Whole muscle contractile parameters and thickness loss during 35-day bed rest

Extended exposure to microgravity leads to significant musculoskeletal adaptations. Contractile parameters of four skeletal muscles (biceps brachii-BB, vastus medialis-VM, biceps femoris-BF and gastrocnemius medialis-GM) were measured in ten healthy males (aged 22.3 +/- 2.2 years) during 35 days of horizontal bed rest by a mechanomyography-based method termed 'tensiomyography' (TMG). Two contractile parameters: contraction time (Tc) and maximal displacement (Dm) were individually measured from electrically evoked maximal single twitch TMG response of all four muscles before and after bed rest. Significant changes in Tc were found after bed rest, as shown by an increase in GM muscle Tc by 18% (p < 0.01). Dm values significantly increased (p < 0.01) after bed rest, by 24, 26 and 30% in the VM, BF and GM muscles, respectively. In the GM, the change in Dm significantly correlated with the decrease in muscle thickness (r = -0.70, p < 0.01). In conclusion, bed rest induced changes in both Dm and Tc of the TMG signal; changes in Dm being inversely related to those of muscle thickness. Amongst the investigated muscles, most affected, in terms of atrophy and mechanical alterations, were those of the lower limbs. The observed increase in Dm may be attributed to a decrease in muscle, as well as tendon stiffness, causing larger muscle fibre and non-contractile tissue oscillations following contraction.