Motor cortical and corticospinal function differ during an isometric squat compared with isometric knee extension

New Findings: What is the central question of this study? In order to discern information about testing modalities when assessing neuroplastic responses to squat resistance training, the present study investigated whether corticospinal and intracortical function was different between a joint-angle-matched isometric squat and isometric knee extension. What is the main finding and its importance? The present data show poor agreement of corticospinal and intracortical function between the isometric squat and isometric knee extension. The data reinforce the notion that task specificity is of the utmost importance for assessing neuroplasticity. Abstract: It has been suggested that task-specific changes in neurophysiological function (neuroplasticity) should be assessed using testing modalities that replicate the characteristics of the intervention. The squat is a commonly prescribed resistance exercise that has been shown to elicit changes in CNS function. However, previous studies have assessed squat-induced neuroplasticity using isometric knee extension, potentially confounding the results. The aim of the present study was to assess the agreement between corticospinal and intracortical activity relating to the knee extensors during isometric knee extension compared with an isometric squat task. Eleven males completed a neurophysiological assessment in an isometric squat (IS) and knee-extension (KE) task matched for joint angles (hip, knee and ankle). Single- and paired-pulse transcranial magnetic stimulation was delivered during isometric contractions at a range of intensities to assess short-interval cortical inhibition (SICI) and corticospinal excitability. Group mean values for SICI (70 ± 14 versus 63 ± 12% of unconditioned motor evoked potential during IS and KE, respectively) and corticospinal excitability (mean differences 2–5% of the maximal compound muscle action potential at 25, 50, 75 and 100% maximal voluntary contraction between the IS and KE) were not different between the two tasks (P > 0.05) in the vastus lateralis. However, limits of agreement were wide, with poor-to-moderate average intraclass correlation coefficients (ICCs) (SICI, ICC_3,1 = 0.15; corticospinal excitability, average ICC_3,1 range = 0.0–0.63), indicating disparate corticospinal and intracortical activity between the IS and KE. These data highlight the importance of task specificity when assessing the modulation of corticospinal excitability and SICI in response to interventions resulting in neuroplastic changes.