# Changes in Spinal-Reflex Excitability during Static Stretch and/or Explosive Contraction
## 1 Introduction  
Performance enhancement after warm-up activity could be explained by thermal and non-thermal effects. While the term \"warm-up\" is derived from the thermal effects due to any given activity (e.g., increased core body and local muscle temperatures), \"pre-conditioning\" [1], the working of muscles by performing sports movements (e.g., explosive movements), is considered to contribute the non-thermal effects of warming-up [2]. Assuming the thermal effects are similar, the level of performance among different types of warm-up would be affected by the ability to utilize elastic energy [3]. Elastic energy is associated with muscle spindle activity [4], mytotic reflex [5], and elasticity of contractile components [6]. Measures of elastic energy are difficult because they are derived from various structures (e.g., actin, myosin, sarcolemma, tendon, etc.) that are instantiated through different neural pathways (e.g., \(\alpha\) and \(\gamma\) motoneuron). Spinal-reflex excitability, Hoffmann (H)-reflex [7] normalized by the motor (M)-response (H:M ratio), is an autonomic homonymous response to a given peripheral (especially Ia afferent) stimulus [8]. Exogenous electrical stimuli should directly evoke afferents; the H-reflex bypasses the signals of muscle spindles and \(\gamma\)-motoneurons [9]. It has been suggested that the magnitude of the stretch-reflex is related to the amount of stored elastic energy [10, 11]. The amplitude of this value is indicative of synaptic transmission [2], spindinhibition [12], and an estimate of \(\alpha\)-motoneuron activity [13]. Additionally, the assessment of the H-reflex and M-response latency time are also considered to be parameters affecting synaptic transmission along with performance change [14]. Therefore, the non-thermal effects of warming-up in terms of enhancing elastic potential energy could be assessed by spinal-reflex excitability and the latency times of the H-reflex and the M-response. Previous studies concerning the acute change in the soleus H-reflex reported a reduction after static stretch [15; 16] and an increase after muscle contractions [17; 18]. Although the existing data inform us how the H-reflex responds to stretch or contraction, several limitations still need to be addressed. First, spinal-reflex excitability in the quadriceps has not been examined even though it is the primary muscle for functional movements [19; 20]. Second, subjects in previous reports examined isometric contractions [17; 18], which are not commonly performed during warming-up or training. Third, the combined effect of stretch and contraction is unknown. For example, reduced spinal-reflex excitability after performing static stretch might be offset by performing muscle contractions such as maximal vertical jumps. This indicates that static stretch does not produce performance hinderance if vertical jumps are followed. Knowing the direction and magnitude of change in spindally mediated muscle activation of quadriceps responding to static stretch and/or explosive contraction would help coaches and athletes to plan and execute pre-exercise activities. While the general hesitance of static stretch to hamper muscle power in the field of exercise science still exists, comparing jump heights before and after static stretch would also provide a comprehensive understanding of the relationships among static stretch, quadriceps spinal-reflex excitability, and athletic performance. A comparison of the individual and combined effects of static stretch (e.g., hip and knee extensors) and explosive contraction (e.g., maximal vertical jump) could address the limitations above, and thus provide information on the relationship between spinal-reflex excitability and athletic performance as well as the interaction effect between muscle stretch and contraction.