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.

No abstract

Harber MP, Konopka AR, Douglass MD, Minchev K, Kaminsky LA, Trappe TA, Trappe S. Aerobic exercise training improves whole muscle and single myofiber size and function in older women. Am J Physiol Regul Integr Comp Physiol 297: R1452-R1459, 2009. First published August 19, 2009 doi:10.1152/ajpregu.00354.2009.-To comprehensively assess the influence of aerobic training on muscle size and function, we examined seven older women (71 Ϯ 2 yr) before and after 12 wk of cycle ergometer training. The training program increased (P Ͻ 0.05) aerobic capacity by 30 Ϯ 6%. Quadriceps muscle volume, determined by magnetic resonance imaging (MRI), was 12 Ϯ 2% greater (P Ͻ 0.05) after training and knee extensor power increased 55 Ϯ 7% (P Ͻ 0.05). Muscle biopsies were obtained from the vastus lateralis to determine size and contractile properties of individual slow (MHC I) and fast (MHC IIa) myofibers, myosin light chain (MLC) composition, and muscle protein concentration. Aerobic training increased (P Ͻ 0.05) MHC I fiber size 16 Ϯ 5%, while MHC IIa fiber size was unchanged. MHC I peak power was elevated 21 Ϯ 8% (P Ͻ 0.05) after training, while MHC IIa peak power was unaltered. Peak force (Po) was unchanged in both fiber types, while normalized force (Po/cross-sectional area) was 10% lower (P Ͻ 0.05) for both MHC I and MHC IIa fibers after training. The decrease in normalized force was likely related to a reduction (P Ͻ 0.05) in myofibrillar protein concentration after training. In the absence of an increase in Po, the increase in MHC I peak power was mediated through an increased (P Ͻ 0.05) maximum contraction velocity (Vo) of MHC I fibers only. The relative proportion of MLC1s (Pre: 0.62 Ϯ 0.01; Post: 0.58 Ϯ 0.01) was lower (P Ͻ 0.05) in MHC I myofibers after training, while no differences were present for MLC 2s and MLC 3f isoforms. These data indicate that aerobic exercise training improves muscle function through remodeling the contractile properties at the myofiber level, in addition to pronounced muscle hypertrophy. Progressive aerobic exercise training should be considered a viable exercise modality to combat sarcopenia in the elderly population.

The purpose of this investigation was to examine the activation (phosphorylation) and total protein content of MAPK signalling cascade proteins (ERK 1/2, p90RSK, Mnk 1, eIF4E, p38 MAPK, JNK/SAPK, MKP 1) at rest and following exercise, in sedentary young and old men. Eight young (22 +/- 1 years; YM) and eight old (79 +/- 3 years; OM) men underwent a resting muscle biopsy of the vastus lateralis; they then performed a knee extensor resistance exercise session (29 contractions at approximately 70 % of max), followed by a post-exercise biopsy. Western immunoblot analysis demonstrated that the OM had higher resting phosphorylation of ERK 1/2, p90RSK, Mnk 1, p38 MAPK and JNK/SAPK proteins versus YM (P < 0.05). The resistance exercise bout caused an increase in phosphorylation of the ERK 1/2, p90RSK and Mnk 1 proteins (P < 0.05) in the YM. Conversely, the OM had a decrease in ERK 1/2, p90RSK, Mnk 1, p38 MAPK and JNK/SAPK phosphorylation (P < 0.05) after the exercise bout. Neither group showed a change in eIF4E phosphorylation. The total amount of protein expression of the MAPK signalling proteins was not different between the YM and OM, except that there was a higher (P < 0.05) MKP 1 protein content in the OM. This investigation is the first to provide evidence that MAPK proteins are differentially activated at rest and in response to a bout of resistance exercise in skeletal muscle of young and old men. These findings may have implications for other processes (e.g. transcription and translation) involved in skeletal muscle type and growth, when examining the changes occurring with ageing muscle before and after resistance exercise/training.