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Chapter 2 – Literature Review
entrainment. Heroux, et al. (2010) investigated the effect of muscle
contraction under progressive loads (5%, 10%, 20% and 30% of a
maximum load) measuring EMG activity and tremor variables. The
authors found EMG activity and force fluctuations increased with
increasing loads however tremor spectral power did not, supporting
their earlier findings. In this study the authors had used two sub-groups
of ET based on tremor severity. This grouping allowed them to find that
greater force fluctuations occurred in the more severe ET subgroup. The
authors postulated that this may be because of an association between
greater motor unit entrainment and a greater tremor severity.
Altered co-contraction of the antagonist muscle has been proposed to
possibly have a weak role in ET. Lundervold, et al. (2004a) found a small
correlation between agonist and antagonist co-contraction in one of
three participants in a bio-behavioural intervention study. Interestingly
increased levels of antagonist co-contraction are seen with aging, with
greater levels of co-contraction seen in healthy older adults compared to
younger adults (Laursen, Jensen, & Ratkevicius, 2001; Tracy & Enoka,
2002). As physiological tremor increases with age it is conceivable that
this co-contraction may have a role in the peripheral contribution to
tremor (Morrison, Mills, & Barrett, 2006).
Pathophysiology - Neurodegeneration
Post-mortem research on donated ET brains is providing some insight
into possible degeneration that may occur in ET patients. In a recent
review, Louis (2010b) discussed recent post-mortem investigations of ET
brains and the differences that were observed in the cerebellum. In
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