Mouse plantar flexor muscle size and strength after inactivity and training

Introduction: Losses in muscle mass and strength may affect an astronaut's safety; therefore, it is of utmost importance to optimize countermeasures to minimize atrophy and strength loss during spaceflight. The main purpose of this study was to determine if high force eccentric or isometric contractions performed by the plantar flexor group during hind limb suspension would preserve muscle mass and strength. Methods: Plantar flexor muscles of mice were trained with either eccentric or isometric contractions every other day during a 10-d hind limb suspension period. Pre- and post-suspension stimulation frequency- and angular velocity-dependent measurements of torque of the plantar flexors, soleus twitch (P_t) and tetanic (P_o) force, bodyweight, and muscle wet weight measurements were made. Results: The 19 and 26% losses in gastrocnemius and soleus muscle wet weights, respectively, were not attenuated with eccentric or isometric contractions. Neither eccentric nor isometric contractions attenuated the soleus muscle's 30% isometric force loss after hind limb suspension. Despite losses in muscle mass, there was no decrease in the force produced by the plantar flexor muscle group after hind limb suspension. Discussion: Hind limb suspension decreased both gastrocnemius and soleus mass, and in vitro soleus force production. However, in vivo force production of the plantar flexor muscle group did not decrease, which may be explained by a shift in the isometric torque: ankle angle relationship. The use of eccentric or isometric contractions as a countermeasure to offset muscle mass and strength requires further investigation as neither was capable of maintaining soleus muscle force production, or gastrocnemius and soleus muscle mass during hind limb suspension.