Corticospinal excitability during walking in humans with absent and partial body weight support Maria Knikou a,b,c,⇑ , Nupur Hajela b , Chaithanya K. Mummidisetty b a Graduate Center/Department of Physical Therapy, City University of New York, NY, USA b Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL, USA c Department of Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, Chicago, IL, USA article info Article history: Accepted 6 June 2013 Available online xxxx Keywords: Body unloading Corticospinal excitability Motor evoked potentials Transcranial magnetic stimulation Walking highlights  MEPs are modulated in a phase-dependent pattern under conditions of reduced body loading.  The phase-dependent modulation pattern of the MEPs recorded from ankle muscles is reproducible over time.  Reduced body loading utilized for gait rehabilitation will not change the strength of corticospinal drive. abstract Objective: To establish changes in corticospinal excitability with absent and partial body weight support (BWS), and determine test–retest reliability of motor evoked potentials (MEPs) recordings during step- ping in healthy humans. Methods: The tibialis anterior (TA) and soleus MEPs during stepping at 0 and at 25 BWS were recorded in two experimental sessions in the same subjects. Transcranial magnetic stimulation was delivered ran- domly across the step cycle at 1.2  TA MEP resting threshold. The non-stimulated associated electro- myogram (EMG) was subtracted from the TA and soleus MEPs at identical time windows and bins of the step cycle, and the resultant values were normalized to the maximal homologous EMG activity during stepping. The relationship between MEPs and background EMG activity was determined for each BWS level and session tested. Results: The TA MEPs were facilitated at heel contact, progressively decreased during the stance phase, and facilitated throughout the swing phase of the step cycle. In contrast, the soleus MEPs were progres- sively increased at early-stance, depressed at the stance-to-swing transition, and remained depressed throughout the swing phase. The TA and soleus MEPs were modulated in a similar pattern across sessions at 0 and at 25 BWS, and were linearly related to the associated background EMG activity. Conclusions: These results provide evidence that reduced body weight loading does not alter the strength of corticospinal excitability, and that MEPs can be reliably recorded at different sessions during stepping in healthy humans. Significance: A rehabilitation strategy to restore gait in neurological disorders utilizes BWS during step- ping on a motorized treadmill. Based on our findings, the strength of corticospinal drive will not be affected negatively during stepping under conditions of partial body loading. Ó 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. 1. Introduction Spinal neuronal networks integrating sensory afferent feedback interact continuously with supraspinal centers to produce efferent activity appropriate to the task and to the phase of the step during walking (task- and phase-dependent neuronal activity) (Nielsen, 2003; Knikou, 2010, 2012). However, most of evidence on the role of motor cortex in the neural control of locomotion comes from experiments conducted in the cats (Armstrong and Drew, 1984; Drew et al., 2002). Consequently, studying how corticospinal drive 1388-2457/$36.00 Ó 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinph.2013.06.004 ⇑ Corresponding author. Address: Graduate Center of the City University of New York, Department of Physical Therapy, College of Staten Island, 2800 Victory Blvd, Bldg 5N- 207 Staten Island, NY 10314, USA. Tel.: +1 718 982 3316; fax: +1 718 982 2984. E-mail addresses: Maria.Knikou@csi.cuny.edu, m-knikou@northwestern.edu (M. Knikou). URL: http://www.smpp.northwestern.edu/research/eagp/ (M. Knikou). Clinical Neurophysiology xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph Please cite this article in press as: Knikou M et al. Corticospinal excitability during walking in humans with absent and partial body weight support. Clin Neurophysiol (2013), http://dx.doi.org/10.1016/j.clinph.2013.06.004