Effects on Contralateral Muscle after Unilateral Electrical Muscle Stimulation and Exercise
Song Y, Forsgren S, Yu J, Lorentzon R, Stal PS. PLoS One 2012;7(12):e52230
Some studies have shown that unilateral exercise can also affect the contralateral limb. For example, strength training one limb also improves the opposite, untrained limb. Additionally, many afflictions, like painful Achilles tendinopathy, present bilaterally, but the mechanism behind this is unknown. This study sought to examine whether unilateral muscle overuse will produce bilateral muscle damage and how the tissue in both the exercised and contralateral limbs compares to control. Twenty-four rabbits were divided into control or experimental groups. The experimental group received unilateral electrical stimulation to their right triceps surae muscle and simultaneous flexion of their foot using a kicking machine that gave an ankle range of motion up to 65°, of which 20-25° was dorsiflexion and 35-40° was plantarflexion to cause eccentric loading (previously described model). This motion was continued for 2 hours and repeated every other day for 1, 3, and 6 weeks. The contralateral limb was left alone. Upon sacrifice, muscle tissue from the soleus and gastrocnemius was dissected from both limbs and prepared for histological analysis. Tissue was examined for an inflammatory response (macrophages, neutrophils/T-cells, eosinophils), neural changes (axons, Schwann cells, motor-endplates), muscle fiber degeneration and necrosis, and muscle fiber regeneration. Overall, the authors found that the soleus muscle was more negatively affected than the gastrocnemius, which could be due to fiber type differences. At all time points (1, 3, 6 wk), for all parameters, the exercised soleus muscle of the experimental group was more damaged than the control group, as expected. Additionally, the damage at 6 weeks was greater than the earlier time points. Interestingly, the non-exercised contralateral soleus muscle was also significantly different from control at 3 and 6 weeks, with even greater damage at 6 weeks. The gastrocnemius changes in both the exercised and contralateral limbs peaked at 3 weeks. In addition to the muscle, nerve tissue in both the gastrocnemius and soleus muscles of the exercised and contralateral limbs showed changes at 3 and 6 weeks. Muscle and nerve tissue changes were focal.
These findings of increased turnover and damage in muscle and nerve tissue in the uninvolved limb suggest that unilateral muscle overuse also impairs the contralateral limb. Changes to the contralateral muscles are preceded by changes to the exercised muscles. The authors speculate that the nervous system may be one of the underlying mechanisms of the “cross-transfer” effect; however, because this model produces overuse via two mechanisms (electrical stimulation and mechanical manipulation), we cannot identify the origin of the damage. Similarly, we cannot ascertain cause-effect relationships between the nerve and muscle damage. While the authors did not note any functional compensation in the contralateral limb following overuse, they could have used gait analysis or EMG to monitor potential differences. Finally, this study is limited in testing only the triceps surae; the authors did not investigate other potentially affected contralateral muscles. While this is a basic science animal study, the findings have implications for clinical practice. If left untreated, unilateral injury may transition to the contralateral side thereby creating a more substantial clinical problem. In comparison to unilateral injuries, how frequently do you come across bilateral injuries in your practice? Have you considered treating the uninjured contralateral limb in the presence of a unilateral injury to prevent a negative cross-transfer effect?
Written by: Sarah Ilkhani-Pour
Reviewed by: Stephen Thomas