High Force Unimanual Handgrip Contractions Increase Ipsilateral Sensorimotor Activation and Functional Connectivity

Abstract

Imaging and brain stimulation studies seem to correct the classical understanding of how brain networks, rather than contralateral focal areas, control the generation of unimanual voluntary force. However, the scaling and hemispheric-specificity of network activation remain less understood. Using fMRI, we examined the effects of parametrically increasing right-handgrip force on activation and functional connectivity among the sensorimotor network bilaterally with 25%, 50%, and 75% maximal voluntary contractions (MVC). High force (75% MVC) unimanual handgrip contractions resulted in greater ipsilateral motor activation and functional connectivity with the contralateral hemisphere compared to a low force 25% MVC condition. The ipsilateral motor cortex activation and network strength correlated with relative handgrip force (% MVC). Increases in unimanual handgrip force resulted in greater ipsilateral sensorimotor activation and greater functional connectivity between hemispheres within the sensorimotor network.

Ipsilesional motor cortex activation with high-force unimanual handgrip contractions of the less-affected limb in participants with stroke

“…Using a partial eta squared (ηp 2 ) effect size of ηp 2 = 0.439 based on a main effect of condition for the modulation of ipsilateral M1 (iM1) cortical activation between 25%, 50%, and 75% maximum voluntary contraction (MVC) from previously published work (Andrushko et al, 2021), it was determined that an estimated total sample size of eight was needed (G*Power 3.1.9.2; 1-β = 0.95, α = 0.05). Eleven participants with stroke volunteered to participate in a larger intervention (clinicaltrials.gov: NCT02948725), of which their baseline data was used in the present study (Data are presented as mean ± standard deviation; 3 female, 8 male, 10 right-handed, 1 left-handed, age: 60 ± 11 yrs, height: 173.3 ± 6.4 cm, mass: 81.7 ± 18.3 kg).…”

“…Eleven participants with stroke volunteered to participate in a larger intervention (clinicaltrials.gov: NCT02948725), of which their baseline data was used in the present study (Data are presented as mean ± standard deviation; 3 female, 8 male, 10 right-handed, 1 left-handed, age: 60 ± 11 yrs, height: 173.3 ± 6.4 cm, mass: 81.7 ± 18.3 kg). Additionally, 13 neurologically intact controls (11 right-handed, 2 left-handed, age: 28 ± 6 yrs, height: 170.9 ± 9.8 cm, mass: 75.1 ± 16.7 kg) from a previously published study were used to conduct between group comparisons (Andrushko et al, 2021).…”

“…During each contraction force condition, participants were instructed to relax their non-active more-affected arm and hand to prevent mirror activity. Using this same experimental paradigm, the muscle activity of non-active arm was not significantly active compared to baseline noise in an electromyography control experiment with young neurologically intact adults (Andrushko et al, 2021).…”

“…The cross-activation hypothesis suggests unilateral voluntary movements facilitate bilateral brain activation, which results in motor training-related neuroplasticity in both hemispheres (Lee et al, 2010;Ruddy and Carson, 2013;. In neurologically intact humans, previous research has observed ipsilateral brain activation with high-force (Andrushko et al, 2021), fatiguing (Benwell et al, 2006;Jiang et al, 2012), or complex (Verstynen et al, 2005) unilateral voluntary movements. Further, greater cross-education is known to occur when the exercise is performed at high intensities (Urbin et al, 2015), high velocities (Farthing and Chilibeck, 2003), and when eccentric muscle actions are incorporated into the unilateral training regimen (Farthing and Chilibeck, 2003;Manca et al, 2017).…”

“…The purpose of this study is to determine if high-force unilateral handgrip contractions performed with the less-affected limb in participants with stroke result in increased cortical activation and functional connectivity between the primary motor cortex (M1) and the supplementary motor area (SMA) within the ipsilesional hemisphere. Further, utilizing previously published data from neurologically intact controls (Andrushko et al, 2021), a secondary purpose is to determine if unilateral handgrip contractions performed with the less-affected limb result in differences in ipsilateral/ipsilesional brain activation and functional connectivity between participants with and without a history of stroke. The primary hypotheses are that high-force unilateral handgrip contractions will result in greater brain activation and functional connectivity within the ipsilesional hemisphere in both groups.…”