Abstract
It is well established that unilateral exercise can produce contralateral effects. However, it is unclear whether unilateral exercise that leads to muscle injury and inflammation also affects the homologous contralateral muscles. To test the hypothesis that unilateral muscle injury causes contralateral muscle changes, an experimental rabbit model with unilateral muscle overuse caused by a combination of electrical muscle stimulation and exercise (EMS/E) was used. The soleus and gastrocnemius muscles of both exercised and non-exercised legs were analyzed with enzyme- and immunohistochemical methods after 1, 3 and 6 weeks of repeated EMS/E. After 1 w of unilateral EMS/E there were structural muscle changes such as increased variability in fiber size, fiber splitting, internal myonuclei, necrotic fibers, expression of developmental MyHCs, fibrosis and inflammation in the exercised soleus muscle. Only limited changes were found in the exercised gastrocnemius muscle and in both non-exercised contralateral muscles. After 3 w of EMS/E, muscle fiber changes, presence of developmental MyHCs, inflammation, fibrosis and affections of nerve axons and AChE production were observed bilaterally in both the soleus and gastrocnemius muscles. At 6 w of EMS/E, the severity of these changes significantly increased in the soleus muscles and infiltration of fat was observed bilaterally in both the soleus and the gastrocnemius muscles. The affections of the muscles were in all three experimental groups restricted to focal regions of the muscle samples. We conclude that repetitive unilateral muscle overuse caused by EMS/E overtime leads to both degenerative and regenerative tissue changes and myositis not only in the exercised muscles, but also in the homologous non-exercised muscles of the contralateral leg. Although the mechanism behind the contralateral changes is unclear, we suggest that the nervous system is involved in the cross-transfer effects.
Highlights
There is a wide range of examples in the literature showing that unilateral intervention produces bilateral effects
The mechanism for the contralateral increase in strength after unilateral exercise may differ from the symmetrical distribution in inflammation and pain and the bilateral effects after unilateral tendon treatment, the effects have been proposed to be mediated through the central nervous system [1,9,10,11,12,16,17,18,19,20]
General Findings Our results show that the used experimental model caused severe muscle changes and inflammatory cell infiltration in focal regions of the exercised soleus muscles after 1 w of electrical muscle stimulation and exercise (EMS/E)
Summary
There is a wide range of examples in the literature showing that unilateral intervention produces bilateral effects. Previous studies have shown that unilateral strength training can increase the strength in the trained muscle and in the homologous muscle of the contralateral limb [2,3,4] This crosstransfer effect has been reported both for small and large limb muscles and in response to different types of exercises as well as after unilateral electrical muscle stimulation (EMS) [5,6,7,8]. The mechanism for the contralateral increase in strength after unilateral exercise may differ from the symmetrical distribution in inflammation and pain and the bilateral effects after unilateral tendon treatment, the effects have been proposed to be mediated through the central nervous system [1,9,10,11,12,16,17,18,19,20]
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