ORIGINAL ARTICLE Short-term plasticity of human spinal inhibitory circuits after isometric and isotonic ankle training Traci Jessop • Alyssa DePaola • Lauren Casaletto • Chaya Englard • Maria Knikou Received: 15 February 2012 / Accepted: 29 May 2012 / Published online: 9 June 2012 Ó Springer-Verlag 2012 Abstract The purpose of this study was to determine to what extent one session of isotonic and isometric ankle dorsi and plantar flexion training induces changes in the frequency-dependent depression of the soleus H-reflex. Further, adaptation of reciprocal Ia inhibition exerted from tibialis anterior flexor group I afferents on soleus moto- neurons, and presynaptic inhibition of Ia afferent terminals induced by a conditioning afferent volley following stim- ulation of the antagonist nerve were established with sub- jects seated before and after training. The soleus H-reflexes evoked at the inter-stimulus intervals of 1, 2, 3, 5, and 8 s were normalized to the mean amplitude of the H-reflex evoked every 10 s. Conditioned H-reflexes were normal- ized to the associated control H-reflex evoked with subjects seated before and after training. Twenty-six subjects were randomly assigned to one or more of the 4 exercise groups. Isometric ankle dorsi flexion training decreased the reci- procal and presynaptic inhibition, while isotonic ankle dorsi flexion had no significant effects. Isotonic plantar flexion training decreased only the reciprocal inhibition, whilst isometric plantar flexion had no significant effects on the reciprocal or presynaptic inhibition. None of the training exercise protocols affected the amount of homo- synaptic depression of the soleus H-reflex. Our findings support the notion that plastic changes of reciprocal and presynaptic inhibition due to exercise are transferrable to a resting state, and that homosynaptic depression remains unaltered after a single session of ankle training. Further research is needed to outline the time-course of plastic changes of spinal inhibitory mechanisms in humans. Keywords H-reflex Á Human Á Homosynaptic depression Á Neural plasticity Á Randomized exercise Á Reciprocal inhibition Á Presynaptic inhibition Á Ankle training Introduction Activity-dependent neural plasticity accounts for the development or preservation of motor behavior and improvement of motor performance (Wolpaw 2007, 2010). Neural circuits have the ability to alter their structure and function in response to motor training (Adkins et al. 2006), and reorganize simultaneously at multiple sites of the central nervous system (Wolpaw and Tennissen 2001). Various training protocols in humans induce reorgani- zation of spinal neural excitability. For example, 30-min ankle co-contraction training decreased the ratio of maxi- mal H-reflex versus maximal M-wave (H max /M max ), and Communicated by Fausto Baldissera. T. Jessop Á A. DePaola Á L. Casaletto Á C. Englard Á M. Knikou (&) Department of Physical Therapy, Graduate Center, College of Staten Island, City University of New York, 2800 Victory Blvd, Bldg 5N-207, Staten Island, NY 10314, USA e-mail: Maria.Knikou@csi.cuny.edu; m-knikou@northwestern.edu URL: http://www.smpp.northwestern.edu/research/eagp/ M. Knikou Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL, USA M. Knikou Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA M. Knikou Electrophysiological Analysis of Gait and Posture Laboratory, 345 East Superior Street, Suite 1406, Chicago, IL 60611, USA 123 Eur J Appl Physiol (2013) 113:273–284 DOI 10.1007/s00421-012-2438-1