Peripheral effects on the amplitude of monopolar and bipolar H-reflex potentials from the soleus muscle.

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.

Is the Mmax/Hmax ratio a reliable index of the monosynaptic excitability in man? Recruitment curves for H- and M-responses using monopolar and bipolar recording techniques.

The amplitude changes of M- and H-potentials, recorded monopolarly and bipolarly from different leading-off areas on the surface of m. triceps surae upon varying the stimulus intensity from threshold to supramaximal values, in a state of rest and fixed muscle length, are investigated. It is shown that: a) the Mmax/Hmax amplitude ratio cannot serve as a reliable index for a quantitative evaluation of the monosynaptic reflex excitability of the motoneurone pool and b) recording along the medial line of m. soleus in the region from 1.5 to 8.0 cm under the distal insertion of the two gastrocnemii into the Achilles tendon in a state of rest and submaximal values of the stimulus intensity, both mono- and bipolar recording techniques can equivalently be used in studying the changes in the monosynaptic reflex excitability.