Objectives:This study was designed to determine the effects of the absence of weight support on flexor and extensor muscles of the hind limb. These effects were assessed morphologically and biochemically from muscle biopsies taken from a slow extensor: the soleus; a fast extensor: the medial gastrocnemius (MG); and a fast flexor: the tibialis anterior (TA). A second objective was to determine the relative importance of activity (by EMG) and force (by joint torque) on the adaptation of muscle.
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Approach:
Pre- and postflight EMGs were analyzed using amplitude histograms, scatterplots, and joint probability density distributions of rectified, smoothed EMGs. Of the two flight animals only one provided a complete data set from implanted soleus, MG and TA muscles. Muscle biopsies of 8-14 mg were taken from two independent sites with a Bergstrom needle. For each biopsy mean fiber cross-sectional area and succinic dehydrogenase (SDH) activity were measured. Joint probability distribution was generated on a logarithmic scale using three consecutive trials and totaling 20,000 data points.
Results:
Activity of the TA muscles appeared unchanged after the flight, while the amplitude of soleus EMG was reduced, and MG amplitude was elevated, possibly to compensate for the soleus. Two weeks postflight, MG amplitude had declined to normal values but recovery of soleus was incomplete. Joint probability distribution showed a similar correlation between soleus and MG. This redistribution may be caused by a reduction of activity in the vestibulospinal pathways which normally excites motorneurons innervating the slow motor units of extensor muscles. Mean cross-sectional area of MG and soleus fibers were increased postflight, while TA fibers were significantly smaller. Mean SDH activity was not significantly different in soleus, but decreased in postflight biopsies of the MG and TA.
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Activity, EMG, Gastrocnemius, muscle, primate, Implanted EMG electrodes (recording)
Activity, EMG, Soleus, muscle, primate, Implanted EMG electrodes (recording)
Activity, EMG, Tibialis anterior, muscle, primate, Implanted EMG electrodes (recording)
Activity, enzymic, Succinate dehydrogenase (SDH), gastrocnemius, muscle, primate, Assay
Activity, enzymic, Succinate dehydrogenase (SDH), soleus, muscle, primate, Assay
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Activity, enzymic, Succinate dehydrogenase (SDH), tibialis anterior, muscle, primate, Assay
Area, cross section, Fiber, gastrocnemius, muscle, primate, Immunochemistry, microscopy
Area, cross section, Fiber, gastrocnemius, muscle, rat, Immunochemistry, microscopy
Area, cross section, Fiber, soleus, muscle, primate, Immunochemistry, microscopy
Area, cross section, Fiber, soleus, muscle, rat, Immunochemistry, microscopy
Area, cross section, Fiber, tibialis anterior, muscle, primate, Immunochemistry, microscopy
Area, cross section, Fiber, tibialis anterior, muscle, rat, Histochemistry, microscopy