1. During maintained ankle extension with background torques ranging from 0 to 70 N.m, the ankle extensors were stretched by a 5-degrees rotation of the ankle joint. Maximal stretch velocity was 170-degrees/s. Regression analysis of simultaneous measurements of total torque and needle and surface electromyograms (EMG) from the soleus and gastrocnemius muscles showed that the soleus muscle generates about two-thirds of the maximal torque (approximately 120 Nm) with the subjects in sitting position. In addition, it was found that there is considerable cross talk between the soleus and gastrocnemius muscles when EMGs are recorded by surface electrodes. 2. The soleus EMG response to stretch began with a "phasic" response (latency 41 +/- 4 ms, mean +/- SD), consisting of two peaks, labeled M1 and M2. The phasic response ended 120-140 ms after stretch onset and was followed by a period of reduced EMG activity, ending at 170-210 ms. After this "silent period," a smaller "tonic" response was seen. The phasic responses of the soleus muscle were much larger than the corresponding responses in the anterior tibial muscle. In contrast, the tonic responses were comparable in the soleus and anterior tibial muscles. 3. The amplitudes of the phasic M1 and M2 responses were independent of the level of the background contraction. This disagrees with the "automatic gain principle," according to which the amplitudes of M1 and M2 should increase proportionally with the background EMG. In contrast to the phasic responses, the amplitude of the tonic EMG response, measured 200-400 ms after stretch onset, followed the automatic gain principle. 4. Nonreflex stiffness was measured during electrical stimulation of the soleus nerve. In the relaxed extensor muscles, approximately two-thirds of a total stiffness of 1.4 N.m/deg arose from the reflex-mediated stiffness when measured 200 ms after stretch onset. When measured 400 ms after stretch onset, approximately one-third of a total stiffness of 1.2 N.m/degree arose from the reflex-mediated stiffness. At a background torque of 70 N.m, the reflex-mediated stiffness only contributed with approximately 10% of the total stiffness. The total stiffness at this contraction level was 5.8 and 5.2 N.m/degree 200 and 400 ms after stretch onset. 5. We conclude that the total stiffness is sufficient to play an important role, both during active movement and during postural tasks, and that the ankle extensors have a larger reflex-mediated stiffness than the ankle flexors at low contraction levels, but interestingly a less important reflex component at intermediate levels of contraction. 6. Reflex-mediated torque increments were predicted from the phasic and tonic EMG responses. The reflex-mediated torque increments predicted from the tonic EMG response agreed with the measured torque increments, whereas those predicted from the phasic EMG responses were approximately 3 times larger than the measured torque increments. We suggest that the discrepancy arises from the high degree of synchronization of the phasic EMG response, which is characteristic for the ankle extensors, but not for the flexors.
