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

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.

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?

by: Sarah Ilkhani-Pour
by: Stephen Thomas


Song Y, Forsgren S, Yu J, Lorentzon R, & Stål PS (2012). Effects on contralateral muscles after unilateral electrical muscle stimulation and exercise. PloS one, 7 (12) PMID: 23284946