Physiological and Biomechanical Analyses of Rigidity in Parkinson's Disease

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