## 3 Experimental Results
### Muscle Activation  
SOL and GAS contract to generate positive ankle joint torque. The activation of SOL and GAS last for about 1.5 s. TA becomes inactive (around 0.005, as shown in Figure 5f) when SOL and GAS are activated. TA is re-activated when activations of SOL and GAS are below 2 to 3%. This phenomenon is the so-called reciprocal inhibition. The stretch of the extensor muscles inhibits the activity of flexor motor neurons, and vice versa. When TA is re-activated, the ankle joint angle is still located within the lean forward range (above 0.02 rad from 2 to 3 s). We assume that the behavior of this part is controlled by the high-level nervous system interference.  
This alternate-active phenomenon appears in all five subjects. We separate the push-recovery process into two parts: the calf control part, in which SOL and GAS are active, and the TA control part, in which TA is active. The activation ratios between SOL and GAS of the five subjects in calf control  
Figure 5: Experimental results for subject A. Each color represents one trial. There are six trials. The x-axis is the time in ms. (**a**) shows the measurement of ankle joint angle. (**b**) represents the ankle joint angular velocity. (**c**) shows the ankle joint torque. (**d**) shows the muscle activation level of SOL. (**e**) shows the muscle activation level of GAS. (**f**) shows the muscle activation level of TA.  
part are shown in Figure 6. For subject A, the ratio increases. For subjects B and C, the ratio varies around a constant level. For subject D, the ratio is like a 'valley'. For subject E, the ratio decreases. Given that SOL is the uniarticular muscle and GAS is the biarticular muscle, it seems that different movement manners are adopted among subjects. However, the ratios are greater than 1, which means that the muscle activation level of SOL is larger than that of GAS. The maximum muscle force of SOL is larger than GAS. SOL contributes more to torque production during this period. The purpose of the calf control part seems to be to stop the leaning forward trend in the trunk quickly and to produce a gross pull-back torque. As the body leans forward, the calf muscles are likely to be stretched. The reflex control model described in Equation (3) is applied in this part to predict the calf muscle activations.  
In the TA control parts, the activation level of TA varies from 2 to 10%. The calf muscles and TA are co-contracted. Co-contraction of antagonistic muscles can increase joint stiffness while keeping the same torque level [43]. Thus, joint stability increases. It is helpful to increase joint stiffness to prevent CoM moving to the \"joint-heel\" side. As the supporting area of the human foot from the ankle joint to the heel is smaller than that from the ankle joint to the toe, there is less room left in the \"joint-heel\" part to adjust CoP (or ankle joint torque) to counteract gravity while maintaining ankle strategy. No significant ankle joint angle overshoot (below 0.02 rad) can be observed in the experiment (as shown in Figure 5a). The purpose of the TA control part seems to be to smoothly adjust body movement to stop at an upright posture, preventing too much overshoot in the opposite direction.