It is known that the force per cross-sectional area (F/CSA) as well as the force of isolated muscle fibers decreases following actual and simulated space flight. Recent evidence shows that a loss of muscle strength may not only be related to neural drive and muscular changes, but also to tendon and extracellular matrix alterations affecting the mechanical output of the muscle and its ability to transduce mechanical signals into chemical processes driving protein synthesis. Sarcolab is trying to investigate this phenomenon further, using an in vivo and in vitro approach, by studying the plantar flexors in vivo, and isolated muscle fibers of the soleus in vitro.

The contractile characteristics of the plantar flexors are studied during static (isometric contractions at different joint angles) and dynamic (isokinetic contractions at different angular velocities) contractions performed on the MARES dynamometer. Muscle architectural features, such as fiber fascicle length and pennation angle are assessed using ultrasound imaging and Magnetic Resonance Imaging (MRI). Tendon mechanical properties such as stiffness and hysteresis are assessed by ultrasound imaging and during a slow ramp isometric contraction.

Furthermore, Sarcolab evaluates the changes in the elastic properties of the musculo-tendionous system by measuring ankle flexibility (range of motion), quick-release tests and sinusoidal perturbations and the stretch reflex. For this purpose, EMG electrodes will be placed on the tested leg during maximal and submaximal isometric contraction. This will also provide information about muscle fatigability.

Before and after the flight a biopsy sample of the Soleus will be obtained to study single muscle fiber properties. The force will be related to the myosin concentration to clarify whether the loss of specific force is related to myosin and whether it occurs both in slow and fast muscle fibers. Additional tests include maximum shortening velocity to determine if the loss of velocity is caused by changes in the function of myosin itself or if other muscle components are involved. For further analysis of the involved mechanisms the quantity and type of costameric proteins is determined using two different analysis methods.

Combining the results obtained on whole muscle, single fibers and tendon, will clarify whether the decrease in F/CSA of locomotor muscles following spaceflight is due to a) an impairment of the contractile capacity of the muscle tissue itself, b) a change in tendon mechanical properties, c) modifications in muscle architecture and/or d) alterations in the extracellular matrix.

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