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Chapter 2 – Literature Review
Adding a load to a limb dampens the mechanical resonance of the limb
and alters the mechanical reflex tremor frequency. With this in mind
Heroux, Pari and Norman (2009) examined the effect of an unsupported,
light to moderate load on wrist tremor in ET. The authors found that
applying a load equal to 15% of a maximum load, to the hand, resulted
in significant decreases in tremor amplitude. This loading also resulted in
postural tremor spectral power frequency separation of the mechanical
and central elements. From this the authors suggested that peripheral
factors may contribute to the amplitude of the tremor via interaction
with the central oscillations. Interestingly Heroux, et al. (2009) found an
increase in wrist EMG activity without a corresponding increase in EMG
tremor spectral power. As a result the authors raised the possibility that
a component of ET may relate to entrainment of motor units. Further
evidence of the potential role of the stretch reflex was found when the
authors compared results from 21 participants that had been part of
both, the unsupported loading Heroux, et al. (2009) study and the
supported loading Heroux, Pari and Norman (2010) study. By using the
method of supported loading, mechanical reflex influences were
reduced (M. E. Héroux, et al., 2010). When the two results were
compared the authors noted that tremor amplitude was lower in the
supported loading method. They suggested that this difference may
further highlight the contribution of the stretch reflex to tremor (M. E.
Héroux, et al., 2010).
Motor unit entrainment may occur at the centrally driven 8-12Hz
frequency seen in ET and can share the same oscillation frequency of the
wrist (Elble, 2003). Interestingly ET severity may relate to motor unit
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