**1. Introduction**

484 Etiology and Pathophysiology of Parkinson's Disease

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Parkinson's disease is one of the most common movement disorders characterized by bradykinesia, rigidity, resting tremor and postural instability (Fahn, 2003). It affects nearly five million elderly people worldwide (de Lau & Breteler, 2006). As the population ages, the incidence and prevalence of Parkinson's disease are expected to increase dramatically (Dorsey et al., 2007; Tanner & Goldman, 1996; Tanner & Ben-Shlomo, 1999). Rigidity is one of the clinical hallmark symptoms that characterize and define Parkinson's disease. Rigidity is one form of the increased muscle tone, which is defined as a resistance to a passive movement. Rigidity is clinically characterized by an increase in muscle tone, and is felt as a constant and uniform resistance to the passive movement of a limb persisting throughout its range (Bantam, 2000; Fung & Thompson, 2002; Hallett, 2003). There are two types of rigidity: plastic or lead-pipe rigidity, in which resistance remains uniform, constant and smooth, such as experienced when bending a piece of lead; and cogwheel rigidity, in which tremor is superimposed on increased tone, giving rise to the perception of intermittent fluctuation in muscle tone. The latter is principally attributable to the combination of plastic rigidity and tremor.

In addition to being a key element of parkinsonian rigidity, increased muscle tone also characterizes spasticity which is a common motor symptom in a few other neurological disorders, such as multiple sclerosis, stroke and cerebral palsy. Spasticity is clinically described as an increased resistance to passive movement due to hyperexcitability of stretch reflex (Lance, 1980; Rymer & Katz, 1994). Rigidity and spasticity share the characteristic feature of the increased muscle tone to a passive movement. However, the unique lead-pipe resistance can distinguish the increased muscle tone in rigidity from that associated with spasticity. In particular, the differentiation between rigidity and spasticity is not straightforward in a clinical scenario (Fung & Thompson, 2002).

Rigidity generally responds well to dopaminergic medication and surgical intervention. Thus, it is used as a diagnostic criterion and to evaluate the efficacy of therapeutic interventions (Prochazka et al., 1997). Clinical examination and assessment of rigidity is determined by an examiner's perception of resistance while rotating the limb at major joints, based upon the Unified Parkinson Disease Rating Scale (Fahn & Elton, 1987; Goetz et al., 2008). A better understanding of the physiological and biomechanical characteristics of rigidity merits scientific significance and clinical implication. In this chapter, studies on

Physiological and Biomechanical Analyses of Rigidity in Parkinson's Disease 487

they cannot account for the constancy and uniformity of resistance which is uniquely associated with rigidity. Recent studies have shed light on the underlying mechanism of the uniform nature of parkinsonian rigidity (Xia & Rymer, 2004; Xia et al., 2011). Evidence indicates that shortening reaction and stretch-induced inhibition play pivotal roles in the

Besides abnormal muscle responses to stretch, anomalous reactions in the shortened muscles during a passive joint motion have also been described in Parkinson's disease. More than a century ago, Westphal (1877, 1880) observed muscular contraction in the passively shortened skeletal muscles. Before he observed this phenomenon, he had already studied the muscular contraction in lengthening or stretched muscles. Thus, he named it 'paradoxer Muskel-contraction'. At that time, he also described enhanced activation of tibialis anterior corresponding to the shortening phase in patients who had great difficulty in passively aiding imposed movement. The phenomenon he observed is often referred to as "Westphal's phenomenon". Later, Sherrington (1909) described analogous findings in both the spinal dog and the decerebrate cat under the name "shortening reaction'. This term has

Application of electromyographic (EMG) recording method has demonstrated that inappropriate activation of shortened muscles occurs widely in basal ganglia disorders (Rondot & Metral, 1973), and most prominently in Parkinson's disease (Andrews et al., 1972; Angel, 1983; Berardelli & Hallett, 1984; Rondot & Metral, 1973; Xia & Rymer, 2004). An example of shortening reaction is illustrated in Fig. 1B (Xia et al., 2011). During the passive wrist flexion movement, flexor muscles were progressively shortened. There occurred strong muscle activations in the wrist flexor muscles. Shortening reaction has been reported to be manifested in both upper and lower limb muscles. Some investigators suggested that shortening reaction plays an important role in the pathophysiology of rigidity (Angel, 1983; Rondot & Metral, 1973), given that it represents a reflex action that agonistically assists with a passive movement in contrast to antagonistic opposition of a motion caused by stretch reflex. Correlational analysis showed that there was no direct relationship between shortening reaction and changes in muscle tone (Berardelli &

Nevertheless, the neural mechanism of the shortening reaction was virtually unknown. Since shortening reaction was first reported a century ago, very little attention has been paid to exploring its underlying physiology. Shortening reaction is the opposite of stretch reflex, a topic that has been extensively studied for a long period of time. In contrast, only a limited number of studies were conducted to understand and characterize shortening reaction, and these previous studies simply monitored muscle electromyographic activity (Andrews et al. 1972; Angel, 1983; Berardelli et al. 1983; Berardelli and Hallett, 1984; Rondot and Metral, 1973). The importance of shortening reaction in the pathophysiology of parkinsonian rigidity is undoubtedly underestimated. Utilization of both EMG recording and joint torque measure has provided us with useful information to reveal the role of shortening reaction in mediating rigidity in Parkinson's disease (Xia & Rymer, 2004; Xia et al., 2011). A parallel mechanism responsible for mediating parkinsonian rigidity is stretch-induced inhibition

genesis of lead-pipe characteristics of rigidity.

been used since then.

Hallett, 1984).

which will be discussed in the next section.

**2.2 Responses to passive shortening – Shortening reaction** 

elucidation of the physiological mechanisms and biomechanical quantification of parkinsonian rigidity will be reviewed and the latest research on this topic will be presented.
