Does the left leg know what the right leg is doing?
It is known that unilateral resistance exercise or electromyostimulation (EMS) training can increase muscular strength not only in the trained limb, but also in the homologous muscle of the contralateral limb, a phenomenon termed cross education. It has been suggested that unilateral exercise might be useful in rehabilitation for one limb injury, planned surgical operations or stroke. The principle of using unilateral therapy to treat conditions of the contralateral limb has been applied in traditional Chinese medicine, eg. with acupuncture or electroacupuncture (EA). The mechanisms and clinical significance of cross education are still to be critically examined. The primary mechanism of cross education is believed to reside in the CNS because there was no solid evidence of muscle hypertrophy found in the unexercised limb. However other mechanisms such as regulatory factors via circulation or intrinsic factors should not be excluded.
AIMS: A series of experiments has been conducted in our laboratories with the objective of elucidating the potential mechanisms of cross education, while a specific focus has beeb given to the effects of EMS and EA. The aim of this brief review is to present a summary of our current works in this area.
METHODS: Experimental trials had been performed
(1) to compare the bilateral effects of 4 weeks of unilateral voluntary resistance training and EMS training on knee extensors in healthy human subjects;
(2) to compare the bilateral effects of 4 weeks of unilateral voluntary resistance training, EMS, acupuncture and EA on ankle dorsiflexors in healthy human subjects;
(3) functional MRI investigation of brain activities during unilateral voluntary contraction and EMS in healthy human subjects;
(4) bilateral responses of IGF-1 and other regulatory factors in leg muscles to unilateral EA in rats; and
(5) bilateral responses of IGF-1 and other regulatory factors in the brain and leg muscles to unilateral EA using an ischemic stroke model in rats.
RESULTS: (1) All of the trials in humans have demonstrated that the unilateral voluntary exercise could improve the muscular strength of both the exercised and the contralateral limbs.
(2) The EMS, manual acupuncture and EA could induce bilateral effects on muscle strength similar to voluntary training (approximately 20% strength gain).
(3) The fMRI trial demonstrated that there were bilateral brain activities in the primary motor area, secondary somatosensory area and cingulate gyrus area during both voluntary and EMS tasks; while EMS also increased bilateral activity in primary somatosensory area, ipsilateral premotor cortex, and contralateral supplementary motor area. These results provided further evidence that supports the hypothesis that inter-cortical activities are involved in manifestation of cross education, and indicated the potentially significant role of somatosensory inputs in causing the contralateral effects.
(4) In respect of potential peripheral mechanisms, the investigation on the effects of 1-4 week EA and EMS intervention on IGF-1 mRNA in the animal studies found that the expression of IGF-1 was significantly elevated in the stimulated muscle as well as in the contralateral muscle of the EA group after 3 weeks of intervention, as compared with the manual acupuncture, EMS and control groups. However, IGF-1 and MyoD protein synthesis significantly increased only in the stimulated leg while no significant changes in contractile proteins in both legs. These results indicated that unilateral EA could affect the local secretion, but there is also a possibility of affecting systemic level of IGF-l.
(5) To investigate clinical implications of these findings, an animal model of unilateral ischemic stroke was used to evaluate the effects of unilateral EA (on either the affected limb, AL, or the unaffected limb UAL) on the expression of IGF-1 in leg muscles and the brain during three weeks after middle cerebral artery occlusion. The brain IGF-1 level demonstrated a significant decline in the order of 7d>14d>21d, while the IGF-1 protein level in the non-ischemia cortex was significantly lower compared to the ischemic cortex. The expressions of IGF-1 mRNA in the ischemic cortex of the non-intervention controls increased at 7d but decreased at 14d, and increased again at 21d. There was a delay in the expression of IGF-1 mRNA in the non-ischemia cortex, with an increase only from 14d to 21d. The IGF-1 protein level in the cortex of both sides in the EA groups was significantly higher than that of the controls. The IGF-1 protein in both cortices of the AL was significantly higher than that of the controls at each of the time points, while that in the non-ischemic cortex of the NAL was higher than that of the AL at 7d and 21d. There was also a quickly increased expression of IGF-1 mRNA in the ischemic cortex of the UAL and AL, which was higher than that of the controls at the 7d point.
CONCLUSIONS: The experimental trials in our laboratories have demonstrated robust cross education effect in response to both voluntary and EMS training. It was interesting to find that unilateral EA could produce similar or even better contralateral effects. The evidence from fMRI demonstrated bilateral brain activities in response to both voluntary contraction and EMS, indicating the involvement of bilateral cortical processes during unilateral activity. There was evidence that the IGF-1 and some other regulatory factors increased bilaterally in the muscles of rats, while there was no significant changes in the contractile protein synthesis in the contralateral muscles. The unilateral EA on the affected limb and unaffected limb of the ischemic stroke animal model demonstrated different temporal effects on the expression of IGF-1 mRNA and proteins, that may have clinical implications. Further clinical trials on humans need to be conducted to confirm the therapeutic value of cross education.
Zhou, S, Huang, LP, Lu, Z, Li, X, Cao, LJ, Yu, JH, Liu, J, Hu, X, Xiang, H, Ma, XF, Xiao, N, Wang, ZH, Tian, Q, Oakman, A, Bezerra, P & Davie, AJ 2009, 'Does the left leg know what the right leg is doing?', paper presented to Motor Control Conference VI: From basic motor control to functional recovery, Varna, Bulgaria, 5-8 October.
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