This chapter discusses the various forms of tremor and their characteristics. Section 2.1 defines the conditions under which a tremor is present. Section 2.2 describes the possible origins of tremor. Finally, a small overview on the characteristics of pathological tremors is provided in Section 2.3.
Tremors can be categorized by the conditions under which tremor is activated (Rubchinsky, Kuznetsov, Wheelock, & Sigvardt, 2007) (Smaga, 2003) (Deuschl, Bain, & Brin, 1998). The first category is rest tremors, which are active when there is no voluntary muscle activation. The amplitude of rest tremor reduces with activity. The second category is postural tremors, which are active when a body part is held in a position where muscle activity is required. The third category is kinetic tremors. This category of tremors includes simple kinetic tremor and intention tremor. Simple kinetic tremor occurs when a limb is moved voluntarily. Intention tremor is active when a body part is moved to a specific target and the amplitude of intention tremor increases as the target gets closer. A tremor can have more than one condition under which it is active.
2.2 Tremor origins
Several tremor origins are possible (Grimaldi & Manto, Tremor: From Pathogenesis to Treatment, 2008) (Ahmed & Sweeney, 2002) (McAuley & Marsden, 2000). These origins are mechanical resonances, feedback (or reflex) oscillators and central oscillators. These three origins will be described in more detail.
2.2.1 Mechanical properties and resonances
The mechanical properties of a limb segment determine its dynamics. These mechanical properties cannot initiate a tremor, but they determine the motions resulting from input forces. A limb segment of the human body has viscous and elastic properties, allowing the limb segment to be modeled as a mass-spring-damper system (Grimaldi & Manto, Tremor: From Pathogenesis to Treatment, 2008). Figure 1 shows an example of the response of a mass-spring-damper system to an applied vibration. Figure 1 illustrates that a system has a tendency to oscillate with larger amplitude at some frequencies than others. The largest amplitude is found at the natural frequency, which is determined by the mass and the stiffness of the system. A higher stiffness increases the natural frequency and a higher mass reduces the natural frequency. The amplitude at the natural frequency is affected by the damping ratio, which is determined by the mass, damping and stiffness of the system.
Figure 1 Example of damped forced vibration.M is the amplitude ratio bewteen input and output, Ï‰ is the frequency of the driving motion, Ï‰n is the natural frequency and Î¶ is the damping ratio. (Meriam & Kraige, Engineering mechanics)
As a result of the dynamics of a limb segment, tremor generators at frequencies beyond the natural frequency of the limb segment needs far more power to result in notable tremor. Some natural frequencies of limb segments are: 25-27 Hz for a finger, 9 Hz for a wrist and 2 Hz for an elbow (McAuley & Marsden, 2000).
A limb segment can resonate with muscle activation as well as with vibrations originating from connected limb segments. The term mechanical resonance is used in this report to indicate that the source of the oscillation lies in another limb segment.
Mechanical resonance by itself does not involve muscle activation and thus does not result in EMG activity. However, when a limb is oscillating due to resonances, the muscles are passively stretched, which can result in reflexive activity.