The aim of this investigation was to document the exercise program used by crewmembers (n = 9; 45 ± 2 yr) while aboard the International Space Station (ISS) for 6 mo and examine its effectiveness for preserving calf muscle characteristics. Before and after spaceflight, we assessed calf muscle volume (MRI), static and dynamic calf muscle performance, and muscle fiber types (gastrocnemius and soleus). While on the ISS, crewmembers had access to a running treadmill, cycle ergometer, and resistance exercise device. The exercise regimen varied among the crewmembers with aerobic exercise performed ∼5 h/wk at a moderate intensity and resistance exercise performed 3–6 days/wk incorporating multiple lower leg exercises. Calf muscle volume decreased (P < 0.05) 13 ± 2% with greater (P < 0.05) atrophy of the soleus (−15 ± 2%) compared with the gastrocnemius (−10 ± 2%). Peak power was 32% lower (P < 0.05) after spaceflight. Force-velocity characteristics were reduced (P < 0.05) −20 to −29% across the velocity spectrum. There was a 12–17% shift in myosin heavy chain (MHC) phenotype of the gastrocnemius and soleus with a decrease (P < 0.05) in MHC I fibers and a redistribution among the faster phenotypes. These data show a reduction in calf muscle mass and performance along with a slow-to-fast fiber type transition in the gastrocnemius and soleus muscles, which are all qualities associated with unloading in humans. Future long-duration space missions should modify the current ISS exercise prescription and/or hardware to better preserve human skeletal muscle mass and function, thereby reducing the risk imposed to crewmembers.

The purpose of this study was to determine whether there was an age‐related decline in the isometric and isotonic contractile function of permeabilized slow (MHC I) and fast (MHC IIa) single muscle fibres. Vastus lateralis muscle fibres from six young men (YM; 25 ± 1 years), six young women (YW; 25 ± 1 years), six old men (OM; 80 ± 4 years) and six old women (OW; 78 ± 2 years) were studied at 15 °C for in vitro force‐velocity properties, peak force and contractile velocity. Peak power was 23‐28 % lower (P < 0.05) in MHC I fibres of YW compared to the other three groups. MHC IIa peak power was 25–40 % lower (P < 0.05) in OW compared to the other three groups. No difference was found in MHC I and IIa normalized peak power among any of the groups. Peak force was lower (P < 0.05) in the YW (MHC I fibres) and OW (MHC IIa fibres) compared to the other groups. Differences in peak force with ageing were negated when normalized to cell size. No age‐related differences were observed in single fibre contractile velocity of MHC I and IIa fibres. These data show that YW (MHC I) and OW (MHC IIa) have lower single fibre absolute peak power and peak force compared to men; however, these differences are negated when normalized to cell size. General muscle protein concentrations (i.e. total, sarcoplasmic and myofibrillar) from the same biopsies were lower (4–9 %, P < 0.05) in the OM and OW. However, myosin and actin concentrations were not different (P > 0.05) among the four groups. These data suggest that differences in whole muscle strength and function that are often observed with ageing appear to be regulated by quantitative rather than qualitative parameters of single muscle fibre contractile function.

Muscle biopsies were obtained from the vastus lateralis before and after 84 days of bed‐rest from six control (BR) and six resistance‐exercised (BRE) men to examine slow‐ and fast‐twitch muscle fibre contractile function. BR did not exercise during bed‐rest and had a 17 and 40% decrease in whole muscle size and function, respectively. The BRE group performed four sets of seven maximal concentric and eccentric supine squats 2–3 days per week (every third day) that maintained whole muscle strength and size. Slow (MHC I) and fast (MHC IIa) muscle fibres were studied at 15°C for diameter, peak force (P_o), contractile velocity (V_o) and force–power parameters. SDS‐PAGE was performed on each single fibre after the functional experiments to determine MHC isoform composition. MHC I and IIa BR fibres were, respectively, 15 and 8% smaller, 46 and 25% weaker (P_o), 21 and 6% slower (V_o), and 54 and 24% less powerful after bed‐rest (P < 0.05). BR MHC I and IIa P_o and power normalized to cell size were lower (P < 0.05). BRE MHC I fibres showed no change in size or V_o after bed‐rest; however, P_o was 19% lower (P < 0.05), resulting in 20 and 30% declines (P < 0.05) in normalized P_o and power, respectively. BRE MHC IIa fibres showed no change in size, P_o and power after bed‐rest, while V_o was elevated 13% (P < 0.05). BRE MHC IIa normalized P_o and power were 10 and 15% lower (P < 0.05), respectively. MHC isoform composition shifted away from MHC I fibres, resulting in an increase (P < 0.05) in MHC I/IIa (BR and BRE) and MHC IIa/IIx (BR only) fibres. These data show that the contractile function of the MHC I fibres was more affected by bed‐rest and less influenced by the resistance exercise protocol than the MHC IIa fibres. Considering the large differences in power of human MHC I and IIa muscle fibres (5‐ to 6‐fold), the maintenance of whole muscle function with the resistance exercise programme is probably explained by (1) the maintenance of MHC IIa power and (2) the shift from slow to fast (MHC I → MHC I/IIa) in single fibre MHC isoform composition.

The purpose of this study was to 1) examine single cell contractile mechanics of skeletal muscle before and after 12 wk of progressive resistance training (PRT) in older women ( n = 7; 74 ± 2 yr) and 2) to compare these results to our previously completed single cell PRT work with older men ( n = 7; 74 ± 2 yr) (Trappe S, Williamson D, Godard M, Porter D, Rowden G, and Costill D. J Applied Physiol 89:143–152, 2000). Knee extensor PRT was performed 3 days/wk at 80% of one-repetition maximum. Muscle biopsies were obtained from the vastus lateralis before and after the PRT. Chemically skinned single muscle fibers ( n = 313) were studied at 15°C for peak tension (P_o), unloaded shortening velocity ( V _o), and power. Due to the low number of hybrid fibers identified post-PRT, direct comparisons were limited to MHC I and IIa fibers. Muscle fiber diameter increased 24% (90 ± 2 to 112 ± 6 μm; P < 0.05) in MHC I fibers with no change in MHC IIa fibers. P_o increased ( P< 0.05) 33% in MHC I (0.76 ± 0.04 to 1.01 ± 0.09 mN) and 14% in MHC IIa (0.73 ± 0.04 to 0.83 ± 0.05 mN) fibers. Muscle fiber V _o was unaltered in both fiber types with PRT. MHC I and IIa fiber power increased ( P< 0.05) 50% [11 ± 2 to 17 ± 2 μN · fiber length (FL) · s⁻¹] and 25% (40 ± 8 to 51 ± 6 μN · FL · s⁻¹), respectively. However, when peak power was normalized to cell size, no pre- to postimprovements were observed. These data indicate that PRT in elderly women increases muscle cell size, strength, and peak power in both slow and fast muscle fibers, which was similar to the older men. However, in contrast to the older men, no change in fiber V _o or normalized power was observed in the older women. These data suggest that older men and women respond differently at the muscle cell level to the same resistance-training stimulus.

The purpose of this investigation was to characterize the effects of marathon training on single muscle fiber contractile function in a group of recreational runners. Muscle biopsies were obtained from the gastrocnemius muscle of seven individuals (22 ± 1 yr, 177 ± 3 cm, and 68 ± 2 kg) before, after 13 wk of run training, and after 3 wk of taper. Slow-twitch myosin heavy chain [(MHC) I] and fast-twitch (MHC IIa) muscle fibers were analyzed for size, strength (P_o), speed ( V_o), and power. The run training program led to the successful completion of a marathon (range 3 h 56 min to 5 h 35 min). Oxygen uptake during submaximal running and citrate synthase activity were improved ( P < 0.05) with the training program. Muscle fiber size declined ( P < 0.05) by ∼20% in both fiber types after training. P_o was maintained in both fiber types with training and increased ( P < 0.05) by 18% in the MHC IIa fibers after taper. This resulted in >60% increase ( P < 0.05) in force per cross-sectional area in both fiber types. Fiber V_o increased ( P < 0.05) by 28% in MHC I fibers with training and was unchanged in MHC IIa fibers. Peak power increased ( P < 0.05) in MHC I and IIa fibers after training with a further increase ( P < 0.05) in MHC IIa fiber power after taper. These data show that marathon training decreased slow-twitch and fast-twitch muscle fiber size but that it maintained or improved the functional profile of these fibers. A taper period before the marathon further improved the functional profile of the muscle, which was targeted to the fast-twitch muscle fibers.