Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC297
Human tremor size reduces with ischaemia due to decreased EMG to muscle gain
C. Vernooij1, M. Lakie1
1. University of Birmingham, Birmingham, United Kingdom.
In almost every person there is a clear oscillation at ~10 Hz in the limbs under postural conditions. This postural physiological tremor has been investigated and explained in terms of either mechanical influences (e.g. mechanical resonance of the limb), or neural influences (e.g. rhythmicities of central neural firing) (McAuley and Marsden 2000). We recently showed that human physiological tremor has a strongly mechanical basis (Lakie et al, 2010) and largely results from broad band forcing (EMG) of a resonant system. Here we investigate the effect of altering the forcing by making the muscles ischaemic. With ethical permission we recorded tremor of the outstretched splinted middle finger in 12 subjects (11 male) using a 3 axis accelerometer. One slow movement condition and three postural conditions were studied. Ischaemia was produced by an arm cuff inflated to 150 mm Hg for up to 8 minutes. Surface EMG was recorded from the forearm extensor muscles. Records lasted 60 s each and were repeated 4 times over 2 days. Cross spectral gain between rectified EMG and vertical acceleration was subsequently calculated. Acceleration size was greatly reduced by ischaemia. EMG was if anything slightly increased. Accordingly gain was greatly reduced (Fig 1). Ischaemia is known to substantially reduce tremor size (Christakos et al, 2006). This reduction has been attributed to the effect of ischaemia on muscle spindles (Lippold, 1970) or to its effect on muscle contractile properties (Lakie et al, 2004). The reduction in gain that we see in the present experiments strongly supports the latter explanation.
Where applicable, experiments conform with Society ethical requirements