Vol.:(0123456789) 1 3 Experimental Brain Research https://doi.org/10.1007/s00221-019-05520-3 RESEARCH ARTICLE Contribution of corticospinal drive to ankle plantar flexor muscle activation during gait in adults with cerebral palsy Rasmus Feld Frisk 1,2,3  · Jakob Lorentzen 1,3  · Jens Bo Nielsen 1,3 Received: 27 August 2018 / Accepted: 13 March 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Impaired plantar flexor muscle activation during push-off in late stance contributes importantly to reduced gait ability in adults with cerebral palsy (CP). Here we used low-intensity transcranial magnetic stimulation (TMS) to suppress soleus EMG activity during push-off as an estimate of corticospinal drive in CP adults and neurologically intact (NI) adults. Ten CP adults (age 34 years, SD 14.6, GMFCS I–II) and ten NI adults (age 33 years, SD 9.8) walked on a treadmill at their preferred walking speed. TMS of the leg motor cortex was elicited just prior to push-off during gait at intensities below threshold for motor-evoked potentials. Soleus EMG from steps with and without TMS were averaged and compared. Control experiments were performed while standing and in NI adults during gait at slow speed. TMS induced a suppression at a latency of about 40 ms. This suppression was similar in the two populations when differences in control EMG and gait speed were taken into account (CP 18%, NI 16%). The threshold of the suppression was higher in CP adults. The findings suggest that corticospinal drive to ankle plantar flexors at push-off is comparable in CP and NI adults. The higher threshold of the suppression in CP adults may reflect downregulation of cortical inhibition to facilitate corticospinal drive. Interventions aiming to facilitate excitability in cortical networks may contribute to maintain or even improve efficient gait in CP adults. Keywords Cerebral palsy · Corticospinal drive · Plantar flexor muscles · Push-off Introduction Cerebral palsy (CP) is caused by a non-progressive lesion of the developing brain primarily characterized by disordered movement and posture (Graham et al. 2016). Gait impair- ment has a crucial impact on physical independence, social participation and quality of life in adults with CP (Anders- son and Mattsson 2001; Jahnsen et al. 2004; Morgan and McGinley 2014; Verschuren et al. 2018). Common gait characteristics in persons with CP are reduced gait speed, reduced and asymmetrical step length with increased energy expenditure (Winters et al. 1987; Roche et al. 2014; Morgan et al. 2016). Weak plantar flexor muscles have been found to be associated with impaired propulsive force and reduced gait speed in both children (Olney et al. 1990; Barber et al. 2017) and adults with CP (Riad et al. 2012; Roche et al. 2014). Propulsion is gen- erated during the push-off phase in late stance of the gait cycle, where force generated by plantar flexor muscles is transferred to the ground propelling the body upward and forward (Winters et al. 1987; Neptune et al. 2001; Honeine et al. 2014). The neural drive to the plantar flexors is the result of integration of the input to spinal motor neurons from a number of different sources all of which may poten- tially be affected in CP. Transmission in specific spinal pathways contributes to the coordination of muscle activity during movement, including gait, and have been shown to be impaired in adults with CP (Achache et al. 2010). Sensory feedback mechanisms, which contribute to the activation of the motor neurons during gait, have also been found to be impaired in persons with CP (Willerslev-Olsen et al. 2014; Frisk et al. 2017). This likely reflects that sensory feedback is not sufficiently integrated with central motor commands to generate an efficient muscle activation at push-off (Nielsen 2002, 2003). Among the descending pathways, the corti- cospinal tract has received the most interest because of its * Rasmus Feld Frisk frisk@sund.ku.dk 1 Institute of Neuroscience, University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen N, Denmark 2 Professionshøjskolen Absalon, Roskilde, Denmark 3 Elsass Institute, Charlottenlund, Denmark