**2.3 Lengthening reaction or stretch-induced inhibition**

488 Etiology and Pathophysiology of Parkinson's Disease

Fig. 1. Kinematic and EMG recordings during passive flexion movement obtained from a patient after an overnight withdrawal of medication (Off-medication). **A.** Wrist joint position during the passive flexion movement; the subject's wrist joint was externally rotated from 30 degree to -30 degree at 50°/s. **B.** Shortening reaction was recorded in shortened flexors in the Off-medication state in a parkinsonian subject. There was an increased EMG activation in passively shortened muscles. **C.** Stretch-induced inhibition was observed in the stretched extensor muscles during the same movement. There was an EMG reduction, when the stretch exceeded the neutral position and the muscle length was

elongated [from Xia et al. (2011) with permission].

In addition to shortening reaction, Sherrington (1909) also observed in the above-noted animal preparations that "*… when an examiner bent the knee against the knee-extensor's contraction, the examiner felt the opposition offered by the extensor gave away almost abruptly at a certain pressure; the knee could then be flexed without opposition ...*". He named this phenomenon "lengthening reaction". Lengthening reaction was demonstrated in both spinal dog and in decerebrate rigidity of the cat, yet the reaction was recognized to have differential features in the two preparations. In his monograph, he also pointed out that muscles, exhibiting shortening reaction and lengthening reaction, were all extensor muscles. The distinction of the flexor and extensor muscle groups has been documented in parkinsonian rigidity (Mera et al., 2009; Xia et al., 2006).

The well-know clasp-knife phenomenon associated with human spasticity, appears to be the equivalent of the lengthening reaction (Burke et al., 1970, 1971). The clasp-knife reflex is characterized by an abrupt decline in muscle force that occurs when a spastic limb is moved beyond a certain joint angle. There is a common ground between lengthening reaction in animal preparations (Burke et al., 1972b; Rymer et al., 1979) and clasp-knife reflex in human spasticity in that the essential feature of both phenomena is the sudden release of the resistance due to continuous stretch of the elongated muscle, hence also referred to as "stretch-induced inhibition" (Rymer et al., 1979). The physiological framework previously established or explored in the context of the lengthening reaction or stretch-induced inhibition has recently been investigated in parkinsonian rigidity (Xia & Rymer, 2004; Xia et al., 2011).

Fig. 1C illustrates a stretch-induced inhibition recorded from a parkinsonian patient in the Off-medication state. During the passive flexion movement (Fig. 1A), there was a large initial stretch reflex in the wrist extensor muscles. The initial stretch reflex was followed by a period of sustained activity and curtailed by an evident decline when the progressive movement approached at almost the neutral position and the muscle length of the extensors was elongated, demonstrating the stretch-induced inhibition (Fig. 1C). It is noted that both shortening reaction and stretch-induced inhibition occur during the same movement phase (Fig. 1). The importance and functional role stretch-induced inhibition and the above described shortening reaction may have played in parkinsonian rigidity will be explained and discussed in Section 3.
