Fig. 7.

Qualitative explanations of results in terms of threshold position control. A: 3 possible patterns (a–c) of central changes in the threshold muscle length (λ) in response to passive changes in muscle length (x) are shown. Pattern a: λ is constant, i.e., the control system tolerates the muscle stretching without intervening. Pattern b: the system continuously increases λ to make it overrun and remain ahead of x during the entire muscle stretching. Pattern c: in response to the stretch onset, the system triggers a set of discrete ramp-and-hold increases in λ, each time overrunning the muscle stretching. B: dynamics of muscle activation during pattern a. Muscle activation is roughly proportional to the difference between x and λ* (vertical distance between upper and lower curves). Before the stretch onset, the muscle activation is defined by distance e. Although λ remains constant, the dynamic threshold (λ*) decreases with stretch velocity (i.e., deviates down from λ line). After the stretch onset, the shaded area below λ line represents the phasic, velocity-dependent SR component; the shaded area above λ line is the tonic, length-dependent SR component. After the end of stretching, the phasic component vanishes but the tonic component (E) of muscle activation remains and substantially exceeds the initial, prestretch activation level (E >> e). C: during pattern b, the phasic component of the SR is suppressed and the muscle becomes deactivated, thus ceasing active muscle resistance during the entire muscle stretching. During pattern c, the phasic SR component and the ongoing muscle activation are also suppressed, producing an IP. During the holding phase of each λ ramp, the continuing stretching reaches threshold λ* and the muscle is reactivated but generates much less activity (defined by the small shaded areas below curve x) than during pattern a, thus minimizing active resistance to muscle stretching. After learning to generate pattern c, the system can use it, without further learning, to minimize resistance to stretching of preliminary not stretched muscles (generalization of learning).