Human skeletal muscle type 1 fibre distribution and response of stress-sensing proteins along the titin molecule after submaximal exhaustive exercise.

Early responses of stress-sensing proteins, muscle LIM protein (MLP), ankyrin repeat proteins (Ankrd1/CARP and Ankrd2/Arpp) and muscle-specific RING finger proteins (MuRF1 and MuRF2), along the titin molecule were investigated in the present experiment after submaximal exhaustive exercise. Ten healthy men performed continuous drop jumping unilaterally on a sledge apparatus with a submaximal height until complete exhaustion. Five stress-sensing proteins were analysed by mRNA measurements from biopsies obtained immediately and 3 h after the exercise from exercised vastus lateralis muscle while control biopsies were obtained from non-exercised legs before the exercise. Decreased maximal jump height and increased serum creatine kinase activities as indirect markers for muscle damage and HSP27 immunostainings on muscle biopsies as a direct marker for muscle damage indicated that the current exercised protocol caused muscle damage. mRNA levels for four (MLP, Ankrd1/CARP, MuRF1 and MuRF2) out of the five studied stress sensors significantly (p < 0.05) increased 3 h after fatiguing exercise. The magnitude of MLP and Ankrd2 responses was related to the proportion of type 1 myofibres. Our data showed that the submaximal exhaustive exercise with subject's own physical fitness level activates titin-based stretch-sensing proteins. These results suggest that both degenerative and regenerative pathways are activated in very early phase after the exercise or probably already during the exercise. Activation of these proteins represents an initial step forward adaptive remodelling of the exercised muscle and may also be involved in the initiation of myofibre repair.

Determinants of lower-body muscle power in early postmenopausal women.

OBJECTIVES: To investigate the association between muscle size, density, and fiber composition; body composition; maximal isometric knee extension strength (KES); and lower-body muscle power in healthy postmenopausal women. DESIGN: Cross-sectional analysis of baseline data from a 1-year randomized controlled experiment. SETTING: University-based research laboratory. PARTICIPANTS: Seventy-eight healthy postmenopausal women aged 50 to 57. MEASUREMENTS: Maximal lower-body muscle power was assessed using vertical jump height (VJH). Maximal isometric KES was measured on a dynamometer chair. Computed tomography scans were used to determine lean-tissue cross-sectional area and density of the thigh and lower leg muscles. Relative area occupied by type I, IIa, IIax, and IIx muscle fibers was assessed from the vastus lateralis muscle. lean body mass and total body fat mass were assessed using bioelectrical impedance. RESULTS: High VJH was associated with low body fat mass, high KES, and high density of thigh and lower leg muscles. Multivariate linear regression modeling revealed that high thigh muscle density (beta=0.242; P=.019), relative area occupied by the fastest muscle fiber types (IIax+IIx; beta=0.246; P=.007), KES (beta=0.247; P=.007), and low body fat mass (beta=-0.455; P<.001) were independently associated with high VJH, accounting for 45% of the variability in VJH. CONCLUSION: This study showed that thigh muscle composition, muscle strength, and body fat mass are important determinants of lower-body muscle power production during weight-bearing activity in healthy postmenopausal women.

Short-term effects of forced eccentric contractions on collagen synthesis and degradation in rat skeletal muscle.

Acute downhill running has been shown to activate matrix metalloproteinase- (MMP-) 2 and to change type IV collagen concentration in some muscle types. In order to study the influence of more intense exercise on total collagen and type IV collagen concentrations, molecules regulating their synthesis and degradation were investigated after forced lengthening contractions in rat skeletal muscle. Tibialis anterior (TA) muscle of 24 male Wistar rats was subjected to 240 forced lengthening contractions. TA muscle was excised at consecutive time points (0 and 6 h, 2, 4, and 7 days) after stimulation. With immunohistochemistry, types I, III and IV collagen were located in the swollen, necrotic and regenerated fibres in a similar manner as in intact undamaged skeletal muscle fibre. An increase in the activity of prolyl 4-hydroxylase was indicative of an overall elevated collagen biosynthesis. No change was demonstrated in total collagen concentration, whereas type IV collagen concentration increased after exercise. MMP-2 and MMP-9, which are the proteins that degrade type IV collagen, elevated after exercise. In conclusion, the increase in type IV collagen concentration seems to be the result of an increase in both the synthesis and activation of degrading enzymes and their inhibitors during recovery after forced lengthening contractions.