Bilateral deficit in plantar flexion: relation to knee joint position, muscle activation, and reflex excitability.

Six male subjects made maximal isometric plantar flexions unilaterally (UL) and bilaterally (BL), with the knee joint angle positioned at 90 degrees and 0 degrees (full extension) and the ankle joint kept at 90 degrees. Plantar flexion torque and electromyogram (EMG) of the lateral gastrocnemius (LG) and the soleus (Sol) muscles were recorded. There was a deficit in torque in BL compared to UL (P<0.05), and the deficit was greater when the knee was extended than when bent to 90 degrees (13.9% vs 6.6%). The integrated EMG (iEMG) of UL and BL did not differ when the knee was at 90 degrees. On the other hand, when the knee was extended iEMG of LG was smaller for BL than for UL, suggesting that the larger bilateral deficit when the knee was extended was due to a reduced activity of the LG motor units. In addition, the H-reflex recorded from Sol when the contralateral leg was performing a maximal unilateral plantarflexion was reduced. This would indicate that the force deficit was associated with a reduction of motoneuron excitability.