Abstract
Transcranial magnetic stimulation (TMS) studies have demonstrated that unilateral muscle contractions in the upper limb produce motor cortical activity in both the contralateral and ipsilateral motor cortices. The increase in excitability of the corticomotor pathway activating the resting limb has been termed “cross-activation”, and is of importance due to its involvement in cross-education and rehabilitation. To date, very few studies have investigated cross-activation in the lower limb. Sixteen healthy participants (mean age 29 ± 9 years) took part in this study. To determine the effect of varying contraction intensities in the lower limb, we investigated corticomotor excitability and intracortical inhibition of the right rectus femoris (RF) while the left leg performed isometric extension at 0%, 25%, 50%, 75% and 100% of maximum force output. Contraction intensities of 50% maximal force output and greater produced significant cross-activation of the corticomotor pathway. A reduction in silent period duration was observed during 75% and 100% contractions, while the release of short-interval intracortical inhibition (SICI) was only observed during maximal (100%) contractions. We conclude that increasing isometric contraction intensities produce a monotonic increase in cross-activation, which was greatest during 100% force output. Unilateral training programs designed to induce cross-education of strength in the lower limb should therefore be prescribed at the maximal intensity tolerable.
Highlights
It is well established that unilateral muscle contractions of the upper limb are associated with bilateral activity of the motor cortex (M1; Carson, 2005)
Our results demonstrate that cross-activation of the ipsilateral M1 (iM1) during unilateral contractions of the lower limb occurs at isometric contraction intensities of 50% maximum voluntary contraction (MVC) and above
When considered alongside the unchanged measure of short-interval intracortical inhibition (SICI), these results indicate that unilateral contractions produced a reduction in inhibition downstream of the iM1, for example, reduced gamma-aminobutyric acid B (GABAB) activity at the level of the spinal cord
Summary
It is well established that unilateral muscle contractions of the upper limb are associated with bilateral activity of the motor cortex (M1; Carson, 2005). Whilst unilateral movements are primarily driven by activity in the contralateral M1 (cM1), activation of the ipsilateral M1 (iM1) has been demonstrated with transcranial magnetic stimulation (TMS; Muellbacher et al, 2000; Perez and Cohen, 2008; Howatson et al, 2011) and functional magnetic resonance imaging (Kobayashi et al, 2003; van Duinen et al, 2008) This increase in iM1 excitability measured by TMS has been termed ‘‘cross-activation’’, and is of particular interest due to its potential role in the phenomenon of cross-education (Ruddy and Carson, 2013). This knowledge is critical when considering the prescription of unilateral exercise to induce cross-education (Dragert and Zehr, 2013; Kim et al, 2015)
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