Hindlimb unloading-induced muscle atrophy and loss of function: protective effect of isometric exercise.

The primary objective of this study was to determine the effectiveness of isometric exercise (IE) as a countermeasure to hindlimb unloading (HU)-induced atrophy of the slow (soleus) and fast (plantaris and gastrocnemius) muscles. Rats were assigned to either weight-bearing control, 7-day HU (H7), H7 plus IE (I7), 14-day HU (H14), or H14 plus IE (I14) groups. IE consisted of ten 5-s maximal isometric contractions separated by 90 s, administered three times daily. Contractile properties of the soleus and plantaris muscles were measured in situ. The IE attenuated the HU-induced decline in the mass and fiber diameter of the slow-twitch soleus muscle, whereas the gastrocnemius and plantaris mass were not protected. These results are consistent with the mean electromyograph recordings during IE that indicated preferential recruitment of the soleus over the gastrocnemius and plantaris muscles. Functionally, the IE significantly protected the soleus from the HU-induced decline in peak isometric force (I14, 1.49 +/- 0.12 vs. H14, 1.15 +/- 0.07 N) and peak power (I14, 163 +/- 17 vs. H14, 75 +/- 11 mN.fiber length.s-1). The exercise protocol showed protection of the plantaris peak isometric force at H7 but not H14. The IE also prevented the HU-induced decline in the soleus isometric contraction time, which allowed the muscle to produce greater tension at physiological motoneuron firing frequencies. In summary, IE resulted in greater protection from HU-induced atrophy in the slow soleus than in the fast gastrocnemius or plantaris.