Effects of a 5-day bed rest on muscle size, T2-relaxation and neuromuscular function

Introduction: The study was a part of a series of short-term bed rest studies organized by the European Space Agency (ESA) involving several teams of scientists. The bed rest studies are used to simulate microgravity environment and to help to find out countermeasures for adverse effects on human physiology that are caused e.g. by inactivity and unloading of skeletal muscles. Data from these studies can also help general population e.g. to recover faster after hospitalization. Objectives: The purpose of the study was to investigate effects of a 5-day bed rest on upper leg muscle size (vasti compared to sartorius), T2-relaxation in those muscles, and changes in neuromuscular performance. Objectives were to find out if initial muscle volume changes are caused by changes in the content of free non-macromolecular bound water, and evaluate possible consequences to neuromuscular performance. Methods: Ten healthy male subjects (age 29.4 ± 5.9 years; height 178.8 ± 3.7 cm; body mass 77.7 ± 4.1 kg) took part in three 6º head-down tilt bed rest campaigns each lasting five days and including 5-day preparatory (baseline) and 5-day recovery periods. The subjects were randomly assigned either in a control group (CON; strict bed rest), in a standing group (STA, daily 25 min upright standing) or in a locomotion replacement training group (LRT, daily 25 min exercise protocol). All subjects performed the protocols with approximately 2-3 months between the campaigns. The muscle volume changes were determined by segmenting MRI of vasti and sartorius, and a MRI parameter T2 was analyzed from the same muscles to determine if it correlates with volume changes. MRIs were taken two days before the bed rest (BDC-2), after the bed rest (R+0) and after 5-day recovery (R+5). Knee extensor strength was measured via testing isometric maximum voluntary contraction (MVC), and changes in maximum jump height were tested using a force platform. Muscle fatigability and electrical conduction properties were also analyzed via M-wave and EMG measurements. Results: Muscle volume reduction (BDC-2 to R+0) were observed in sartorius (-1.38% in LRT; -0.38% in STA and -0.46 % in CON groups; p<0.05), and in vasti (-0.20% in LRT; - 2.21 % in STA and -1.75 % in CON groups; p<0.001). The muscle volume kept on declining or remained at R+0 level during the recovery period in both muscles. T2 increased in sartorius if all subjects were observed regardless of intervention (LRT, STA, CON) and recovered to BDC-2 values after recovery (BDC-2: T2=42.10; R+0: T2=48.22 ms; R+5: T2=46.61 ms; p<0.05). There was no difference between different interventions. In vasti there were no statistical significances in T2 between interventions or between different measurement days (BDC-2, R+0, R+5). MVC decreased by 8 % for CON (p<0.05), was maintained for STA, and increased with 12 % for LRT (p < 0.05). Maximum jump height declined approximately 1.5 cm for all groups (p < 0.001). Neural activation and knee extensor fatigability did not change. Conclusions: It can be concluded that the 5-day bed rest with this sample size was not adequate to draw conclusions regarding correlation of T2 and muscle volume changes. The LRT training program was able maintain muscle size and MVC. Future studies are required to validate the methodology.