Abstract
Limited rehabilitation strategies are available for movement restoration when paresis is too severe following stroke. Previous research has shown that high-intensity resistance training of one muscle group enhances strength of the homologous, contralateral muscle group in neurologically intact adults. How this “cross education” phenomenon might be exploited to moderate severe weakness in an upper extremity muscle group after stroke is not well understood. The primary aim of this study was to examine adaptations in force-generating capacity of severely paretic wrist extensors resulting from high intensity, dynamic contractions of the non-paretic wrist extensors. A secondary, exploratory aim was to probe neural adaptations in a subset of participants from each sample using a single-pulse, transcranial magnetic stimulation (TMS) protocol. Separate samples of neurologically intact controls (n = 7) and individuals ≥4 months post stroke (n = 6) underwent 16 sessions of training. Following training, one-repetition maximum of the untrained wrist extensors in the control group and active range of motion of the untrained, paretic wrist extensors in the stroke group were significantly increased. No changes in corticospinal excitability, intracortical inhibition, or interhemispheric inhibition were observed in control participants. Both stroke participants who underwent TMS testing, however, exhibited increased voluntary muscle activation following the intervention. In addition, motor-evoked potentials that were unobtainable prior to the intervention were readily elicited afterwards in a stroke participant. Results of this study demonstrate that high-intensity resistance training of a non-paretic upper extremity muscle group can enhance voluntary muscle activation and force-generating capacity of a severely paretic muscle group after stroke. There is also preliminary evidence that corticospinal adaptations may accompany these gains.
Highlights
Fatiguing muscle contractions reduce neural output from corticomotor regions [1,2,3]
Inclusion criteria for stroke participants were [1] clinical diagnosis of ischemic or hemorrhagic stroke as determined by a stroke neurologist; [2] ≥3 months post stroke; and [3] Medical Research Council Scale for Strength score of 0 to 2 in the paretic wrist extensors. Exclusion criteria for both control and stroke participants were [1] neurological conditions; [2] presence of musculoskeletal conditions affecting the bones and/or soft tissues of the upper extremity; [3] history of resistance training involving the wrist extensors; [4] presence of aphasia; and [5] contraindications to transcranial magnetic stimulation (TMS) including history of seizure, prescribed medications that increase the risk of seizure, and/or presence of metal implants
Stroke participants were between 4 months and ~2 years post stroke at the time they enrolled in the study with half affected on their dominant side
Summary
Fatiguing muscle contractions reduce neural output from corticomotor regions [1,2,3] This compromises the ability to drive spinal motor neurons to threshold and activate skeletal muscle. Repeated bouts of high-intensity resistance training evoke lasting adaptations that function to maintain muscle activation and force output [4]. These adaptations are not exclusive to the musculature targeted by training. Unilateral resistance training at sufficiently high-intensity enhances force-generating capacity of the homologous, untrained musculature, a phenomenon known as the cross education effect [5,6,7,8,9,10]. Effective treatment alternatives are needed for individuals who cannot participate in retraining programs that leverage the nervous system’s adaptive capacity to reorganize following neurological damage [20]
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