Changes in functional magnetic resonance imaging cortical activation with cross education to an immobilized limb.
Farthing JP, Krentz JR, Magnus CR, Barss TS, Lanovaz JL, Cummine J, Esopenko C, Sarty GE, Borowsky R. Med Sci Sports Exerc. 2011 Aug;43(8):1394-405.
It has been well established that strength training one limb can lead to increases of strength in the opposite untrained limb. A few studies have also indicated that this cross education of strength may be useful in attenuating the loss of strength and muscle mass associated with immobilization. While there is agreement that neural mechanisms are responsible for the cross education the precise mechanisms are still unclear. To clarify possible mechanisms Farthing et al used functional magnetic resonance imaging (fMRI) to assess cortical activation associated with the cross-education effect to an immobilized limb. Fourteen healthy, young, right-handed individuals with little previous resistance training experienced volunteered to have their nondominant (left) forearm casted for 3 weeks. The casts were similar to a short arm cast but also included the thumb and proximal interphalangeal joints of the other fingers. The first 7 participants performed progressive strength training of the dominate arm (isometric handgrip contractions 5 days/week) while the last 7 participants were controls and did not strength train. Isometric maximal handgrip strength, muscle thickness, electromyography of wrist flexors and extensors (muscle activation during handgrip strength assessment), as well as fMRI (during handgrip contractions) were assessed for both arms before and immediately after the 3 weeks of immobilization. Handgrip strength in the immobilized arm was maintained among participants that completed the strength training but control participants lost strength in the immobilized arm. Muscle thickness decreased over time in both groups. The wrist flexor muscle activation (flexor carpi ulnaris) increased over time among the strength training participants but decreased over time among the controls (regardless of arm). Among the control participants there was no change over time in cortical activation but among the strength training group there was an increase in contralateral (left) motor cortex activation after training. The authors concluded that contralateral handgrip strength training could attenuate strength losses during forearm immobilization and that these outcomes may be associated with increased motor cortex activation.
This study is exciting because in a small cohort it demonstrates the clinical benefit of handgrip strength training the contralateral limb during immobilization and a possible mechanism for the clinical benefit. The authors acknowledge that their findings were limited by not measuring muscle activation of the immobilized forearm during strength training activities. Therefore, the study cannot conclusively rule out that the immobilized muscle were contracting during the contralateral strength training. The authors also note that their findings in healthy patients may not be applicable to an injured population. It will be interesting to see a randomized controlled trial of casted patients doing handgrip strength training compared to normal treatment protocols. Regardless, the risks associated with handgrip strength training of the contralateral arm during immobilization are low and it would seem prudent to advise our patients to do these easy exercises. How many of you are already using contralateral strength training with your injured patients?
Written by: Jeffrey Driban
Reviewed by: Stephen Thomas