Cross-education of muscular strength is facilitated by homeostatic plasticity

Abstract

The novel findings of this study include: priming the ipsilateral M1 via anodal tDCS prior to a single bout of strength training augments the cross-transfer of strength which is manifested by an increase in corticospinal excitability and cross-activation. These findings provide insight into how priming methods that induce homeostatic plasticity may be used to enhance the cross-education phenomenon.

Ipsilateral corticomotor responses are confined to the homologous muscle following cross-education of muscular strength

“…Specifically, cross-education studies have reported increased corticomotor excitability , decreased corticomotor inhibition (Coombs et al 2016;Zult et al 2016;Hendy and Kidgell 2014;Hendy et al 2015), reduced interhemispheric inhibition (IHI) Zult et al 2016) and increased voluntary activation (Lee et al 2009) in the M1 ipsilateral to the training limb. In support of this, several imaging and transcranial magnetic stimulation (TMS) studies have revealed increased excitability of the ipsilateral M1 of a resting limb during unilateral voluntary contractions (increased cross-activation) (Hortobágyi et al 2003;Liepert et al 2001;Muellbacher et al 2000;Frazer et al 2017;Hendy and Kidgell 2014;Zult et al 2016). Therefore, the elevated activity in the ipsilateral M1 that is detected during unilateral motor practice appears to be, in part a mediator for the crosseducation of muscular strength Leung et al 2015;Hortobágyi et al 2011;Hendy et al 2015;Ruddy et al 2017;Zult et al 2016;Lee et al 2010).…”

“…Certainly, given that there was an increase in the AURC for the motor pathway of the untrained contralateral homologous agonist muscle, the overall contribution of force production from the synergist wrist flexors has restricted the activation of the motor pathway of the synergist wrist flexors and has subsequently had a limited spatial effect on corticomotor excitability. This is important, because cross-activation appears to be the primary mechanism for increased excitability within the ipsilateral motor pathway following a unilateral strength training intervention (Hortobágyi et al 2003;Ruddy and Carson 2013;Hendy and Kidgell 2014;Frazer et al 2017) and it seems that cross-activation is limited to the homologous agonist muscle. This finding is consistent with the increases in cross-activation reported in this study (Figure 5a-b).…”

“…The neural mechanism mediating cross-education, appear to be related to bilateral cortical activity, whereby, during unilateral strength training, there is concurrent activation in both cerebral hemispheres that are involved in motor output (Lee et al 2010;Frazer et al 2017;Hendy and Kidgell 2014). The primary motor cortex (iM1) ipsilateral to the training limb has been shown to play an important role in mediating the cross-education effect Lee et al 2010;Hendy et al 2015;Frazer et al 2017).…”

“…The neural mechanism mediating cross-education, appear to be related to bilateral cortical activity, whereby, during unilateral strength training, there is concurrent activation in both cerebral hemispheres that are involved in motor output (Lee et al 2010;Frazer et al 2017;Hendy and Kidgell 2014). The primary motor cortex (iM1) ipsilateral to the training limb has been shown to play an important role in mediating the cross-education effect Lee et al 2010;Hendy et al 2015;Frazer et al 2017). Specifically, cross-education studies have reported increased corticomotor excitability , decreased corticomotor inhibition (Coombs et al 2016;Zult et al 2016;Hendy and Kidgell 2014;Hendy et al 2015), reduced interhemispheric inhibition (IHI) Zult et al 2016) and increased voluntary activation (Lee et al 2009) in the M1 ipsilateral to the training limb.…”

The ipsilateral corticospinal responses to cross-education are dependent upon the motor-training intervention

“…Concurrent activation of both cerebral cortices, a process known as cross-activation or bilateral activation, has been reported during both unilateral skill and strength training (Ruddy and Carson, 2013;Hendy and Kidgell, 2014;Carroll et al 2008;Lee et al 2010;Hinder et al 2011;Hinder et al 2013;Frazer et al 2017). Moreover, transcranial magnetic stimulation (TMS) studies have shown increased excitability of the ipsilateral primary motor cortex (iM1) of a resting limb during unilateral voluntary contractions (increased cross-activation) (Muellbacher et al 2000;Liepert et al 2001;Hortobágyi et al 2003;Hendy and Kidgell, 2014;Zult et al 2016;Frazer et al 2017;Mason et al 2017). The elevated activity in the iM1 appears to be associated with the changes in motor function of the untrained limb following unilateral training (Kidgell et al 2011;Latella et al 2012;Kidgell et al 2015;Coombs et al 2016;Mason et al 2017;Christiansen et al 2017).…”