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J Appl Physiol (June 20, 2003). doi:10.1152/japplphysiol.00516.2002
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Submitted on June 13, 2002
Accepted on June 19, 2003

Hindlimb unloading induced muscle atrophy and loss offunction: protective effect of isometric exercise

James E Hurst1 and Robert H Fitts1*

1 Department of Biological Sciences, Marquette Unversity, Milwaukee, Wisconsin, USA

* To whom correspondence should be addressed. E-mail: robert.fitts{at}mu.edu.

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: weightbearing control (WB), 7d HU (H7), 7d HU plus IE (I7), 14d HU (H14), or 14d HU plus IE (I14) groups. IE consisted of 10, 5s maximal isometric contractions separated by 90s, administered 3 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, while the gastrocnemius and plantaris mass were not protected. These results are consistent with the mean EMG 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 P0 (I14, 1.49±0.12 N vs. H14, 1.15±0.07 N) and peak power(I14,163±17mN.FL.s-1 vs H14,75±11mN.FL.s-1). The exercise protocol showed protection of the plantaris P0 at 7 but not 14 d of HU. The IE also prevented the HU-induced decline in the soleus isometric contraction time which allowed the muscle to produce greater tension at physiological motor neuron firing frequencies. In summary, IE resulted in greater protection from HU-induced atrophy in the slow soleus than for the fast gastrocnemius or plantaris.




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