Rev. Neurosci. 2016; aop Francesco Budini* and Markus Tilp Changes in H-reflex amplitude to muscle stretch and lengthening in humans DOI 10.1515/revneuro-2016-0001 Received January 4, 2016; accepted March 10, 2016 Abstract: Spinal reflex excitability is traditionally assessed to investigate neural adjustments that occur during human movement. Different experimental procedures are known to condition spinal reflex excitability. Among these, lengthening movements and static stretching the human triceps have been investigated over the last 50 years. The purpose of this review is to shed light on several appar- ent incongruities in terms of magnitude and duration of the reported results. In the present review dissimilarities in neuro-spinal changes are examined in relation to the methodologies applied to condition and measure them. Literature that investigated three different conditioning procedures was reviewed: passive dorsiflexion, active dorsiflexion through antagonists shortening and eccen- tric plantar-flexors contractions. Measurements were obtained before, during and after lengthening or stretch- ing. Stimulation intensities and time delays between conditioning procedures and stimuli varied consider- ably. H-reflex decreases immediately as static stretching is applied and in proportion to the stretch degree. During dorsiflexions the inhibition is stronger with greater dor- siflexion angular velocity and at lower nerve stimulation intensities, while it is weaker if any concomitant muscle contraction is performed. Within 2 s after a single passive dorsiflexion movement, H-reflex is strongly inhibited, and this effect disappears within 15 s. Dorsiflexions repeated over 1 h and prolonged static stretching training induce long-lasting inhibition. This review highlights that the apparent disagreement between studies is ascribable to small methodological differences. Lengthening move- ments and stretching can strongly influence spinal neural pathways. Results interpretation, however, needs careful consideration of the methodology applied. Keywords: reflex depression; reflex excitability; reflex inhibition. Introduction The effects induced by stretching a human muscle in vivo have been extensively investigated in relation to its influ- ences on different parameters such as range of motion (Decoster et al., 2005; Radford et al., 2006; Konrad and Tilp, 2014b), muscular and athletic performance (Behm and Chaouachi, 2011; Obst et al., 2013) and mechanical and morphological parameters (Kay and Blazevich, 2012; Konrad and Tilp, 2014a; Konrad et al., 2015). Compared to these topics, the neural implications of muscle stretch- ing have been certainly underinvestigated (Guissard and Duchateau, 2006) although these aspects are often called upon when it comes to discussing results related to changes in performance and range of motion (Kay and Blazevich, 2009). Moreover, some stretching techniques commonly used in rehabilitation and sport practice base the rationale of their efficacy upon neuromuscular mech- anisms (Sharman et al., 2006). This assumption is legiti- mate since even a single lengthening movement induces remarkable neural changes as quantifiable by monitoring the spinal reflex excitability (Romano and Schieppati, 1987; Hultborn et al., 1996). Surely, lengthening a muscle induces reflex activation of the Ia afferents, as a con- sequence of this size of the H-reflex evoked within few seconds since such conditioning is expected to decrease because of post-activation depression. However, variations in H-reflex amplitude can be ascribable to many factors, and it would be simplistic to consider only one element. Before any speculation, though, it is essential to ensure the accuracy and reproducibility of the measurements. H-reflex should in fact be elicited either at a percentage of the maximal compound muscle action potential or in co- presence of a small compound muscle action potential to control consistency in stimulation intensity (Zehr, 2002). Assuming that the methodology is technically sound, variations in H-reflex amplitude can be influenced by neurotransmitter availability in Ia afferents and intrinsic properties of the motoneurons. If also these elements are constant, then variations in H-reflex amplitude represent neural adjustments controlled by presynaptic inhibitions and/or excitatory and inhibitory postsynaptic influences (Misiaszek, 2003). *Corresponding author: Francesco Budini, Institute for Sport Science, Graz University, Mozartgasse 14/I, A-8010 Graz, Austria, e-mail: francesco.budini@uni-graz.at Markus Tilp: Institute for Sport Science, Graz University, Mozartgasse 14/I, A-8010 Graz, Austria Unauthenticated Download Date | 4/19/16 1:52 PM