Changes in H - reflex and V - wave following short - term 3 endurance and strength training 4 5

30 This study examined the effects of 3 weeks of either endurance or strength training on plasticity of 31 the neural mechanisms involved in the soleus H-reflex and V-wave. Twenty-five sedentary healthy 32 subjects were randomized into an endurance group (n = 13) or strength group (n = 12). Evoked V33 wave, H-reflex and M-wave recruitment curves, maximal voluntary contraction (MVC) and time34 to-task-failure (isometric contraction at 40% MVC) of the plantar flexors were recording before and 35 after training. Following strength training, MVC of the plantar flexors increased by 14.4 ± 5.2% in 36 the strength group (P < 0.001), whereas time-to-task-failure was prolonged in the endurance group 37 (22.7 ± 17.1%; P < 0.05). The V/Mmax ratio increased significantly (55.1 ± 28.3 %; P < 0.001) 38 following strength training but the Hmax/Mmax ratio remained unchanged. Conversely, in the 39 endurance group the V/Mmax ratio was not altered whereas Hmax/Mmax ratio increased by 30.8 ± 40 21.7% (P < 0.05). The endurance training group also displayed a reduction in the H-reflex 41 excitability threshold while the H-reflex amplitude on the ascending limb of the recruitment curve 42 increased. Strength training only elicited a significant decrease in H-reflex excitability threshold 43 while H-reflex amplitudes over the ascending limb remained unchanged. These observations 44 indicate that the H-reflex pathway is strongly involved in the enhanced endurance resistance which 45 occurs following endurance training. On the contrary, the improvements in MVC following strength 46 training are likely attributed to increased descending drive and/or modulation in afferents other than 47 Ia afferents. 48