The purpose of this research was to evaluate the effects of a long-term static stretching program on flexibility and spinal reflexes in the human soleus muscle. Day-to-day reliability over two days of H-reflex gain (Hslp/Mslp) and pre-synaptic inhibition of spinal reflexes in the human soleus was measured in an initial study. Thirty subjects (age=23.4 ± 3.9 yr, height=175.64 ± 10.87 cm, weight=84.5 ± 24.18 kg) with no history of lower leg pathology and/or injury within the previous 12 months volunteered. The slopes of the ascending portions of the H-reflex and M-wave recruitment curves were used to evaluate H-reflex gain (Hslp/Mslp). The mean soleus H-reflex from 10 conditioning stimulations and 10 unconditioned stimulations was used to calculate the amount of pre-synaptic inhibition. Measurements of H-reflex gain and pre-synaptic inhibition yielded test-retest reliability of R=.95 and R=.91, respectively. The contribution of pre-synaptic and post-synaptic reciprocal mechanisms in flexibility adaptations has not been measured during the same study. A long-term static stretching program (5 times per week for 6 weeks) of the soleus implemented within an experimental group of 20 subjects was used as an intervention to measure both spinal control flexibility changes. Additionally, 20 control subjects were measured that did not participate in the stretching program. Passive ankle dorsiflexion, Hmax/Mmax, H-reflex gain (Hslp/Mslp), pre-synaptic and post-synaptic reciprocal mechanisms were measured at baseline, 3 weeks, and 6 weeks for all 40 subjects. A 2 (Group) X 2 (Sex) X 3 (Time) mixed MANOVA with Tukey HSD with (a<0.05) was used. Only ROM had a significant interaction between Group and Time, whereas, a significant difference was not detected in the other dependent variables. The experimental group demonstrated significantly improved dorsiflexion ROM from baseline to 3 weeks (mean=6.2 ± .88, P<0.001), 3 weeks to 6 weeks (mean=4.9 ± .84, P<0.001), and baseline to 6 weeks (mean=11.2 ± .91, P<0.001). Consequently, the increase in flexibility by 42% after 30 stretching sessions was not the result of spinal reflex changes. / Graduation date: 2006