In five healthy males sustained isometric torques during elbow flexion, knee extension, and plantar flexion correlated positively with intramuscular tissue pressure (MTP) in the range 0-80% of the maximal voluntary contraction (MVC). During passive compression of the muscle at rest 133-Xenon muscle clearance stopped when MTP reached diastolic arterial pressure (DAP) indicating that the muscle vascular bed was occluded. However, during sustained contraction this relation between DAP, flow and MTP was not seen. In two cases 133-Xenon clearance from M. soleus did not stop in spite of an 80% maximal contraction and MTP stayed below DAP. In other cases MTP would reach as high as 240 mm Hg before clearance was zero. In the deeper parts of the muscles MTP during contraction was increased in relation to the more superficial parts. The means values for the % MVC that would stop MBF varied between 50 and 64% MVC for the investigated muscles. Mean rectified EMG (MEMG) showed a high correlation to MTP during sustained exhaustive contractions: When MEMG was kept constant MTP also remained constant while the exerted force decreased; when force was kept constant both MEMG and MTP increased in parallel. This demonstrated that muscle tissue compliance is decreasing during fatigue. Muscle ischemia occurring during sustained isometric contractions is partly due to the developed MTP, where especially the MTP around the veins in the deeper parts of the muscle can be considered of importance. However, ischemia is also affected by muscle fiber texture and anatomical distorsion of tissues.

Nine healthy men carried out head-down bed rest (BR) for 20 days. five subjects (TR) performed isometric, bilateral leg extension exercise every day, while the other four (NT) did not. Before and after BR, maximal isometric knee extension force was measured. Neural activation was assessed using a supramaximal twitch interpolated over voluntary contraction. From a series cross-sectional magnetic resonance imaging scans of the thigh, physiological cross-sectional areas (PCSA) of the quadriceps muscles were estimated (uncorrected PCSA, volume/estimated fibre length). Decrease in mean muscle force after BR was greater in NT [-10.9 (SD 6.9)%, P < 0.05] than in TR [0.5 (SD 7.9)%, not significant]. Neural activation did not differ between the two groups before BR, but after BR NT showed smaller activation levels. Pennation angles of the vastus lateralis muscle, determined by ultrasonography, showed no significant changes in either group. The PCSA decreased in NT by -7.8 (SD 0.8)% (P < 0.05) while in TR PCSA showed only an insignificant tendency to decrease [-3.8 (SD 3.8)%]. Changes in force were related more to changes in neural activation levels than to those in PCSA. The results suggest that reduction of muscle strength by BR is affected by a decreased ability to activate motor units, and that the exercise used in the present experiment is effective as a countermeasure.

The present study aimed to investigate the effect of dynamic leg press training on the physiological cross-sectional areas (PCSAs) of human lower limb muscles during 20 days of 6 degrees head-down tilt bed rest. Five healthy men comprised the resistance training group (BR-Tr) and data from two previous studies were used to derive a 10-man control group (BR-Cont). The BR-Tr performed two sessions (morning and afternoon session) of dynamic leg press action including knee extension and plantar flexion daily for the bed rest period: (1) three sets of 10 repetitions at 90% of maximum load and (2) 40% of maximum load to exhaustion. The PCSAs of the knee extensor (KE), knee flexor (KF), plantar flexor (PF), and dorsiflexor muscle groups were estimated using serial axial magnetic resonance (MR) images of the right-thigh and leg. After the bed rest period, the BR-Tr showed a significant increase in the PCSA of the KE. Although PCSA of the KF in two groups significantly decreased after bed rest, percentage of change in PCSA of the biceps femoris (long head) and semitendinosus muscles in the BR-Tr, which occupied approximately 70% of the KF, was significantly higher than those in the BR-Cont. Both the BR-Tr and BR-Cont groups showed significant decreases in the PCSA of PF with similar magnitude of 11.6% (P < 0.001) and 11.9% (P < 0.001), respectively. These results suggest that dynamic leg press training during bed rest can prevent deteriorating of the KE and a part of KF, but not the calf muscles.

The purpose of this study was to investigate the effects of resistance training on the morphological and functional properties of human lower limb muscles during 20 days of 6 degrees head-down-tilt bed rest. Nine men were randomly assigned to the resistance training group (BR-Tr, n = 5) or the non-training, control group (BR-Cont, n = 4). Isometric leg-press exercises were performed: 3 s x 30 repetitions (30 s rest between repetitions) daily for 20 days during the bed-rest period. Serial axial magnetic resonance images were taken from the right thigh and leg muscles, and muscle volume, muscle length, and fibre length were estimated. The physiological cross-sectional areas (PCSAs) of the knee extensor, knee flexor, ankle plantarflexor, and ankle dorsiflexor (tibialis anterior) muscle groups were determined as muscle volume multiplied by the cosine of the angle of fibre pennation divided by fibre length. Maximum voluntary contraction (MVC) during knee extension was measured. No significant changes were observed in the PCSA of the knee extensor muscles in BR-Tr group, whereas the PCSA in the BR-Cont group decreased by 7.8%. The PCSA of the knee flexor and plantarflexor muscles in the BR-Tr group and BR-Cont group significantly decreased after bed rest (knee flexors, 10.2% and 11.5%; plantarflexors, 13.0% and 12.8%, respectively). However, in both groups bed rest had no effect on the muscle volume and PCSA of the tibialis anterior. MVC was maintained by resistance training in the BR-Tr group (decreased by 1%). In contrast, a decline of strength was observed in the BR-Cont group (-16%), but this result was not statistically significant. These results suggest that isometric leg-press training prevents the deconditioning (i.e. atrophy and decline of strength) of the knee extensor muscle group.

Changes in bacterial diversity during the field experiment on biostimulation were monitored by denaturing gradient gel electrophoresis (DGGE) analysis of PCR-amplified 16S rDNA fragments. The results revealed that the bacterial community was disturbed after the start of treatment, continued to change for 45 days or 60 days and then formed a relatively stable community different from the original community structure. DGGE analysis of soluble methane monooxygenase (sMMO) hydroxylase gene fragments, mmoX, was performed to monitor the shifts in the numerically dominant sMMO-containing methanotrophs during the field experiment. Sequence analysis on the mmoX gene fragments from the DGGE bands implied that the biostimulation treatment caused a shift of potential dominant sMMO-containing methanotrophs from type I methanotrophs to type II methanotrophs.