This study employed longitudinal measures of evoked spinal reflex responses (Hoffman [H] reflex, V wave) to investigate changes in the activation of muscle, and determine if there are "linked" neural adaptations in the motor pathway following isometric resistance training. Twenty healthy, sedentary males were randomly assigned to either the trained (n = 10) or control group (n = 10). The training protocol consisted of 12 sessions of isometric resistance training of the plantar flexor muscles over a 4 week period. All subjects were tested prior to, and after the 4 week period. To estimate changes in spinal excitability, soleus H reflex and M wave recruitment curves were produced at rest, and during submaximal contraction. Recruitment curves were analyzed using the slope method (Hslp/Mslp). Modulation of efferent neural drive was assessed through evoked V wave responses (V/Mmax) at 50, 75, and 100% MVC. After 4 weeks, maximal voluntary contraction (MVC) torque increased 20.0+/-13.9% (mean+/-SD) in the trained group. The increase in MVC was accompanied by significant increases in the rate of torque development (42.5+/-13.3%), the soleus surface electromyogram (EMGRMS, 60.7+/-30.8%), voluntary activation (2.8+/-0.1%), and the rate of activation (48.7+/-24.3%). Hslp/Mslp was not altered by training, however V/Mmax increased 57.3+/-34.2% during MVC. These results suggest increases in MVC observed in the first few weeks of isometric resistance training can be accounted for by an increase in the rate of activation at the onset of muscle contraction. Augmentation of muscle activation may be due to increased volitional drive from supraspinal centers. Key words: H reflex, V wave, soleus.
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