Variations in the amplitude of mono- and bipolarly measured H-reflex potentials can be influenced by muscle architecture and changes in muscle length. In the passive soleus muscle with the ankle joint fixed at 90 degree, the maximal-amplitude bipolar H-potentials were obtained along the midline of soleus at a distance of 2.0-4.0 cm below the insertion of the gastrocnemii on the Achilles tendon. In contrast, the optimal location of monopolar H-potentials was 5.0-8.0 cm below the gastrocnemii insertion. Stepwise passive shortening of soleus resulted in an increase in the amplitude of both H- and motor-unit potentials. This correspondence implicates peripheral factors, such as changes in muscle fibre diameter and inclination to the skin surface, as mechanisms mediating the changes in the amplitude of the potentials. Such effects necessitate caution in interpretation of the association between H-potential amplitude and monosynaptic reflex excitability.

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