No evidence of neural adaptations following chronic unilateral isometric training of the intrinsic muscles of the hand: a randomized controlled study
PurposeTo test whether long term cortical adaptations occur bilaterally following chronic unilateral training with a simple motor task.MethodsParticipants (n 34) were randomly allocated to a training or control groups. Only the former completed a 4 week maximal intensity isometric training of the right first dorsal interosseus muscle through key pinching. Maximal strength was assessed bilaterally in four different movements progressively less similar to the training task: key, tip and tripod pinches, and handgrip. Transcranial magnetic stimulation was used to probe, in the left and right primary hand motor cortices, a number of standard tests of cortical excitability, including thresholds, intra cortical inhibition and facilitation, transcallosal inhibition, and sensory motor integration.ResultsTraining increased strength in the trained hand, but only for the tasks specifically involving the trained muscle (key +8.5 p 0.0005; tip +7.2 p 0.02) However, the effect size was small and below the cutoff for meaningful change. Handgrip and tripod pinch were instead unaffected. There was a similar improvement in strength in the untrained hand, i.e. a cross education effect (key +6.4 p 0.02; tip +4.7 p 0.007) Despite these changes in strength, no significant variation was observed in any of the neurophysiological parameters describing cortico spinal and intra cortical excitability, inter hemispheric inhibition, and cortical sensory motor integration.ConclusionsA 4 week maximal intensity unilateral training induced bilaterally spatial and task specific strength gains, which were not associated to direct or crossed cortical adaptations. The observed long term stability of neurophysiological parameters might result from homeostatic plasticity phenomena, aimed at restoring the physiological inter hemispheric balance of neural activity levels perturbed by the exercise.Trial registration numberClinicalTrials.gov identifier NCT02010398.
European Journal of Applied Physiology