Corticomotor Network Activity Does Not Contribute To The Bilateral Deficit Phenomenon

Abstract

The bilateral deficit (BLD) phenomenon is an inability to maximally contract bilaterally as compared to the sum of the corresponding unilateral contractions, and is expressed as an index (BI). The underlying mechanism is unknown, but altered transcallosal inhibition (TCI) and diminished voluntary activation (VA) during bilateral homologous (BH), as compared to bilateral non-homologous (BNH) and unilateral tasks, is considered the most likely mechanism. PURPOSE: To examine corticospinal activity during BH, BNH, and unilateral maximal contractions, and resultant changes from task practice. METHODS: Eleven healthy adults (6 women/5 men, 25.6±3.7years; 171.81±11.44cm; 74.4±21.2kg) participated in the counterbalanced repeated measures study. TCI and VA were assessed with transcranial magnetic stimulation for BH, BNH, and unilateral dominant flexion (DF) sessions 1 and 7. For sessions 2-6, five of each BH elbow flexion, BNH flexion/extension, and DF maximal voluntary isometric contraction (MVIC) tasks were practiced. Paired t-tests or Wilcoxon signed-rank tests, as appropriate, were used to test TCI and VA between sessions 1 and 7. Pearson or Spearman correlation coefficients, as appropriate, between VA and TCI measures and BLD and peak force measures were used to assess any association between neurological and performance variables on test days 1 and 7. RESULTS: There were no differences in VA or TCI between BH, BNH, and DF. BH VA decreased from day 1 (95.14±4.3%) to day 7 (92.2±4.4%, p = 0.014), but did not exceed the minimal detectable change (6.47). VA for BH (r = -0.655; p = 0.039) and DF (r = -0.636; p = 0.035) was associated with BH BI on day 1. When associated with peak force, day 7 BH (r = -0.627, p = 0.039) and BNH (r = -0.682, p = 0.021) TCI was correlated. CONCLUSION: BI was similar to prior research, but neurophysiological measures did not explain these differences. This is similar to previous equivocal research between BH and unilateral maximal force and TCI and/or VA, suggesting differences in BH and BNH BI are not solely influenced by neural drive modifications. Of interest, TCI was highly correlated with force, revealing neurophysiological influence between tasks, but differences in bilateral and unilateral force may be influenced by outside mechanisms.