MECHANICAL AND ELECTROMYOGRAPHIC ANALYSIS OF RECIPROCAL INHIBITION AT THE HUMAN ANKLE JOINT

1. The purpose of the present study is to investigate how reciprocal inhibition influences the mechanical and electromyographic (EMG) properties of the ankle plantar flexors in humans during a voluntary contraction. 2. At different levels of maintained plantar flexion contractions ranging from 0 to 20 Nm, the size of the soleus EMG stretch reflex and the ankle joint stiffness (ratio between the torque increment and the amplitude of the stretch) were measured in response to an imposed dorsiflexion. At matched plantar flexion contraction levels, stretch responses were compared before and after reversible block of the common peroneal nerve (CPN). Stretch responses were also measured during an attempted voluntary fictive dorsiflexion after CPN block. 3. In the preactivated soleus muscles, the phasic EMG response to stretch consisted of two peaks labeled M1 and M2. After CPN block, the M1 short-latency stretch reflex on average increased by 25 +/- 5.7%, mean +/- SD (P < 0.001), and the M2 stretch reflex increased on average by 29 +/- 13.0% (P = 0.002). 4. The total stiffness of the ankle joint during a stretch is the sum of the nonreflex and the reflex mediated stiffness. The total stiffness after CPN block increased on average by 13 +/- 2.7% (P = 0.002) and the estimated reflex stiffness by 33 +/- 6.5% (P < 0.001). 5. When the subjects were asked to make a strong dorsiflexion after CPN block, the soleus stretch reflex was depressed to the extent that the reflex mediated mechanical effect around the ankle joint was abolished. 6. The results demonstrate that transmission in the pathways, which mediates a mechanically important reciprocal inhibition, is facilitated by descending neural control signals. 7. The increases in the EMG and mechanical responses after CPN block were independent of the contraction level, which is consistent with a decreased presynaptic inhibition or a change in recruitment gain of the soleus motoneuron pool.