the 3-week retraining period, isometric torque, PCSA, QF vol , Lf and θ of all four heads of QF were all fully restored to pre ULLS values. CSA of individual muscle fibres and myosin content of muscle samples decreased by 26% and 35% respectively (post-ULLS) and recovered to almost pre-ULLS values following retraining. There were no significant changes in voluntary activation of the quadriceps muscles in response to either ULLS or subsequent retraining. These results indicate that, 1) the loss of muscle force with 3-week unloading in humans is mostly explained by muscle atrophy and by a decrease in myosin content and, 2) all the neuromuscular changes induced by this model of disuse can be fully restored after a resistance training intervention of equal duration. KeywordsDisuse, Skeletal muscle, atrophy, recovery, resistance training 2 Introduction Deficits in skeletal muscle mass and function are primary outcomes of chronic unloading, with antigravity muscles such as the knee extensors (KE) being particularly affected (Narici and De Boer 2011). An extensive knowledge of these adverse conditions is paramount, for they are inevitably linked to clinical situations such as prolonged best rest or ...

New Findings r What is the central question of this study?Do the contractile properties of single muscle fibres differ between body-builders, power athletes and control subjects? r What is the main finding and its importance?Peak power normalized for muscle fibre volume in power athletes is higher than in control subjects. Compared with control subjects, maximal isometric tension (normalized for muscle fibre cross-sectional area) is lower in body-builders. Although this difference may be caused in part by an apparent negative effect of hypertrophy, these results indicate that the training history of power athletes may increase muscle fibre quality, whereas body-building may be detrimental.We compared muscle fibre contractile properties of biopsies taken from the vastus lateralis of 12 body-builders (BBs; low-to moderate-intensity high-volume resistance training), six power athletes (PAs; high-intensity, low-volume combined with aerobic training) and 14 control subjects (Cs). Maximal isotonic contractions were performed in single muscle fibres, typed with SDS-PAGE. Fibre cross-sectional area was 67 and 88% (P < 0.01) larger in BBs than in PAs and Cs, respectively, with no significant difference in fibre cross-sectional area between PAs and Cs. Fibres of BBs and PAs developed a higher maximal isometric tension (32 and 50%, respectively, P < 0.01) than those of Cs. The specific tension of BB fibres was 62 and 41% lower than that of PA and C fibres (P < 0.05), respectively. Irrespective of fibre type, the peak power (PP) of PA fibres was 58% higher than that of BB fibres (P < 0.05), whereas BB fibres, despite considerable hypertrophy, had similar PP to the C fibres. This work suggests that high-intensity, low-volume resistance training with aerobic exercise improves PP, while low-to moderate-intensity high-volume resistance training does not affect PP and results in a reduction in specific tension. We postulate that the decrease in specific tension is caused by differences in myofibrillar density and/or post-translational modifications of contractile proteins.

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